Synthetic and Degradative Pathways
Catecholamines are molecules that contain a catechol nucleus with an ethylamine group attached at the 1 position 6–2. 5HT is an indolamine with a hydroxy group at the 5 position and a terminal amine group on the carbon chain 6–3. The catecholamine neurotransmitters, NE, DA, and E, are sequential products of a single biosynthetic pathway that originates with the amino acid tyrosine 6–4. 5HT and melatonin are synthesized from the amino acid tryptophan 6–5. The catecholamine neurotransmitters and serotonin are discussed together because they have similar widely projecting anatomic organizations, interact with each other functionally, have related mechanisms of clearance from synapses and some shared pathways of metabolism, and are jointly targeted by several important classes of drugs, including psychostimulants (DA, NE, and 5HT), tricyclic antidepressants (TCAs; NE and 5HT), serotonin and NE reuptake inhibitor antidepressants (NE and/or 5HT), and monoamine oxidase inhibitor (MAOI) antidepressants (DA, NE, and 5HT).
Chemical structure of catecholamines. An ethylamine group is attached to a catechol nucleus at the 1 position.
Chemical structure of indolamines. Serotonin is an indolamine where R1 and R2 are hydrogen atoms, and a hydroxy group is substituted at the 5 position.
Biosynthetic pathway of catecholamines. Dopamine, norepinephrine, and epinephrine are derived from the multistep processing of tyrosine, a dietary amino acid that is actively transported across the blood–brain barrier and concentrated in catecholaminergic neurons. Region-specific expression of the enzymes shown here determines which neurotransmitters are expressed in a given cell; for example, both dopaminergic and noradrenergic cells express tyrosine hydroxylase (TH) and L-amino acid decarboxylase (AADC), but only noradrenergic cells express dopamine-β-hydroxylase (DBH). The principal metabolites of dopamine and norepinephrine are HVA, VMA, and MHPG. MAO, monoamine oxidase; COMT, catechol-O-methyl transferase; HVA, homovanillic acid; VMA, 3-methoxy-4-hydroxy-mandelic acid (also known as vanillylmandelic acid); MHPG, 3-methoxy-4-hydroxy-phenylglycol.
Biosynthetic pathway of serotonin and melatonin. Both serotonin, also known as 5-hydroxytryptamine or 5HT, and melatonin are derived from the multistep processing of the dietary amino acid tryptophan. Serotonin is synthesized neuronally in various brainstem nuclei and is converted by monoamine oxidase and aldehyde dehydrogenase into its primary metabolite, 5-HIAA. Serotonin is also produced in cells of the pineal gland, which contain two enzymes—5HT N-acetylase and 5-hydroxyindole-O-methyltransferase—not expressed in serotonergic cells. These enzymes rapidly convert serotonin to melatonin.
Catecholamine biosynthesis begins with dietary tyrosine, which is actively transported into the brain (or peripheral sympathetic neurons; Chapter 9). It is hydroxylated within neurons at the 3 position by the enzyme tyrosine hydroxylase (TH) to form dihydroxyphenylalanine (dopa; known as levodopa orL-dopa). TH requires Fe2+ as a cofactor, as well as molecular oxygen and tetrahydrobiopterin (a hydrogen donor). An inhibitor of TH, α-methylparatyrosine (AMPT), has been used historically as an experimental tool to study catecholamine function; more recent alternatives include mice genetically engineered to lack TH or other enzymes in the biosynthetic pathway.
In dopaminergic neurons, one additional enzyme in this pathway is expressed, L-aromatic amino acid decarboxylase (AADC), which converts dopa to DA. AADC is a cytoplasmic enzyme that requires pyridoxal phosphate, a cofactor derived from vitamin B6. AADC was originally known as dopa decarboxylase until it was recognized that it decarboxylates other substrates, including 5-hydroxytryptophan, the precursor of serotonin 6–5.
In noradrenergic neurons, an additional enzyme, dopamine-β-hydroxylase (DBH), is expressed that catalyzes the conversion of DA to NE. DBH requires Cu2+ and ascorbic acid (vitamin C) as cofactors. DBH is associated with synaptic vesicles that store NE. In the adrenal medulla and in brainstem neurons that produce E, an additional enzyme, phenylethanolamine-N-methyltransferase (PNMT), is expressed that converts NE to E. S-Adenosyl-L-methionine (SAM), a methyl donor, is a required cofactor for this step.
TH, the rate-limiting enzyme in catecholamine synthesis, is regulated by multiple mechanisms. Increased catecholamine release leads to increased TH activity that results from regulation at the transcriptional, translational, and posttranslational levels. Rapid activation of TH activity occurs via its phosphorylation at four serine residues in the N terminus of the protein by several protein kinases, including protein kinase A, Ca2+/calmodulin-dependent protein kinase II (CaM-kinase II), and protein kinase C. It is believed that such phosphorylation induces a conformational change in the protein that results in a higher affinity for its tetrahydrobiopterin cofactor and a lower affinity for catecholamines that trigger end-product inhibition of TH. The end result is an increase in the TH catalytic activity. Longer-term changes in TH activity can occur through transcriptional regulation of the TH gene by extracellular stimuli. Stimuli that upregulate TH expression include chronic environmental stress and drugs such as caffeine, nicotine, and morphine; drugs that downregulate TH expression include many antidepressants (which initially increase synaptic concentrations of NE and/or 5HT).
The ability of tyrosine to penetrate the blood–brain barrier depends on an active transport process. With normal dietary consumption of tyrosine, both active transport and TH activity are fully saturated. Thus, the administration of supplemental tyrosine does not produce significant increases in catecholamine synthesis in the central nervous system (CNS). However, increased catecholamine synthesis can be achieved by peripheral administration of L-dopa, which bypasses this rate-limiting enzymatic step and penetrates the blood–brain barrier, so long as its peripheral metabolism is blocked. For this reason, L-dopa is used in the treatment of Parkinson disease (Chapter 18).
5HT is synthesized from the amino acid tryptophan 6–5, which is actively transported across the blood–brain barrier and hydroxylated by tryptophan hydroxylase (TPH) to produce 5HT. This product is then decarboxylated to form 5HT by AADC, the same enzyme involved in the biosynthesis of catecholamines. In the pineal gland, additional enzymatic steps convert 5HT to melatonin, which is discussed further in Chapter 13.
TPH is the rate-limiting enzyme for 5HT biosynthesis. There are two closely related genes: TPH1, predominantly expressed in the periphery, and TPH2, expressed preferentially in the brain. TPH is subject to short-term and long-term regulatory processes similar to those described for TH, a related amino acid hydroxylase. Like TH, TPH requires molecular oxygen and tetrahydrobiopterin as cofactors, and can be activated by protein kinase A and CaM-kinase II. Although genetic variations in TPH2 have been linked weakly with depression, such findings are not significant in large genome-wide association studies.
An additional member of the amino acid hydroxylase family is phenylalanine hydroxylase, which converts phenylalanine to tyrosine. Mutations of this enzyme that decrease its catalytic activity result in phenylalaninemias (eg, phenylketonuria) that, among other ill effects, can damage the developing brain and cause generalized intellectual disability. Interference with the metabolism of phenylalanine causes the buildup of oxidized derivatives such as phenylketones, which exert toxic effects on neurons. Individuals with such mutations can prevent phenylketonuria by eliminating phenylalanine from the diet, a remarkable example of preventing a genetic disease with an environmental intervention.
Levels of 5HT in the brain can be altered by several means. Drugs such as p-chlorophenylalanine (PCPA), for example, can irreversibly inhibit TPH to produce a long-lasting depletion of 5HT. Experimental manipulation of tryptophan intake also can reduce levels of 5HT in the brain. Individuals who are asked to follow a low-tryptophan diet and subsequently are challenged with a beverage containing other amino acids but lacking tryptophan typically experience not only a dramatic reduction in blood tryptophan levels but also a substantial reduction of 5HT in the brain. In nonhuman primates, where direct measures are possible, a 90% reduction can be achieved. Among patients who have recovered from depression, tryptophan depletion induces a return of depressive symptoms in those who were successfully treated with a selective serotonin reuptake inhibitor (SSRI); however, depressive symptoms do not occur in healthy individuals or in those who were treated with antidepressants that influence NE reuptake. Thus, depletion of 5HT most likely does not cause depression; instead, patients treated with SSRIs may experience withdrawal symptoms on tryptophan depletion that include transient return of depressive symptoms (Chapter 15).
Degradation of catecholamines and serotonin
The most significant mechanism by which the synaptic actions of catecholamines, 5HT, and histamine are terminated is by reuptake into the nerve terminal via neurotransmitter-specific transporters expressed on the plasma membranes of presynaptic terminals. In addition, these monoamines are enzymatically catabolized by monoamine oxidase (MAO). The catecholamines, but not 5HT or histamine, are also metabolized by catechol-O-methyltransferase (COMT).
MAO has both intracellular and extracellular forms. The intracellular form is associated with the outer membrane of mitochondria; given that mitochondria are plentiful in presynaptic terminals, the primary action of MAO is to metabolize catecholamines, 5HT, and histamine after they are taken up into presynaptic terminals. However, the extracellular form may also act to metabolize neurotransmitter while in the synapse. Two major forms of MAO have been described: MAOA and MAOB. These forms are derived from distinct genes on the X chromosome and differ with regard to several biochemical properties, including their substrate specificity, cellular localization, and regulation by pharmacologic agents. MAOA mRNA is expressed almost exclusively in noradrenergic neurons, such as those in sympathetic ganglia and locus ceruleus (LC; described below and shown in 6–7). MAOB mRNA is detected predominantly in serotonergic and histaminergic neurons. There are conflicting reports about the expression of MAO genes and protein in dopaminergic neurons, although the evidence favors expression of MAOA.
Both enzymes oxidize monoamines but differ somewhat in their affinity for substrates. MAOA displays a strong affinity for NE and 5HT, even though it is not expressed in serotonergic neurons. The function of MAOB may be not to oxidize 5HT, but rather to metabolize DA or trace amines that might act as false neurotransmitters, such as β-phenylethylamine, for which it has highest affinity. Extracellular 5HT appears to be oxidized by MAOA derived from sources other than 5HT neurons.
MAOIs, such as phenelzine and tranylcypromine, are used to treat depression and anxiety disorders; the MAOIs selegiline and rasagiline are used to treat Parkinson disease (Chapters 15 and 18). However, clinical use of MAOIs as antidepressants and antianxiety agents has been limited by their side effects 6–1. Unsuccessful efforts to link naturally occurring variations in MAO genes with risk for certain complex behaviors are discussed in 6–2.
6–1 Monoamine Oxidase Inhibitors
Most MAOIs, such as phenelzine, tranylcypromine, and isocarboxazid, that have been used clinically are nonselective, blocking both MAOA and MAOB. The first MAOI used in the clinic, iproniazid, was tested in the 1950s as a treatment for tuberculosis. Although it was ineffective against mycobacteria, it relieved the depression that was common among patients hospitalized with TB. Its actions on MAO were only subsequently recognized. Iproniazid is no longer used clinically because it is hepatotoxic, but the other MAOIs proved highly efficacious in the treatment of depression and diverse anxiety disorders. Today, serotonin and norepinephrine reuptake inhibitors are far more widely used clinically because of their superior tolerability (Chapter 15).
MAOs are expressed not only in brain but also in peripheral tissues. MAOA, found in gut and liver, catabolizes biogenic amines present in foods. Some aged or fermented foods, including many wines and cheeses, have particularly high levels of biogenic amines such as tyramine. When MAOA is inhibited, as in response to the therapeutic use of nonselective MAOIs, biogenic amines in foods can enter the general circulation and can be taken up into sympathetic nerve terminals by norepinephrine transporters. This process can lead to the displacement and release of norepinephrine from sympathetic nerve terminals and the release of epinephrine from the adrenal medulla. Such release can produce a hyperadrenergic crisis, which is characterized by headache, hypertension that can be severe, and chest pain. To prevent a potentially dangerous syndrome, individuals who take nonselective MAOIs must eliminate tyramine-containing foods from their diet. Despite their efficacy, it is this complexity of use that has relegated MAOIs to limited clinical utilization.
Because the inhibition of MAOA appears to be required for antidepressant action and also necessitates dietary restrictions, there has been considerable interest in the development of reversible inhibitors of MAOA (so-called RIMAs such as meclobemide). Unfortunately, RIMAs may be less efficacious than other antidepressants.
MAOIs also have been used to treat Parkinson disease. They were initially tested for this purpose after investigators discovered that the dopamine neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which can cause Parkinson disease, must be converted to MPP+ by MAOB before it can exert its toxic effects. MPTP was discovered when an illicit drug laboratory, attempting to make the opiate meperidine, left MPTP as a contaminant. The individuals who injected it became acutely and severely Parkinsonian and were found to have destroyed their SNc dopamine neurons, likely by extreme oxidative damage. As a result, the MAOB-selective inhibitors selegiline (also known as deprenyl) or rasagiline were administered to patients with early Parkinson disease in clinical trials as a putative neuroprotective agent (presumably preventing the activation of endogenous or exogenous MPTP-like neurotoxins). Although these drugs are efficacious, their mechanism of action remains unclear; their modest benefits might be related to their ability to increase levels of synaptic dopamine rather than to any putative neuroprotective effect.
6–2 Alleged Association of the MAOA Gene With Aggression: Lessons for Psychiatric Genetics
In a large Dutch family, multiple males with mild intellectual disability and aggression were found to have a loss-of-function mutation in the MAOA gene. Given the small numbers involved in a single family, and the complexity of the aggression phenotype, causality could not be proved. Nonetheless, a large number of studies have investigated possible associations between MAOA gene variants with aggression and antisocial behavior. These have extended beyond simple association studies to studies of gene–environment (G × E) interactions in which an MAOA gene variant that results in lower MAOA levels than other common variants is associated with aggression conditional on a history of childhood abuse. Despite the inconclusive nature of these studies, they have influenced findings of culpability in the sentencing phase of murder trials. In 2009 and 2011, two separate Italian courts reduced the duration of prison sentences based in part on the finding that the defendants had the low-activity allele of MAOA presumed to increase risk of reactive aggression given childhood histories of maltreatment.
From a genetics perspective, the existing evidence that attempts to associate a complex behavioral phenotype either with a single genetic variant or with a G × E interaction remains unconvincing. Except for extreme behavioral phenotypes (typically characterized by severe intellectual disability), in which single genes can explain a significant fraction of the variance, mental illnesses and normal behavioral variation have proven to be genetically highly complex. The study of environmental risk has generally relied on epidemiologic studies that have identified risk factors (eg, child maltreatment) that are often associated with increased probability of many negative outcomes, and where the causal mechanisms are yet to be understood. Given these complexities, there are no studies of candidate genes or environments related to aggression that have been adequately powered to test the association hypothesis.
Perhaps the most important question to ask in judging the relevance of a putative association between a particular allele and a disease phenotype or behavior is how much of the phenotypic variance it explains (relative to other genetic and nongenetic factors), and what is the magnitude of the effect in a given individual. For virtually all candidate gene and G X E studies in psychiatry (eg, variants in MAOA, the 5HT transporter, and COMT) significance is fragile at best, and even if the associations survive methodologically stronger studies, their effect size is likely to be very small and to be swamped by the aggregate influence of many other factors. With the advent of large-scale genetic studies of mental illness and eventually other behavioral phenotypes (assuming that they can be objectively defined), it is at long last becoming possible to identify bona fide risk genes for some of these conditions (see Chapters 14 and 17 for further discussions related to autism, schizophrenia, and bipolar disorder).
Catecholamines are also catabolized by COMT. Peripherally the major isoform is soluble, but, in the brain, a longer, membrane-bound isoform predominates, which is found in catecholamine synapses. COMT methylates catecholamines using S-adenosyl-methionine as a methyl donor. COMT inhibitors, such as entacapone and tolcapone, increase levels of catecholaminergic neurotransmitter in synapses and prolong receptor activation. In general, COMT appears to play a far smaller role in terminating the synaptic action of DA and NE than their specific membrane transporters, but in the prefrontal cortex, where the dopamine transporter (DAT) is expressed at relatively low levels, COMT may exert a more significant effect.
The major products that emerge from the enzymatic breakdown of catecholamines by MAO and COMT are shown in 6–4. Historically, these metabolites were investigated as indirect measures of brain catecholaminergic function in depression and schizophrenia. They were measured in cerebrospinal fluid, blood, and urine; however, interpretation of metabolite levels was significantly confounded by activity of the sympathetic nervous system and adrenal medulla and by many other factors. Thus, their usefulness as markers of CNS catecholamine function proved quite limited. 5HT metabolites were also historically investigated. After 5HT is oxidized by MAO, aldehyde dehydrogenase acts to produce 5-hydroxyindoleacetic acid (5-HIAA) as an end product 6–5. Reduction of 5-HIAA in cerebrospinal fluid has been reported to correlate with impulsive violence in some circumstances, most notably among individuals who have attempted suicide by violent means. Despite considerable research, the significance of these findings remains unclear. Overall, it may have been naïve to believe that major neuropsychiatric disorders reflected global abnormalities in levels of one neurotransmitter, rather than disorders of more specific neural circuits that could be influenced by DA, NE, or 5HT.
The overall anatomy of DA in the brain is shown in 6–1. Historically, the first recognition of the importance of DA in the brain came from the investigation of Parkinson disease, which results from degeneration of SNc DA neurons. Death of these neurons results in denervation of the neostriatum (composed of the caudate and putamen in the human brain and the dorsal striatum in the rat and mouse brains), resulting in a movement disorder characterized by tremor, rigidity, abnormal gait, and difficulty initiating voluntary movement (Chapter 18). The major strategy for treatment is DA replacement, but DA is polar and cannot penetrate the blood–brain barrier. Instead, L-dopa is administered. After it crosses the blood–brain barrier it is taken up by dopaminergic nerve terminals via DAT and subsequently converted into DA by AADC 6–6. However, because AADC also resides in peripheral tissues, a significant fraction of L-dopa is decarboxylated into DA before it can be transported into the brain. Among other side effects, this produces nausea because there are DA receptors in the area postrema of the medulla, an area of the brain that controls nausea and vomiting, and which lies outside the blood–brain barrier so that it can sample the systemic environment (Chapter 2). L-Dopa is thus coadministered with an AADC inhibitor such as carbidopa that cannot penetrate into the brain.
Model of a dopaminergic synapse. Presynaptic and postsynaptic molecular entities involved in the synthesis, release, signaling, and reuptake of dopamine are shown. Although MAO is shown extracellularly, it also exists in glia proximate to DA synapses and possibly in mitochondria within dopaminergic nerve terminals. VMAT, vesicular monoamine transporter; Gi and Gs, inhibitory and stimulatory guanine nucleotide–binding proteins; MAO, monoamine oxidase; COMT, catechol-O-methyltransferase; HVA, homovanillic acid; AC, adenylyl cyclase.
Given the role of DA receptors in vomiting, D2 antagonists are effective antiemetic medications. However, due to their risk of causing Parkinson-like side effects, alternatives are often used. 5HT3 antagonists such as ondansetron, and neurokinin 1 (NK1) receptor antagonists (the NK1 receptor recognizes the neuropeptide, substance P; Chapter 7) such as aprepitant, are effective antiemetics and lack the side effects of D2 receptor antagonists.
VTA projections to the limbic forebrain, as noted earlier, are required for reward-related behaviors. VTA DA projections also form the substrate on which certain drugs produce addiction, which is described in detail in Chapter 16. Under normal circumstances VTA DA neurons fire prior to behaviors that are elicited by cues predictive of reward. In order to coordinate responses to such cues, the VTA receives inputs from diverse brain regions. These include reciprocal inputs with regions involved in valuation of rewards and also inputs from other widely projecting systems involved in arousal, attention, and memory. Different subregions of the VTA receive glutamatergic inputs from the prefrontal cortex, orexinergic inputs from the lateral hypothalamus (see 6–25), cholinergic and also glutamatergic and GABAergic inputs from the laterodorsal tegmental nucleus and pedunculopontine nucleus 6–18, noradrenergic inputs from the LC 6–7, serotonergic inputs from the raphe nuclei 6–10, and GABAergic inputs from the nucleus accumbens and ventral pallidum. The activity of VTA DA neurons is also suppressed by activation of the lateral habenula, which sends glutamatergic inputs to GABAergic neurons in the vicinity of the VTA, which normally inhibit VTA DA neurons.
Noradrenergic projections in the CNS. The locus ceruleus (LC), which lies in the rostral pons, is the largest noradrenergic nucleus. It provides virtually all norepinephrine (NE) to the cerebral cortex and innervates many other forebrain areas (thick arrows). Other sources of NE come from clusters of cells within other brainstem nuclei. In the rat, the LC is often designated as area A6 and the other noradrenergic brainstem nuclei as A1, A2, A5, and A7, based on the Falck–Hillarp method of mapping (see 6–1).
DA has multiple actions in the prefrontal cortex. It promotes the “cognitive control” of behavior: the selection and successful monitoring of behavior to attain chosen goals. Aspects of cognitive control in which DA plays a role include working memory, the ability to hold information “on line” in order to guide actions, suppression of prepotent behaviors that compete with goal-directed actions, and control of attention and thus the ability to overcome distractions (Chapter 14). Cognitive control is impaired in several disorders, including attention deficit hyperactivity disorder (ADHD), which is treated with psychostimulants, a term used to describe indirect DA agonists such as methylphenidate and amphetamines that block DAT or cause reverse transport of DA into synapses. Cognitive control is also deficient in addiction and a range of poorly described conditions known as impulse control disorders. Working memory is impaired in schizophrenia, to a lesser degree in some nonpsychotic relatives of individuals with schizophrenia, and in patients with bipolar disorder with psychotic symptoms (Chapter 17). Antipsychotic drugs do not have a therapeutic benefit for working memory or for any of the other cognitive impairments of schizophrenia.
Antipsychotic drugs produce their therapeutic effects (diminishing psychotic symptoms, eg, delusions and hallucinations) by blocking D2 receptors, presumably those that reside in the terminal fields of VTA DA neurons in subcortical structures in the limbic forebrain. The mechanism by which reducing dopaminergic stimulation of these structures ameliorates psychotic symptoms is unclear. Because antipsychotic drugs block D2 receptors in the caudate and putamen as well as in the neurons where they exert therapeutic effects, these drugs produce side effects that are similar to the symptoms of Parkinson disease. (These are often called extrapyramidal side effects to distinguish striatally based motor systems from the corticospinal motor system, the fibers of which descend in a brainstem structure called the “pyramids.”) Because these are “on-target” side effects, that is, side effects that arise from the same molecular target—the D2 DA receptor—that is required for their therapeutic effects, attempts to mitigate the side effects must depend on stimulation or blockade of other receptors. Antipsychotic drug–induced Parkinson-like effects are not treated with L-dopa or indirect DA agonists because D2 DA receptors are already blocked and, were DA or DA agonists to successfully compete for D2 DA receptors, the likely result would be worsening of psychotic symptoms. Instead extrapyramidal side effects are treated with anticholinergic drugs (ie, drugs that block muscarinic cholinergic receptors). Clozapine and second-generation antipsychotic drugs have less tendency to produce these side effects than first-generation drugs partly as a result of lower affinity for D2 receptors, but also as a result of having other receptor actions, including antagonism of 5HT2A receptors, antagonism of muscarinic cholinergic receptors, or some combination (Chapter 17).
The tuberoinfundibular DA system, arising in the arcuate nucleus of the hypothalamus, inhibits prolactin synthesis and release from the anterior pituitary. Antipsychotic drugs, which antagonize D2 receptors, elevate levels of prolactin. Conversely, D2 receptor agonists, such as bromocriptine, can be used to suppress hyperprolactemia, which is most commonly caused by prolactin-secreting pituitary adenomas (Chapter 10).
NE is produced by neurons contained within multiple nuclei of the pons and medulla 6–7. The LC, which is located on the floor of the fourth ventricle in the rostral pons, contains more than 50% of all noradrenergic neurons in the brain; it innervates both the forebrain (eg, it provides virtually all of the NE to the cerebral cortex) and regions of the brainstem and spinal cord. Yet in the human brain, the LC contains only about 12,500 neurons per side, illustrating its remarkably wide projections. The other noradrenergic neurons in the brain occur in loose collections of cells in the brainstem, including the lateral tegmental regions. These neurons project largely within the brainstem and spinal cord.
NE, along with 5HT, ACh, histamine, and orexin, is a critical regulator of the sleep–wake cycle and of levels of arousal (Chapter 13). Not surprisingly, the LC receives significant input from other widely projecting systems involved in regulating sleep and arousal, such as orexin-containing neurons 6–25. LC neurons fire at a basal (tonic) rate during the waking state; the firing rate decreases during slow-wave sleep, and the LC does not fire during paradoxical sleep, the rodent equivalent of rapid eye movement (REM) sleep in humans. Transient increases in LC firing (phasic firing) are correlated with the onset of sensory stimulation; the highest rates of firing are associated with stimuli that portend threat. In addition to increasing arousal, the LC influences diverse aspects of attention and vigilance. In response to threat, LC firing may also increase anxiety, by releasing NE in the amygdala and other regions of the limbic forebrain. Stimulation of β-adrenergic receptors (β-ARs) or α1-adrenergic receptors in the amygdala results in enhanced memory for information encoded under strong emotion, facilitating the recall of stimuli that predict danger. However, this mechanism likely contributes to posttraumatic stress disorder (PTSD) in humans. β-Adrenergic (eg, propranolol) and α1-adrenergic (eg, prazosin) receptor antagonists have been investigated as interventions to treat PTSD, but their efficacy appears to be modest. Part of the challenge is that administration generally follows a traumatic experience; the drugs might be more effective if taken before—but this would require something implausible under most circumstances, prediction of trauma (Chapter 15). In the laboratory, administration of α2-adrenergic receptor antagonists, such as yohimbine, which increase firing of LC neurons by blocking inhibitory autoreceptors 6–8, induces fear and anxiety in laboratory animals and in humans. Opiates, which bind to μ opioid receptors, inhibit LC firing and are thus anxiolytic, among the many other actions of these drugs. With chronic administration, cellular adaptations within LC neurons lead to tolerance and dependence. With cessation of opiate administration, very high rates of LC firing contribute to the opiate physical withdrawal syndrome. These mechanisms are discussed fully in Chapter 16.
Autoreceptors. Autoreceptors typically function to inhibit activity in cell bodies and in synaptic terminals. Activation of α2-adrenergic receptors (α2-ARs) in a noradrenergic cell body, illustrated by the micropipette release of norepinephrine (NE), leads to a decrease in the firing rate of the cell, which can be recorded experimentally with an extracellular electrode. In the synaptic terminal, release of NE into the synaptic cleft allows for the diffusion of transmitter and the activation of presynaptic α2-ARs. Such activation can inhibit further synthesis of NE and block the release of more transmitter. Thus, autoreceptors function in negative feedback loops to modulate signaling between neurons. Tyr, tyrosine; TH, tyrosine hydroxylase.
Electrophysiologic studies in monkeys suggest that the LC may have relatively specific roles in arousal and attention-related behaviors, that is, survival-related behaviors, including exploring the environment for sources of reward (eg, food, water, or safety), exploiting these resources, and disengaging with satiety or when the source of reward is depleted or otherwise becomes problematic. The LC receives inputs from the orbitofrontal cortex, which is involved in valuation of rewards, and which responds both to anticipation of rewards and to satiety. It also receives inputs from the anterior cingulate cortex, which is involved in monitoring task performance, and providing information as to whether performance is successfully approaching the selected goal. It has been argued that phasic LC firing optimizes task performance (exploitation), while tonic firing returns when task utility wanes and correlates with disengagement from the task and a search for alternative behaviors (exploration). In response to these altered firing patterns, NE released in the prefrontal cortex and other brain regions would modulate task-related behaviors. If correct, this model of LC function would contribute to our understanding of ADHD and other disorders of “top-down” cognitive control. In ADHD, there is impaired control of engagement and disengagement with tasks, as well as impaired ability to resist distractions. Noradrenergic projections from the LC thus interact with dopaminergic projections from the VTA to regulate cognitive control. Drugs that increase synaptic NE 6–9 by blocking the NE transporter (NET), such as the antidepressants desipramine or atomoxetine, exhibit some efficacy in treating ADHD. Psychostimulants have greater efficacy in most patients, however, probably because they increase DA as well as NE. Indeed stimulants act not only on DAT and NET but also on the serotonin transporter (SERT), although it has not been shown that 5HT makes a therapeutic contribution to treatment of ADHD. This model also suggests limits to current selective NE reuptake inhibitor or stimulant treatment because these produce sustained elevations in synaptic NE and DA, rather than optimizing the phasic and tonic firing patterns of monoamine neurons.
Model of a noradrenergic synapse. Presynaptic and postsynaptic molecular entities involved in the synthesis, release, signaling, and reuptake of norepinephrine are shown. MAO is shown extracellularly and in mitochondria within noradrenergic nerve terminals. MHPG, 3-methoxy-4-hydroxyphenylglycol; PLC, phospholipase Cβ; IP3, inositol triphosphate; DAG, diacylglycerol. See 6–6 for other abbreviations.
E occurs in only a small number of central neurons, all located in the medulla. E is involved in visceral functions, including control of respiration. It is also produced by the adrenal medulla.
It has been estimated that there are several hundred thousand serotonergic neurons in the human brain. These neurons are confined almost exclusively to discrete nuclei in the brainstem raphe (raphe refers to their midline location; 6–10). The most caudal clusters of the raphe innervate the medulla as well as the spinal cord. The dorsal and median raphe are located in the midbrain and innervate much of the rest of the CNS by means of numerous and sometimes diffuse projection pathways. The dorsal raphe forms the ventral-most portion of the periaqueductal gray. The median raphe is located ventral to the dorsal raphe in roughly the same anterior–posterior position of the midbrain. Although these two nuclei have overlapping terminal fields, the dorsal raphe preferentially innervates the cerebral cortex, thalamus, striatal regions (caudate–putamen and nucleus accumbens), and dopaminergic nuclei of the midbrain (eg, the substantia nigra and VTA), while the median raphe innervates the hippocampus, septum, and other structures of the limbic forebrain. Projections from these and other raphe nuclei are so extensive that virtually every neuron in the brain may be contacted by a serotonergic fiber.
Serotoninergic projections in the CNS. Serotonin is produced by several discrete brainstem nuclei, shown here in rostral and caudal clusters. The rostral nuclei, which include the nucleus linearis, dorsal raphe, medial raphe, and raphe pontis, innervate most of the brain, including the cerebellum. The caudal nuclei, which comprise the raphe magnus, raphe pallidus, and raphe obscuris, have more limited projections that terminate in the cerebellum, brainstem, and spinal cord. Together, the rostral and caudal nuclei innervate most of the CNS. In the rat, the raphe nuclei are termed areas B1 to B9 based on the original histochemical studies conducted by Dahlstrom and Fuxe using the Falck–Hillarp method (see 6–1). The large dorsal raphe nucleus is designated as area B7 and the median raphe nucleus as area B8.
The function of serotonergic projections has been challenging to study partly because they are remarkably extensive, partly because the raphe nuclei contain a mixed population of neurons of which only a minority produce 5HT (making physiologic recordings difficult), and partly because of the large number of 5HT receptors for which truly selective antagonists have only recently become available. Well-formulated, testable hypotheses on the overall effects of 5HT are also difficult to construct because some of the actions of serotonin mediated by different receptors produce effects in opposite directions. Nevertheless, 5HT affects numerous functions including sleep, arousal, attention, processing of sensory information, and important aspects of emotion (likely including aggression) and mood regulation.
Storage, Release, and Reuptake
Most biosynthesis of catecholamines and 5HT does not occur in the cell bodies of their neurons; rather, the synthetic enzymes are transported to nerve terminals, where transmitter synthesis predominantly takes place (6–6, 6–9, and 6–11). DA is synthesized in the cytoplasm and is packaged in storage vesicles by means of the vesicular monoamine transporter protein (VMAT). In noradrenergic terminals, DA is converted to NE by DBH, which also is located in storage vesicles. DA, NE, and 5HT are all transported by the same VMAT protein, which spans the vesicle membrane. Vesicular storage not only permits rapid release of neurotransmitter in response to action potentials but also protects a reservoir of neurotransmitter from metabolism by MAO. VMAT2 is the form of VMAT that operates in the brain, while VMAT1 is the form found in adrenal medulla.
Model of a serotonergic synapse. Presynaptic and postsynaptic molecular entities involved in the synthesis, release, signaling, and reuptake of serotonin are shown. MAO is shown extracellularly and in mitochondria within serotonergic nerve terminals. 5-HIAA, 5-hydroxyindoleacetic acid. See 6–6 for other abbreviations.
Reserpine, a compound initially derived from Rauwolfia, a plant used in Indian herbal medicine, disrupts catecholamine and 5HT reuptake into storage vesicles by blocking VMAT; the monoamine transmitters are then subject to metabolism by MAO. Based on its ability to deplete NE, reserpine was once used as an antihypertensive agent. Based on its ability to deplete DA, it can act as an antipsychotic drug. Indeed, it was used empirically for this purpose in the 1950s. Clinical use of reserpine was abandoned, however, because of its side effects. Tetrabenazine is another VMAT inhibitor that depletes monoamine stores. It is now approved for the treatment of chorea in Huntington Disease.
Plasma membrane transporters
As mentioned above, the DA, NE, and 5HT transporters (DAT, NET, and SERT, respectively) are related transmembrane proteins that move neurotransmitter from the synapse into the cytoplasm of the presynaptic terminal, where it is either reloaded into vesicles by VMAT2 or degraded by MAO. Each of the plasma membrane transporters has 12 hydrophobic membrane-spanning domains 6–12. The rapid reuptake of synaptic transmitter by these transporters has several consequences that are vitally important to signaling among neurons. First, reuptake limits the duration of presynaptic and postsynaptic receptor activation. Second, it limits the diffusion of transmitter molecules to other synapses. Third, it permits recycling and reuse of unmetabolized transmitter. The critical role played by these transporters (compared with the action of MAO or COMT) is illustrated by DAT knockout mice. These mice exhibit extreme hyperactivity, and exhibit other evidence of a hyperdopaminergic state, despite multiple compensatory adaptations in enzyme and receptor function that help to counter the elevated synaptic levels of DA.
Two-dimensional model of a monoamine transporter. This beads-on-a-string model illustrates several features of these proteins. Both the N terminus (N) and the C terminus (C) reside intracellularly. Second, the protein possesses 12 hydrophobic, membrane-spanning domains with intervening extracellular and intracellular loops. The second extracellular loop is the largest and contains several potential glycosylation sites (indicated with tree-like symbols). Third, the color coding of the beads reveals the similarity between norepinephrine and dopamine transporters in terms of constituent amino acids. The most highly conserved regions of these transporters are located in transmembrane domains; the most divergent areas occur in N and C termini. (Adapted with permission from Buck KJ, Amara S: Structural domains of catecholamine transporter chimeras involved in selective inhibition by antidepressants and psychomotor stimulants. Mol Pharmacol. 1995;48(6):1030–1037.)
NET, DAT, and SERT are targets for two major classes of psychotropic drugs, antidepressants and psychostimulants 6–2. The TCAs and newer serotonin–norepinephrine reuptake inhibitors (SNRIs) block NET, SERT, or both with differing selectivity. The norepinephrine reuptake inhibitors (NRIs) and SSRIs are selective for NET or SERT, respectively. Less commonly, antidepressants such as bupropion may also block DAT (Chapter 15). The most significant clinical difference between SSRIs, SNRIs, and NRIs versus TCAs is that the TCAs are also antagonists of α1-adrenergic receptors, H1 histamine receptors, and muscarinic cholinergic receptors resulting in postural hypotension (due to α1 receptor blockade), sedation (due to H1, α1, and muscarinic cholinergic receptor blockade), and dry mouth, failure of pupillary accommodation, constipation, and urinary retention (due to muscarinic antagonism) (Chapter 9). While the newer antidepressants also have significant side effects (eg, sexual dysfunction), they are safer in overdose and far better tolerated. The clinical utility of these antidepressants in the treatment of depression and chronic pain syndromes is discussed further in Chapters 11 and 15.
6–2Binding Properties of Monoamine Plasma Membrane Transporters ||Download (.pdf) 6–2 Binding Properties of Monoamine Plasma Membrane Transporters
Psychostimulants interact with DAT, NET, and SERT. Cocaine blocks these three transporters and thereby blocks reuptake of all three neurotransmitters after normal vesicular release. Amphetamines are a family of related drugs that have a more complex mode of action. They enter monoaminergic nerve terminals via DAT, NET, and SERT, and disrupt the action of VMAT2 to cause leakage of neurotransmitter out of synaptic vesicles. This causes cytoplasmic levels of the neurotransmitters to rise, which triggers the “reverse transport” of the neurotransmitters by DAT, NET, and SERT into the synapse. The rewarding and addictive actions of the psychostimulants result primarily from their actions on DAT (Chapter 16).
Halogenated amphetamines such as fenfluramine are selective for SERT, and stimulate rapid “reverse transport” of 5HT into the synapse. Fenfluramine was prescribed alone or in combination with phentermine, a sympathomimetic drug with amphetamine-like actions, to suppress appetite. However, fenfluramine was removed from the market because it caused cardiac valvular disease and primary pulmonary hypertension in some patients, an action mediated via its peripheral effects on 5HT and the activation of 5HT2B receptors. (In blood, platelets are a significant source of 5HT.) There is evidence that the weight loss caused by these agents is mediated via activation of 5HT2C receptors in the medial hypothalamus (Chapter 10).
The brain expresses low levels of additional types of plasma membrane transporters, which may also contribute to the removal of monoamines from the synapse, although this remains speculative. There is evidence that certain members of the SLC29 (equilibrative nucleoside transporter) family, discussed in Chapter 8, can transport DA and 5HT nearly as effectively as DAT and SERT. Organic cation transporters (OCTs), which are expressed in discrete brain areas, are reported to transport E and histamine. While the physiologic role of OCTs requires further investigation, they may be involved in maintenance of salt and fluid homeostasis.
All receptors for DA and NE belong to the G protein–coupled receptor (GPCR) superfamily. E does not have its own receptors, but utilizes the same receptors as NE. As described in Chapter 4, binding of neurotransmitters to GPCRs initiates a conformational change in the receptor such that it activates a G protein, which in turn is coupled to regulation of an ion channel or second messenger–generating enzyme. Although DA and NE each has only one plasma membrane transporter, each has numerous receptors encoded by different genes. Each of these receptors has unique pharmacologic properties and localization, features that can be exploited in the design of drugs. There are more receptors than G proteins; thus, there is some convergence of actions at the level of intracellular signaling (eg, D1 DA receptors, β1- and β2-adrenergic receptors, and 5HT4 serotonin receptors activate the stimulatory G protein Gs), but specificity of action is maintained by the cell types on which these receptors are expressed, the location of the receptors on these cells (eg, distal or proximal dendrites, dendritic spines or shafts, perikarya, or presynaptic terminals), and their interactions with other receptor-activated signaling systems in the cells expressing them.
DA receptors 6–3 are divided into the D1 family (D1 and D5 receptors, coupled to Gs or the related Golf) and the D2 family (D2, D3, and D4, coupled to Gi/Go and to beta-arrestin signaling). D2 and D3 receptors function as inhibitory presynaptic autoreceptors and as postsynaptic receptors (ie, heteroreceptors expressed on noncatecholamine neurons). D2 receptors have two splice variants, D2short and D2long; however, distinct functional roles of these isoforms have not been identified. Numerous medications produce their clinical effects via actions on DA receptors. All antipsychotic drugs are antagonists of D2 receptors (Chapter 17). By contrast, D2 agonists (eg, bromocriptine) are useful in the treatment of Parkinson disease and hyperprolactinemia (Chapters 10 and 18). Pramipexole and ropinirole are agonists at D2-like receptors, with some selectivity toward D3 receptors, and show some promise in the treatment of Parkinson disease and restless leg syndrome.
6–3The Dopaminergic Receptor Family ||Download (.pdf) 6–3 The Dopaminergic Receptor Family
|Receptor ||Agonists1 ||Antagonists1 ||G Protein Coupling ||Areas of Localization |
|D1 ||SKF82958*; SKF81297* ||SCH23390*; SKF83566; haloperidol ||Gs ||Neostriatum; cerebral cortex; olfactory tubercle; nucleus accumbens |
|D2 ||Bromocriptine* ||Raclopride; sulpiride; haloperidol ||Gi/o ||Neostriatum; olfactory tubercle; nucleus accumbens2 |
|D3 ||Quinpirole*; 7-OH-DPAT ||Raclopride ||Gi/o ||Nucleus accumbens; islands of Calleja |
|D4 || ||Clozapine ||Gi/o ||Midbrain; amygdala |
|D5 ||SKF38393 ||SCH23390 ||Gs ||Hippocampus; hypothalamus |
Adrenergic receptors are divided into α and β families, both of which have multiple receptor subtypes 6–4. Each subtype responds in varying degrees to both NE and E. All β receptors are Gs-coupled, and most α1 receptors are Gq-coupled; α2 receptors, which are generally Gi-coupled, function as inhibitory autoreceptors and as postsynaptic receptors. Many medications on the market act via adrenergic receptors; most of their actions are in the autonomic nervous system, as discussed in Chapter 9.
6–4The Adrenergic Receptor Family
The physiologic responses elicited by receptor stimulation vary not only among receptors but also for any given receptor, depending on the neuronal cell type involved. The latter phenomenon occurs because the ion channels regulated by GPCRs are separate molecules (as described in Chapters 2–4), unlike the situation for receptors that are ligand-gated channels, and different signaling molecules and ion channels are found in different cells types. For example, an ion channel that is inhibited by protein kinase A phosphorylation may predominate in one cell type, but a channel activated by protein kinase A phosphorylation might predominate in another, leading to very different responses to activation of the same receptor. Because the physiologic responses elicited by catecholamine receptor activation are numerous and complex, only general themes related to these responses are presented here.
The electrophysiologic consequences of activation of D1-like receptors have been extremely difficult to pin down because of conflicting reports. Such inconsistencies likely reflect the heterogeneity of the cell types that have been studied, and also the complex cascades of protein kinases and protein phosphatases that are influenced by D1-like receptors and that regulate many types of ion channels. The consequences of D2 receptor activation appear to be more uniform, and frequently give rise to inhibitory responses, like most Gi/Go-linked receptors, caused by the activation of inwardly rectifying K+ channels (Chapter 4).
β-ARs 6–13, coupled to Gs, lead to the activation of adenylyl cyclase, the synthesis of cAMP, and the activation of protein kinase A (Chapter 4). Such activation in turn leads to excitatory or inhibitory effects in neurons, depending on the protein kinase A substrates expressed in the particular cell type. In addition, cAMP can, independently of protein kinase A, activate cyclic nucleotide–gated channels called HCN (hyperpolarization- and cyclic nucleotide–activated) channels (Chapter 2). In the cerebral cortex and hippocampus, β-AR activation facilitates the excitation of pyramidal cells by blocking the activity of a Ca2+-activated K+ channel. In contrast, in cardiac muscle, β-AR activation leads to the phosphorylation and activation of voltage-gated Ca2+ channels, representing the mechanism by which NE released from sympathetic neurons and circulating E released from the adrenal medulla increase the force and rate of cardiac contraction.
The activation of α1 receptors by means of Gq coupling triggers the phosphatidylinositol cascade, which can have multiple effects on neuronal excitability. In contrast, the activation of α2 receptors, typically through Gi coupling, causes inhibitory responses in many cells; indeed α2 receptors serve as autoreceptors on NE neurons as stated earlier.
The brain and certain peripheral tissues also express receptors for so-called “trace amines,” which refer to endogenous amines (eg, tryptamine) that are structurally related to catecholamines and present in mammalian brain at extremely low levels. Trace amines can activate trace amine–associated receptors (TAARs) that are G protein–coupled. There is some evidence that amphetamine and related drugs can bind to TAARs; however, the functional importance of trace amines and TAARs remains unknown.
There are 13 known human 5HT receptors 6–5, 12 of which are GPCRs and 1, the 5HT3 receptor, is ionotropic. Activation of the 5HT3 receptor by 5HT opens a nonselective cation channel and triggers a rapid, transient depolarizing current that is carried by Na+ and K+. Like the nicotinic cholinergic and GABAA receptors, 5HT3 receptors are pentamers with subunits arrayed around a central pore. Several variants of the 5HT3 have been reported, but little is known about their differing functions and pharmacology.
6–5The Serotonergic Receptor Family ||Download (.pdf) 6–5 The Serotonergic Receptor Family
|Receptor1 ||Agonists ||Antagonists ||G protein ||Localization2 |
|5HT1A ||8-OH-DPAT; buspirone, gepirone ||WAY 100135 ||Gi/o ||Hippocampus; septum; amygdala; dorsal raphe; cortex |
|5HT1B ||Sumatriptan and related triptans || ||Gi/o ||Substantia nigra; basal ganglia |
|5HT1D ||Sumatriptan and related triptans ||GR 127935 ||Gi/o ||Substantia nigra; striatum nucleus; accumbens; hippocampus |
|5HT1E || || ||Gi/o || |
|5HT1F || || ||Gi/o ||Dorsal raphe; hippocampus; cortex |
|5HT2A ||DMT and related psychedelics3 ||Ketanserin; cinanserin; MDL900239 ||Gq/11 ||Cortex; olfactory tubercle; claustrum |
|5HT2B ||DMT; fenfluramine4 || ||Gq/11 ||Not located in brain |
|5HT2C ||DMT; MCPP; lorcaserin ||Mesulergine; fluoxetine ||Gq/11 ||Basal ganglia; choroid plexus; substantia nigra |
|5HT35 ||2-Methyl-5HT ||Ondansetron; granisetron ||Ligand-gated channel ||Spinal cord; cortex; hippocampus; brainstem nuclei |
|5HT4 ||Metoclopramide; cisapride; mosapride; tegaserod ||GR 113808; piboserod ||Gs ||Hippocampus; nucleus accumbens; striatum substantia nigra |
|5HT5A6 ||Valerenic acid ||Methiothepin ||Gs ||Cortex; hippocampus; cerebellum |
|5HT6 ||EMD-386,088; EMDT; WAY-181,187 ||Methiothepin; clozapine; amitriptyline; SB-399,885; Lu AE58054; MS-245 ||Gs ||Striatum; olfactory tubercle; cortex; hippocampus |
|5HT7 ||AS-19; E-55888 ||Methiothepin; clozapine; amitriptyline; SB-269,673 ||Gs ||Hypothalamus; thalamus; cortex; suprachiasmatic nucleus |
5HT1A receptors function as somatodendritic autoreceptors because they reside on the cell bodies and dendrites of 5HT neurons; their activation reduces cell firing and inhibits the synthesis and release of 5HT. The activation of serotonergic autoreceptors expressed on presynaptic nerve terminals, 5HT1B and 5HT1D receptors, decreases local synthesis and release of transmitter. 5HT1A, 5HT1B, and 5HT1D receptors are highly homologous, and signal by coupling to the inhibitory G protein, Gi. It is believed that their inhibitory effects on serotonergic neurons, like those of other Gi-linked receptors, are mediated by the activation of inwardly rectifying K+ channels, the inhibition of voltage-gated Ca2+ channels, and the inhibition of adenylyl cyclase. The actions of these autoreceptors are represented in 6–14. These various 5HT receptors are also expressed on nonserotonergic neurons where they mediate the postsynaptic actions of the neurotransmitter.
Serotonergic autoreceptors. The 5HT1A receptor is a somatodendritic autoreceptor. It resides on cell bodies and dendrites and regulates not only cell firing and therefore release of serotonin but also serotonin synthesis, by influencing second messenger systems that regulate tryptophan hydroxylase. The 5HT1B/1D receptors are localized on presynaptic terminals and regulate the synthesis and release of serotonin.
Pharmacology of serotonin receptors Extensive attention has been given to the development of agents that selectively target individual 5HT receptors either as probes of neural function or as putative drugs. Partial 5HT1A receptor agonists, for example, buspirone and gepirone, have been used to treat generalized anxiety disorder (GAD). They lack the risks of dependence, sedation, or abuse that characterize benzodiazepines, but exhibit only modest efficacy (Chapter 15). Sumatriptan and a large number of related triptan drugs, which activate 5HT1D receptors, are important treatments for migraine headaches (Chapter 20). Clozapine, and many second-generation antipsychotic drugs, such as risperidone and olanzapine, are antagonists at 5HT2A receptors in addition to their D2 antagonist properties. As stated earlier, 5HT2A receptor antagonism appears to counteract some of the risk of Parkinson-like side effects caused by D2 receptor antagonism that is required for their therapeutic effects. Hopes that 5HT2A receptor antagonism also contributes to the therapeutic effects of antipsychotic drugs have not been borne out. The idea that 5HT2A receptor blockade might be antipsychotic derived partly from the observation that the hallucinogens, sometimes also described as “psychedelic drugs,” are partial agonists at 5HT2A receptors 6–3. As was mentioned above, 5HT3 antagonists, such as ondansetron and granisetron, are effective antiemetics that lack the Parkinson-like side effects of D2 receptor antagonists. There is great interest in the therapeutic potential of drugs that activate or inhibit the other 5HT receptor subtypes, for example, 5HT2C agonists as antiobesity drugs (Chapter 10) or 5HT4 agonists as antidepressants (Chapter 15), but clinical experience with most of these molecules remains in early stages.
Hallucinogens, also called psychedelic drugs, are most notable for altering perception. The first scientifically characterized hallucinogen is the indolamine D-lysergic acid diethylamide (LSD; “acid”; see figure). However, others such as mescaline (from the peyote cactus) and psilocybin (from psilocybin mushrooms) have been used in non-Western religious ceremonies and herbal medicine well before such drugs were chemically synthesized. Aside from LSD and psilocybin, another indolamine hallucinogen is N,N-dimethylamine (DMT). Phenethylamine hallucinogens include mescaline and dimethoxymethylamphetamine (DOM).
Altered perceptions include illusions and hallucinations. These are most commonly visual, but can occur in any sensory modality. In addition, the drugs can cause synesthesia, in which stimulation of one sensory modality can produce experiences in another (eg, “hearing colors” or “tasting sounds”). Hallucinogens also produce cognitive effects, such as dissociation or depersonalization, and emotional effects, which can range from an experience of pleasant intoxication to severe anxiety.
LSD is a nonselective serotonin receptor agonist; however, its effects and those of all other hallucinogenic drugs on sensory perception are mediated through partial agonism of 5HT2A receptors. There are reports that hallucinogens cause unique behavioral effects, compared with nonhallucinogenic 5HT2A acting drugs, through ligand-directed or biased signaling where they target distinct intracellular signaling cascades (Chapter 1). Hallucinogens produce their sensory effects because of the dense innervation of the cerebral cortex, including sensory cortex, by serotonin fibers and high levels of expression of 5HT2A receptors in these regions (see 6–10). Under normal circumstances, serotonin modulates sensory processing and attention in response to changes in the levels of arousal and to environmental stimuli.
LSD and other hallucinogens were tested as potential therapeutic agents for a range of disorders including schizophrenia and drug addiction (eg, alcoholism). Given their potent effects, blinding was not possible in clinical trials design. In any case, efficacy was not convincingly demonstrated. Some hallucinogens have been tried as adjuncts to psychotherapy, but it has been difficult to study them given their illegal status in most countries.
Alternative signaling cascades
Studies over the past decade have shown that the activation of GPCRs leads to the stimulation of so-called alternative signaling cascades mediated, not by G proteins, but by a family of proteins called arrestins (or β-arrestins based on their initial description in conjunction with the β-AR). Early studies showed that the phosphorylation of the β-AR, bound by ligand, by G protein receptor kinases (GRKs) triggers the recruitment of β-arrestin, which then mediates downregulation of the receptor (Chapter 4) via its internalization 6–15. More recently, however, such recruitment of β-arrestin has been shown to activate several additional signaling cascades, including MAP-kinase and Ca2+ pathways. Interestingly, receptor agonists can differ in their ability to activate “canonical” (G protein–mediated) versus “noncanonical” (β-arrestin–mediated) pathways—a phenomenon referred to as ligand-directed or biased signaling—and such diversity of signaling has been documented for many types of GPCRs. An example of biased signaling at DA receptors, which might be relevant to antipsychotic drug action, is discussed in Chapter 17.
Receptor trafficking. G protein–coupled receptor trafficking involves a series of molecular events after ligand binding takes place (step 1). Phosphorylation of the ligand-bound receptor occurs (step 2); arrestin then binds to the phosphorylated receptor–ligand complex (step 3). Bound arrestin complexes with clathrin lattices to bring about receptor sequestration in clathrin-coated pits and internalization by means of a dynamin-mediated process (step 4). After the receptor is internalized in a vesicle, acidification alters its conformation and triggers the release of the ligand (step 5). The receptor is dephosphorylated before it is recycled to the cell membrane (step 6).