Serotonin (5-HT, 5-OH-tryptamine) is an indole alkylamine found throughout nature (animals, plants, venoms) and has the most complex receptor family of all known neurotransmitters. In the CNS, several hundred thousand serotonergic neurons lie in or in juxtaposition to numerous midline nuclei in the brainstem (raphe nuclei), from which they project to virtually all areas of the brain and spinal cord. Serotonin is involved with mood, emotion, learning, memory, personality, affect, appetite, aggression, motor function, temperature regulation, sexual activity, pain perception, sleep induction, and other basic functions. Serotonin is not essential for any of these processes, but modulates their quality and extent. A number of psychiatric disorders, including depression, anxiety, obsessive-compulsive disorder, dementia, schizophrenia, and eating disorders, are linked to altered serotonin function. Consequently, modification of serotonergic neurotransmission is an integral part of the treatment plan for most of these conditions.165
The serotonergic system is extremely diverse, with 14 types of receptors that act to stimulate or inhibit neurons, including those of other neurotransmitter systems. Serotonin is also the precursor for the pineal hormone, melatonin. Despite the important role 5-HT plays in the CNS, less than 5% of the body’s 5-HT is found within the CNS, with the great majority of 5-HT being located within enterochromaffin cells of the intestine and a small amount of serotonin sequestered by platelets.15
Peripherally, 5-HT is released by enterochromaffin cells in response to intestinal stimulation, which contributes to peristalsis and fluid secretion. Platelets take up 5-HT while passing through the enteric circulation. Serotonin is released from activated platelets to interact with other platelet membranes (promote aggregation) and with vascular smooth muscle.15
Experimentally, 5-HT exhibits diverse effects on the cardiovascular and peripheral nervous systems, although the importance of these actions remains uncertain in the normal physiologic state. Serotonin-induced vasoconstriction or vasodilation found in experimental studies involves multiple types of 5-HT receptors and, in turn, is influenced by multiple other factors. 5-HT1B receptor agonists (eg, sumatriptan) produce coronary artery vasoconstriction in some patients as an adverse event.147,210
Centrally, it is particularly difficult to ascribe a specific symptom or physical finding to serotonergic neurons because of the diversity of their physiologic actions. However, 5-HT definitely plays an important role in the action of many hallucinogenic or illusionogenic drugs, which act as partial agonists at cortical 5-HT2 receptors. Proserotonergics are used to treat depression, whereas 5-HT receptor (5-HT2) antagonists have greater importance in the management of schizophrenia.64
Generally, in areas where they overlap, 5-HT acts in opposition to DA. For example, 5-HT serves to increase prolactin, adrenocorticotropic hormone (ACTH), and growth hormone secretion, whereas DA decreases prolactin secretion. As another example, activation of basal ganglial 5-HT2A receptors inhibits DA release.64 However, well-known exceptions exist, such as cortical 5-HT3 receptors31 and 5-HT1A receptors that are capable of promoting DA release under certain circumstances.144
Synthesis, Release, and Reuptake
Serotonin does not cross the blood–brain barrier. It is synthesized from the amino acid L-tryptophan, which passes through the blood–brain barrier using a neutral amino acid transporter. Figure 14–8 illustrates 5-HT synthesis. Tryptophan-5-hydroxylase is the rate-limiting enzyme of 5-HT synthesis. Increases in tryptophan are predictably accompanied by increased 5-HT production. l-Amino acid decarboxylase (dopa decarboxylase) converts 5-hydroxytryptophan to 5-HT. Cytoplasmic 5-HT is transported into vesicles by VMAT2, where it is concentrated by ion trapping before release by Ca2+-dependent exocytosis. In contrast to vesicles containing DA or NE, 5-HT vesicles contain almost no ATP. After release into the synapse, a transporter (SERT) in the neuronal membrane moves 5-HT back into the neuron, where it reenters vesicles or is degraded by MAO.165
A serotonergic nerve ending and postsynaptic membrane. Tryptophan hydroxylase  converts tryptophan to 5-hydroxytryptophan (5-OH-tryptophan). Aromatic l-amino acid decarboxylase (AADC) then metabolizes 5-OH-tryptophan to serotonin (5-HT). Serotonin is concentrated within vesicles through uptake by VMAT2 before exocytosis . After uptake into the neuron by SERT , 5-HT is transported back into vesicles or undergoes degradation by monoamine oxidase (MAO) to an intermediate compound, which is converted to 5-hydroxyindoleacetic acid (5-HIAA) . 5-HT1,2,4,6,7 receptors [3,9,10] are coupled to G proteins, while 5-HT3 receptors  are ligand-gated cation channels that may conduct Na+ and/or Ca2+ (only Na+ is illustrated). 5-HT3 cation channels also appear to be blocked by Mg2+ until the cell is depolarized, allowing Mg2+ to dissociate—a mechanism similar to that found at NMDA glutamate receptors. In addition to residing on postsynaptic membranes, 5-HT1A, 5-HT1B, and 5-HT1D receptors serve as presynaptic autoreceptors that, when stimulated, decrease further release of 5-HT [9,10]. Presynaptic 5-HT1A receptors mainly serve as somatodendritic autoreceptors, whereas presynaptic 5-HT1B, and 5-HT1D receptors serve as terminal autoreceptors. Xenobiotics in Table 14–7 act to enhance 5-HT synthesis ; inhibit VMAT2 to prevent vesicle uptake of 5-HT ; raise cytoplasmic concentrations of 5-HT, resulting in reverse transport of 5-HT into the synapse by SERT ; by displacing 5-HT from vesicles  or inhibiting MAO ; activate or antagonize 5-HT receptors [3,4,9,10]; or by inhibiting 5-HT reuptake . G = G protein; SERT = membrane 5-HT uptake transporter; VMAT2 = vesicle membrane uptake transporter.
Serotonin is preferentially metabolized by the MAO-A isozyme. Paradoxically, the serotonergic nerve terminal is almost devoid of MAO-A, but contains abundant amounts of MAO-B. It has been hypothesized that the large amounts of MAO-B metabolize other xenobiotics that might inappropriately promote 5-HT release (eg, dopamine). However, the small amount of MAO-A found in serotonergic neurons provides adequate degradation of 5-HT.142
There are seven major functioning receptor types (5-HT1–5-HT7) and numerous subtypes.
Receptors in the 5-HT1 class are coupled to G proteins and commonly increase K+ efflux and decrease cAMP concentrations. Members of the 5-HT1 receptor class express greatest affinity for 5-HT and are thus biologically active under normal physiologic conditions. 5-HT1A receptors reside predominantly on raphe nuclei, where they act as somatodendritic autoreceptors.101,132,151 Hippocampal 5-HT1A receptors reside postsynaptically, where they also inhibit through similar mechanisms.101
Central 5-HT1D and 5-HT1B receptors primarily act as inhibitory terminal autoreceptors and heteroreceptors. They are found less commonly on postsynaptic membranes.165 Originally 5-HT1B receptors were not believed to exist in humans. However, most of the actions described in older literature regarding 5-HT1D receptors can now be attributed to 5-HT1B receptors. Cranial blood vessels (eg, meninges) possess 5-HT1D and 5-HT1B receptors, whose activation produces vasoconstriction and decreased inflammation.147,165
5-HT1E and 5-HT1F receptors are more recently discovered members of the 5-HT1 receptor class. Their functional activity is yet to be determined.
The three subtypes of 5-HT2 receptors are coupled to G proteins, thus serving to decrease K+ efflux and/or increase intracellular Ca2+ concentration by raising concentrations of inositol triphosphate and diacylglycerol.165 The three subtypes of 5-HT2 receptors are so similar in characterization that investigational probes have great difficulty in distinguishing the subtypes. 5-HT2A receptors are most concentrated in the cerebral cortex, where they serve as excitatory postsynaptic receptors. Their activation increases glutamate release from pyramidal cells, but also can lead to release of GABA.130 5-HT2A receptors also reside on platelets, where their activation produces platelet aggregation. 5-HT2C receptors (previously 5-HT1C) reside on the choroid plexus, where they regulate cerebrospinal fluid production. Activation of 5-HT2B receptors in the GI tract promotes stomach contraction.165 At least some xenobiotics that activate 5-HT2B receptors on cardiac valves cause a valvulopathy identical to that of carcinoid syndrome, though both sides of the heart can be involved following 5-HT2B agonists, whereas carcinoid syndrome usually affects the right sided valves.24
5-HT3 receptors are isopentameric ligand-gated cation channels that are structurally similar to ACh nicotinic receptors, GABAA Cl– channels, glycine, and NMDA glutamate receptors.31 They are localized to both presynaptic and postsynaptic membranes. Upon activation, postsynaptic receptors stimulate the neuron by opening the channel to cause depolarization through Na+ and/or Ca2+ influx. In addition, these channels are normally blocked by Mg2+ in a voltage-dependent manner similar to glutamatergic NMDA receptors (see Glutamate later). Centrally, 5-HT3 receptors are expressed diffusely, but are especially concentrated in the CTZ, where their activation induces emesis.165 In the cerebral cortex, their activation leads to increased release of DA and decreased release of ACh.31 Cortical 5-HT3 receptors are frequently identified on GABA interneurons where they increase inhibitory, GABAergic tone. In contrast to cerebral actions, activation of peripheral 5-HT3 receptors on cholinergic nerves in the gut enhances ACh release to increase gastrointestinal motility.140
5-HT4 receptors are coupled to G proteins (Gs). Their activation leads to increased cAMP concentrations. 5-HT4 receptors are scattered diffusely throughout the brain, and their exact role remains undefined, although they are known to increase the release of ACh.140 Peripheral 5-HT4 receptors reside in the heart, intestines, and adrenal gland where their activation produces tachycardia, aldosterone and cortisol release, and contraction of gut and bladder smooth muscle. Whether these actions are important under normal physiologic conditions is not clear, but peripheral 5-HT4 receptors promote the release of ACh and increase gut motility.140
5-HT5 receptors exist in two subtypes: 5-HT5a and 5-HT5b. Humans only have the 5-HT5a subtype, which is coupled to Gi/o.165 This receptor may act as a somatodendritic autoreceptor but the functionality of this role is unknown, in contrast to the predominant importance of 5-HT1a autoreceptors.
5-HT6 and 5-HT7 Receptors.
5-HT6 and 5-HT7 receptors are positively coupled to cAMP formation through G proteins.165 Their distribution is poorly defined. However, many antidepressants and antipsychotics antagonize these receptors. They are currently a source of great interest because of the possibility of avoiding DA blockade to achieve antipsychotic activity. The 5-HT7 receptor may be particularly important in regulating circadian rhythms.106
Table 14–7 provides examples of xenobiotics that affect serotonergic neurotransmission.
TABLE 14–7.Examples of Xenobiotics Affecting Serotonergic Neurotransmission ||Download (.pdf) TABLE 14–7. Examples of Xenobiotics Affecting Serotonergic Neurotransmission
The body rapidly metabolizes orally administered 5-HT via intestinal and hepatic MAO. However, l-tryptophan is an amino acid precursor to 5-HT that is readily absorbed by the intestinal tract. This method of augmenting CNS 5-HT production was previously used as an unproved sleep aid until it was associated with the eosinophilia myalgia syndrome in 1990. 5-Hydroxytryptophan (5-HTP) is the immediate precursor to 5-HT. 5-HTP is commonly available without a prescription. The anxiolytics, buspirone, gepirone, and ipsapirone act as partial agonists at somatodendritic and postsynaptic 5-HT1A receptors. Sumatriptan, an antimigraine medication, mainly activates 5-HT1D and 5-HT1B receptors.147 The action of sumatriptan may result from vasoconstriction of meningeal and other cranial, extracerebral vasculature; no impairment of cerebral blood flow follows its use. Other members of the triptan class include rizatriptan, zolmitriptan, and naratriptan. Vilazodone is an antidepressant with both partial 5-HT1A receptor agonism and SERT inhibition.44
Metoclopramide and tegaserod are prokinetic drugs that activate 5-HT4 receptors to increase gut motility.140 Because 5-HT4 receptors are also found in the heart and urinary bladder detrusor muscle, 5-HT4 agonists occasionally produce urinary incontinence and tachycardia.
Numerous indoles and phenylalkylamines, including ergot alkaloids, lysergic acid diethylamide (LSD), psilocybin, and mescaline, exhibit both agonistic and antagonistic properties at multiple 5-HT receptors. Their hallucinogenic/illusionogenic action is best explained by partial agonism at 5-HT2A receptors.64 Some substituted amphetamines (eg, methylenedioxymethamphetamine, XTC) directly stimulate 5-HT receptors.198
Cocaine and indirect-acting sympathomimetics, especially amphetamines, cause 5-HT release as previously described.198 Centrally, DA undergoes uptake into serotonergic neurons to displace 5-HT from the neuron. Ingestion of l-dopa or other xenobiotics that increase synaptic DA concentrations can cause 5-HT release.132
Inhibitors of 5-HT reuptake include amphetamines, cocaine, various antidepressants, meperidine, tramadol, and dextromethorphan. Several antidepressants specifically inhibit 5-HT reuptake. Examples of selective 5-HT reuptake inhibitors (SSRIs) include fluoxetine, sertraline, paroxetine, and citalopram. The use of SSRIs sometimes produces extrapyramidal side effects for reasons that remain unclear because of the numerous actions of 5-HT in the basal ganglia.74 Two anticonvulsants, carbamazepine and lamotrigine, appear to inhibit 5-HT reuptake.191 Again, reserpine and tetrabenazine prevent 5-HT uptake into vesicles.
MAO-A accounts for most 5-HT degradation, and nonspecific MAOIs and MAO-A inhibitors (clorgyline, moclobemide) raise 5-HT concentrations and, through indirect action, probably cause 5-HT release. Some medications (eg, methylene blue, procarbazine, linezolid) have the undesired side effect of causing MAO inhibition, which occasionally produces serotonin toxicity when combined with other serotonergic drugs.
Trazodone and nefazodone act mainly as antagonists at 5-HT2 receptors, but are also weak reuptake inhibitors. Both undergo metabolism to m-chlorophenylpiperazine (mCPP), which activates most 5-HT receptors, but is especially active at 5-HT2C receptors. Agomelatine is a novel antidepressant that antagonizes 5-HT2C receptors and stimulates melatonin MT1 and MT2 receptors.50 Ketanserin and ritanserin specifically antagonize 5-HT2C receptors, while methysergide and cyproheptadine antagonize 5-HT1 and 5-HT2 receptors.142
Mirtazapine exhibits complex actions, including antagonism of 5-HT2A, 5-HT2C, and 5-HT3 receptors.66 It also indirectly increases 5-HT1A activity and enhances release of NE through antagonism of α2-adrenergic receptors. Mirtazapine demonstrates potent antagonism of histaminic and muscarinic receptors.66
Most antipsychotics and tricyclic antidepressants antagonize 5-HT2A and, to a lesser extent, 5-HT2C receptors. In fact, investigators are interested in developing antipsychotics similar to risperidone that possess potent antagonistic properties at 5-HT2 receptors, without accompanying potent DA receptor antagonism, in order to limit extrapyramidal side effects. These investigations have resulted in the introduction of olanzapine, sertindole, ziprasidone, zotepine, quetiapine, and amisulpride.130
Ondansetron, granisetron, tropisetron, dolasetron, and alosetron antagonize 5-HT3 receptors.140 Their antiemetic action is thought to be explained by several mechanisms. Central antagonism at the CTZ lessens vomiting. Peripheral 5-HT3 receptor antagonism in the gut prevents ACh release, decreasing gut motility. Finally, antagonism of vagal 5-HT3 receptors decreases afferent stimulatory signals to the vomiting center in the brainstem. Metoclopramide antagonizes 5-HT3 and D2 receptors. Ondansetron and some experimental 5-HT3 antagonists are being studied in the treatment of schizophrenia because of their ability to prevent DA release.
Tianeptine is an antidepressant with several pharmacologic effects, including enhancement of 5-HT reuptake, thus lowering synaptic 5-HT concentrations.163
Serotonin toxicity (formerly called serotonin syndrome) represents an iatrogenic and largely idiosyncratic condition that is most commonly caused by the combination of two or more proserotonergic xenobiotics, although it can happen following single 5-HT agonists in overdose or at therapeutic dosages.136 Animal models indicate that serotonin toxicity can be prevented by the blockade of 5-HT1A receptors and 5-HT2A receptors.81,142,144 Serotonin toxicity is characterized by alterations in mentation and cognition, autonomic nervous system dysfunction, and neuromuscular abnormalities. Symptoms may include confusion, agitation, convulsions, coma, tachycardia, diaphoresis, hyperthermia, hypertension, shivering, myoclonus, tremor, hyperreflexia, and muscle rigidity (especially of legs).136 Serotonin toxicity is often confused with neuroleptic malignant syndrome in its more severe presentation due to their similar manifestations. Serotonin toxicity usually responds to supportive care alone but may improve with 5-HT1A and 5-HT2A receptor antagonists such as cyproheptadine.22,136 Serotonin toxicity can be caused by xenobiotics, increasing CNS 5-HT neurotransmission (Table 14–7). In addition, xenobiotics that act to increase CNS DA concentrations, such as levodopa and bromocriptine, have potential to precipitate serotonin syndrome by indirectly causing 5-HT release (Chap. 75).
GABA is one of two main inhibitory neurotransmitters of the central nervous system (glycine is discussed later). Xenobiotics that enhance GABA activity are generally used as anticonvulsants, sedative-hypnotics, anxiolytics, and general anesthetics. Xenobiotics that antagonize GABA activity typically produce CNS excitation and convulsions. GABA is synthesized from glutamate, the brain’s main excitatory neurotransmitter.
In general, GABA inhibition predominates in the brain. In the spinal cord, through monosynaptic and polysynaptic reflex pathways, GABA mediates a number of physiologically minor peripheral effects outside the CNS (eg, vasodilation, bladder relaxation). Spinal cord GABA is important in attenuating skeletal muscle reflex arcs.122
Synthesis, Release, and Reuptake
Figure 14–9 illustrates GABA synthesis. Glutamate is converted to GABA via glutamic acid decarboxylase (GAD), which requires pyridoxal phosphate (PLP) as a cofactor. Pyridoxal phosphate is synthesized from pyridoxine (vitamin B6) by the enzyme pyridoxine kinase (PK).133 VGAT, a vesicle-bound transporter, transports GABA into vesicles from where it is released through Ca2+-dependent exocytosis into the synapse.122 Reuptake of GABA from the synapse back into the presynaptic neurons is mediated by the Na+-dependent transporter, GAT-1, whereas uptake into glial cells and possibly postsynaptic neurons is mediated by GAT-2, GAT-3, and GAT-4. Evidence also suggests that GABA is released into the synapse from cytoplasm by reverse transport under some conditions. In glial cells, cytoplasmic GABA can undergo degradation by GABA-transaminase (GABA-T) to succinic semialdehyde (SSA), part of which then undergoes oxidation to succinate. GABA-T also requires PLP as a cofactor.150 The transamination of GABA to SSA by GABA-T results in the conversion of α-ketoglutarate to glutamate, which then moves back into neurons to be used for resynthesis of GABA.
GABAergic neurotransmission. γ-aminobutyric acid (GABA) released from a presynaptic neuron (B) binds to postsynaptic GABAA or GABAB receptors to hyperpolarize and inhibit neuron D [5,6] or to presynaptic GABAB heteroreceptors on neuron C  to inhibit neurotransmitter release by blocking Ca2+ influx (an excitatory glutamatergic neuron is shown as an example). Stimulation of GABAB autoreceptors on neuron B  also reduces further release of GABA. Synaptic GABA undergoes reuptake into the presynaptic neuron by GAT-1, and uptake into glial cells and possibly postsynaptic neurons by GAT-2, GAT-3, and GAT-4 (GAT-2 is shown mediating uptake into glial cell A as an example.) Acute falls in pyridoxal phosphate (PLP) lead to impaired glutamic acid decarboxylase (GAD) activity and low GABA concentrations. Although GABA-transaminase (GABA-T) also requires PLP, acute falls in PLP do not affect this enzyme as dramatically because of tight PLP binding to the GABA-T complex. Xenobiotics in Table 14–9 act to impair PLP formation by inhibiting pyridoxine kinase (PK) ; to increase GABA concentrations by either stimulating GAD  or inhibiting SSAD ; to inhibit GABA reuptake ; to stimulate or block GABA receptors [5–8]; to cause GABA release ; or to inhibit GABA-T . Glutamic-oxaloacetic transaminase (GOT), GABA-T, and SSAD are mitochondrial enzymes. G = G protein; GAT = membrane GABA reuptake transporter; SA = succinic acid; SSA = succinic semialdehyde; SSAD = SSA dehydrogenase; VGAT = vesicle membrane GABA uptake transporter.
There are two main types of GABA receptors (Table 14–8). GABAA receptors are Cl– channels that mediate inhibition by allowing Cl– to move into and hyperpolarize the neuron. Most GABAA receptors are located postsynaptically and mediate fast or phasic inhibition. About 5% to 10% of GABAA receptors are located outside the synapse and are responsible for slower tonic current that is present at resting membrane potential.73,115,141,175 Situated at various sites in relation to the GABA recognition site on the Cl– channel are sites for exogenous and endogenous modulators where numerous excitatory and depressant xenobiotics bind, and through which GABAA receptor responsiveness is regulated under normal physiologic conditions (Fig. 14–10). The common denominator for modulation at the GABAA complex is an increase or decrease in inward Cl– current.
TABLE 14–8.GABA Receptors and Their Characteristics ||Download (.pdf) TABLE 14–8. GABA Receptors and Their Characteristics
| ||GABAA ||GABAB || |
|Receptor ||Cl– Channel ||G Protein-Coupled || |
|Bicuculline antagonism ||Yes ||No || |
|Baclofen agonism ||No ||Yes || |
|Benzodiazepine agonism ||Yes ||No || |
|Barbiturate agonism ||Yes ||No || |
|Picrotoxin antagonism ||Yes ||No || |
Representation of the GABAA Cl− channel receptor complex. Benzodiazepines (BENZOs), barbiturates, picrotoxin, steroids, and GABA (γ-aminobutyric acid) clearly bind to different sites on the channel. Although separate circles represent different agents capable of binding to and of modulating Cl− influx through the GABAA receptor complex, it is not always apparent where these xenobiotics bind on the channel. Chloral hydrate undergoes metabolism to trichloroethanol, which interacts with the GABAA receptor complex. Zolpidem, zopiclone, and zaleplon are nonbenzodiazepines that bind to the benzodiazepine site. Given the structural similarity of glutethimide and methyprylon to barbiturates, it is speculated that their action may be mediated at GABAA receptors.
GABAA receptors exist as pentamers, composed of various subunits, and throughout the CNS there are regional variations in expressions of multiple subunit genes for GABAA complexes. To date 19 subunits have been identified (α1 to α6, β1- β3, γ1- γ3, δ, ε, θ, π, ρ1- ρ3). GABAA receptors are composed most commonly of two α subunits, two β subunits, and either a γ or δ subunit. Multiple isoforms of subunits exist, but within a single receptor, the isoforms of individual subunits appear to be identical. While the large numbers of isoforms and different combinations of subunits could theoretically produce more than 2000 different GABAA Cl– channels, only a few dozen combinations exist naturally.42,141 The most common is α1β2γ2.11,186 Previously, GABAC receptors were classified as a separate type of GABA receptor, but are now classified as GABAA receptors comprising homo-oligomers and hetero-oligomers formed by ρ subunits (ρ1 to ρ3).
The second type of GABA receptor, GABAB, is a G protein–coupled receptor found presynaptically, postsynaptically, and on extrasynaptic membranes. GABAB receptors are heterodimers formed by two subunits, GABAB1 and GABAB2. The GABAB1 subunit binds to GABA or other ligands such as baclofen, and the GABAB2 subunit couples the receptor with the effector Gi/o protein.13 GABAB receptors are distributed both in the CNS and peripheral nervous system and mediate both presynaptic and postsynaptic inhibition.21 Presynaptic inhibition results from preventing Ca2+ influx so as to impair exocytosis of neurotransmitter vesicles, including those containing excitatory glutamate. Through presynaptic actions, GABAB receptors serve not only as heteroreceptors for glutamatergic and other nerve terminals, but also as autoreceptors, where their activation in response to synaptic GABA provides feedback inhibition of further neurotransmitter release (Fig. 14–9). Postsynaptic inhibition is mediated by increasing K+ efflux through K+ channels, resulting in hyperpolarization of the membrane away from threshold. In addition to their effects on ion channels, GABAB receptors, via the Gi/o protein, inhibit adenylate cyclase and, thus, the cAMP-protein kinase A pathway, which is necessary for the phosphorylation and upregulation of NMDA receptors.13
Table 14–9 provides examples of xenobiotics that affect GABAergic neurotransmission.
TABLE 14–9.Examples of Xenobiotics Affecting GABAergic Neurotransmission ||Download (.pdf) TABLE 14–9. Examples of Xenobiotics Affecting GABAergic Neurotransmission
Modulation of GABA Production and Degradation.
Isoniazid (INH) and other hydrazines (eg, monomethylhydrazine from mushrooms) lower CNS GABA concentrations by several mechanisms. Most important, they compete with pyridoxine for binding to PK, impairing PLP production.133 Pyridoxal phosphate binding to the GAD complex is easily reversible.150 The acute decrease in PLP concentration is rapidly accompanied by impaired GABA synthesis and a decrease in GABA concentration. Lack of normal GABA inhibition produces seizures typical of hydrazine toxicity. Although PLP is also required for GABA degradation by GABA-T, acute decreases in PLP do not affect this enzyme nearly as much, because PLP is more tightly bound to the GABA-T complex and remains associated with the enzyme.150 To a lesser extent, isoniazid binds to the GAD-PLP complex to prevent GABA formation.
Large ingestions of Ginkgo biloba seeds have resulted in recurrent seizures that may result from decreased GABA concentrations. Ginkgo seeds contain 4-methoxypyridoxine, which acts as a competitive antagonist of pyridoxal phosphate, thereby inhibiting glutamate decarboxylase and impairing GABA synthesis.91,138
Cyanide inhibits numerous enzymes besides cytochrome oxidase. Domoic acid (see Glutamate later) may also inhibit GAD.43
In vitro studies demonstrate the ability of valproate to increase brain GABA concentrations, either by inhibition of succinic semialdehyde dehydrogenase or by activation of GAD.88 Gabapentin may increase the rate of GABA synthesis in the brain by stimulating GAD, although the main mechanism of action of gabapentin is to bind to calcium channels.202 Vigabatrin, an anticonvulsant, acts by irreversibly inhibiting GABA-T.194
Figure 14–10 schematically illustrates the GABAA receptor complex. In general, GABAA agonists cause CNS depression, ranging from mild sedation and nystagmus to ataxia, stupor, and coma. Many indirect agonists that bind to the GABAA complex have no activity in the absence of GABA. With some exceptions, their pharmacologic actions require the binding of GABA to its receptor and do not result from a direct effect on Cl– conductance exclusive of GABA binding. Many of these xenobiotics demonstrate additional actions that are not mediated through the GABAA complex.
The main direct GABA agonist of toxicologic interest is muscimol, found in some poisonous mushrooms. Muscimol binds at the GABA binding site on the GABAA complex to mimic the action of GABA.152 Ibotenic acid, a direct glutamate agonist found in the same mushrooms, is decarboxylated to muscimol just as glutamate is decarboxylated to GABA.
Benzodiazepines bind to GABAA complexes to increase the affinity of GABA for its receptor and to increase the frequency of Cl– channel opening in response to GABA binding.185 The benzodiazepine binding site on the GABAA receptor is located in a pocket between an α subunit and a γ2 subunit.211 Benzodiazepines also inhibit adenosine uptake apart from GABAA activity (see Adenosine later). The historical terms “benzodiazepine receptor” and “omega receptor” (for benzodiazepines) are being abandoned, and benzodiazepine-binding sites on subtypes of GABAA receptors are categorized as high, intermediate, or low affinity benzodiazepine-binding sites, based on zolpidem binding.174
It follows that various isoforms of GABAA Cl– channels differ in their affinity for different benzodiazepines. GABAA receptors containing γ2 subunits are more sensitive to benzodiazepines than are GABAA receptors containing γ1 and γ3 subunits. Sensitivity and response to benzodiazepine binding is also highly dependent on the specific α subunit composition of the GABAA receptor. GABAA receptors containing an α4 or α6 subunit are completely insensitive to and will not bind benzodiazepines, whereas GABAA receptors containing α1, α2, α3, or α5 subunits are sensitive to benzodiazepine binding. This has important implications in that the development of tolerance to ethanol confers cross-tolerance to benzodiazepines through a change in α subunits. In addition, specific subunits may mediate different effects of benzodiazepines. For example, sedating and amnestic effects are mediated through binding to α1 subunits while anxiolytic effects appear to be mediated by binding to α2 subunits.57
Zolpidem, zaleplon, and zopiclone are non-benzodiazepines that act as agonists at the benzodiazepine binding site on the GABAA receptor. They exhibit a high selectivity for the α1 subunit and low selectivity for α2, α3, and α5 subunits.189,211 This selective binding to α1 subunits is thought to account for their relatively selective sedative properties at therapeutic doses, and lack of anxiolysis, as compared to benzodiazepines.
Barbiturates bind to the GABAA complex to produce several effects.98,185 All barbiturates enhance the action of GABA by producing more Cl– influx for a given amount of GABA binding by increasing the duration of Cl– channel opening. Whereas phenobarbital does not change the affinity of GABA or benzodiazepines for their binding sites, depressant barbiturates, such as pentobarbital, do increase GABA and benzodiazepine binding site affinities for their ligands, further enhancing inward Cl– currents. At high concentrations, at least some barbiturates directly open Cl– channels to cause Cl– influx.98 Phenobarbital can directly open the Cl– channel at antiepileptic concentrations. In addition, barbiturates possess other actions that depress all excitable membranes, including cardiac and smooth muscle.
The intravenous anesthetics propofol and etomidate enhance inward GABAA Cl– currents, and at high concentrations they directly open chloride channels in the absence of GABA.10 The respiratory depressant and immobilizing effects of etomidate and propofol are mediated by β3 subunits, while the sedative effects are mediated through agonism at β2 subunits.190,221 Volatile general anesthetics also directly activate GABAA Cl– channels.
Some of ethanol’s action is mediated through binding to the GABAA complex. The degree to which ethanol enhances the effect of GABA on Cl– influx depends on the GABAA receptor subunit composition. For example, receptors with an α4 or α6 subunit and a δ subunit respond to very low concentrations of ethanol.199,213
Methaqualone produces at least part of its pharmacologic effect through indirect GABAA activity. Little is known of the mechanisms of action of glutethimide and methyprylon. Their structural similarities to barbiturates suggest that they have activity at the GABAA receptor. Trichloroethanol, a metabolite of chloral hydrate, and clomethiazole interact at the GABAA complex in a manner similar to barbiturates, although it is not clear whether they are binding to an identical site on the Cl– channel.216 Ivermectin, an anthelmintic, activates GABAA Cl– channels by increasing GABA binding. Meprobamate displays barbituratelike action at the GABAA receptor and, at high concentrations, is able to cause Cl– influx in the absence of GABA.169 High concentrations of felbamate also cause inward Cl– currents in the presence of GABA, although this seems unimportant at therapeutic doses.169 Part of the anticonvulsant action of topiramate may result from enhanced Cl– influx through binding to GABAA receptors.180
Inhibition of GABA reuptake.
Valproate and the anticonvulsants guvacine and tiagabine work, in part, by inhibiting GABA reuptake. Although valproate is structurally similar to GABA, its inhibition of the GABA transporter does not appear to be competitive.148
Direct GABAA antagonists.
Xenobiotics that act by any mechanism to decrease GABAA activity can cause CNS excitation and convulsions by decreasing inhibitory inward Cl– currents. Direct antagonists bind to the same site as GABA to prevent GABA binding, the prototype being the convulsant bicuculline. Various antibiotics interact with the GABAA receptor to antagonize the action of GABA. In a dose-dependent manner, both imipenem and cephalosporins appear to directly antagonize GABA binding and can produce seizures at high doses or at therapeutic doses in susceptible individuals.212 Evidence suggests that penicillin may also directly antagonize GABA binding. Electrophysiologic and radioligand-binding studies indicate that norfloxacin, ciprofloxacin, ofloxacin, and enoxacin interact with the GABA binding site to prevent GABA binding.212 Theophylline and at least some NSAIDs markedly enhance GABA antagonism by some fluoroquinolones in vitro.212 Virol A, from Cicuta virosa, appears to directly antagonize binding of GABA to its receptor on the GABAA complex.206
Indirect GABAA antagonists.
Penicillin is well known for producing convulsions at high doses (eg, >20 million units of penicillin per day with renal insufficiency), and both penicillin and aztreonam, a monobactam, appear to block the Cl– channel to prevent GABA-mediated inward Cl– currents.212
Picrotoxin, from Anamirta cocculus (fish berries), and the experimental convulsant, pentylenetetrazol, bind to the picrotoxin site of the GABAA receptor complex to inhibit the action of GABA. Excessive doses produce CNS excitation and convulsions. Some organochlorine insecticides (eg, lindane) also inhibit the action of GABA by binding to what appears to be the picrotoxin site and cause convulsions.117 Both α-thujone, the active component in wormwood oil, and cicutoxin from the water hemlock noncompetitively antagonize GABAA activity.78,207
Flumazenil competitively antagonizes benzodiazepines, zolpidem, zaleplon, and zopiclone at their binding sites to reverse their pharmacologic effects.20,186 Paradoxically, large doses of flumazenil exhibit anticonvulsant activity in animals. This is explained, at least in part, by the ability of flumazenil to inhibit adenosine reuptake.159,195
Cyclic antidepressants, including amoxapine and maprotiline, and at least two MAOIs (isocarboxazid and tranylcypromine) inhibit GABA-mediated Cl– influx at GABAA receptors.121,193 Their potency at inhibiting Cl– influx correlates with the frequency of seizures that occur in patients taking therapeutic doses of these medications. Impaired GABAA activity may contribute to or be primarily responsible for seizures that occur in patients who overdose on these xenobiotics. Their exact binding on the GABAA receptor complex remains unknown, although some evidence suggests at least indirect activity at the picrotoxin-binding site.
Some subtypes of GABAA receptors are susceptible to inhibition by zinc ions.185 What role this plays in normal physiology or toxicology is not established.
Acute withdrawal from all GABAA direct and indirect agonists appears almost identical except for time course; the common denominator is impaired Cl– influx. Withdrawal of all GABAA agonists can cause tremor, hypertension, tachycardia, respiratory alkalosis, diaphoresis, agitation, hallucinations, and convulsions. When GABAA receptors are chronically exposed to an agonist, changes in gene expression of receptor subunits occur, which lessens Cl– influx in response to GABA or drug binding, producing tolerance. Importantly, withdrawal of the agonist produces yet further changes in subunit expression. For example, benzodiazepine-insensitive α4-subunit expression is increased following discontinuation of many GABA agonists, including benzodiazepines, zolpidem, zopiclone, zaleplon, neurosteroids, and ethanol. Expression of other subunits, including α1, γ2, β2, and β1 also change in response to exposure and/or withdrawal of GABAA agonists.57 Alterations in GABAA receptor subunit composition following chronic exposure to and withdrawal of an agonist can, therefore, affect the ability to successfully treat withdrawal symptoms. While any GABAA receptor agonist may be used to treat withdrawal from another, some agents work better than others in different clinical settings. For example, patients experiencing severe alcohol withdrawal may have an increased proportion of GABAA receptors containing benzodiazepine-insensitive α4 subunits, and contain fewer GABAA receptors with benzodiazepine-sensitive α1 subunits.27 Even extremely high doses of benzodiazepines in these patients may not effectively control severe alcohol withdrawal. A better treatment option in such a setting would be GABAA agonists such as propofol or phenobarbital that either act on a different site on the GABAA receptor or directly open the Cl– channel.10,27 Phenytoin and carbamazepine do not stop GABAA withdrawal seizures because their pharmacologic effects are independent of GABAA agonism.
The main GABAB receptor agonist of toxicologic significance is baclofen, which is used for treatment of spasticity and some types of neuropathic pain. Coma, hypothermia, hypotension, bradydysrhythmias, and seizures characterize its toxicity. The convulsions that occur in patients with baclofen overdose are proposed to result from disinhibition (inhibition of inhibitory neurons). Carbamazepine’s activation of GABAB receptors has been demonstrated, although this is not thought to explain most of its anticonvulsant action. Some of the actions of γ-hydroxybutyrate following pharmacologic doses may be mediated through activation of GABAB receptors.
Baclofen withdrawal is similar clinically to GABAA withdrawal. Hallucinations, agitation, tremor, increased sympathetic activity, and convulsions are the main characteristics of baclofen withdrawal. Withdrawal from chronic intrathecal baclofen administration may also be accompanied by large swings in autonomic tone (hypotension, hypertension, tachycardia, bradycardia) and transient cardiomyopathy and shock. Reinstitution of oral baclofen therapy following oral withdrawal, or intrathecal baclofen following intrathecal withdrawal is the treatment of choice when possible.122