Multiple receptors contribute to the behavioral effects of indoleamine hallucinogens

Serotonergic hallucinogens produce profound changes in perception, mood, and cognition. These drugs include phenylalkylamines such as mescaline and 2,5-dimethoxy-4-methylamphetamine (DOM), and indoleamines such as (+)-lysergic acid diethylamide (LSD) and psilocybin. Despite their differences in chemical structure, the two classes of hallucinogens produce remarkably similar subjective effects in humans, and induce cross-tolerance. The phenylalkylamine hallucinogens are selective 5-HT₂ receptor agonists, whereas the indoleamines are relatively non-selective for serotonin (5-HT) receptors. There is extensive evidence, from both animal and human studies, that the characteristic effects of hallucinogens are mediated by interactions with the 5-HT_2A receptor. Nevertheless, there is also evidence that interactions with other receptor sites contribute to the psychopharmacological and behavioral effects of the indoleamine hallucinogens. This article reviews the evidence demonstrating that the effects of indoleamine hallucinogens in a variety of animal behavioral paradigms are mediated by both 5-HT₂ and non-5-HT₂ receptors.     

Evidence that 5-HT1A receptors are involved in the adrenaline-releasing effects of 8-OH-DPAT in the conscious rat

Summary 8-Hydroxy-2-(di-n-propylamino)tetralin (8OH-DPAT) is a 5-HT_1A receptor-selective agonist that has recently been reported to trigger adrenal catecholamine release and hyperglycemia. The aim of this study was to analyze in the conscious rat whether the 5-HT_1A receptor subtype is involved in these effects.8-OH-DPAT (0.1–1 mg/kg, i.v.) evoked dose-dependent increases in plasma adrenaline and glucose concentrations. Increases in plasma adrenaline levels peaked 5 min after administration of 8-OH-DPAT; in contrast, plasma glucose levels rose throughout the 20 min period of analysis. Prior administration of (–)pindolol, a beta-adrenoceptor antagonist that blocks 5-HT_1A receptors, markedly diminished the rise in plasma adrenaline levels and abolished the hyperglycemia triggered by 8-OHD-PAT. On the other hand, neither the selective beta 1-adrenoceptor antagonist, betaxolol, the selective beta 2-adrenoceptor antagonist, ICI 118.551, nor the 5-HT₂ receptor antagonist, ketanserin, affected 8-OH-DPAT-induced increases in plasma adrenaline levels. In addition, except for ICI 118.551 pretreatment, which delayed the hyperglycemic effect of 8-OH-DPAT, none of these antagonists affected the rise in glycaemia evoked by 8-OHD-PAT.The data suggest that the adrenaline-releasing and a major part of the hyperglycemic effects of 8-OH-DPAT are mediated by activation of 5-HT_1A receptors. Send offprint requests to F. Chaouloff at the above address     

Behavioral and neurochemical effects of the serotonin (5-HT)1A receptor ligand spiroxatrine

The effects of spiroxatrine, a putative antagonist with selectivity for the serotonin (5-HT)_1A receptor, were compared with compounds believed to function as agonists at the 5-HT_1A receptor. Schedule-controlled responding of pigeons was maintained under a multiple 30-response fixed-ratio (FR), 3-min fixed-interval (FI) schedule or under a schedule in which responding was suppressed by electric shock (conflict procedure). Under the multiple schedule, spiroxatrine (0.3–1.0 mg/kg) decreased FR responding but did not affect FI responding; responding was decreased in both schedule components at 3.0 mg/kg. When administered alone, buspirone, a compound believed to produce its anxiolytic effects through 5-HT_1A agonist actions, produced effects similar to those of spiroxatrine; in combination, the two drugs produced greater effects than when either was administered alone. As with 5-HT_1A agonists such as buspirone and 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT) in the pigeon, spiroxatrine (0.01–1.0 mg/kg) increased punished responding. Spiroxatrine and buspirone were potent inhibitors of [³H]8-OH-DPAT binding to pigeon cerebral membranes with IC₅₀ values in the nM range. Neurochemical analyses of metabolite changes produced by spiroxatrine in pigeon cerebrospinal fluid showed buspirone-like effects, with increases in MHPG, DOPAC and HVA at doses that decreased 5-HIAA levels. Spiroxatrine dose-dependently blocked the behavioral effects of the dopamine agonist piribedil indicating that, like buspirone, it also is a potent dopamine antagonist. Spiroxatrine most likely functions as an agonist at the 5-HT_1A receptor. As with buspirone, however, spiroxatrine has a prominent dopamine antagonist component.     

Circadian rhythm in the response of central 5-HT1A receptors to 8-OH-DPAT in rats

Circadian rhythm in the behavioral responsiveness to the selective 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was studied in rats. Rats were subcutaneously injected with 8-OH-DPAT at one of the following times of day: 0000, 0400, 0800, 1200, 1600, 2000 hours. The postsynaptic 5-HT_1A receptor behavioral syndrome, that is, forepaw treading, head weaving, and flat body posture, were measured after the administration of 8-OH-DPAT. Circadian rhythms were found in each of the behavioral responses to 8-OH-DPAT. Peak responses were observed in the mid-dark phase (1200 hours) while the weakest responses were observed in the midlight phase (0000 hours). In a subsequent experiment, 8-OH-DPAT was administered intracerebroventricularly during the mid-dark phase and the mid-light phase. The behavioral responses to the drug in the middark phase were significantly higher than those in the mid-light phase. These results suggest that the function of central postsynaptic 5-HT_1A receptor exhibits circadian rhythm.     

Distinct temporal phases in the behavioral pharmacology of LSD: dopamine D<Subscript>2</Subscript> receptor-mediated effects in the rat and implications for psychosis

Rationale The effect of LSD in humans has been described as occurring in two temporal phases. The behavioral effects in rats also occur in two temporal phases: an initial suppression of exploration followed by increased locomotor activity.Objectives We decided to investigate this phenomenon from the perspective that the pharmacology might have relevance to the neurochemical mechanisms underlying psychosis.Methods Twenty-five male Sprague–Dawley rats were trained to discriminate LSD (186 nmol/kg, 0.08 mg/kg, i.p.) with a 30-min preinjection time (LSD-30, N=12) and LSD (372 nmol/kg, 0.16 mg/kg, i.p.) with a 90-min preinjection time (LSD-90, N=13) from saline, using a two-lever, food-reinforced operant conditioning task.Results LSD (186 or 372 nmol/kg, 0.08 or 0.16 mg/kg) given 30 min prior to training produced a cue that was completely antagonized by 5-HT_2A antagonists and lasted no longer than 1 h. LSD (372 nmol/kg, 0.16 mg/kg) injected 90 min before training produced a cue that was not fully blocked by 5-HT_2A antagonists, but instead was significantly inhibited by haloperidol. In these rats, substitution no longer occurred with the 5-HT₂ agonists DOI or LSD (30 min preinjection), but full substitution was obtained with the D₂ agonists apomorphine, N-propyldihydrexidine, and quinelorane.Conclusion The discriminative stimulus effect of LSD in rats occurs in two phases, and these studies provide evidence that the later temporal phase is mediated by D₂ dopamine receptor stimulation. A second temporal phase that involves dopaminergic pathways would be consistent with the widespread belief that excessive dopaminergic activity may be an underlying cause of paranoid psychosis.     

Somatodendritic 5-HT1A receptors are critically involved in the anxiolytic effects of 8-OH-DPAT

In the rat shock-induced ultrasonic vocalization test, the anxiolytic effects of the 5-HT_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) obtained after systemic (IP) and intracerebral injection into the dorsal raphe nucleus (DRN) were selectively abolished by pretreatment with the 5-HT_1A receptor antagonist WAY-100635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide trihydrochloride]. This blockade was demonstrated both after systemic and DRN application of WAY-100635. Therefore, it is concluded that the anxiolytic effects of 8-OH-DPAT are mediated by activation of somatodendritic 5-HT_1A receptors.     

Effects of serotonergic manipulations on cocaine self-administration in rats

Rats were trained to self-administer cocaine (0.5 mg/kg/infusion) and were then pretreated with the 5-HT_1A agonist 8-OH-DPAT (0.125, 0.25 or 0.5 mg/kg, SC). 8-OH-DPAT pretreatment produced a decrease in reinforced response rates. When the effect of 8-OH-DPAT (0.5 mg/kg, SC) on responding for a range of cocaine doses was assessed, the drug produced a decrease in response rates when lower doses of cocaine served as the reinforcer. Fluoxetine (10 mg/kg, IV), an indirect 5-HT agonist, also reduced reinforced response rates for a low dose infusion of cocaine. Rates of responding for infusions of higher cocaine doses were not affected by fluoxetine pretreatment during an FR1 schedule of reinforcement. When an FR10 schedule of reinforcement was imposed, reinforced response rates for infusions of higher doses of cocaine were also reduced. Thus, under conditions that produce high rates of responding (low dose infusion or high ratio requirements for an infusion) fluoxetine reduced responding. This effect may be due to the effects at the 5-HT_1A receptor, since 8-OH-DPAT produced a similar effect on cocaine self-administration. Given that the effects of these 5-HT agonists are observed only when low doses of cocaine serve as the reinforcer or when task demands are high, it is possible that the suppression of responding reflects an effect that is not specific to the reinforcing impact of cocaine. An alternative explanation for these effects incorporates a concept of unit cost/cocaine infusion that allows for direct comparison across studies that employ different reinforcement schedules.     

Functional crosstalk and heteromerization of serotonin 5-HT2A and dopamine D2 receptors

The serotonin 5-HT_2A receptor (5-HT_2AR) and dopamine D₂ receptor (D₂R) are high-affinity G protein-coupled receptor targets for two different classes of antipsychotic drugs used to treat schizophrenia. Interestingly, the antipsychotic effects are not based on the regulation of same signaling mediators since activation of the 5-HT_2AR and of the D₂R regulate G_q/11 protein and G_i/o protein, respectively. Here we use radioligand binding and second messenger production assays to provide evidence for a functional crosstalk between 5-HT_2AR and D₂R in brain and in HEK293 cells. D₂R activation increases the hallucinogenic agonist affinity for 5-HT_2AR and decreases the 5-HT_2AR induced inositol phosphate production. In vivo, 5-HT_2AR expression is necessary for the full effects of D₂R antagonist on MK-801-induced locomotor activity. Co-immunoprecipitation studies show that the two receptors can physically interact in HEK293 cells and raise the possibility that a receptor heterocomplex mediates the crosstalk observed. The existence of this 5-HT_2AR-D₂R heteromer and crosstalk may have implications for diseases involving alterations of serotonin and dopamine systems and for the development of new classes of therapeutic drugs.     

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of hallucinogenic drugs III: the mechanistic basis for supersensitivity to the LSD stimulus following serotonin depletion

The present study was designed to determine the effects ofp-chlorophenylalanine (PCPA) andp-chloroamphetamine (PCA) administration on (1) the levels of serotonin (5-hydroxytryptamine, 5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in rat brain, (2) the sensitivity of LSD-trained rats to the stimulus effects of LSD, and (3) the maximal levels of 5-HT_2A and 5-HT_2C receptor mediated phosphoinositide (PI) hydrolysis in rat brain. PCA and PCPA both produced a significant depletion of whole brain 5-HT and 5-HIAA concentrations. The depletion of serotonin with PCPA, but not PCA, resulted in supersensitivity of LSD-trained subjects to the stimulus effects of LSD. Neither PCPA nor PCA treatment altered the maximal level of 5-HT_2A receptor-mediated PI hydrolysis. However, PCPA, but not PCA, treatment resulted in a significant upregulation (46%,P<0.05) of the maximal level of 5-HT_2C receptor mediated PI hydrolysis. These data suggest that upregulation of the 5-HT_2C receptor mediates the supersensitivity to LSD discriminative stimulus which follows the depletion of central nervous system serotonin by PCPA.This study was supported in part by US Public Health Service grant DA 03385 (J.C.W., R.A.R.), by National Research Service Award MH 10567 (D.F.), and by a fellowship from Schering-Plough Research Institute (D.F.). Animals used in these studies were maintained in accordance with the Guide for Care and Use of Laboratory Animals of the Institute of Laboratory Animal Resources, National Research Council.     

Complex discriminative stimulus properties of (+)lysergic acid diethylamide (LSD) in C57Bl/6J mice

Rationale The drug discrimination procedure is the most frequently used in vivo model of hallucinogen activity. Historically, most drug discrimination studies have been conducted in the rat. With the development of genetically modified mice, a powerful new tool has become available for investigating the mechanisms of drug-induced behavior. The current paper is part of an ongoing effort to determine the utility of the drug discrimination technique for evaluating hallucinogenic drugs in mice.Objective To establish the training procedures and characterize the stimulus properties of (+)lysergic acid diethylamide (LSD) in mice.Methods Using a two-lever drug discrimination procedure, C57Bl/6J mice were trained to discriminate 0.45 mg/kg LSD vs saline on a VI30 sec schedule of reinforcement, with vanilla-flavored Ensure serving as the reinforcer.Results As in rats, acquisition was orderly, but the training dose was nearly five-fold higher for mice than rats. LSD lever selection was dose-dependent. Time-course studies revealed a rapid loss of the LSD stimulus effects. The 5-HT_2A/2C receptor agonist, 2,5-dimethoxy-4-bromoamphetamine [(–)DOB] (1.0 mg/kg), substituted fully for LSD and the 5-HT_1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) (1.6 mg/kg), substituted partially for LSD. Pretreatment with the 5-HT_2A receptor-selective antagonist, MDL 100907, or the 5-HT_1A-selective antagonist WAY 100635, showed that each antagonist only partially blocked LSD discrimination. Substitution of 1.0 mg/kg (–)DOB for LSD was fully blocked by pretreatment with MDL 100907 but unaltered by WAY 100635 pretreatment.Conclusions These data suggest that in mice the stimulus effects of LSD have both a 5-HT_2A receptor and a 5-HT_1A receptor component.     

The 5-HT1A receptor and behavioral stimulation in the rat: effects of 8-OHDPAT on spontaneous and cocaine-induced behavior

Rationale The contribution of the 5-HT_1A somato-dendritic autoreceptor populations to spontaneous and cocaine-induced locomotion is unclear.Objectives To use a low dose range of ±8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT) to preferentially stimulate 5-HT_1A autoreceptors and a medium 8-OHDPAT dose range to stimulate both 5-HT_1A autoreceptors and postsynaptic receptors as pretreatments prior to either saline or cocaine.Methods In experiment 1, either a medium dose of 8-OHDPAT (0.4 mg/kg) or a low dose (0.05 mg/kg) was given as pretreatments 20 min before five separate 20-min open-field tests. In experiment 2, the pretreatments were changed to a low dose range of 8-OHDPAT (0.01–0.05 mg/kg), with or without WAY 100635 (0.01–0.05 mg/kg). In experiment 3, the 8-OHDPAT pretreatments (0.01, 0.025 or 0.05 mg/kg) were administered 20 min prior to saline or cocaine (10 mg/kg) tests. In experiment 4, a medium dose range (0.2–0.3 mg/kg) was given 20 min prior to saline or cocaine (10 mg/kg) tests.Results Experiment 1 showed that 8-OHDPAT (0.4 mg/kg) tended to increase locomotor activity but that pretreatment with 0.05 mg/kg severely suppressed locomotor activity. In experiment 2, 8-OHDPAT in the low dose range inhibited locomotor activity and this effect was reversed by co-administration of WAY 100635. Experiment 3 showed that the low-dose 8-OHDPAT pretreatment reduced locomotor activity in saline but not cocaine tests. In experiment 4, 8-OHDPAT in the medium dose range enhanced locomotor activity in cocaine tests.Conclusions It is suggested that the facilitatory effect of 8-OHDPAT on cocaine-induced locomotor stimulation is mediated by inhibition of 5-HT_1A somato-dendritic autoreceptors.     

Hallucinogenic drug interactions at human brain 5-HT2 receptors: implications for treating LSD-induced hallucinogenesis

It has been shown that the hallucinogenic potencies of LSD, the phenylisopropylamines, such as DOB (4-bromo-2,5-dimethoxyphenylisopropylamine) and DOI (4-iodo-2,5-dimethoxyphenylisopropylamine), and the indoleaklylamines, such as DMT (dimethyltryptamine) and 5-OMe-DMT (5-methoxy-dimethyltryptamine), strongly correlate with their in vitro 5-HT₂ receptor binding affinities in rat cortical homogenates. In order to ascertain if this correlation applies to human 5-HT₂ receptors as well, we examined the affinities of 13 psychoactive compounds at ³H-ketanserin-labelled 5-HT₂ receptors in human cortical samples. Both radioligand binding and autoradiographical procedures were used. As in rat brain d-LSD was the most potent displacer of ³H-ketanserin specific binding with a K _i of 0.9 nM. The phenylisopropylamine DOI also displayed high affinity (K _i of 6 nM). Stereospecific interactions were found with DOB; (-_ DOB had a K _i of 17 nM while (+) DOB had a K _i of 55 nM. The behaviorally active compound DOM (4-methyl-2,5-phenylisopropylamine) had an affinity of 162 nM while its behaviorally less active congener iso-DOM had an affinity of 6299 nM. The indolealkylamines 5-OMe-DMT and DMT competed with moderate affinities (207 and 462 nM, respectively). In general, Hill coefficients were significantly less than unity which is consistent with an agonist interaction with 5-HT₂ receptors. MDMA, a substituted amphetamine analog was inactive with a K _i of greater than 10 M. A strong correlation was found for the hallucinogen affinities and human hallucinogenic potencies (r=0.97). Also, human and rat brain 5-HT₂ receptor affinities were strongly correlated (r=0.99). These results strongly support the hypothesis that the hallucinogenic effects of these drugs in humans are mediated in whole or in part via 5-HT₂ receptors. Furthermore, these studies imply that treatment with 5-HT₂ receptor antagonists may be effective in reversing the hallucinogenic effects caused by the ingestion, of LSD and LSD-like drugs.     

Serotonin receptors represent highly favorable molecular targets for cognitive enhancement in schizophrenia and other disorders

Rationale Current treatments for schizophrenia adequately treat the positive symptoms of schizophrenia but only modestly improve cognitive deficits. This review provides evidence for and against the use of selective 5-HT receptor drugs as cognition enhancing agents for schizophrenia and other disorders.Methods Pre-clinical and clinical literature concerned with the role of the serotonergic system in cognition and memory as it relates to schizophrenia is reviewed. Individual 5-HT receptor subtypes for which selective drugs are available that are likely to improve cognition are reviewed. Recommendations for clinical testing are proposed.Results and conclusions Four 5-HT receptor systems (5-HT_1A, 5-HT_2A, 5-HT₄, 5-HT₆) are highlighted as suitable targets for enhancing cognition and memory. Because many clinically available antipsychotic drugs already target 5-HT_1A, 5-HT_2A and 5-HT₆ receptors, design of clinical trials will need to take into account the serotonergic pharmacology of concurrently administered antipsychotic medications. 5-HT_1A partial agonists and 5-HT_2A antagonists have shown modest effectiveness in improving cognition in schizophrenia. 5-HT₆-selective compounds for cognition enhancement are in late-stage clinical trials, while 5-HT₄ compounds have not yet been tested in humans for cognition enhancement.Recommendations For stand-alone therapy for enhancing cognition, 5-HT_1A partial agonists, 5-HT_2A antagonists, 5-HT₄ partial agonists and 5-HT₆ antagonists are all likely to induce at least modest improvement in cognition in schizophrenia. If add-on therapy is contemplated, antipsychotic drugs with weak affinities for serotonin receptors should be used to avoid confounds. It is likely that serotonergic drugs will soon be available as cognition enhancing medications for disorders other than schizophrenia (e.g. dementia).     

Chronic lithium treatment enhances the postsynaptic 5-HT1A receptor-mediated 5-HT behavioral syndrome induced by 8-OH-DPAT in rats via catecholaminergic systems

The effects of antimanic agents, including lithium, carbamazepine, clonazepam and zotepine, on the postsynaptic 5-HT_1A receptor-mediated behavioral and hypothermic responses induced by 8-OH-DPAT in rats, and on [³H]8-OH-DPAT binding to 5-HT_1A receptors in the rat hippocampus were examined. Treatment with lithium (3 mEq/kg, IP) for 14 days enhanced forepaw treading, one component of the 5-HT behavioral syndrome induced by 8-OH-DPAT, and this enhancement by lithium was abolished by catecholamine depletion with reserpine or -methyl-p-tyrosine, but not by 5-HT depletion withp-chlorophenylalanine. These data suggest that lithium enhances 5-HT_1A-mediated behavior via catecholaminergic systems. In contrast, chronic lithium treatment did not alter the hypothermic response to 8-OH-DPAT in untreated rats, as well as in rats treated with reserpine. These findings strengthen the suggestion that lithium has no direct influence on the postsynaptic 5-HT_1A-mediated response. Other antimanic agents had no effect on either forepaw treading or hypothermia induced by 8-OH-DPAT. Radioligand binding studies using [³H]-8-OH-DPAT demonstrated that chronic lithium treatment, but not other antimanic agents, caused 5-HT_1A receptor down-regulation in rat hippocampus. A discrepancy therefore exists between 5-HT_1A receptor down-regulation and unaltered 5-HT_1A-mediated behavioral and hypothermic responses in catecholamine-depleted rats after chronic lithium treatment.     

Depletion of brain serotonin by 5,7-DHT: effects on the 8-OH-DPAT-induced changes of sleep and waking in the rat

The effect of 5-HT_1A receptor agonist 8-OH-DPAT on sleep and wakefulness was studied in rats with selective serotonin depletion after ICV administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). Injection of 8-OH-DPAT to vehicle-treated animals induced biphasic effects, such that low doses (0.010 mg/kg) which act perferentially on the somatodendritic autoreceptor decreased wakefulness (W) and increased slow wave sleep (SWS), while higher doses (0.375 mg/kg) which stimulate postsynaptic receptors caused opposite effects. REM sleep was suppressed irrespective of the dosage given. Injection of the 0.010 mg/kg dose in the 5,7-DHT-treated rats did not result in significant changes in sleep or W. On the other hand, the 0.375 mg/kg dose produced changes in sleep variables which were similar to those described in the vehicle-treated animals. Our findings tend to indicate that increased SWS after low doses of 8-OH-DPAT depends upon the activation of inhibitory somatodendritic 5-HT_1A receptors, while increased W after higher doses of the compound is related to stimulation of postsynaptic receptors.     

Interactions of selective serotonin reuptake inhibitors with the serotonin 5-HT2C receptor

Interactions of the selective serotonin reuptake inhibitors (SSRIs) citalopram, fluoxetine and its main metabolite norfluoxetine, and the tricyclic anti-depressant (TCA) imipramine with the rat serotonin 5-HT_2C receptor in a clonal cell line and in the rat choroid plexus were investigated by radioligand binding and phosphoinositide (PI) hydrolysis assays. For comparison, the affinities of a variety of other antidepressants of different chemical classes for the cloned rat 5-HT_2C and 5-HT_2A receptors were also determined by radioligand binding assays. Fluoxetine displayed relatively high affinity for the 5-HT_2C receptor in the choroid plexus, with a K_i value for inhibition of [³H]mesulergine binding of 55.4 nM. The K_i values for imipramine, norfluoxetine and citalopram were 136 nM, 203 nM, and 298 nM, respectively. Similar rank order of potency was detected in PI hydrolysis assays, which showed that these drugs are antagonists at the 5-HT_2C receptor without exhibiting inverse agonist activity. [³H]Ketanserin (5-HT_2A) binding assays revealed that the SSRIs fluoxetine, norfluoxetine and citalopram show 10- to 23-fold selectivity for the 5-HT_2C receptor in vitro, whereas the TCA imipramine does not. Many other TCAs also had high to intermediate affinity for both 5-HT_2A and 5-HT_2C receptors. The present data provide evidence that fluoxetine, norfluoxetine and citalopram, along with many other antidepressant compounds, interact directly with the 5-HT_2C receptor.     

Pharmacological studies of 8-OH-DPAT-induced pupillary dilation in anesthetized rats

Serotonin (5-HT)(1A) receptor agonists have been reported to produce mydriasis in mice, and miosis in rabbits and humans. However, the underlying mechanisms for this action are unclear. This study was undertaken in an attempt to explore the mechanism by which 5-HT(1A) receptors are involved in the modulation of pupillary size in pentobarbital-anesthetized rats. Intravenous administration of the 5-HT(1A) receptor agonist, (2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OH-DPAT; 0.003-3 mg/kg), elicited dose-dependent pupillary dilation, which was not affected by section of the preganglionic cervical sympathetic nerve. 8-OH-DPAT-elicited mydriatic responses were attenuated by the selective 5-HT(1A) receptor antagonist, N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate (WAY 100635; 0.3-1 mg/kg, i.v.), as well as by the selective alpha(2)-adrenoceptor antagonist, (8aR,12aS,13aS)-5,8,8a,9,10,11,12,12a,13,13a-dechydro-3-methoxy-12-(ethylsulfonyl)-6H-isoquino[2,1-g][1,6]naphthyridine hydrochloride (RS 79948; 0.3 mg/kg, i.v.), but not by the selective alpha(1)-adrenoceptor antagonist, prazosin (0.3 mg/kg, i.v.). Mydriatic responses elicited by the alpha(2)-adrenoceptor agonist, guanabenz (0.003-0.3 mg/kg, i.v.), were not antagonized by WAY 100635 (0.3-1 mg/kg, i.v.). To determine whether central nervous system (CNS) 5-HT(1A) receptors, like alpha(2)-adrenoceptors, are involved in reflex mydriasis, voltage response curves of pupillary dilation were constructed by stimulation of the sciatic nerve in anesthetized rats. WAY 100635 (1 mg/kg, i.v.) did not antagonize the evoked reflex mydriasis, which, however, was blocked by RS 79948 (0.3 mg/kg, i.v.). Taken together, these results suggest that 8-OH-DPAT produces pupillary dilation in anesthetized rats by stimulating CNS 5-HT(1A) receptors, which in turn trigger the release of norepinephrine, presumably from the locus coeruleus. The latter reduces parasympathetic neuronal tone to the iris sphincter muscle by stimulation of postsynaptic alpha(2)-adrenoceptors within the Edinger-Westphal nucleus. Unlike alpha(2)-adrenoceptors, 5-HT(1A) receptors in the CNS do not mediate reflex mydriasis evoked by sciatic nerve stimulation.     

Role of 5-HT2A and 5-HT2C receptors in the stimulus effects of hallucinogenic drugs II: reassessment of LSD false positives

In the context of animal studies of hallucinogens, an LSD-false positive is defined as a drug known to be devoid of hallucinogenic activity in humans but which nonetheless fully mimics LSD in animals. Quipazine, MK-212, lisuride, and yohimbine have all been reported to be LSD false positives. The present study was designed to determine whether these compounds also substitute for the stimulus effects of the more pharmacologically selective hallucinogen (–)DOM (0.56 mg/kg, 75-min pretreatment time). The LSD and (–)DOM stimuli fully generalized to quipazine (3.0 mg/kg) and lisuride (0.2 mg/kg), but only partially generalized to MK-212 (0.1–1.0 mg/kg) and yohimbine (2–20 mg/kg). In combination tests, pirenpirone (0.08 mg/kg), a compound with both D₂ and 5-HT_2A affinity, blocked the substitution of quipazine and lisuride for the (–)DOM stimulus. Ketanserin (2.5 mg/kg), an antagonist with greater than 1 order of magnitude higher affinity for 5-HT_2A receptors than either 5-HT_2C or D₂ receptors, also fully blocked the substitution of these compounds for the (–)DOM stimulus, while the selective D₂ antagonist thiothixene (0.1–1.0 mg/kg) failed to block the substitution of lisuride for the (–)DOM stimulus. These results suggest that quipazine and lisuride substitute for the stimulus properties of the phenylalkglamine hallucinogen (–)DOM via agonist activity at 5-HT_2A receptors. In addition, these results suggest that 5-HT_2A agonist activity may be required, but is not in itself sufficient, for indolamine and phenylalkglamine compounds to elicit hallucinations in humans. Finally, it is concluded that MK-212 and yohimbine are neither LSD nor (–)DOM false positives.This study was supported in part by US Public Health Service grant DA 03385 (J.C.W., R.A.R.), by National Research Service Award MH 10567 (D.F.), and by a fellowship from Schering-Plough Research Institute (D.F.). Animals used in these studies were maintained in accordance with the Guide for Care and Use of Laboratory Animals of the Institute of Laboratory Animal Resources, National Research Council.     

Cholesterol reduction attenuates 5-HT1A receptor-mediated signaling in human primary neuronal cultures

Lipid rafts regulate functions of various G protein-coupled receptors and signaling proteins. We show that human primary neuronal cultures contain high levels of 5-HT(1A) receptors. Stimulation with the 5-HT(1A/7) receptor agonist, 8-OH-DPAT, reduced P-T(185)/Y(187)-ERK2. This reduction could be blocked by the 5-HT(1A) receptor antagonist, WAY100635. Pretreatment with the cholesterol sequestering agent, methyl-beta-cyclodextrin, before adding 8-OH-DPAT, significantly counteracted the inhibitory influence of 8-OH-DPAT on P-T(185)/Y(187)-ERK2 and P-S(133)-CREB. These data indicate that reduction of cholesterol levels significantly influence signaling via 5-HT(1A) receptors in intact neurons.     

Role of serotonin 5-HT<Subscript>1A</Subscript> and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats

Rationale Tramadol (1RS, 2RS)-2-[(dimethylamino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol) is an atypical centrally acting analgesic agent with weak opioid receptor affinity that, like some antidepressants, enhances the extraneuronal concentrations of the monoamine neurotransmitters, noradrenaline and serotonin, by interfering with their re-uptake and release mechanisms. Objectives The present study was undertaken to evaluate the potential role of 5-HT_1A receptors and opioids receptors in the analgesic effect of tramadol in neuropathic pain. With this aim, the effect of either a selective 5-HT_1A receptor antagonist (WAY-100635, N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT_1A receptor agonist (8-OH-DPAT, 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) or an opioid receptor antagonist (naloxone; naloxone hydrochloride dihydrate) was investigated in combination with tramadol by means of the cold-plate test in the chronic constriction injury model in rats. Results The results showed that WAY-100635 (0.8 mg/kg) significantly enhanced the antiallodynic effect of non-effective doses of tramadol (5–10 mg/kg). In contrast, 8-OH-DPAT (0.5 mg/kg) counteracted the antiallodynic effect of an effective dose of tramadol (22 mg/kg). Naloxone (0.5 mg/kg) partially counteracted the antiallodynic effect of tramadol (22 mg/kg). Conclusions These findings suggest the involvement of opioid and 5-HT_1A receptors in the antinociceptive effect of tramadol and support the idea that the combination of tramadol with compounds having 5-HT_1A antagonist properties could be a new strategy to improve tramadol-induced analgesia in neuropathic pain.