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.     

The time-dependent stimulus effects of R(-)-2,5-dimethoxy-4-methamphetamine (DOM): implications for drug-induced stimulus control as a method for the study of hallucinogenic agents

The pharmacodynamic characteristics of the stimulus effects of the hallucinogensd-LSD and (–)DOM were investigated in the rat. The stimulus control induced by (–)DOM (0.56 mg/kg) was significantly less stable at the 15-min pretreatment time than at the 75-min pretreatment time. In addition, (–)DOM (0.8 mg/kg) produced a time-dependent substitution for the LSD stimulus in LSD trained subjects (0.1 mg/kg, 15-min pretreatment time). As pretreatment times were increased, the substitution of (–)DOM (0.8 mg/kg) for the LSD stimulus increased, culminating in a maximal level of 99.5% LSD-appropriate responding at the 75-min pre-treatment time. A dose-response relationship for the substitution of (–)DOM (75-min pretreatment time) for the LSD stimulus, indicated that 0.2 mg/kg (–)DOM was the minimum dose which elicited greater than 90% LSD-appropriate responding. LSD (0.32 mg/kg, 15-min pretreatment time) fully substituted for (–)DOM in the (–)DOM trained subjects (0.56 mg/kg, 75-min pretreatment time). These findings suggest that the pharmacodynamic parameters ofd-LSD and (–)DOM-induced stimulus control differ. The time of onset for the stimulus effects of (–)DOM is markedly longer than that of LSD in the rat.This study was supported in part by U.S. 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     

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects ofm-chlorophenylpiperazine

m-Chlorophenylpiperazine (mCPP), a major metabolite of the atypical antidepressant trazadone, has been observed to produce marked physiological and behavioral effects in both humans and animals. These effects have been attributed to the interaction of mCPP with serotonergic receptors. The present study was designed to characterize those interactions of mCPP with central serotonergic receptors which mediate mCPP-induced stimulus control. A series of serotonergic antagonists (mesulergine, pizotyline, ketanserin, spiperone, risperidone, ritanserin, metergoline, pirenpirone, and LY53857) was tested for the ability to block the mCPP stimulus. The affinity of these antagonists for 5-HT_2A and 5-HT_2C receptors was then correlated with maximal percent inhibition of the mCPP stimulus. K_d at the 5-HT_2C receptor was inversely proportional (r=–0.75,P<0.05), and K_d at the 5-HT_2A receptor directly proportional (r=+0.67,P<0.05) to the maximal percent inhibition of the mCPP stimulus. The 5-HT_2C selectivity ratio [K_d(5-HT_2A)/K_d(5-HT_2C)] of the antagonists was directly proportional (r=+0.86,P<0.01) to maximal percent inhibition of the mCPP stimulus. A multiple regressions analysis indicated that 81% of the variance in the ability of a given antagonist to block the mCPP stimulus could be predicted on the basis of its affinity for 5-HT_2A and 5-HT_2C receptors. It is concluded that the stimulus effects of mCPP are mediated predominantly by a combination of agonist activity at 5-HT_2C receptors and antagonist activity at 5-HT_2A receptors.This study was supported in part by U.S. 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.     

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.     

5-HT2A and 5-HT2C receptor antagonists have opposing effects on a measure of impulsivity: interactions with global 5-HT depletion

Rationale Global serotonin (5-HT) depletion increases the number of premature responses made on the five-choice serial reaction time task (5CSRT) in rats. In contrast, the 5-HT_2A receptor antagonist M100907 decreases this measure of impulsivity. Mounting evidence suggests that 5-HT_2A and 5-HT_2C receptors have opposing effects on behaviour, and that the 5-HT_2C receptor antagonist SB 242084 produces a pattern of behaviour similar to 5-HT depletion.Objectives To assess the effects of 5-HT_2A and 5-HT_2C receptor antagonists on performance of the 5CSRT, to directly compare the effects of these drugs with those of ICV 5,7-dihydroxytryptamine (5,7-DHT) lesions and to investigate whether 5-HT depletion affects the action of these agents.Methods The effects of M100907 (0, 0.01, 0.03, 0.1 mg/kg IP) and SB 242084 (0, 0.1, 0.25, 0.5 mg/kg IP) were investigated on performance of the 5CSRT in both ICV 5,7-DHT-lesioned and sham-operated rats.Results ICV 5,7-DHT lesions, which significantly decreased forebrain levels of 5-HT by around 90%, increased levels of premature responding, decreased omissions and the latency to respond correctly, yet did not affect performance accuracy. M100907 decreased premature responding in sham-operated controls but not in 5-HT-depleted rats. In contrast, SB 242084 increased premature responding in all animals, and also decreased the latency to make a correct response in sham-operated controls.Conclusions These data support the view that serotonergic regulation of impulsive behaviour through different members of the 5-HT₂ receptor family is functionally heterogeneous. Although both 5-HT_2A and 5-HT_2C receptors participate in controlling this form of impulsive action, their relative contribution may depend on the endogenous state of the 5-HT system.     

Acute and chronic tryptophan depletion differentially regulate central 5-HT1A and 5-HT2A receptor binding in the rat

Rationale Tryptophan depletion is used to reduce central serotonergic function and to investigate its role in psychiatric illness. Despite widespread clinical use, its effects on serotonin (5-HT) receptors have not been well characterized. Objective The aim of this study was to examine the effect of acute (ATD) and chronic tryptophan depletion (CTD) on free-plasma tryptophan (TRP), central TRP and 5-HT and brain 5-HT_1A and 5-HT_2A receptor binding in the rat. Methods TRP and 5-HT were measured by high-performance liquid chromatography and receptor levels determined by homogenate radioligand binding and in-vitro receptor autoradiography. Results Free-plasma TRP, central TRP and central 5-HT levels were significantly and similarly reduced by ATD and 1- and 3-week CTD compared to controls. ATD significantly reduced 5-HT_1A binding in the dorsal raphe (14%) but did not significantly alter postsynaptic 5-HT_1A binding (frontal cortex, remaining cortex and hippocampus) or 5-HT_2A binding (cortex and striatum). One-week CTD did not significantly alter cortical 5-HT_2A binding or postsynaptic 5-HT_1A binding. Furthermore, 3-week CTD did not significantly alter 5-HT_1A binding but significantly increased cortical 5-HT_2A binding without affecting striatal or hippocampal levels. In the CTD 1 and 3-week groups, rat body weight was significantly decreased as compared to controls. However, weight loss was not a confounding factor for decreased cortical 5-HT_2A-receptor binding. Conclusion ATD-induced reduction in somatodendritic 5-HT_1A autoreceptor binding may represent an intrinsic ‘homeostatic response’ reducing serotonergic feedback in dorsal raphe projection areas. In contrast, the increase in 5-HT_2A receptor after CTD may be a compensatory response to a long-term reduction in 5-HT.     

Diltiazem potentiates pentobarbital-induced hypnosis via 5-HT1A and 5-HT2A/2C receptors: role for dorsal raphe nucleus

It has been reported that the sedative component of pentobarbital is mediated by GABA receptors in an endogenous sleep pathway and the ventrolateral preoptic area (VLPO)-tuberomammillary nucleus (TMN) or VLPO-dorsal raphe nucleus (DRN) neural circuit is important in the sedative response to pentobarbital. Our previous findings indicated that the VLPO-TMN neuronal circuit may play crucial part in the augmentative effect of diltiazem on pentobarbital sleep and the serotonergic system may be involved. This study was designed to investigate the role of DRN and the serotonergic receptors 5-HT_1A and 5-HT_2A/2C in the augmentative effect of diltiazem on pentobarbital-induced hypnosis in rats. The results showed that diltiazem (5 mg/kg, i.g.) significantly reversed pentobarbital-induced (35 mg/kg, i.p.) reduction of c-Fos expression in 5-HT neurons of DRNV (at − 7.5 mm Bregma), DRND, DRNVL and MRN (at − 8.0 mm Bregma). However it did not influence this reducing effect of pentobarbital on non-5-HT neurons either in DRN or in MRN. Moreover, the effect of diltiazem (1 or 2 mg/kg, i.g.) on pentobarbital-induced (35 mg/kg, i.p.) hypnosis was significantly inhibited by 5-HT_1A agonist 8-OH-DPAT (0.5 mg/kg, i.p.) and 5-HT_2A/2C agonist DOI (0.5 mg/kg, i.p.), and potentiated by 5-HT_1A antagonist p-MPPI (2 mg/kg, i.p.) and 5-HT_2A/2C antagonist ritanserin (2 mg/kg, i.p.), respectively. From these results, it should be presumed that the augmentative effect of diltiazem on pentobarbital-induced sleep may be related to 5-HT_1A and 5-HT_2A/2C receptors, and DRN may be involved. In addition, it also suggested that the DRN may play a multi-modulating role in sleep-wake regulation rather than being recognized simply as arousal nuclei.     

Molecular structural basis of ligand selectivity for 5-HT2 versus 5-HT1C cortical receptors

Summary A molecular structural criterion of ligand selectivity for the 5-HT₂ versus 5-HT_1C receptor was hypothesized on the basis of radioligand binding data. Despite the large number of compounds which have been tested at both receptors, analysis of published data led to the identification of only five agents which are greater than 10-fold selective for the 5-HT₂ versus the 5-HT_1C receptor. Comparison of the two-dimensional structures revealed that, although these five compounds represent three distinct structural classes, they share a common structural feature located in the region hypothesized to be involved in receptor binding: a carbonyl or carboxyl oxygen interposed spatially between an aromatic ring and nitrogen atom. This structural feature was used to predict the relative selectivity of compounds that had not previously been analyzed at both the 5-HT₂ and 5-HT_1C receptors.All six drugs tested which contain the identified reactive carbonyl or carboxyl group were found to be selective for the 5-HT₂ versus the 5-HT_1C receptor with selectivity ratios ranging from 26 to 380. By contrast, three agents which are structurally similar but do not contain the reactive carbonyl or carboxyl group displayed equally high affinity for both receptor binding sites. Since the physiological roles of the 5-HT₂ and 5-HT_1C receptor are markedly different, it would be of potential clinical and scientific value to utilize this molecular structural feature to further identify chemical compounds which would selectively interact with only one of the two receptors. Send offprint requests to S. J. Peroutka at the current address     

Comparative desensitization of the human 5-HT2A and 5-HT2C receptors expressed in the human neuroblastoma cell line SH-SY5Y

1. We have used previously characterized clones of the human neuroblastoma cell line, SH-SY5Y, constitutively expressing either the human 5-HT_2A or 5-HT_2C receptor to compare their desensitization profiles after exposure to 5-HT.
2. 5-HT stimulated [³H]-inositol phosphate ([³H]-IP_x) production at both the 5-HT_2C (pEC₅₀=8.03±0.15) and 5-HT_2A receptors (pEC₅₀=7.15±0.08), with maximal responses occurring after exposure to 1 μM and 10 μM 5-HT, respectively.
3. Exposure of cells to maximally effective concentrations of 5-HT caused time- and concentration-dependent desensitization of [³H]-IP_x formation. The 5-HT_2A response desensitized slower (t_1/2=110 min) and with lower sensitivity than that of the 5-HT_2C receptor (t_1/2=12.5 min). In each case, desensitization was blocked by co-administration of a specific receptor antagonist. Following exposure to 10 μM 5-HT for 2 h, both receptors exhibited extensive desensitization, with subsequent responses to 5-HT reduced by more than 80%.
4. 5-HT stimulated Ins(1,4,5)P₃ production with a potency similar to that for [³H]-IP_x production at each receptor. In both cases Ins(1,4,5)P₃ levels peaked rapidly then returned to basal level within a short time. This peak consistently occurred earlier for the 5-HT_2C receptor (5 s) than for the 5-HT_2A receptor (20 s).
5. Prior exposure of SH-SY5Y/5-HT_2C cells to 5-HT (1 μM/15 min) caused a significant decrease in the 5-HT-stimulated peak in Ins(1,4,5)P₃ levels whereas no such change occurred in SH-SY5Y/5-HT_2A cells following exposure to 10 μM 5-HT for 15 min.
6. These results indicate that the human 5-HT_2A and 5-HT_2C receptors both exhibit desensitization at the level of inositol phosphate formation when expressed in the same cellular environment, with the 5-HT_2C receptor being more sensitive to 5-HT-mediated desensitization than the 5-HT_2A receptor.

     

5-HT2C receptors are involved in the discriminative stimulus effects of citalopram in rats

 Rats were trained on a fixed ratio 10, food-reinforced schedule to recognize a discriminative stimulus (DS) elicited by the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram (2.5 mg/kg, IP). The preferential, high efficacy agonist at 5-HT_2C receptors, Ro60-0175, dose-dependently generalized to citalopram with an ED₅₀ of 0.3 mg/kg, IP. Further, the selective 5-HT_2C receptor antagonist, SB242,084, dose-dependently (ED₅₀=0.1 mg/kg, IP) blocked the citalopram DS. These data suggest that 5-HT_2C receptors are involved in the DS properties of the SSRI, citalopram, in rats. They do not, however, exclude a potential role of other 5-HT receptor types. Received: 15 October 1998 / Final version: 10 December 1998     

Citalopram-induced hypophagia is enhanced by blockade of 5-HT1A receptors: role of 5-HT2C receptors

1. The selective 5-hydroxytryptamine reuptake inhibitor citalopram (10 and 20 mg kg⁻¹, i.p.) significantly reduced food intake in male rats (CD-COBS) habituated to eat their daily food during a 4-h period.
2. The 5-HT_1A receptor antagonist WAY100635 (0.3 mg kg⁻¹) administered systemically did not modify feeding but significantly potentiated the reduction in food intake caused by 10 mg kg⁻¹ i.p. citalopram. The dose of 5 mg kg⁻¹ i.p. citalopram was not active in animals pretreated with vehicle but significantly reduced feeding in animals pretreated with WAY100635.
3. WAY100635 (0.1 μg 0.5 μl⁻¹) injected into the dorsal raphe significantly potentiated the hypophagic effect of 10 mg kg⁻¹ citalopram.
4. WAY100635 (1.0 μg 0.5 μl⁻¹) injected into the median raphe did not modify feeding or the hypophagic effect of 10 mg kg⁻¹ citalopram.
5. The 5-HT_2B/2C receptor antagonist SB206553 (10 mg kg⁻¹, p.o.) slightly reduced feeding by itself but partially antagonized the effect of WAY100635 administered systemically (0.3 mg kg⁻¹, s.c.) or into the dorsal raphe (0.1 μg 0.5 μl⁻¹) in combination with 10 mg kg⁻¹ i.p. citalopram. The hypophagic effect of 10 mg kg⁻¹ i.p. citalopram alone was not significantly modified by SB206553.
6. Brain concentrations of citalopram and its metabolite desmethylcitalopram in rats pretreated with SB206553, WAY100635 and their combination were comparable to those of vehicle-pretreated rats, 90 min after citalopram injection.
7. The hypophagic effect of citalopram was potentiated by blocking 5-HT_1A receptors. Only the effect of the WAY100635/citalopram combination seemed to be partially mediated by central 5-HT_2C receptors.

     

Anxiolytic-like effect of a serotonergic ligand with high affinity for 5-HT1A, 5-HT2A and 5-HT3 receptors

S-(-)-2-[[4-(napht-1-yl)piperazin-1-yl]methyl]-1,4-dioxoperhydropyrrolo[1,2-alpha]-pyrazine (CSP-2503) is a serotonin (5-HT) receptor ligand with selectivity and high affinity for 5-HT1A, 5-HT2A and 5-HT3 receptors. CSP-2503 reduced rectal temperature and 5-HT neuronal hypothalamic activity in mice, decreased electrical activity of raphe nuclei cells in rats and blocked the enhancement of adenylate cyclase activity induced by forskolin in HeLa cells transfected with the human 5-HT1A receptor. This compound also blocked head-twitches induced by the 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI). Contractions of guinea pig ileum induced by the 5-HT3 receptor agonist 2-methyl-5-HT were prevented by CSP-2503. Moreover, it reduced the bradycardia reflex induced by 2-methyl-5-HT in anaesthetized rats. In the light/dark box and social interaction tests, CSP-2503 presented anxiolytic activity, an action shared by 5-HT1 agonists and 5-HT3 antagonists. Taken together, these results suggest that CSP-2503 is a new 5-HT1 receptor agonist with 5-HT2A and 5-HT3)receptor antagonist activities that might be useful in a number of conditions associated with anxiety.     

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.     

Comparisons of hallucinogenic phenylisopropylamine binding affinities at cloned human 5-HT2A, 5-HT2B and 5-HT2C receptors

Since the classical hallucinogens were initially reported to produce their behavioral effects via a 5-HT₂ agonist mechanism (i.e., the 5-HT₂ hypothesis of hallucinogen action), 5-HT₂ receptors have been demonstrated to represent a family of receptors that consists of three distinct subpopulations: 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors. Today, there is greater support for 5-HT_2A than for 5-HT_2C receptor involvement in the behavioral effects evoked by these agents. However, with the recent discovery of 5-HT_2B receptors, a new question arises: do classical hallucinogens bind at 5-HT_2B receptors? In the present study we examined and compared the binding of 17 phenylisopropylamines at human 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors. Although there was a notable positive correlation (r>0.9) between the affinities of the agents at all three populations of 5-HT₂ receptors, structural modification resulted only in small differences in 5-HT_2B receptor affinity such that the range of affinities was only about 50-fold. As with 5-HT_2A and 5-HT_2C receptor affinity, there is a significant correlation (r>0.9, n=8) between 5-HT_2B receptor affinity and human hallucinogenic potency. Nevertheless, given that 5-HT_2A and 5-HT_2A/2C antagonists – antagonists with low affinity for 5-HT_2B receptors – have been previously shown to block the stimulus effects of phenylisopropylamine hallucinogens, it is likely that 5-HT_2A receptors play a more prominent role than 5-HT_2B and 5-HT_2C receptors in mediating such effects despite the affinity of these agents for all three 5-HT₂ receptor subpopulations.     

Neuropharmacological reassessment of the discriminative stimulus properties ofd-lysergic acid diethylamide (LSD)

The neuropharmacological mechanisms underlying the behavioral effects ofd-lysergic acid diethylamide (LSD) were assessed by comparing the discriminative stimulus properties of LSD with those of agonists and antagonists that act selectively at putative serotonin (5-hydroxytryptamine; 5-HT) receptor subtypes (5-HT₁ and 5-HT₂). Male Sprague-Dawley rats (N=23) were trained to discriminate LSD (0.08 mg/kg) from saline and given substitution tests with the following agents: 8-hydroxy-2(di-n-propylamino) tetralin (8-OHDPAT; 0.02–0.64 mg/kg), Ru 24969 (0.2–3.2 mg/kg),m-chlorophenylpiperazine (MCPP; 0.1–1.6 mg/kg), 1-(m-trifluoromethylphenyl)piperazine (TFMPP; 0.1–1.6 mg/kg), and quipazine (0.2–3.2 mg/kg). Only quipazine mimicked LSD. In combination tests, BC 105 (0.2–3.2 mg/kg), 2-bromolysergic acid diethylamide (BOL; 0.1–1.6 mg/kg), Ly 53857 (0.4–3.2 mg/kg), metergoline (0.05–0.8 mg/kg), ketanserin (0.2–3.2 mg/kg), and pipenperone (0.0025–0.08 mg/kg), all of which act as 5-HT₂ antagonists, blocked the LSD cue; only spiperone (0.02–0.32 mg/kg) was without effect. Although commonalities may exist among 5-HT agonists, the present results demonstrate that such agonists are not identical. Since putative 5-HT₁ agonists do not mimic LSD and the LSD cue is potently blocked by 5-HT₂ antagonists, it appears that 5-HT₂ neuronal systems are of greater importance than 5-HT₁ systems in mediating the discriminative stimulus and, perhaps, other effects of LSD.     

The discriminative stimulus properties of 2,5-dimethoxy-4-methylamphetamine (DOM): Differentiation from amphetamine

Rats were trained in a two-lever operant procedure to discriminate either 1.0 mg/kg(+)amphetamine or 1.5 mg/kg DOM from saline. Rats trained to discriminate DOM from saline showed generalization with the DOM training condition when tested with mescaline or 2,5-dimethoxy-4-ethylamphetamine (DOET), but not when tested with (+)amphetamine or methylphenidate. Both isomers of DOM generalized with racemic training compound, the (-)isomer being more potent. The DOM stimulus was completely blocked by the serotonin (5-HT) antagonists cinanserin and methysergide, but not by the peripheral 5-HT antagonist xylamidine nor the dopamine antagonist haloperidol. Rats trained to discriminate (+)amphetamine from saline generalized with the amphetamine training condition when tested with methylphenidate but not when tested with mescaline, DOET, racemic DOM, or either isomer of DOM. The amphetamine stimulus was blocked by pretreatment with haloperidol but not by cinanserin, methysergide, or xylamidine. The results show that, despite their structural similarity, amphetamine and DOM induce pharmacologically distinct stimuli.     

Agonist activity of LSD and lisuride at cloned 5HT2A and 5HT2C receptors

Evidence from studies with phenylisopropylamine hallucinogens indicates that the 5HT_2A receptor is the likely target for the initiation of events leading to hallucinogenic activity associated with LSD and related drugs. Recently, lisuride (a purported non-hallucinogenic congener of LSD) was reported to be a potent antagonist at the 5HT_2C receptor and an agonist at the 5HT_2A receptor. LSD exhibited agonist activity at both receptors. These data were interpreted as indicating that the 5HT_2C receptor might be the initiating site of action for hallucinogens. To test this hypothesis, recombinant cells expressing 5HT_2A and 5HT_2C receptors were used to determine the actions of LSD and lisuride. LSD and lisuride were potent partial agonists at 5HT_2A receptors with EC₅₀ values of 7.2 nM and 17 nM, respectively. Also, LSD and lisuride were partial agonists at 5HT_2C receptors with EC₅₀ values of 27 nM and 94 nM, respectively. We conclude that lisuride and LSD have similar actions at 5HT_2A and 5HT_2C receptors in recombinant cells. As agonist activity at brain 5HT_2A receptors has been associated with hallucinogenic acitivity, these results indicate that lisuride may possess hallucinogenic activity, although the psychopharmacological effects of lisuride appear to be different from the hallucinogenic effects of LSD. Received: 19 September 1997/Final version: 31 October 1997     

Blockade of serotonin 5-HT1B and 5-HT2A receptors suppresses the induction of locomotor activity by 5-HT reuptake inhibitors,citalopram and fluvoxamine,in NMRI mice exposed to a novel environment: a comparison to other 5-HT receptor subtypes

Rationale Though 5-HT plays an important role in the modulation of motor function, which is perturbed in depressive states, little is known concerning the influence of serotonin reuptake inhibitors (SSRIs) on locomotor activity (LA). Recently, we demonstrated that SSRIs, such as citalopram, enhance LA in mice exposed to a novel environment. Objectives This study examined the role of multiple classes of 5-HT receptor in citalopram-induced LA. Methods The most selective antagonists currently available were used. Results Citalopram-induced LA was dose-dependently attenuated by the 5-HT_1B/1D receptor antagonists, S18127, GR125,743 and GR127,935, and by the selective 5-HT_1B antagonist, SB224,289, but unaffected by the selective 5-HT_1A antagonist, WAY100,635. The selective antagonists at 5-HT_2A receptors, MDL100,907 and SR46,349 also dose-dependently attenuated induction of locomotion by citalopram, whereas the 5-HT_2B antagonist, SB204,741, and the 5-HT_2B/2C antagonist, SB206,553 were ineffective. Further, the selective 5-HT_2C antagonist, SB242,084, potentiated the response to citalopram. Selective antagonists at 5-HT₃ (ondansetron), 5-HT₄ (GR125,487), 5-HT₆ (SB271,046) and 5-HT₇ (SB269,970) receptors did not significantly modify the action of citalopram. Underpinning these findings, SB224,289, GR125,743, MDL100,907 and SR46,349 likewise attenuated induction of locomotion by a further SSRI, fluvoxamine. Conclusions The locomotor response to SSRIs of mice exposed to a novel environment is mediated via 5-HT_1B and 5-HT_2A receptors. In view of the importance of motor function to the etiology and treatment of depression, the significance of these observations to the clinical actions of SSRIs will be of interest to elucidate.     

mCPP-induced hyperactivity in 5-HT2C receptor mutant mice is mediated by activation of multiple 5-HT receptor subtypes

The serotonin receptor agonist mCPP induces hyperlocomotion in 5-HT2C receptor knockout (KO) mice or in the presence of a 5-HT2C receptor antagonist. In the present group of experiments, we evaluate the role of 5-HT1A, 5-HT1B and 5-HT2A receptors in mCPP-induced hyperactivity in 5-HT2C KO mice. We also assess the ability of agonists at these receptors to induce hyperactivity in wildtype (WT) mice pre-treated with a selective 5-HT2C receptor antagonist. As previously reported, mCPP (3 mg/kg) induced hyperactivity in 5-HT2C KO mice. A combination of the 5-HT1B receptor agonist CP-94,253 (20 mg/kg) and the 5-HT1A receptor agonist 8-OH-DPAT (0.5 mg/kg) induced marked hyperactivity in WT but not in 5-HT2C KO mice, nor in mice treated with the selective 5-HT2C receptor antagonist, SB 242084 (1.5 mg/kg). Neither CP-94,253 nor 8-OH-DPAT had any intrinsic effect on locomotion in WTs. mCPP-induced hyperactivity was attenuated in 5-HT2C KO mice by the 5-HT1B receptor antagonist SB 224289 (2.5 mg/kg), and the 5-HT2A receptor antagonists ketanserin (0.3 mg/kg) and M100907 (0.01 mg/kg) but not by the 5-HT1A receptor antagonist WAY 100635 (1 mg/kg). The 5-HT(2A/2B/2C) receptor agonist, Ro 60-0175 (3 mg/kg), induced a modest increase in locomotor activity in WT mice pre-treated with SB 242084. However, the combination of Ro 60-0175 with CP-94,253 induced a substantial increase in activity in 5-HT2C KO mice, an effect comparable to mCPP-induced hyperactivity. Thus, joint activation of 5-HT1A and 5-HT1B receptors stimulates locomotion in WT mice but this response is dependent on a functional 5-HT2C receptor population and hence is absent in 5-HT2C KO mice. By contrast, mCPP-induced hyperactivity depends on the inactivation of a separate 5-HT2C receptor population and is mediated by 5-HT2A and 5-HT1B receptor activation.     

Opposing effects of 5-HT2A and 5-HT2C receptor antagonists in the rat and mouse on premature responding in the five-choice serial reaction time test

Rationale Serotonin (5-HT) has been linked to impulsivity with recent data suggesting that different receptor sub-types exert opposing influences on this behaviour. Objectives This work characterised the effects of 5-HT_2A (ketanserin, (±)2,3-dimethoxyphenyl-1-[2–4-(piperidine)-methanol] [M100907]), 5-HT_2B (6-chloro-5-methyl-1-(5-quinolylcarbamoyl) indoline [SB215505]) and 5-HT_2C (6-chloro-5-methyl-1-[2-(2-methylpyridyl-3-oxy)-pyrid-5-yl carbomyl] indoline [SB242084]) receptor antagonists on impulsive behaviour, measured in the five-choice serial reaction time test (5CSRTT), in rats and mice. The effects of (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) and (S)-2-(chloro-5-fluoro-indol-1-yl)-1-methylethylamine 1:1 C₄H₄O₄ (Ro60-0175), two compounds that have been used extensively as agonists for the 5-HT_2A and 5-HT_2C receptor, were also measured. Materials and methods Rats and mice were trained on the 5CSRTT in which reinforcement is earned for detecting and correctly responding to brief presentations of a stimulus light. Impulsivity in this task is measured as premature responding, before stimulus presentation. Several variants of the task were used in which the inter-trial interval (ITI) length was manipulated to alter basal levels of premature responding. Results In the rat, ketanserin and M100907 reduced and SB242084 enhanced premature responding. SB215505 had no effect. DOI generally disrupted responding, while Ro60-0175 reduced premature responding when a long ITI was used. In mice, M100907 reduced and SB242084 increased premature responding when the ITI was lengthened. The effects of these drugs on other aspects of performance were less robust. M100907 and ketanserin did not affect response accuracy but tended to slow speed of responding; SB242084 occasionally increased speed of responding and slightly reduced accuracy. Conclusions Serotonin exerts both excitatory and inhibitory influences on motor impulsivity via 5-HT_2A and 5-HT_2C receptors in both rats and mice.