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

Depression of mGluR-mediated IPSCs by 5-HT in dopamine neurons of the rat substantia nigra pars compacta

Dopamine neurons of the substantia nigra pars compacta receive a prominent serotonin (5-HT) projection from the dorsal raphe nucleus and important functional interactions between the serotonergic and the dopaminergic system have been postulated. In the present report we examined the role of 5-HT in the modulation of the metabotropic glutamate receptor-mediated inhibitory postsynaptic current (mGluR-IPSC) in midbrain dopamine neurons, and we found a reversible depression of this synaptic response at concentrations of 5-HT ranging from 100 nm to 30 microm (EC50 1.06 microm). This resulted in a shift towards excitation of the overall dopamine neuron response to glutamatergic synaptic input. This effect was not because of a direct modulation of the Ca2+-sensitive K+ conductances underlying the mGluR-IPSC, but was associated with a decrease in the intracellular calcium signal triggered by mGluR stimulation. Similar results were obtained with alpha-methyl-5-hydroxytryptamine and 5-methoxytryptamine, but not with 5-carboxamidotryptamine or 1-(3-chlorophenyl) piperazine. No significant depression of the mGluR-IPSC by 5-HT was observed in the presence of the 5-HT2 antagonist cinanserin or the 5-HT4 receptor antagonist RS 23597-190, whereas the 5-HT2C antagonist RS 102221 was ineffective. Our results demonstrate a powerful inhibition of the mGluR-IPSC by 5-HT in midbrain dopamine neurons, most probably through stimulation of 5-HT2A and 5-HT4 receptors.

5-HT2A and 5-HT2C/5-HT1B receptors are differentially involved in alcohol preference and consummatory behavior in cAA rats

The present study aimed to investigate the role of serotonin 5-HT2A and 5-HT2C receptors in the control of alcohol preference and consummatory behavior in alcohol-preferring cAA rats. Effects of the 5-HT(2A/2C) receptor agonist, DOI, the 5-HT(2C/1B) receptor agonist, mCPP, the 5-HT(2A/2C) receptor antagonist, ritanserin, and the 5-HT2A receptor antagonist, MDL 100,907, on ethanol (EtOH, 10% v/v) preference and intake, as well as total fluid and food intake were evaluated in a 12-h limited-access two-bottle paradigm. DOI (0.3-3 mg/kg, i.p.) reduced EtOH intake and preference, but not total fluid or food intake; whereas mCPP (1-10 mg/kg, s.c.) reduced EtOH, total fluid, and food intake. Therefore, it is concluded that DOI induces a specific and selective antialcohol effect, whereas mCPP rather induces a general suppressive effect on consummatory behavior. Ritanserin (1-10 mg/kg, i.p.) did not affect EtOH intake and preference, nor total fluid and food consumption. MDL 100,907 (0.1-1 mg/kg, i.p.) induced only a small reduction of food intake at the highest dose tested. Pretreatment with ritanserin (3 mg/kg, i.p.) and MDL 100,907 (0.3 mg/kg, i.p.) blocked the effects of DOI (3 mg/kg, i.p.), but not those of mCPP (10 mg/kg, i.p.). It is suggested that activation of 5-HT2C and/or 5-HT1B receptors results in a general decrease of consummatory behavior, whereas activation of 5-HT2A receptors selectively decreases EtOH intake and preference, as assessed in the cAA rat model of alcoholism.

Facilitation of female rat lordosis behavior by hypothalamic infusion of 5-HT(2A/2C) receptor agonists

Ovariectomized rats were hormonally primed with 0.5 microg estradiol benzoate and 500 microg progesterone to produce two groups of rats differing in their lordosis behavior. Females with a lordosis to mount (L/M) ratio < 0.5 were used to test the hypothesis that 5-HT(2A/2C) receptor agonists could facilitate lordosis behavior. Females with L/M ratios > or = 0.5 were used to evaluate the potential suppressive effect of 5-HT(2A/2C) receptor compounds. Lordosis behavior was examined following bilateral infusion of drugs into the ventromedial nucleus of the hypothalamus (VMN). Drugs examined were the 5-HT(2A/2C) receptor agonist, (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI), the 5-HT(2A/2C) receptor antagonist, 3-[2-[4-(4-fluorobenzoyl)-1-piperdinyl]ethyl]-2,4(1H,3H)-quinazoli nedione tartrate (ketanserin tartrate), and the non-selective 5-HT receptor agents, 2-(1-piperazinyl)quinoline dimaleate (quipazine) and N-(3-trifluoromethylphenyl)piperazine HCl (TFMPP). Drugs with agonist action at 5-HT(2A/2C) receptors increased lordosis behavior in rats with low sexual receptivity. The 5-HT(2A/2C) receptor antagonist, ketanserin, inhibited lordosis behavior in sexually receptive rats. DOI attenuated the lordosis-inhibiting effect of ketanserin, but ketanserin was less effective in preventing DOI from increasing lordosis behavior. These results strengthen prior inferences that activation of 5-HT(2A/2C) receptors can facilitate lordosis behavior and that the VMN is one site at which such facilitation can occur.

Dual role of 5-HT in defense and anxiety

Pharmacological results obtained in animals tested in approach/avoidance conflict situations have led to the suggestion that 5-HT enhances anxiety by acting on forebrain structures. In contrast, results with intracerebral drug injection associated with aversive electrical brain stimulation indicate that 5-HT inhibits aversion in the dorsal periaqueductal gray (DPAG). To reconcile this evidence, it has been suggested that 5-HT may enhance conditioned fear in the amygdala while inhibiting innate fear in the DPAG. To test this hypothesis, we used three drug treatments known to increase the release of 5-HT from terminals of the dorsal raphe nucleus (DR): (1) intra-DR microinjection of the benzodiazepine inverse agonist FG 7142, (2) intra-DR microinjection of the excitatory amino acid kainic acid and (3) intraperitoneal injection of the 5-HT releaser and uptake blocker D-fenfluramine. All drug treatments enhanced inhibitory avoidance (learned fear) in the elevated T-maze, a new animal model of anxiety. Intra-raphe kainate and D-fenfluramine also decreased one-way escape (innate fear) in the T-maze. In contrast, reduction of 5-HT release by intra-DR injection of 8-OH-DPAT impaired inhibitory avoidance without affecting one-way escape. Overall, these results agree with the above hypothesis. Clinical implications are discussed, especially with regard to panic disorder.

Dihydrobenzofuran analogues of hallucinogens. 4. Mescaline derivatives

Dihydrobenzofuran and tetrahydrobenzodifuran functionalities were employed as conformationally restricted bioisosteres of the aromatic methoxy groups in the prototypical hallucinogen, mescaline (1). Thus, 4-(2-aminoethyl)-6,7-dimethoxy-2,3-dihydrobenzofuran hydrochloride (8) and 1-(8-methoxy-2,3,5,6-tetrahydrobenzo[1,2-b:5,4-b']difuran-4-yl)-2- aminoethane hydrochloride (9) were prepared and evaluated along with 1 for activity in the two-lever drug discrimination (DD) paradigm in rats trained to discriminate saline from LSD tartrate (0.08 mg/kg). Also, 1, 8, and 9 were assayed for their ability to displace [3H]ketanserin from rat cortical homogenate 5-HT2A receptors and [3H]8-OH-DPAT from rat hippocampal homogenate 5-HT1A receptors. In addition, these compounds were evaluated for their ability to compete for agonist and antagonist binding to cells expressing cloned human 5-HT2A, 5-HT2B, and 5-HT2C receptors. Finally, agonist efficacy was assessed by measurement of phosphoinositide hydrolysis in NIH 3T3 cells expressing the rat 5-HT2A or 5-HT2C receptors. Although 1 fully substituted for LSD in the DD assays (ED50 = 33.5 mumol/kg), neither 8 nor 9 substituted for LSD, with just 50% of the rats administered 8 selecting the drug lever, and only 29% of the rats administered 9 selecting the drug lever. All of the test compounds had micromolar affinity for the 5-HT1A and 5-HT2A receptors in rat brain homogenate. Curiously, the rank order of affinities of the compounds at 5-HT2A sites was opposite their order of potency in the behavioral assay. An evaluation for ability to stimulate phosphoinositide turnover as a measure of functional efficacy revealed that all the compounds were of approximately equal efficacy to serotonin in 5-HT2C receptors. At 5-HT2A receptors, however, 8 and 9 were significantly less efficacious, eliciting only 61 and 45%, respectively, of the maximal response. These results are consistent with the proposed mechanism of action for phenethylamine hallucinogens, that such compounds must be full agonists at the 5-HT2A receptor subtype. In contrast to the 2,5-dimethoxy-substituted phenethylamines, where rigidification of the methoxy groups had no deleterious effect on activity, the loss of activity in the 3,4,5-trioxygenated mescaline analogues may suggest that the 3 and 5 methoxy groups must remain conformationally mobile to enable receptor activation.

Auditory-evoked potentials as indicator of brain serotonergic activity--first evidence in behaving cats

Due to the increasing importance of the central serotonergic neurotransmission for pathogenetic concepts and as a target of pharmacotherapeutic interventions in psychiatry, reliable indicators of this system are needed. Several findings from basic and clinical research suggest that the stimulus intensity dependence of auditory evoked potentials (AEP) may be such an indicator of behaviorally relevant aspects of serotonergic activity (Hegerl and Juckel 1993, Biol Psychiatry 33:173-187). In order to study this relationship more directly, epidural recordings over the primary and secondary auditory cortex were conducted in chronically implanted cats under intravenous (i.v.) administration of drugs influencing the serotonergic and other modulatory systems (8-OH-DPAT, m-CPP, ketanserin, DOI, apomorphine, atropine, clonidine). The intensity dependence of the cat AEP component with the highest functional similarity to this of the N1/P2-component in humans was significantly changed by influencing 5-HT1a and 5-HT2 receptors, but not 5-HT1c receptors. This serotonergic modulation of the intensity dependence was only found for the primary auditory cortex which corresponds to the known different innervation of the primary and secondary auditory cortex by serotonergic fibers. Our study supports the idea that the intensity dependence of AEP could be a valuable indicator of brain serotonergic activity; however, this indicator seems to be of relative specificity because at least cholinergic effects on the intensity dependence were also observed.

The effects of paroxetine given repeatedly on the 5-HT receptor subpopulations in the rat brain

Effects of paroxetine (10 mg/kg PO, twice daily, 14 days) on 5-HT receptor subpopulations in the brain were evaluated pharmacologically, electrophysiologically and biochemically in male Wistar rats. Imipramine was used for comparison. Repeated paroxetine antagonized the 8-OH-DPAT-induced behavioural syndrome (a 5-HT1A effect); imipramine showed similar, yet weaker, activity. The 5-HT-or 8-OH-DPAT-induced inhibition of population spikes in hippocampal slices was increased by both those repeated antidepressants. Repeated (or acute) paroxetine decreased the density of and increased the affinity for 5-HT1A receptors ([3H]-8-OH-DPAT used as ligand) in the hippocampus, while imipramine induced opposite effects. m-Chlorophenyl piperazine (m-CPP)-evoked exploratory hypoactivity, a 5-HT2C effect, was reduced by repeated paroxetine, but not by imipramine. Either of the antidepressants given repeatedly antagonized TFMPP-induced hyperthermia (another putative 5-HT2C effect). 5-HTP-induced head twitches (a 5-HT2A effect) were inhibited by repeated paroxetine or imipramine. Either antidepressant given repeatedly decreased the density of 5-HT2A receptors ([3H]-ketanserin as a ligand) in the brain cortex, but did not change their affinity. The present results indicate that paroxetine given repeatedly induces secondary changes in 5-HT2 receptors, which lead to reduction of the 5-HT2 neurotransmission (reduced responsiveness of 5-HT2 postsynaptic receptors). The consequences of the secondary changes in 5-HT1A receptors, found here still await clarification.

Molecular modeling of serotonin, ketanserin, ritanserin and their 5-HT2C receptor interactions

Molecular modeling techniques were used to build a three-dimensional model of the rat 5-HT2C receptor, which was used to examine receptor interactions for protonated forms of serotonin, ketanserin and ritanserin. Molecular dynamics simulations which were started with the fluoro benzene moiety of ketanserin and ritanserin oriented towards the cytoplasmic side of the receptor model, produced the strongest antagonist-receptor interactions. The fluoro bezene ring(s) of the antagonists interacted strongly with aromatic residues in the receptor model, which predicts slightly different orientations and ligand-receptor interactions of ketanserin and ritanserin at a putative binding site. The model suggests that Asn333 (transmembrane helix 6) is involved in a hydrogen-bonding interaction with ketanserin, but not with ritanserin. The model also also suggests that the position corresponding to Cys362 (transmembrane helix 7) may be an important determinant for specifying 5-HT2A receptor selectivity in ketanserin binding.

Activities of novel aryloxyalkylimidazolines on rat 5-HT2A and 5-HT2C receptors

Using transfected NIH 3T3 mouse fibroblast cell lines expressing the rat 5-HT2A and rat 5-HT2C receptor subtypes, and techniques of 2-[125I](+)-iodolysergic acid diethylamide ([125I]LSD) binding and serotonin (5-hydroxytryptamine, 5-HT)-stimulated phosphoinositide hydrolysis, we have characterized a new structural class of 5-HT receptor ligands, the aryloxyalkylimidazolines. These compounds were found to be potent competitors of [125I]LSD binding at both receptor subtypes (Ki approximately 5-200 nM) and to have efficacy ranging from potent competitive antagonists (IC50 approximately 25 nM) to moderately potent full agonists (EC50 approximately 200 nM). Some of these compounds are agonists at both receptor subtypes, while others are 5-HT2C receptor agonists with 5-HT2A receptor antagonist activity. None of the aryloxyalkylimidazolines reported here have 5-HT2A or 5-HT2C receptor selective antagonist activity. Since these compounds are novel structures, we compared them with a variety of reference 5-HT receptor ligands selected from other chemical classes that have previously been studied at 5-HT2A and 5-HT2C receptors in native tissues.

Behavioural interactions between 5-hydroxytryptophan, neuroleptic agents and 5-HT receptor antagonists in modifying rodent responding to aversive situations

1. The ability of 5-hydroxytryptophan, 5-HT2 receptor antagonists and typical and atypical neuroleptic agents to modify behavioural responding to aversive situations was investigated in the mouse light/dark test and rat social interaction. 2. The administration of 5-hydroxytryptophan inhibited rat social interaction and the exploratory behaviour of mice in the light/dark test. 3. The 5-HT2 receptor antagonists, ketanserin, ritanserin, MDL11939, methysergide and RP62203, the neuroleptic agents, spiperone, haloperidol and benperidol, and the atypical neuroleptic agent, clozapine, when administered alone failed to modify mouse or rat behaviour. In contrast, when administered alone, sulpiride in rats and mice and thioridazine in rats disinhibited behaviour. 4. Methysergide, RP62203, ketanserin, ritanserin and MDL11939 antagonized the inhibitory effects of 5-hydroxytryptophan or reversed the inhibitory effects to one of disinhibition. 5. Low doses of spiperone (but not haloperidol or benperidol) also antagonized the inhibitory effects of 5-hydroxytryptophan in the rat but not the mouse. Higher doses of the three neuroleptic agents caused locomotor depression in both rats and mice which obscured any specific changes in behavioural responding to the aversive situations. 6. The disinhibitory profile of sulpiride in both mice and rats and thioridazine in rats was evident during their interaction with 5-hydroxytryptophan. Thioridazine in the mouse and clozapine in rats and mice also reversed the inhibitory effects of 5-hydroxytryptophan to one of disinhibition. 7. In summary, we present evidence that the atypical neuroleptic agents, thioridazine and clozapine, with their known affinity for the 5-HT2 receptors, can mimic the actions of reference 5-HT2 receptor antagonists to antagonize the inhibitory effects of 5-hydroxytryptophan in rodent models of anxiety. The results are intepreted in terms of drug action on different 5-HT2 and other 5-HT receptor subtypes. In addition, thioridazine and sulpiride have disinhibitory effects in their own right which remain to be explained.

Role of 5-HT receptor subtypes in the modulation of dorsal periaqueductal gray generated aversion

To explore the role of 5-HT receptor subtypes in controlling aversion, we measured the effect of 5-HT1A and 5-HT2A/2C receptor agonists microinjected into the dorsal periaqueductal gray (DPAG) of rats on aversive behavior induced by electrical stimulation of the same brain area. The 5-HT1A agonists 8-OH-DPAT (4-16 nmol) and BAY-R-1531 (4-16 nmol) raised the threshold of aversive electrical stimulation in a dose-dependent way. Similarly, microinjection of the 5-HT2A/2C agonist DOI (4-16 nmol) increased the aversive mCPP (16 and 32 nmol) was ineffective. Previous intra-DPAG administration of the 5-HT1A receptor blocker NAN-190 (40 nmol) antagonized the antiaversive effect of 8-OH-DPAT (8 nmol), whereas pretreatment with the 5-HT2A receptor blocker spiperone (10 nmol) antagonized the effect of DOI (16 nmol). Spiperone also counteracted the effect of 8-OH-DPAT and NAN-190 counteracted the effect of DOI. These results indicate that activation of 5-HT1A and 5-HT2A receptors inhibits aversion in the DPAG and that both receptors have to be functional for the expression of each one's activation to occur.

Further evidence that fluoxetine interacts with a 5-HT2C receptor in glial cells

It is generally believed that the antidepressant drug fluoxetine (Prozac) exerts all its effects by inhibition of serotonin uptake into neurons and an ensuing increase in the extracellular concentration of serotonin. However, these studies have confirmed and expanded our previous observation that fluoxetine on its own exerts agonist effects on astrocytes (a glial cell type), which resemble those exerted by serotonin. Fluoxetine appears to act on a different subtype of receptor (the 5-HT2C receptor [in original terminology the 5-HT1C receptor]) than the one on which micromolar concentrations of serotonin are known to act in astrocytes (the 5-HT2A receptor [in original terminology the 5-HT2 receptor]). However, this study has shown that application of serotonin to these cells stimulates glycogenolysis and causes an increase in free cytosolic concentration of calcium that is not inhibited by the 5-HT2A selective antagonist, ketanserin. Moreover, both effects are pronounced at the low nanomolar level of serotonin and, therefore, by definition, act on the 5-HT2C receptor. The concentration/response correlation is identical for the serotonin effects on free cytosolic calcium concentration and on glycogenolysis. Fluoxetine exerts similar effects, but low nanomolar concentrations have no effect, and the concentration required to obtain half-maximum response is 1-3 microM, a concentration dependence that is consistent with the plasma levels of fluoxetine during treatment with this drug.(ABSTRACT TRUNCATED AT 250 WORDS)

The head-twitch response in the least shrew (Cryptotis parva) is a 5-HT2- and not a 5-HT1C-mediated phenomenon

Our initial studies suggested that the 5-HT2/1C agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl-2-aminopropane [(+/-)-DOI] produces both the head-twitch response (HTR) and the ear-scratch response (ESR) in mice via stimulation of 5-HT2 receptors. However, challenge studies revealed that these behaviors are produced via two different receptors (possibly 5-HT2 and 5-HT1C). Due to a lack of selective agents one cannot designate a particular response for the activation of a specific receptor. The purpose of the present study was to investigate such behaviors in the least shrew, which is more sensitive to (+/-)-DOI than rodents. IP injection of (+/-)-DOI in shrews produced a dose-dependent (bell-shaped) and time-dependent increase in the HTR frequency. The (+/-)-DOI-induced HTR was equipotently and completely attenuated by the 5-HT2/1C antagonists ketanserin and spiperone. The 5-HT1C antagonist with 5-HT2 agonist action, lisuride, also produced the HTR in a bell-shaped dose- and time-dependent fashion. Central injections of both (+/-)-DOI (0.2 microgram) and lisuride (0.5 microgram) also induced the behavior. Both peripheral and central administration of lisuride failed to produce the ESR. (+/-)-DOI significantly induced the ESR only at the highest dose tested (2.5 mg/kg, IP). Centrally administered (+/-)-DOI (0.2 microgram) produced more ESRs relative to vehicle controls; however, the difference did not attain significance. At low doses (0.31 and 0.63 mg/kg), (+/-)-DOI had no effect on locomotor activity, but it significantly attenuated the behavior at larger doses. Both low and high doses of lisuride increased the motor activity. Spiperone dose-dependently suppressed locomotion, whereas ketanserin had no effect. The present results suggest that the HTR is a 5-HT2 receptor-mediated event and changes in locomotor activity do not affect the induced HTR.

Drug discrimination and receptor binding studies of N-isopropyl lysergamide derivatives

Isopropyl (IPLA), N-methyl-N-isopropyl (MIPLA), N-ethyl-N-isopropyl (EIPLA), and N,N-diisopropyl (DIPLA) lysergamides were evaluated for lysergic acid diethylamide (LSD)-like activity. In rats trained to discriminate 0.08 mg/kg LSD tartrate from saline, each of the subject compounds completely substituted, with an ED50 two to three times larger than that of LSD except for DIPLA, which had an ED50 about eightfold greater. Similarly, all the compounds displaced [125I](R)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ([125I]DOI) from rat cortical homogenates and displaced [3H]8-hydroxy-2-(di-n-propylamino)tetralin ([3H]8-OH-DPAT) from rat hippocampal homogenates with KI values similar to those of LSD, again with the exception of DIPLA, which had about nine- and fourfold lower affinities, respectively. Interestingly, all the compounds had four- to fivefold lower affinities than LSD in displacing [3H]ketanserin from 5-HT2 binding sites. Molecular modeling studies found that all the compounds had low energy conformations similar to LSD. No correlation between the activity of the compounds and the preferred conformation of the amide substituents was apparent. In summary, N-alkyl-N-isopropyl analogs of LSD retain LSD-like activity in drug discrimination and 5-HT1A and 5-HT2 agonist binding assays only until the N-alkyl substitution is as large as ethyl; LSD-like activity dramatically drops when the second alkyl substituent is N-isopropyl.