[<Superscript>3</Superscript>H]LY334370, a novel radioligand for the 5-HT<Subscript>1F</Subscript> receptor. I. In vitro characterization of binding properties

[³H]LY334370 was developed as a radioligand to study the characteristics of this compounds interaction with the 5-HT_1F receptor. Monovalent or divalent cations did not enhance the binding of [³H]LY334370 to the cloned human 5-HT_1F receptor. In the presence of MgCl₂, the time to reach equilibrium was approximately 2 h, while in its absence equilibrium was reached in less than 1 h. [³H]LY334370 had high affinity for the cloned human 5-HT_1F receptor (K_d=0.446 nM) and the 5-HT_1F receptor in rat brain (K_d=0.388 nM). The expression density of 5-HT_1F receptors, as determined by binding to homogenates of cortical regions from rat, was low (B_max=79.1 fmol/mg protein). There was a statistically significant correlation between the apparent pK_i for inhibition of [³H]LY334370 binding and the pEC₅₀ for stimulation of [³⁵S]GTPS binding to homogenates of cells expressing the cloned human 5-HT_1F receptor. In addition, there was a statistically significant correlation between the apparent pK_i for inhibition of [³H]LY334370 binding to the cloned human 5-HT_1F receptor and the pID₅₀ for inhibition of trigeminal nerve stimulated dural plasma protein extravasation in the guinea pig. The conclusion from these studies is that [³H]LY334370 is a high affinity radioligand which can be used for the study of the 5-HT_1F receptor in rat brain or in cells transformed with the human 5-HT_1F receptor.     

Cloning,pharmacological characterisation and tissue distribution of a novel 5-HT<Subscript>4</Subscript> receptor splice variant, 5-HT<Subscript>4(i)</Subscript>

5-HT₄ receptor pre-mRNA is alternatively spliced in human (h) tissue to produce several splice variants, called 5-HT_4(a) to 5-HT_4(h) and 5-HT_4(n). Polymerase chain reaction (PCR) with primers designed to amplify both 5-HT_4(a) and 5-HT_4(b) amplified three additional bands in different tissues, two representing different mRNA species both encoding 5-HT_4(g) and one representing mRNA for a novel splice variant named 5-HT_4(i), cloned from testis and pancreas respectively. Primary and nested PCR detected both 5-HT_4(g) and 5-HT_4(i) in multiple tissues. Whereas 5-HT_4(i), was found in all cardiovascular tissues analysed, 5-HT_4(g) was mainly present in atria. However, quantitative RT-PCR indicated 5-HT_4(g) expression also in cardiac ventricle. The pharmacological profiles and ability to activate adenylyl cyclase (AC) were compared between four recombinant h5-HT₄ splice variants (a, b, g and i) expressed transiently and stably in HEK293 cells. Displacement of [³H]GR113808 with ten ligands revealed identical pharmacological profiles (affinity rank order: GR125487, SB207710, GR113808>SB203186>serotonin, cisapride, tropisetron>renzapride, 5-MeOT>5-CT). In transiently transfected HEK293 cells cisapride was a partial agonist compared to serotonin at 5-HT_4(b), 5-HT_4(g) and 5-HT_4(i) receptors. In membranes from HEK293 cells stably expressing 5-HT_4(g) (3,000 fmol/mg protein) or 5-HT_4(i) (500 fmol/mg protein), serotonin and 5-MeOT were full agonists while cisapride was full agonist at 5-HT_4(g) and partial agonist at 5-HT_4(i), probably due to different receptor expression levels. At both 5-HT_4(g) and 5-HT_4(i), the behaviour of 5-HT₄ receptor antagonists was dependent on receptor level. At high receptor levels, tropisetron and SB207710 and to a variable extent SB203186 and GR113808 displayed some partial agonist activity, whereas GR125487 and SB207266 reduced the AC activity below basal, indicating both receptors to be constitutively active. We conclude that the novel 5-HT_4(i) receptor splice variant is pharmacologically indistinguishable from other 5-HT₄ splice variants and that the 5-HT_4(i) C-terminal tail does not influence coupling to AC.     

Pharmacological characterisation of the agonist radioligand binding site of 5-HT<Subscript>2A</Subscript>, 5-HT<Subscript>2B</Subscript> and 5-HT<Subscript>2C</Subscript> receptors

In the present study we compared the affinity of various drugs for the high affinity agonist-preferring binding site of human recombinant 5-HT_2A, 5-HT_2B and 5-HT_2C receptors stably expressed in monoclonal mammalian cell lines. To ensure that the agonist-preferring conformation of the receptor was preferentially labelled in competition binding experiments, saturation analysis was conducted using antagonist and agonist radiolabels at each receptor. Antagonist radiolabels ([³H]-ketanserin for 5-HT_2A receptor and [³H]-mesulergine for 5-HT_2B and 5-HT_2C receptor) bound to a larger population of receptors in each preparation than the corresponding agonist radiolabel ([¹²⁵I]-DOI for 5-HT_2A receptor binding and [³H]-5-HT for 5-HT_2B and 5-HT_2C receptor binding). Competition experiments were subsequently conducted against appropriate concentrations of the agonist radiolabels bound to the agonist-preferring subset of receptors in each preparation. These studies confirmed that there are a number of highly selective antagonists available to investigate 5-HT₂ receptor subtype function (for example, MDL 100907, RS-127445 and RS-102221 for 5-HT_2A, 5-HT_2B and 5-HT_2C receptors respectively). There remains, however, a lack of highly selective agonists. (–)DOI is potent and moderately selective for 5-HT_2A receptors, BW723C86 has poor selectivity for human 5-HT_2B receptors, while Org 37684 and VER-3323 display some selectivity for the 5-HT_2C receptor. We report for the first time in a single study, the selectivity of numerous serotonergic drugs for 5-HT₂ receptors from the same species, in mammalian cell lines and using, exclusively, agonist radiolabels. The results indicate the importance of defining the selectivity of pharmacological tools, which may have been over-estimated in the past, and highlights the need to find more selective agonists to investigate 5-HT₂ receptor pharmacology.     

Repeated administration of the 5-HT<Subscript>1B</Subscript> receptor antagonist SB-224289 blocks the desensitisation of 5-HT<Subscript>1B</Subscript> autoreceptors induced by fluoxetine in rat frontal cortex

Desensitisation of 5-HT_1A and 5-HT_1B autoreceptors is thought to be the mechanism underlying the therapeutic effects of fluoxetine and other selective serotonin re-uptake inhibitors (SSRIs) when these are administered chronically, while blockade of these autoreceptors occurring on administration of an SSRI together with an autoreceptor antagonist is responsible for the acute increase in 5-HT levels in vivo observed under these circumstances. The effects of repeated administration of SSRIs together with 5-HT_1B receptor antagonists on 5-HT levels and autoreceptor activity have not been studied previously with an in vivo method. In this work we found, using in vivo microdialysis that the effect of fluoxetine (5 mg/kg i.p. daily for 7 days) to desensitise 5-HT_1B autoreceptors in frontal cortex, as measured by the action of CP 93129 (10 M) to reduce 5-HT levels, was prevented by concomitant administration of the 5-HT_1B receptor antagonist SB 224289 (2.5 mg/kg s.c.). 5-HT_1B receptor activity in hypothalamus and 5-HT_1A autoreceptor activity, as determined by the effects of s.c. 8-OH-DPAT to reduce 5-HT levels, were not altered either by fluoxetine alone at this dose or by fluoxetine in the presence of SB 224289. We conclude that the effects obtained when 5-HT_1B autoreceptor antagonists are administered acutely together with SSRIs may not be maintained after repeated administration.     

Clozapine decreases [<Superscript>3</Superscript>H] CP 55940 binding to the cannabinoid<Subscript>1</Subscript> receptor in the rat nucleus accumbens

Antipsychotic drugs are effective in the treatment of cannabis-induced psychosis, but only clozapine appears effective in the treatment of comorbid schizophrenia and cannabis use. The unique effects of clozapine on cannabis use could, therefore, be due to an as yet unidentified interaction between clozapine and the endogenous cannabinoid system. To address this hypothesis, we used in situ radioligand binding and quantitative autoradiography with the selective cannabinoid CB₁ receptor agonist, (-)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)phenyl]-trans-4-(3-hydroxypropyl)cyclohexanol (side chain-2,3,4(N)-³H) ([³H]CP 55940) to measure the density of the CB₁ receptor in frontal cortex, hippocampus, nucleus accumbens and striatum from rats treated with a variety of antipsychotic drugs. Clozapine significantly decreased [³H]CP 55940 binding in the nucleus accumbens compared with vehicle after 1 (35.0±14.0 vs. 71.2±8.5 fmol/mg estimated tissue equivalent (ete); P=0.03) and 3 months (42.3±4.0 vs. 71.1±16.3 fmol/mg ete; P<0.04) of treatment, an effect not observed with haloperidol, chlorpromazine or olanzapine. In rats treated with clozapine for 3 months and then left for 1 month without treatment, [³H]CP 55940 binding was not different in the nucleus accumbens (100.5±22.2 vs. 100.9±25.4 fmol/mg ete; P>0.10). By contrast, there were significant increases in accumbal [³H]CP 55940 binding in rats treated with haloperidol (136.5±14.2 fmol/mg ete; P<0.05), chlorpromazine (137.4±12.7 fmol/mg ete; P<0.05) and olanzapine (144.7±10.1 fmol/mg ete; P<0.01). These data indicate that in the nucleus accumbens clozapine differs from other antipsychotic drugs in its effects on [³H]CP 55940 binding. If these results can be extrapolated into humans, then this effect of clozapine on the CB₁ receptor may be a mechanism that makes it uniquely effective in schizophrenia and comorbid cannabis use.     

Differential regulation of rat peripheral 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2B</Subscript> receptor systems: influence of drug treatment

Most studies of 5-HT₂ receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT_2A and 5-HT_2C receptors); very few in vitro studies have addressed the peripheral receptors 5-HT_2A and 5-HT_2B. The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat.The in vitro contractile response elicited by serotonin (5-HT, 10 µM) in the rat gastric fundus (5-HT_2B receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT_2A receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (±)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E_max (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E_max was lower in animals treated with (±)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E_max between experimental and control animals. After chronic treatment with (±)DOI (2.5 mg/kg), clozapine and cyproheptadine, E_max was lower than in controls. In the gastric fundus, E_max 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E_max was significantly lower than in controls for each drug used.These findings suggest first, that regulation of peripheral 5-HT₂ receptors (5-HT_2A and 5-HT_2B) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT₂ receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT_2B receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT_2A rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.     

Aripiprazole (OPC-14597) fully substitutes for the 5-HT<Subscript>1A</Subscript> receptor agonist LY293284 in the drug discrimination assay in rats

Rationale Aripiprazole (OPC-14597) is a novel atypical antipsychotic drug with a low incidence of side effects. The therapeutic action of aripiprazole has been attributed to its unique agonist/antagonist effects at D₂ dopamine receptors; however, aripiprazole also has significant in vitro affinity at 5-HT_1A receptors.Objectives The 5-HT_1A agonist property of aripiprazole has so far not been evaluated in any in vivo assay.Methods Thirteen male Sprague Dawley rats trained to discriminate the 5-HT_1A agonist LY 293284 (75 nmol/kg) from saline, using a fixed ratio (FR) 50 schedule of food-reinforcement in a two-lever operant-conditioning task, were used to evaluate the behavioral effect of aripiprazole at 5-HT_1A receptors.Results Aripiprazole fully mimicked LY 293284 in a drug-discrimination assay with an ED₅₀ of 1.39 mol/kg (0.62 mg/kg). In combination tests, aripiprazole did not block the LY 293284 cue but at 8.92 mol/kg (4 mg/kg) significantly reduced the response rate by lowering the threshold for induction of the 5-HT syndrome produced by the training dose of LY 293284. Moreover, the selective 5HT_1A receptor antagonist WAY 100635 was able to block the substitution of aripiprazole in LY-293284 trained rats.Conclusion Although the efficacy of aripiprazole against the positive symptoms of schizophrenia may be related to its dopamine receptor interactions, it seems possible that its atypical profile may derive, at least in part, from its 5-HT_1A agonist effect, rather than from unusual D₂ receptor properties.     

Further characterization of the 5-HT<Subscript>1</Subscript> receptors mediating cardiac sympatho-inhibition in pithed rats: pharmacological correlation with the 5-HT<Subscript>1B</Subscript> and 5-HT<Subscript>1D</Subscript> subtypes

Serotonin (5-hydroxytryptamine; 5-HT) is capable of inhibiting the tachycardic responses elicited by sympathetic stimulation, but not by exogenous noradrenaline, in pithed rats pre-treated with desipramine. More recently, it has been shown that this cardiac sympatho-inhibitory response to 5-HT, mediated by prejunctional 5-HT₁ receptors as well as putative 5-ht_5A/5B receptors, is mimicked dose-dependently by the agonists CP 93,129 (r5-HT_1B), sumatriptan (5-HT_1B/1D) and PNU-142633 (5-HT_1D). This study analysed further the pharmacological profile of the above 5-HT₁ receptors.Continuous i.v. infusions of CP 93,129, sumatriptan or PNU-142633 (30 µg kg^–1min^–1 each) failed to modify the tachycardic responses to exogenous noradrenaline but inhibited those elicited by preganglionic (C7–T1) stimulation of the cardiac sympathetic outflow. These sympatho-inhibitory responses were unaltered after i.v. administration of physiological saline (1 ml kg^–1) or the 5-HT_1A receptor antagonist WAY 100635 (10 µg kg^–1). In contrast, the antagonist GR 127935 (5-HT_1B/1D; 100 µg kg^–1, i.v.) abolished the responses to CP 93,129, sumatriptan and PNU-142633, whilst SB224289 (5-HT_1B; 300 µg kg^–1, i.v.) abolished the responses to CP 93,129 without affecting those to sumatriptan and PNU-142633. Interestingly, BRL15572 (5-HT_1D; 300 µg kg^–1, i.v.) abolished the responses to PNU-142633 and attenuated those to sumatriptan, but not those to CP 93,129.WAY 100635, GR 127935, SB224289 and BRL15572, given alone at the above doses, failed to modify the sympathetically induced tachycardic responses. The 5-HT₁ receptors producing cardiac sympatho-inhibition in pithed rats thus display the pharmacological profile of the 5-HT_1B and 5-HT_1D receptor subtypes.     

Functional serotonin 5-HT<Subscript>4</Subscript> receptors in porcine and human ventricular myocardium with increased 5-HT<Subscript>4</Subscript> mRNA in heart failure

Serotonin (5-hydroxytryptamine, 5-HT) increases contractile force and elicits arrhythmias through 5-HT₄ receptors in porcine and human atrium, but its ventricular effects are unknown. We now report functional 5-HT₄ receptors in porcine and human ventricle. 5-HT₄ mRNA levels were determined in porcine and human ventricles and contractility studied in ventricular trabeculae. Cyclic AMP-dependent protein kinase (PKA) activity was measured in porcine ventricle. Porcine and human ventricles expressed 5-HT₄ receptor mRNA. Ventricular 5-HT_4(b) mRNA was increased by four times in 20 failing human hearts compared with five donor hearts. 5-HT increased contractile force maximally by 16% (EC₅₀=890 nM) and PKA activity by 20% of the effects of (–)-isoproterenol (200 M) in ventricular trabeculae from new-born piglets in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine. In ventricular trabeculae from adult pigs (3-isobutyl-1-methylxanthine present) 5-HT increased force by 32% (EC₅₀=60 nM) and PKA activity by 39% of (–)-isoproterenol. In right and left ventricular trabeculae from failing hearts, exposed to modified Krebs solution, 5-HT produced variable increases in contractile force in right ventricular trabeculae from 4 out of 6 hearts and in left ventricular trabeculae from 3 out of 3 hearts— range 1–39% of (–)-isoproterenol, average 8%. In 11 left ventricular trabeculae from the failing hearts of four -blocker-treated patients, pre-exposed to a relaxant solution with 0.5 mM Ca²⁺ and 1.2 mM Mg²⁺ followed by a switch to 2.5 mM Ca²⁺ and 1 mM Mg²⁺, 5-HT (1–100 M, 3-isobutyl-1-methylxanthine present) consistently increased contractile force and hastened relaxation by 46% and 25% of (–)-isoproterenol respectively. 5-HT caused arrhythmias in three trabeculae from 3 out of 11 patients. In the absence of phosphodiesterase inhibitor, 5-HT increased force in two trabeculae, but not in another six trabeculae from 4 patients. All 5-HT responses were blocked by 5-HT₄ receptor antagonists. We conclude that phosphodiesterase inhibition uncovers functional ventricular 5-HT₄ receptors, coupled to a PKA pathway, through which 5-HT enhances contractility, hastens relaxation and can potentially cause arrhythmias.The first two authors contributed equally to the present work     

Amisulpride is a potent 5-HT<Subscript>7</Subscript> antagonist: relevance for antidepressant actions in vivo

Rationale  Amisulpride is approved for clinical use in treating schizophrenia in a number of European countries and also for treating dysthymia, a mild form of depression, in Italy. Amisulpride has also been demonstrated to be an antidepressant for patients with major depression in many clinical trials. In part because of the selective D₂/D₃ receptor antagonist properties of amisulpride, it has long been widely assumed that dopaminergic modulation is the proximal event responsible for mediating its antidepressant and antipsychotic properties. Objectives  The purpose of these studies was to determine if amisulpride’s antidepressant actions are mediated by off-target interactions with other receptors. Materials and Methods  We performed experiments that: (1) examined the pharmacological profile of amisulpride at a large number of central nervous system (CNS) molecular targets and, (2) after finding high potency antagonist affinity for human 5-HT_7a serotonin receptors, characterized the actions of amisulpride as an antidepressant in wild-type and 5-HT₇ receptor knockout mice. Results  We discovered that amisulpride was a potent competitive antagonist at 5-HT_7a receptors and that interactions with no other molecular target investigated in this paper could explain its antidepressant actions in vivo. Significantly, and in contrast to their wild-type littermates, 5-HT₇ receptor knockout mice did not respond to amisulpride in two widely used rodent models of depression, the tail suspension test and the forced swim test. Conclusions  These results indicate that 5-HT_7a receptor antagonism, and not D₂/D₃ receptor antagonism, likely underlies the antidepressant actions of amisulpride.     

Reduced hypophagic effects of <Emphasis Type="Italic">d</Emphasis>-fenfluramine and the 5-HT<Subscript>2C</Subscript> receptor agonist <Emphasis Type="Italic">m</Emphasis>CPP in 5-HT<Subscript>1B</Subscript> receptor knockout mice

Rationale The possible role of compensatory changes in 5-HT_2C receptors in the reduced hypophagic action of d-fenfluramine in 5-HT_1B knockout (KO) mice was assessed by comparing their response to d-fenfluramine and the 5-HT_2C receptor agonist mCPP. In addition we measured 5-HT_2C/A receptor binding in 5-HT_1B KO and wild-type (WT) mice and examined the effects of 5-HT_1B receptor antagonists on d-fenfluramine-induced hypophagia in WT mice.Methods Hypophagic responses to d-fenfluramine (1–30 mg/kg) and mCPP (1–5.6 mg/kg) were measured using a behavioural satiety sequence paradigm. The effects of the 5-HT_1B receptor antagonists GR 127,935 and SB 224289 in opposing the hypophagic action of d-fenfluramine were evaluated in WT mice. The binding of [³H]-mesulergine was compared in the brains of both mouse strains.Results The hypophagic effects of moderate doses of d-fenfluramine and mCPP were attenuated in 5-HT_1B KO mice. Pretreatment of WT mice with the 5-HT_1B/1D receptor antagonist GR 127,935, or food-deprived WT mice with the 5-HT_1B receptor antagonist SB 224289, did not reproduce the reduction in sensitivity to the effects of d-fenfluramine on feeding behaviour observed in 5-HT_1B KO mice. Estimates of 5-HT_2C receptor binding were similar in 5-HT_1B KO and WT mice.Conclusions The hypophagic effect of d-fenfluramine in mice is unlikely to be mediated by the 5-HT_1B receptor. Instead, the evidence suggests that an adaptive change in 5-HT_2C receptor function occurs in 5-HT_1B receptor KO mice and contributes to their reduced response to d-fenfluramine.     

5-HT<Subscript>1B</Subscript> receptor inhibition of alcohol-heightened aggression in mice: comparison to drinking and running

Rationale Serotonin 5-HT_1B receptors are promising targets for the management of several mood and impulse disorders. Objective These experiments examine a 5-HT_1B agonist, CP-94,253, and attempt to distinguish between its effects on seeking to perform three rewarding behaviors: aggression, drinking, and wheel running. Materials and methods Male CFW mice perform nose-poke responses that are maintained by a fixed interval schedules of 10-min (FI10) schedule to gain access to one of three rewarding activities. The first experiment studies mice reinforced by the opportunity to confront an intruder mouse after drinking water or alcohol; the second studies mice reinforced by the presentation of alcoholic or non-alcoholic solutions (i.e., 6% ethanol, 0.05% saccharin vs 0.05% saccharin); the third studies mice reinforced by access to a running wheel. Results CP-94,253 (1.0–10 mg/kg i.p.) dose-dependently reduces aggression, drinking, and wheel running. Of these behaviors, alcohol-heightened aggression is the most sensitive to the 5-HT_1B receptor agonist (ED50 = 4.8 mg/kg). Responding for the opportunity to drink or engage in alcohol-heightened aggression is suppressed by the highest dose of CP-94,253, whereas CP-94,253 does not affect responding that is reinforced by wheel running or species-typical aggression. Conclusions These results confirm the inhibitory effects of 5-HT_1B receptor stimulation on aggressive performance and drinking. They also reveal an inhibition of voluntary wheel running, contrary to the stimulation of running in a novel, open arena. 5-HT_1B receptor agonists may be particularly useful for the treatment of aggressive behavioral disorders, but their efficacy and potency appear to be sensitive to the intensity and context of the behavior.     

The 5-HT<Subscript>1A</Subscript> receptor agonist MKC-242 reverses isolation rearing-induced deficits of prepulse inhibition in mice

Rationale Prepulse inhibition (PPI) of startle provides an operational measure of sensorimotor gating in which a weak stimulus presented prior to a startling stimulus reduces the startle response. PPI deficits observed in schizophrenia patients can be modeled in rats by individual housing from weaning until adulthood. The deficits in PPI produced by isolation rearing can be reversed by antipsychotics. We previously found that (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242), a highly potent 5-HT_1A receptor agonist, reduced aggressive behavior selectively in isolation-reared mice. Objective This study examines whether isolation rearing of mice produces PPI deficits and whether PPI deficits are attenuated by 5-HT_1A receptor activation. Methods Male ddY mice, 4 weeks old, were housed for more than 6 weeks singly or in groups of five or six. The PPI of the acoustic startle response was measured using SR-LAB systems. Results The PPI was less in isolation-reared mice than in group-reared mice. Oral administration of MKC-242 at 0.1–0.3 mg/kg reversed PPI deficits in isolation-reared mice, although it did not affect PPI in group-reared mice. MKC-242 did not affect MK-801-induced and apomorphine-induced PPI deficits in group-reared mice. The reversal by MKC-242 of isolation-induced PPI deficits was antagonized by the 5-HT_1A receptor antagonist WAY100635 at low doses. Conclusion These results suggest that isolation rearing produces deficits in sensorimotor gating in mice that are reversible by activation of 5-HT_1A receptors, probably somatodendritic 5-HT_1A autoreceptors.     

Implication of 5-HT<Subscript>2</Subscript> receptor subtypes in the mechanism of action of antidepressants in the four plates test

Rationale The selective serotonin reuptake inhibitors (SSRIs) and the serotonin and noradrenaline reuptake inhibitors (SNRIs) increase synaptic levels of serotonin, leading to an increased activation of a multitude of specific postsynaptic 5-HT receptors. However, it is not yet known which 5-HT receptor subtypes mediate the therapeutic effects of antidepressants.Methods The effects of the SSRI, paroxetine and the SNRI, venlafaxine were evaluated in the mouse four plates test (FPT).Results Paroxetine administered intraperitoneally (IP) (0.5, 2–8 mg/kg) potently augmented the number of punished passages accepted by mice in this paradigm. The effects of paroxetine (8 mg/kg) were not reversed by the selective 5-HT_2C receptor antagonist, RS 10-2221 (0.1 mg/kg and 1 mg/kg) or the selective 5-HT_2B/2C receptor antagonist SB 206553 (0.1 mg/kg and 1 mg/kg), at doses which lack an effect when administered alone. In contrast, the selective 5-HT_2A receptor antagonist, SR 46349B (0.1 mg/kg and 1 mg/kg) completely abolished the paroxetine-induced increase in punished passages. The acute administration of venlafaxine induced an anxiolytic-like effect in the FPT at the doses of 2–16 mg/kg. This effect was reversed by the 5-HT_2B/2C receptor antagonist as did SR 46349B, for both doses administered. Our results strongly suggest that activation of 5-HT_2A receptors is critically involved in the anxiolytic activity of paroxetine, whereas the 5-HT_2A and 5-HT_2B receptors are involved in the anti-punishment action of venlafaxine in the FPT. The co-administration of selective 5-HT_{2A, 2B, 2C} receptor agonists (DOI, 0.06 mg/kg and 0.25 mg/kg; BW 723C86, 0.5 mg/kg and 2 mg/kg and RO 60-0175, 0.06 mg/kg and 0.25 mg/kg), respectively, was subsequently investigated. The effects of sub-active doses of paroxetine (0.25 mg/kg and 1 mg/kg) were augmented by BW 723C86 and RO 60-0175 receptor agonist challenge. The anti-punishment effects of venlafaxine (0.25 mg/kg and 1 mg/kg) were potentialised only by DOI co-administration.Conclusion These results indicate that the co-administration of 5-HT₂ receptor agonists with paroxetine and venlafaxine may provide a powerful tool for enhancing the clinical efficacy of these antidepressants.     

Pro-cognitive effects of 5-HT<Subscript>6</Subscript> receptor antagonists in the social recognition procedure in rats: implication of the frontal cortex

Rationale 5-HT₆ receptor antagonists improve cognitive processes in rodents. However, their site(s) of action remains unexplored and their influence upon social memory has been little investigated. Objectives We examined the influence of 5-HT₆ receptor ligands upon social memory in rats by use of systemic or local administration into the frontal cortex (FCX), striatum, or nucleus basalis magnocellularis (NBM). Materials and methods The social recognition test is based upon the ability of an adult rat to recognize a younger conspecific during the second of two 5-min sessions. In a procedure without an inter-session interval, the actions of drugs alone and the ability to reverse “amnesia” induced by the muscarinic antagonist, scopolamine (1.25 mg/kg, s.c.), were examined. The potential promnesic effect of drugs was also investigated in another procedure where a spontaneous deficit of recognition was induced by a 120-min inter-session interval. Results The 5-HT₆ receptor agonist, WAY-181187 (10.0 mg/kg, i.p.), significantly impaired social recognition. This effect was abolished by the 5-HT₆ receptor antagonists, SB-271046 (20.0 mg/kg, i.p.) and SB-258585 (10.0 mg/kg, i.p.). These agents also abolished scopolamine-induced amnesia (10.0 and 2.5 mg/kg, i.p., respectively) and reversed the delay-induced deficit (10.0–20.0 and 2.5–10.0 mg/kg, i.p., respectively). WAY-181187 into the FCX significantly impaired social recognition (0.16–0.63 μg/side). Conversely, SB-271046 into the FCX (2.5–5.0 μg/side), but neither into the striatum nor the NBM, significantly reversed spontaneous deficit. Conclusion These results indicate that 5-HT₆ receptors modulate social recognition by actions in the FCX and underpin their pertinence as targets for the treatment of psychiatric disorders in which cognitive function is compromised.     

Possible role for the 5-HT<Subscript>1A</Subscript> receptor in the behavioral effects of REM sleep deprivation on free-operant avoidance responding in rat

Rationale REM sleep deprivation (REMSD) has been shown to increase rates of free-operant avoidance responding. Depletion of 5-hydroxytryptamine (5-HT, serotonin) levels produces similar effects on responding.Objective We studied whether the pharmacological activation of the 5-HT_1A receptor would produce effects on avoidance responding similar to REMSD and depleted 5-HT levels.Methods Rats were trained to lever press on a free-operant avoidance task. Dose-effect functions were established for 8-OH-DPAT (a 5-HT_1A receptor agonist) (0.1–1.0 mg/kg) and WAY 100635 (a 5-HT_1A receptor antagonist) (0.1–1.0 mg/kg). Rats were then exposed to REMSD (48 h) or equivalent control conditions, and then administered 8-OH-DPAT (0.6 mg/kg) and/or WAY 100635 (0.025–0.1 mg/kg).Results Injections of 8-OH-DPAT increased rates of avoidance responding in a dose-dependent manner, while WAY 100635 did not alter responding. The effect of 8-OH-DPAT was antagonized by pre-injection of WAY 100635. REMSD and injections of 8-OH-DPAT increased rates of avoidance responding and the effects of both manipulations were reversed by pre-injection of WAY 100635.Conclusions Activation of the 5-HT_1A receptor may be a mechanism by which REMSD increases rates of free-operant avoidance responding.     

SSR181507, a dopamine D<Subscript>2</Subscript> receptor antagonist and 5-HT<Subscript>1A</Subscript> receptor agonist,alleviates disturbances of novelty discrimination in a social context in rats,a putative model of selective attention deficit

Rationale Selective attention deficit, characterised by the inability to differentiate relevant from irrelevant information, is considered to underlie many cognitive deficits of schizophrenia, and appears to be only marginally responsive to treatment with current antipsychotics. Objectives We compared the activity of the putative atypical antipsychotic SSR181507 (a dopamine D₂ receptor antagonist and 5HT_1A receptor agonist) with reference compounds, on disturbances of novelty discrimination in a social context in rats, a behavioural paradigm that putatively models selective attention deficit. Methods A first (familiar) juvenile rat was presented to an adult rat for a period (P1) of 30 min. A second (novel) juvenile was then introduced at the end of P1 for a period (P2) of 5 min. The ability of the adult rat to discriminate between the two juveniles, presented at the same time, was evaluated by measuring the ratio of the time spent in interaction with the novel vs the familiar juvenile during P2. Results Adult rats spent more time exploring the novel than the familiar juvenile. This novelty discrimination capacity was disrupted by: (1) parametric modification of the procedure (reduction of time spent in contact with the familiar juvenile during P1); (2) acute injection of psychotomimetics that are known to induce schizophrenia-like symptoms in humans, such as phencyclidine (PCP; 3 mg/kg, i.p.) and d-amphetamine (1 mg/kg, i.p.) and (3) neonatal treatment with PCP (three injections of 10 mg/kg, s.c.), a model based on the neurodevelopmental hypothesis of schizophrenia. The potential atypical antipsychotic SSR181507 (0.03–3 mg/kg, i.p.) and the atypical antipsychotics clozapine (0.1–1 mg/kg, i.p.) and amisulpride (1–3 mg/kg, i.p.) attenuated deficits in novelty discrimination produced by parametric manipulation and by acute or neonatal treatment with PCP. The typical antipsychotic haloperidol (up to 0.3 mg/kg, i.p.) attenuated only deficits in novelty discrimination produced by parametric modification. Conclusion Collectively, these results suggest that SSR181507 can alleviate disturbances of novelty discrimination in a social context in rats, and that this paradigm may represent a suitable animal model of selective attention deficits observed in schizophrenia.