Discriminative stimulus properties of flesinoxan

Serotonin: a never-ending story

The neurotransmitter serotonin is an evolutionary ancient molecule that has remarkable modulatory effects in almost all central nervous system integrative functions, such as mood, anxiety, stress, aggression, feeding, cognition and sexual behavior. After giving a short outline of the serotonergic system (anatomy, receptors, transporter) the author's contributions over the last 40 years in the role of serotonin in depression, aggression, anxiety, stress and sexual behavior is outlined. Each area delineates the work performed on animal model development, drug discovery and development. Most of the research work described has started from an industrial perspective, aimed at developing animals models for psychiatric diseases and leading to putative new innovative psychotropic drugs, like in the cases of the SSRI fluvoxamine, the serenic eltoprazine and the anxiolytic flesinoxan. Later this research work mainly focused on developing translational animal models for psychiatric diseases and implicating them in the search for mechanisms involved in normal and diseased brains and finding new concepts for appropriate drugs.

5-HT3 receptor ligands lack discriminative stimulus properties

The putative discriminative stimulus of the 5-HT3 receptor antagonists ondansetron and (DL)-11-[(2-methyl-1H-imidazol-1-yl)methyl]-4,5,6,7,10,11,12-octahydroazepinol[3,2,1-jk]-carbazol-12-one hydrochloride (DU122932), and of the 5-HT3 receptor agonists 2-methyl-5-HT and 3,4-dichlorophenylbiguanide (3,4DCPB) were investigated in a standard two-lever, food-reinforced drug-saline discrimination procedure with groups of rats (N= 10 per group). In three groups of rats after 80 sessions with training doses ranging from 0.1 to 4.0 mg/kg po, stimulus control by ondansetron, DU122932 and 2-methyl-5-HT was still absent. The same 30 animals thereafter rapidly learned to discriminate chlordiazepoxide (CDP) from vehicle. In three other groups of rats, stimulus control by CDP was first established. Then, the vehicle was gradually (from 0.1 to 2.0 mg/kg po) replaced by either ondansetron, DU122932 or 2-methyl-5-HT. Finally, the dose of CDP was gradually decreased. In all three groups, stimulus control disappeared. A seventh group was trained to discriminate 3,4DCPB (5.0 mg/kg po) from saline. When training was not successful, dose and route were changed but discrimination was not attained. It is concluded that in the rat, using the classical two lever discrimination procedure, the 5-HT3 receptor ligands ondansetron, DU122932, 2-methyl-5-HT and 3,4DCPB are incapable of producing an internal state that can act as a stimulus to control responding.

Stimulus properties of the selective 5-HT reuptake inhibitor fluvoxamine in conditioned taste aversion procedures

Previous attempts to train pigeons and rats to discriminate between the antidepressant fluvoxamine and its vehicle as assessed in a drug discrimination paradigm have been without success. The present experiments were, therefore, designed to assess in a conditioned taste aversion procedure (CTA) whether or not fluvoxamine possesses stimulus properties. Rats were exposed to a conditioned taste aversion (CTA) procedure. In Experiment I, subjects were given 15 mg/kg fluvoxamine p.o. or vehicle after drinking a novel tasting saccharin solution. In Experiment II, a comparison was made between the effects of 15 mg/kg fluvoxamine i.p., 30 mg/kg fluvoxamine i.p., NaCl, and lithium chloride (LiCl). In Experiment III, subjects were treated with either 10 mg/kg fluoxetine i.p., 30 mg/kg fluvoxamine i.p., or LiCl. CTA was observed after treatment with LiCl, but never after treatment with fluvoxamine or fluoxetine, suggesting that fluvoxamine does not have clear stimulus properties, which can serve as a discriminative stimulus in operant procedures. In a crossfamiliarization CTA procedure in mice, however, fluvoxamine elicited a reliable CTA, suggesting that under certain conditions (species, dose?) selective serotonin reuptake inhibitors (SSRIs) may lead to certain discriminable effects. It is as yet unclear why SSRIs apparently produce such weak and species or situation-dependent discriminable effects.

The 5-HT1A receptor and its ligands: structure and function

An overview is presented on progress made in research on 5-HT1A receptors and their ligands since their discovery in 1983. Molecular biology has offered new tools, for example cloned 5-HT1A receptors, their mutants and chimeras to study structure and function. Many compounds, belonging to different chemical classes, display high affinity and selectivity for 5-HT1A receptors. The majority of these compounds are agonists or partial agonists, full antagonists are still scarce. Agonists and partial agonists are active in various animal models of anxiety and depression. Partial receptor agonists have been proven to be effective in general anxiety disorder and depression in man. Potential therapeutic applications for 5-HT1A receptor antagonists are evaluated, for example, in cognition disorders.

Anxiolytic effects of flesinoxan in the stress-induced hyperthermia paradigm in singly-housed mice are 5-HT1A receptor mediated

In the stress-induced hyperthermia paradigm in singly-housed male mice, two sequential rectal temperature measurements reveal the basal temperature (T1) and, 10 min later, an enhanced body temperature (T2), due to the stress of the first rectal measurement. The difference T2 - T1 (deltaT) is the stress-induced hyperthermia and putatively reflects a stress-induced anxiogenic response. The full 5-HT1A receptor agonist flesinoxan ((+)-enantiomer), its (-)-enantiomer and the racemic mixture reduced stress-induced hyperthermia effects, indicating putative anxiolytic properties. The ratio of their potencies to reduce stress-induced hyperthermia was similar to their potency in receptor binding affinities for 5-HT1A receptors, supporting that the anti-hyperthermia effects are mediated by the 5-HT1A receptor. This was further substantiated when the 5-HT1A receptor antagonists WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclo-hexane carboxamine trihydrochloride) and DU 125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide, monomesylate) both were able to antagonize the anti-stress-induced hyperthermia effects of flesinoxan. The stress-induced hyperthermia paradigm in singly-housed mice represents a simple and robust paradigm to measure putative anxiolytic effects of drugs.

Flesinoxan: a prosexual drug for male rats

Two tests were carried out to compare the stimulatory (i.e., prosexual) effects of the 5-HT1A receptor agonists flesinoxan and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on sexual behavior in male Wistar rats. Two groups of rats were used: normal males and males with impaired masculine sexual behavior due to neonatal treatment with the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD). In Experiment 1, flesinoxan (0.3 and 1.0 mg/kg) stimulated ejaculation frequency and number of animals displaying this behavior, both in controls and ATD males. With 0.3 mg/kg flesinoxan ATD males did not differ from controls in ejaculation frequencies. There was a concomitant decrease in latency to first ejaculation. No 'premature' ejaculations (i.e., at first or second intromission) were observed. In Experiment 2, the effects of 0.4 mg/kg 8-OH-DPAT, 0.3, 1.0 and 3.0 mg/kg flesinoxan and saline were tested in two ejaculation series. 'Premature' ejaculations only occurred during first ejaculation series with 8-OH-DPAT in 8 of 10 controls and in 6 of 9 ATD males; it did not occur during flesinoxan treatment nor in the second ejaculation series with 8-OH-DPAT treatment. Thus, flesinoxan stimulates sexual behavior in control rats and in rats with impaired sexual behavior. Unlike 8-OH-DPAT flesinoxan does not render them into 'premature' ejaculators. Therefore, flesinoxan could be considered a prosexual drug for male rats.

Effects of 5-HT1A receptor ligands in a modified Geller-Seifter conflict model in the rat

In a modified Geller-Seifter conflict procedure, rats were trained to lever-press for food under a multiple variable interval-fixed ratio (VI30: food; FR10: food + shock) schedule of reinforcement. The ability to antagonize response suppression in the punished period is considered a good predictor for anxiolytic activity. Chlordiazepoxide and alprazolam increased punished responding. The 5-HT1A receptor agonists flesinoxan (R(+)-N-[2[4-(2,3-dihydro-2-2-hydroxymethyl-1,4-benzodioxin-5-yl)- 1-piperazinyl]ethyl]-4-fluorobenzoamide; 0.1-10.0 mg/kg) and 8-OH-DPAT (8-hydroxy-2-(di-n-propyl-amino)tetralin; 0.03-0.5 mg/kg) significantly increased punished responding, supporting a role of the 5-HT1A receptor in anxiety. 8-OH-DPAT and flesinoxan also reduced unpunished responding. The anxiolytic effects of 8-OH-DPAT and flesinoxan could only be antagonized with a high dose (1.0 and 3.0 mg/kg respectively) of the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). All doses of WAY-100635 antagonized the 5-HT1A-induced effects on unpunished responding. The dissimilarity in dose-response curve of WAY-100635 on punished and unpunished behaviour poses questions about the mediation of these effects.

The putative 5-HT1A receptor antagonist DU125530 blocks the discriminative stimulus of the 5-HT1A receptor agonist flesinoxan in pigeons

Twelve homing pigeons were trained to discriminate the 5-HT1A receptor agonist flesinoxan (0.25 mg/kg p.o.) from its vehicle in a fixed ratio (FR) 30 two-key operant drug discrimination procedure. Tests for generalization and antagonism showed that compounds with agonistic action at the 5-HT1A receptor, such as 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), buspirone and ipsapirone all substituted for the flesinoxan cue. Compounds with mixed agonistic action at the 5-HT(1A/1B) receptor fully (eltoprazine) or partially (RU24969 (5-methoxy-3-(1,2,3,6-tetrahydropyridin-4-yl-1H-indole)) substituted for flesinoxan. TFMPP (1-(3-trifluoromethylphenyl)piperazine) and mCPP (1-(3-chlorophenyl)piperazine), both acting at the 5-HT(1B/2C) receptor, did not substitute for flesinoxan, neither did the selective 5-HT re-uptake inhibitor fluvoxamine. The results of the antagonism tests showed that the 5-HT1A receptor antagonists NAN-190 (1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]piperazine), WAY 100635 ((N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo-he xane-carboxamide) and the newly developed DU125530 (2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzodioxin-5-yl)-1-piperazinyl ]butyl]-1,2-benzisothiazol-3(2H)-one-1,1-dioxide) fully (more than 80%) blocked the flesinoxan cue without having substantial effects when given alone. WAY100135 (N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide ), (+/-)-pindolol and (S)-UH-301 ((S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin) all partially antagonized the flesinoxan cue. However, both WAY100135 as well as (+/-)-pindolol also partially substituted for flesinoxan in generalization tests. NAN190, (S)-UH-301, WAY100635 and DU125530 were without any activity in the generalization test at the doses tested. The putative 5-HT1A receptor antagonist S15535 (4-benzodioxan-5-yl) 1-(indan-2-yl)piperazine) was identified as a full agonist in the present procedure. Taken together these results suggest that the flesinoxan cue in pigeons is mediated by the 5-HT1A receptor and that DU125530 acts as a full antagonist on the 5-HT1A receptor.

The effects of the 5-HT1A agonist flesinoxan, in three paradigms for assessing antidepressant potential in the rat

5-HT1A receptor agonists have been shown to be effective clinically in the treatment of depression and anxiety. Flesinoxan is an example which is highly selective for the 5-HT1A receptor subtype. The objective of this study was to appraise the antidepressant potential of flesinoxan (1 and 3 mg/kg s.c.) in three tests which are indicative of antidepressant activity. These are (1) the forced swim test, following sub-acute administration, (2) 'open field' activity in the olfactory bulbectomised (OB) rat, following chronic administration, and (3) 8-OH-DPAT-induced hypothemia following chronic treatment. Both doses of flesinoxan significantly reduced the immobility time in the sham and OB groups when compared to their respective controls. In the 'open field', there was a significant increase in the ambulation of the OB control group. The higher dose of flesinoxan significantly reduced this deficit. In addition both doses of flesinoxan significantly attenuated the 8-OH-DPAT-induced hypothermic response. These effects of flesinoxan are quantitatively similar to those seen following the chronic administration of antidepressants. These studies illustrate the potential antidepressant properties of flesinoxan, and hence further emphasise the role of the 5-HT1A receptor in the pathogenesis of depression.

Discriminative stimulus properties of eltoprazine

Rats were trained to discriminate eltoprazine (1-(2,3-dihydro-1,4-benzodioxin-5-yl)-piperazine) (1.0 mg/kg p.o.) from demineralized water in a two lever operant procedure. Eltoprazine generalized to the 5-HT1B receptor agonist anpirtoline (6-chloro-2-[piperidyl-4-thiol]-pyridine hydrochloride), the 5-HT(1A,1B) receptor agonists batoprazine (8-(1-piperazinyl)-2H-1-benzopyran-2-one) and 1-NP (1-(1-naphthyl)piperazine hydrochloride), and to the 5-HT(1B/2C) receptor agonist mCPP (1-(3-chlorophenyl)piperazine dihydrochloride). The 5-HT1A receptor agonist flesinoxan (R(+)-N-[2[4-(2,3-dihydro-2-2-hydroxy-methyl-1,4-benzodioxin-5-yl) -1-piperazinyl]ethyl]-4-fluorobenzoamide) generalized partially and the 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) failed to antagonize the eltoprazine cue, suggesting that 5-HT1A receptors are of limited importance in the discriminative stimulus properties of eltoprazine. Methiothepin, mCPP, mianserin and alprazolam did not antagonize the eltoprazine cue. The 5-HT(1A,1B,1D) receptor agonist GR46611X (3-[3-(2-dimethylamino-ethyl)-1H-indol-6-yl]-N-(4-methoxy-benzyl)acrylam ide) and the 5-HT(1B,1D) receptor antagonist GR127935T (N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1,-biphenyl]-4-carboxamide) did neither generalize to nor antagonize the eltoprazine cue, whereas (-)-alprenolol showed partial antagonism and substitution. These results show that the eltoprazine discriminative stimulus is mediated by the 5-HT1B receptor, although the lack of good 5-HT1B receptor antagonists weakens this conclusion.

Discriminative stimulus properties of flesinoxan: effects of enantiomers, (S)-UH301 and WAY-100635

Rats were trained to discriminate the specific 5-HT1A receptor agonist (+)-flesinoxan (R(+)-N(-)[2[4-(2,3-dihydro-2-2-hydroxy-methyl-1,4- benzodioxin-5-yl)-1-piperazinyl]ethyl]-4-fluorobenzoamide) (1.5 mg/kg p.o.) from water in a two-lever operant procedure. Generalization tests were conducted with the enantiomers and racemate of flesinoxan and the 5-HT1A receptor antagonists (S)-UH301 ((S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin) and WAY-100635 ((N(-)[2(-)[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride). (S)-UH301, WAY-100635 and fentanyl were investigated for their antagonistic properties. The (+)-flesinoxan stimulus generalized to (-)-flesinoxan and the racemate. The ED50 values for generalization corresponded well with the affinities of the enantiomers and the racemate for the 5-HT1A receptor. The flesinoxan cue could not be mimicked by (S)-UH301 or WAY-100635, but (S)-UH301 reduced response rates. Antagonism tests showed that both (S)-UH301 and WAY-100635 dose dependently antagonized the flesinoxan cue, with ID50 values of 0.52 and 0.03 mg/kg s.c., respectively. Fentanyl had no significant antagonistic properties. It is concluded that rats can learn to discriminate orally administered (+)-flesinoxan from water. The generalization of flesinoxan to the (-)-enantiomer and the antagonism of flesinoxan's cue by specific 5-HT1A receptor antagonists are further evidence for the involvement of flesinoxan's 5-HT1A receptor agonistic properties in its discriminative stimulus effects.

The anxiolytic effects of flesinoxan, a 5-HT1A receptor agonist, are not related to its neuroendocrine effects

The effects of flesinoxan, a selective 5-HT1A receptor agonist, were studied under basal non-stress conditions and in the shock-probe burying paradigm. Flesinoxan (1 and 3 mg/kg s.c.) significantly reduced burying and freezing behaviour, indicating clear anxiolytic properties. Under non-stress conditions, injection of 3 mg/kg flesinoxan significantly enhanced plasma corticosterone and glucose levels, whereas prolactin secretion was significantly enhanced after both 1 mg/kg and 3 mg/kg flesinoxan. Flesinoxan (1 and 3 mg/kg) did not suppress shock-probe stress-induced rises in plasma corticosterone and glucose levels. The enhanced plasma prolactin levels induced by flesinoxan were not further affected by shock-probe exposure. Our data show that the anxiolytic effects of flesinoxan in the shock-probe burying paradigm are not related to increases in plasma corticosterone and glucose levels.

Antianxiety and behavioral suppressant actions of the novel 5-HT1A receptor agonist, flesinoxan

Flesinoxan is a potent and selective 5-HT1A receptor agonist. In this study, the effects of this compound on behavior in the murine elevated plus-maze have been assessed using a recently developed ethological scoring method. Results show that, at low doses (0.1-0.5 mg/kg), flesinoxan inhibited risk assessment behaviors (stretched attend postures and closed arm returns) indicative of a reduction in anxiety. These effects were maintained at a higher dose of 1.0 mg/kg, which also increased percent open entries and time spent on the central platform and open arms. However, this more convincing anxiolytic profile was associated with significant reductions in total arm entries and rearing, suggesting a combination of anxiolysis and behavioral suppression at high doses. The plus-maze profile observed with flesinoxan is very similar to that previously reported for 8-OH-DPAT in the same test but, despite superficial similarities, can be distinguished from that seen with buspirone. Data are discussed in relation to behavioral similarities and differences between 5-HT1A receptor agonists, and the advantages of a more detailed approach to the analysis of plus-maze behavior.

Effects of several 5-HT1A agonists on hippocampal rhythmical slow activity in unanesthetized rats

We examined the effect of 5-hydroxytryptamine (5-HT)1A agonists on walking related, atropine-resistant, rhythmical slow activity (wr-RSA) of the hippocampus in rats. Selective 5-HT1A agonists, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), flesinoxan, buspirone and ipsapirone significantly decreased the power value of 7-9 Hz band activity and the median frequency of wr-RSA. The order of potency was 8-OH-DPAT > flesinoxan = buspirone in power reduction. The 5-HT1A antagonists, (-)pindolol, (-)propranolol and spiperone, inhibited the effect of 8-OH-DPAT on wr-RSA. Pretreatment with parachlorophenylalanine did not abolish the effect of 8-OH-DPAT. These results indicate that 5-HT1A agonists reduce both power and median frequency values of wr-RSA through activation of post-synaptic 5-HT1A receptors in the forebrain in unanesthetized rats, in vivo.

Adrenoceptors and dopamine receptors are not involved in the discriminative stimulus effect of the 5-HT1A receptor agonist flesinoxan

Using a two-lever operant drug discrimination procedure, rats were trained to discriminate the 5-HT1A receptor agonist, flesinoxan (0.5 mg/kg i.p.), from saline. Hereafter, several non-serotonergic drugs were tested in generalization and antagonism tests. The flesinoxan stimulus did not generalize to the stimuli of either the alpha 1-adrenoceptor antagonist, prazosin, the alpha 2-adrenoceptor agonist, clonidine, the dopamine receptor agonist, apomorphine, the dopamine receptor antagonists, haloperidol and pimozide, the benzodiazepine receptor agonist, chlordiazepoxide, nor to the peripherally acting vasodilator, hydralazine. In antagonism studies, prazosin, haloperidol, pimozide and the alpha 2-adrenoceptor antagonist, idazoxan, failed to block the flesinoxan stimulus. In substitution tests, however, flesinoxan partially generalized to idazoxan and completely to the alpha 2-adrenoceptor antagonist, yohimbine. The affinities of yohimbine and idazoxan for the 5-HT1A receptor may explain the latter result. The present findings suggest that the central mechanism through which flesinoxan exerts its discriminative stimulus effects does not involve alpha 1- and alpha 2-adrenoceptors, dopamine and benzodiazepine receptors. Finally, the results with the blood pressure lowering agents, hydralazine, clonidine and prazosin do not support the suggestion that the centrally mediated blood pressure lowering effects of flesinoxan contribute to its internal stimulus effect.

Discriminative stimulus effect of flesinoxan: effect of 5-HT1A antagonists and PCPA

Rats were trained to discriminate 0.3 mg/kg (IP) flesinoxan from saline in a standard two-lever operant procedure and thereafter subjected to generalization and antagonism tests with the 5-HT1A receptor agonist ipsapirone and the beta-adrenergic/5-HT1 receptor antagonist pindolol. Ipsapirone (3.0 mg/kg) completely substituted for flesinoxan. Both the flesinoxan (0.3 mg/kg) and the ipsapirone cue (3.0 mg/kg) were dose-dependently blocked by (+/-)-pindolol. In a second group of rats, trained to discriminate 0.5 mg/kg (IP) of flesinoxan from saline, the putative 5-HT1A antagonist NAN-190 (in the dose range of 1.0 to 6.0 mg/kg) partially blocked the cue of flesinoxan. Generalization studies revealed that the flesinoxan cue could not be mimicked by NAN-190 (3.0 mg/kg). Finally, rats were pretreated with the 5-HT depletor parachlorophenylalanine (PCPA) and thereafter tested with the flesinoxan training dose (0.5 mg/kg). PCPA pretreatment did not significantly attenuate the recognition of the flesinoxan cue. The present results are in agreement with previous findings concerning the stimulus effect of flesinoxan and point to a mechanism that involves the activation of 5-HT1A receptors in the brain. Depletion of 5-HT did not significantly affect the stimulus effect of flesinoxan, suggesting that presynaptic 5-HT1A receptors do not play a crucial role in the mechanism underlying the stimulus effect of flesinoxan.

The discriminative stimulus properties of buspirone involve dopamine-2 receptor antagonist activity

To investigate a dopaminergic component in the discriminative stimulus properties of buspirone, rats were trained to discriminate 2.5 mg/kg buspirone from saline, using a two lever, food-rewarded, fixed ratio 10 operant procedure. To test the dopamine-2 (D2) antagonist action of buspirone, a second group of rats was trained to discriminate 0.16 mg/kg apomorphine from saline. In addition to a complete generalization to 8-OH-DPAT, the D2 antagonists haloperidol, R 79598 and sulpiride showed a partial generalization to buspirone. The benzodiazepine ligands chlordiazepoxide and bretazenil did not generalize to the buspirone cue. Buspirone (2.0 mg/kg) completely blocked the apomorphine cue in the apomorphine trained rats. Haloperidol, R 79895 and sulpiride also blocked the apomorphine cue, although at doses much smaller than the doses needed to evoke buspirone responding in the buspirone trained group. 8-OH-DPAT did not antagonize apomorphine. It was concluded that the D2 action of buspirone partially contributes to its discriminative stimulus properties. Mediation of the buspirone cue by 5-HT1a receptor activation seemed predominant. Further, buspirone can act as a full D2 antagonist in drug discrimination. A model was proposed suggesting a compound discriminative stimulus complex of buspirone with a dominant 5-HT1a component that overshadows a less pronounced D2 component.

Discriminative stimulus properties of 8-OH-DPAT: relationship to affinity for 5HT1A receptors

Previous studies have shown that discriminative stimulus control established with the 5HT1A receptor agonist, 8-OH-DPAT, generalizes to other 5HT1A agonists and partial agonists but also to the alpha 2-adrenoceptor antagonist, yohimbine. On the basis of these results it has been proposed that the 8-OH-DPAT cue may be produced by activity at more than one receptor. In the present study rats were trained to discriminate a dose of 8-OH-DPAT (0.05 mg/kg, SC) from saline. Substitution tests showed dose-dependent generalisation with the 5HT1A compounds, buspirone, ipsapirone, MDL 72832 and MDL 73005EF, the alpha 2-adrenoceptor antagonists, yohimbine and idazoxan, and BMY 14802, which is usually described as a sigma ligand. The buspirone metabolite 1-pyrimidinyl piperazine (1-PP) which possesses mainly alpha 2-adrenoceptor antagonist properties produced only partial generalisation which was not dose related. Receptor binding studies showed that all the compounds which substituted for 8-OH-DPAT displaced [3H]-8-OH-DPAT binding to rat hippocampal membranes. Furthermore, there were statistically significant positive correlations between drug affinity for 5HT1A sites and their ED50 values for both substitution for 8-OH-DPAT and potency to decrease response rates. These results are consistent with the view that the 8-OH-DPAT cue, like the ability of the compounds tested to decrease rates of responding, is largely mediated by activity at 5HT1A receptors.

Flesinoxan shows antidepressant activity in a DRL 72-s screen

Schedules which selectively reinforce low rates of responding (DRL, differential reinforcement of low rate) distinguish between antidepressants and other types of drugs. In a DRL schedule a subject is required to pause for a specified minimum period of time between two consecutive responses in order to obtain a reinforcer. The dependent variables are rate of responding and rate of reinforcement. Response patterns of rats treated with clinically effective antidepressant drugs such as imipramine (2.0-32.0 mg/kg) or fluvoxamine (4.0-32.0 mg/kg) are characterized by a decrease in response rate and an increase in reinforcement rate. Treatment with the 5-HT1A agonist flesinoxan (0.1-3.0 mg/kg) also dose-dependently decreased response rates while at the same time increasing reinforcement rates. Chlordiazepoxide (2.5-20.0 mg/kg) and diazepam (0.25-2.0 mg/kg) had no effects in the present experiment. d-Amphetamine increased response rates at low doses (0.5-2.0 mg/kg), and decreased it at the higher doses (4.0 mg/kg), but reinforcement rates were unaltered. Overall analysis of the effects of haloperidol (0.02-0.32 mg/kg) showed decreased responding and increased reinforcement rates. Post hoc analysis, however, clearly differentiated between haloperidol's profile and that of the antidepressants. As such, the results of the present experiment show that flesinoxan might possess antidepressant activity in humans.