Effects of 8-OH-DPAT and WAY-100635 on performance on a time-constrained progressive-ratio schedule

Rationale. Performance on progressive-ratio schedules has been proposed as a means of assessing the effects of drugs on motivation. We have adopted a mathematical model proposed by Killeen to analyse the effects of drugs acting at 5-HT_1A receptors on progressive-ratio performance. According to this model, the relationship between response rate and ratio size is described by a bitonic (inverted-U) function. One parameter of the function, a, expresses the motivational or "activating" effect of the reinforcer (duration of activation of responding produced by the reinforcer), whereas another parameter, δ, expresses the minimum time needed to execute a response and is regarded as an index of "motor capacity". Objective. To examine the effect of the selective 5-HT_1A receptor agonist 8-OH-DPAT [8-hydroxy-2-(di-n-propylamino)tetralin] and the antagonist WAY-100635 [N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclo-hexanecarboxamide] on progressive-ratio schedule performance. Methods. Sixteen rats responded for a food-pellet reinforcer on a time-constrained progressive-ratio schedule (55-min sessions). In phase 1, they received single doses (s.c.) of 8-OH-DPAT (25, 50, 100, 200 μg kg⁻¹, four treatments at each dose) or the vehicle (0.9% saline solution). In phase 2, they received WAY-100635 (30, 100, 300 μg kg⁻¹) according to the same regimen. In phase 3, they received 8-OH-DPAT (100 μg kg⁻¹) alone or in combination with WAY-100635 (30 μg kg⁻¹). 8-OH-DPAT dose dependently increased the value of a, significant increases being seen with the 50, 100 and 200 μg kg⁻¹ doses. The highest dose also increased δ. WAY-100635 did not significantly alter either a or δ. WAY-100635 significantly attenuated the effect of 8-OH-DPAT on both a and δ. Conclusions. The results suggest that 8-OH-DPAT enhanced the activating effect of the reinforcer (the highest dose may also have induced motor debilitation). The finding that the effect of 8-OH-DPAT on a was attenuated by WAY-100635 implicates 5-HT_1A receptors in this effect. The results are consistent with previous reports that 8-OH-DPAT facilitates feeding and food-reinforced operant responding in rats and suggest that these effects may be brought about by an increase in food motivation. Electronic Publication     

Comparison of hippocampal G protein activation by 5-HT<Subscript>1A</Subscript> receptor agonists and the atypical antipsychotics clozapine and S16924

This study employed [³⁵S]guanosine 5-O-(3-thiotriphosphate) ([³⁵S]GTPS) binding to compare the actions of antipsychotic agents known to stimulate cloned, human 5-HT_1A receptors with those of reference agonists at postsynaptic 5-HT_1A receptors. In rat hippocampal membranes, the following order of efficacy was observed (maximum efficacy, E_max, values relative to 5-HT=100): (+)8-OH-DPAT (85), flesinoxan (62), eltoprazine (60), S14506 (59), S16924 (48), buspirone (41), S15535 (22), clozapine (22), ziprasidone (21), pindolol (7), p-MPPI (0), WAY100,635 (0), spiperone (0). Despite differences in species and tissue source, the efficacy and potency (pEC₅₀) of agonists (with the exception of clozapine) correlated well with those determined previously at human 5-HT_1A receptors expressed in Chinese hamster ovary (CHO) cells. In contrast, clozapine was more potent at hippocampal membranes. The selective antagonists p-MPPI and WAY100,635 abolished stimulation of binding by (+)8-OH-DPAT, clozapine and S16924 (p-MPPI), indicating that these actions were mediated specifically by 5-HT_1A receptors. Clozapine and S16924 also attenuated 5-HT- and (+)8-OH-DPAT-stimulated [³⁵S]GTPS binding, consistent with partial agonist properties. In [³⁵S]GTPS autoradiographic studies, 5-HT-induced stimulation, mediated through 5-HT_1A receptors, was more potent in the septum (pEC₅₀~6.5) than in the dentate gyrus of the hippocampus (pEC₅₀~5) suggesting potential differences in coupling efficiency or G protein expression. Though clozapine (30 and 100 µM) did not enhance [³⁵S]GTPS labelling in any structure, S16924 (10 µM) modestly increased [³⁵S]GTPS labelling in the dentate gyrus. On the other hand, both these antipsychotic agents attenuated 5-HT (10 µM)-stimulated [³⁵S]GTPS binding in the dentate gyrus and septum. In conclusion, clozapine, S16924 and ziprasidone act as partial agonists for G protein activation at postsynaptic 5-HT_1A receptors in the hippocampus. These data support a role of postsynaptic 5-HT_1A receptors in the functional profiles of certain antipsychotic agents.     

Occupancy of dopamine D<Subscript>1</Subscript>, D<Subscript>2</Subscript> and serotonin<Subscript>2A</Subscript> receptors in schizophrenic patients treated with flupentixol in comparison with risperidone and haloperidol

Rationale Flupentixol (FLX) has been used as a neuroleptic for nearly 4 decades. In vitro data show comparable affinity to dopamine D₂, D₁ and 5-HT_2A receptors and recently, FLX showed to be not inferior to risperidone in schizophrenic patients with predominant negative symptomatology, which was implicated with flupentixol’s interaction with 5-HT_2A and/or D₁ receptors. Objectives To assess in vivo receptor occupancy (RO) in patients clinically treated with FLX (n = 13, 5.7 ± 1.4 mg/day) in comparison with risperidone (RIS, n = 11, 3.6 ± 1.3 mg/day) and haloperidol (HAL, n = 11, 8.5 ± 5.5 mg/day). Materials and methods Each patient underwent two PET scans with 3-N-[¹¹C]methylspiperone (target: frontal 5-HT_2A), [¹¹C]SCH23390 (striatal D₁) or [¹¹C]raclopride (striatal D₂). RO was calculated as the percentage reduction of specific binding in comparison with healthy controls. Results D₂-RO under FLX was between 50% and 70%, indicating an ED₅₀ of about 0.7 ng/ml serum. 5-HT_2A and D₁-RO was 20 ± 10% and 20 ± 5% (mean, SEM). Under HAL, D₁-RO was 14 ± 6% and under RIS not significantly different from zero. Conclusions We were able to demonstrate a moderate 5-HT_2A and D₁ occupancy under clinically relevant doses of flupentixol, albeit lower than expected from in vitro data and clearly below saturation. Therefore, if flupentixol’s efficacy on negative symptoms is based on its interaction with 5-HT_2A and/or D₁ receptors, it should be highly dependent on serum concentration and thus on dosage and metabolism. However, these data suggest that mechanisms other than D₁ or 5-HT_2A antagonism may contribute to flupentixol’s efficacy on negative symptoms.     

Effects of quipazine and m-chlorophenylbiguanide (m-CPBG) on temporal differentiation: evidence for the involvement of 5-HT2A but not 5-HT3 receptors in interval timing behaviour

Rationale Temporal differentiation refers to animals’ ability to regulate their behaviour during an ongoing interval. Striatal dopaminergic mechanisms are purported to be involved in temporal differentiation, and recent evidence also implicates 5-hydroxytryptaminergic (5-HTergic) mechanisms, possibly mediated by 5-HT_2A receptors. There is evidence that 5-HT₃ receptors contribute to the regulation of dopamine release in the basal ganglia; however, it is not known whether 5-HT₃ receptor stimulation can influence temporal differentiation. Objective We examined the effects of a selective 5-HT₃ receptor agonist m-CPBG, a mixed 5-HT_2A/3 receptor agonist quipazine, and selective 5-HT₃ and 5-HT_2A receptor antagonists (MDL-72222 and ketanserin, respectively) on temporal differentiation in a free-operant psychophysical procedure. Methods Twenty-four rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement (0.6 M, 50 μl) was provided intermittently for responding on A during the first half and on B during the second half of 50-s trials. Logistic psychometric functions were fitted to the relative response rate data [percent responding on B (%B) vs time from trial onset (t)], and quantitative indices of timing performance [T ₅₀ (value of t corresponding to %B=50), Weber fraction, and mean time of switching from A to B, S ₅₀] were derived. Results Quipazine (0.5, 1, and 2 mg kg⁻¹) altered timing performance, dose-dependently reducing T ₅₀ and S ₅₀; m-CPBG (2.5, 5, and 10 mg kg⁻¹) had no significant effect. The effect of quipazine was antagonized by ketanserin (2 mg kg⁻¹), but not by MDL-72222 (1 mg kg⁻¹). Conclusions The present results provide no evidence for the involvement of 5-HT₃ receptors in temporal differentiation and indicate that the effect of quipazine on performance was mediated by 5-HT_2A receptor stimulation. The results are consistent with previous evidence for the involvement of 5-HT_2A receptors in interval timing behaviour. Jonathan Francis Rickard (1977–2003), a gifted and dedicated PhD student, made a major contribution to this work.     

Attenuation of the effects of <Emphasis Type="Italic">d</Emphasis>-amphetamine on interval timing behavior by central 5-hydroxytryptamine depletion

Rationale  Interval timing in the free-operant psychophysical procedure is sensitive to the monoamine-releasing agent d-amphetamine, the D₂-like dopamine receptor agonist quinpirole, and the D₁-like agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzepine (SKF-81297). The effect of d-amphetamine can be antagonized by selective D₁-like and 5-HT_2A receptor antagonists. It is not known whether d-amphetamine’s effect requires an intact 5-hydroxytryptamine (5-HT) pathway. Objective  The objective of this study was to examine the effects of d-amphetamine, quinpirole, and SKF-81297 on timing in intact rats and rats whose 5-hydroxytryptaminergic (5-HTergic) pathways had been ablated. Materials and methods  Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcement was provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic functions were fitted to the data for derivation of timing indices (T ₅₀, time corresponding to %B = 50%; Weber fraction). The effects of d-amphetamine (0.4 mg kg⁻¹ i.p.), quinpirole (0.08 mg kg⁻¹ i.p.), and SKF-81297 (0.4 mg kg⁻¹ s.c.) were compared between intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Results  Quinpirole and SKF-81297 reduced T ₅₀ in both groups; d-amphetamine reduced T ₅₀ only in the sham-lesioned group. The lesion reduced 5-HT levels by 80%; catecholamine levels were not affected. Conclusions   d-Amphetamine’s effect on performance in the free-operant psychophysical procedure requires an intact 5-HTergic system. 5-HT, possibly acting at 5-HT_2A receptors, may play a ‘permissive’ role in dopamine release.

Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT<Subscript>2A</Subscript> receptors

Rationale Temporal differentiation in the free-operant psychophysical procedure is sensitive to the 5-hydroxytryptamine (5-HT)_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) and the 5-HT₂ receptor agonist 2,5-dimethoxy-4-iodo-amphetamine (DOI); both drugs shift the psychophysical curve leftwards, reducing the indifference point, T ₅₀. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation.Objective We examined whether fenfluramines effect on temporal differentiation can be antagonised by the 5-HT_1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT_2A receptor antagonist ketanserin, and compared the effects of fenfluramine, DOI and 8-OH-DPAT in intact rats and rats whose 5-HTergic pathways had been destroyed by 5,7-dihydroxytryptamine.Methods Rats were trained under the free-operant psychophysical procedure to press levers A and B in 50-s trials in which reinforcers were provided intermittently for responding on A in the first half, and B in the second half of the trial. Percent responding on B (%B) was recorded in successive 5-s epochs of the trials; logistic psychophysical curves were fitted to the data for derivation of timing indices (T ₅₀, time corresponding to %B=50%, and Weber fraction). Experiment 1 examined the effects of acute treatment with fenfluramine, and the interaction between fenfluramine and the 5-HT_1A and 5-HT_2A receptor antagonists WAY-100635 and ketanserin; experiment 2 compared the effects of fenfluramine, 8-OH-DPAT and DOI in intact rats and rats whose 5-HTergic pathways had been destroyed by intra-raphe injection of 5,7-dihydroxytryptamine. Concentrations of 5-HT and catecholamines in the brain were measured by high-performance liquid chromatography.Results Experiment 1: fenfluramine (2 mg/kg) reduced T ₅₀; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 g/kg). Experiment 2: 8-OH-DPAT (100 g/kg) and DOI (250 g/kg) reduced T ₅₀ in both groups; fenfluramine reduced T ₅₀ only in the sham-lesioned group. Levels of 5-HT were reduced by 80% in the lesioned group; catecholamine levels were not affected.Conclusions The results suggest that fenfluramine affects temporal differentiation via the release of endogenous 5-HT which acts mainly on postsynaptic 5-HT_2A receptors.     

Enhancement of latent inhibition by two 5-HT2A receptor antagonists only when given at both pre-exposure and conditioning

Rationale. Clozapine-like atypical antipsychotic drugs, such as olanzapine, risperidone and sertindole, bind most strongly to 5-HT_2A receptors, which may contribute to their antipsychotic effects. Antipsychotic drugs, such as clozapine and haloperidol, have been found to enhance latent inhibition (LI) in humans and rats. LI is a process of learning to ignore irrelevant stimuli that is disrupted in acute, positive-symptom schizophrenia, and can be modelled in animals. Objectives. The aim of this study was to determine the effects of two selective 5-HT_2A receptor antagonists, SR 46,349B and ICI 169,369, on LI, as a test of their antipsychotic potential. Methods. Doses of the 5-HT_2A receptor antagonists that were sufficient for receptor blockade were determined in 5-HT behavioural syndrome tests. SR 46,349B and ICI 169,369 were then tested for enhancement of LI and reversal of amphetamine-induced attenuation of LI in a conditioned suppression paradigm. Results. SR 46,349B (0.6–2.4 mg kg^–1 i.p.) and ICI 169,369 (10–40 mg kg^–1 i.p.) antagonised 5-hydroxytryptophan (5-HTP)-induced head twitches and wet dog shakes, which are mediated by 5-HT_2A receptors, but had no effect on mCPP-induced hypolocomotion, which is mediated by 5-HT_2C receptors. Neither SR 46,349B (1.2 mg kg^–1 i.p.) nor ICI 169,369 (40 mg kg^–1 i.p) affected 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT)-induced forepaw treading, suggesting that they were not in vivo 5-HT_1A receptor antagonists. SR 46,349B (2.4 mg kg^–1 i.p.) and ICI 169,369 (40 mg kg^–1 i.p.) enhanced LI when given at both the pre-exposure and conditioning stages of the paradigm, but not when given at either pre-exposure or conditioning only. Both drugs also reversed the disruption of LI induced by D-amphetamine (1 mg kg^–1 i.p.). Conclusions. The profile of SR 46,349B and ICI 169,369 in LI differs from that of clozapine and haloperidol in LI, which both enhance LI when given only at the conditioning stage of the paradigm.     

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.     

The effect of the selective 5-HT1A agonists alnespirone (S-20499) and 8-OH-DPAT on extracellular 5-hydroxytryptamine in different regions of rat brain

1. We have examined the effects of the systemic administration of the selective 5-HT_1A agonist alnespirone (S-20499) on in vivo 5-hydroxytryptamine (5-HT) release in the dorsal raphe nucleus, the median raphe nucleus and four forebrain areas innervated differentially by both (dorsal striatum, frontal cortex, ventral hippocampus and dorsal hippocampus).
2. Alnespirone (0.1–3 mg kg⁻¹, s.c.) dose-dependently reduced extracellular 5-HT in the six areas examined. In forebrain, the maximal reductions occurred in striatum and frontal cortex (maximal reduction to 23 and 29% of baseline, respectively). Those in dorsal and ventral hippocampus were more moderate (to ca 65% of baseline). In contrast, the decrease in 5-HT elicited in the median raphe nucleus was more marked than that in the dorsal raphe nucleus (to ca 30 and 60% of baseline, respectively). The selective 5-HT_1A antagonist WAY-100635 (0.5 mg kg⁻¹, s.c.) prevented the decrease in 5-HT induced by alnespirone (0.3 mg kg⁻¹, s.c.) in frontal cortex.
3. 8-OH-DPAT (0.025, 0.1 and 0.3 mg kg⁻¹, s.c.) also reduced extracellular 5-HT in a regionally-selective manner (e.g., to 32% of baseline in striatum and to 69% in dorsal hippocampus at 0.1 mg kg⁻¹, s.c.). In midbrain, 8-OH-DPAT reduced the dialysate 5-HT slightly more in the median than in the dorsal raphe nucleus at all doses examined.
4. Doses of both compounds close to their respective ED₅₀ values (0.3 mg kg⁻¹ alnespirone, 0.025 mg kg⁻¹ 8-OH-DPAT) reduced 5-HT to a comparable extent in all regions examined. However, the reductions attained at higher doses were more pronounced for 8-OH-DPAT.
5. These data show that the reduction of 5-HT release elicited by alnespirone and 8-OH-DPAT is more important in forebrain areas innervated by 5-hydroxytryptaminergic neurones of the dorsal raphe nucleus. This regional selectivity seems unlikely to be accounted for by differences in the sensitivity of 5-HT_1A autoreceptors controlling 5-HT release, given the dissimilar effects of these two 5-HT_1A agonists in regions rich in cell bodies and nerve terminals. This suggests the presence of complex mechanisms of control of 5-HT release by 5-HT_1A receptors.

     

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

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

Intraseptal injection of the 5-HT<Subscript>1A</Subscript>/5-HT<Subscript>7</Subscript> agonist 8-OH-DPAT and working memory in rats

Rationale In rats, 5-HT_1A receptors are present in the septal region, e.g. on cholinergic neurons of the medial septum, where they might be a substrate for cognitively relevant interactions between cholinergic and serotonergic systems.Objective The present experiment assessed the effects of the stimulation of septal 5-HT_1A receptors on spatial working memory.Methods Stimulation of septal 5-HT_1A receptors was carried out by infusions targetting the medial septum of the 5-HT_1A/5-HT₇ receptor agonist 8-hydroxy-2-(di-n-propyl-amino)-tetralin (8-OH-DPAT; 0.5 or 4 µg). Spatial memory was assessed in a water maze using a protocol placing emphasis on spatial working memory. The location of the hidden platform was changed every day and performance was assessed on two consecutive trials each day.Results In comparison to vehicle injections, the intraseptal infusion of 4 µg 8-OH-DPAT impaired performance significantly: rats treated with 8-OH-DPAT exhibited increased distances to reach the hidden platform on both trials 1 and 2. Rats infused with 0.5 µg showed similar changes that failed to be significant. Such effects were not observed when the platform was visible.Conclusions These results extend those of a previous experiment which showed that intraseptal injections of 8-OH-DPAT impaired spatial reference memory. Based on the characteristics of the observed deficits, it is suggested that the 8-OH-DPAT-induced impairment, rather than being only the result of a true alteration of working memory, might reflect a more global cognitive deficiency in which alteration of general memory capacities may be biased by disrupted search strategies/exploration and/or dysfunctions of attention.H. Jeltsch and F. Bertrand contributed equally to the work.     

Destruction of serotonergic nerve terminals prevents fluoxetine-induced desensitization of hypothalamic 5-HT<Subscript>1A</Subscript> receptors

Abstract Rationale. Selective serotonin (5-HT, 5-hydroxytryptamine) reuptake inhibitors (SSRIs) such as fluoxetine produce a gradual desensitization of hypothalamic post-synaptic 5-HT_1A receptor systems. It is assumed that the effects of SSRIs on post-synaptic 5-HT_1A receptors are mediated by 5-HT reuptake inhibition, leading to an increase of 5-HT in the synapse. However, there is no direct evidence to support this hypothesis. Objectives. The present study determined whether 5-HT_1A receptor desensitization was mediated by fluoxetine's effects on serotonergic nerve terminals. Methods. Serotonergic nerve terminals were destroyed by intracerebroventricular (i.c.v.) injection of the serotonin neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) combined with injection of the norepinephrine/dopamine reuptake inhibitor nomifensine. 5,7-DHT-induced loss of serotonergic terminals was confirmed by a 95% reduction in the density of [³H]paroxetine-labeled 5-HT transporters in the hypothalamus and a 97% reduction in the levels of 5-HT and 5-hydroxyindoleacetic acid in the cortex. Two weeks after the 5,7-DHT injections, rats were injected daily with fluoxetine (5 mg/kg or 10 mg/kg, i.p.) or saline for 14 days. Results. Injections of 10 mg/kg fluoxetine produced a significant decrease in body weight gain. Destruction of serotonergic nerve terminals reduced body weight and potentiated the ability of fluoxetine to further inhibit body weight gain. Increases in plasma levels of adrenal corticotrophic hormone (ACTH, corticotropin), corticosterone, and oxytocin after injection of the 5-HT_1A agonist 8-hydroxy-2-dipropylaminotetralin [(±)8-OH-DPAT] were used as peripheral markers of 5-HT_1A receptor function in the hypothalamus. In vehicle-pretreated rats, fluoxetine produced a dose-dependent reduction in the (±)8-OH-DPAT-induced increase in plasma ACTH and oxytocin levels. Destruction of serotonergic nerve terminals blocked the ability of fluoxetine to produce a desensitization in the ACTH, corticosterone, and oxytocin responses to (±)8-OH-DPAT. Conclusions. The ability of fluoxetine to induce a desensitization of hypothalamic post-synaptic 5-HT_1A receptor systems is dependent on the integrity of serotonergic nerve terminals in the hypothalamus, while its effect on body weight is not mediated by serotonergic nerve terminals in the hypothalamus. Electronic Publication     

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.     

Facilitation of brain stimulation reward by MK-801 (dizocilpine) may be independent of D<Subscript>2</Subscript>-like dopamine receptor stimulation in rats

Rationale Dopamine (DA) and glutamate (Glu) interactions in the mesocorticolimbic pathway may regulate motivation and reward and contribute to schizophrenia and drug abuse. We have recently demonstrated synergistic effects of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor blockade and D_2/3 DA receptor stimulation in brain stimulation reward (BSR). Objectives This study was conducted to explore interactions between DA and Glu systems in BSR using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine. Methods Systemic effects of these compounds were measured in male Sprague–Dawley rats using rate–frequency threshold analysis of ventral tegmental area (VTA) BSR (n=27). Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=10). Results MK-801 (0.03 or 0.13 mg kg⁻¹ i.p. or 0.66 μg intra-NAS) reduced reward thresholds while 7-OH-DPAT (0.03 mg kg⁻¹ s.c. or 5.0 μg intra-NAS) or apomorphine (0.05 mg kg⁻¹, s.c.) increased this measure. MK-801 combined with apomorphine or with 7-OH-DPAT, systemically or in the NAS shell, induced additive effects. Conclusions Lack of interaction between DA agonists and MK-801 in this study contrasts with our previous work showing synergistic reward-decreased effects of AMPA/kainate receptor blockade and D_2/3 DA receptor stimulation in the NAS shell, and indicates possible independence of DA and N-methyl-d-aspartate (NMDA) receptor effects in VTA electrical self-stimulation.     

Clozapine increases cutaneous blood flow and reduces sympathetic cutaneous vasomotor alerting responses (SCVARs) in rats: comparison with effects of haloperidol

Rationale Clozapine inhibits sympathetic outflow to the cutaneous vascular bed. Clozapine reverses hyperthermia and cutaneous vasoconstriction induced by 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) or by lipopolysaccharide (LPS). Clozapine also reverses cutaneous vasoconstriction elicited by exposure to cold. These actions distinguish clozapine from haloperidol. Clozapine could also inhibit sympathetic cutaneous vasomotor alerting responses (SCVARs), vasoconstrictor episodes that reflect emotional/psychological function, and this property might also distinguish clozapine from haloperidol. Objectives Experiments in rats determined whether clozapine and haloperidol inhibit SCVARs, and whether SR46349B (a 5HT_2A receptor antagonist), 8-OH-DPAT (a 5-HT_1A agonist), L741,626 (a dopamine D₂ antagonist) or SCH23390 (a dopamine D₁ antagonist) have clozapine-like effects on SCVARs. Methods Mean level and pulse amplitude of the tail artery Doppler flow signal were recorded in conscious freely moving rats before and after alerting stimuli (e.g. tapping the cage), and expressed as a SCVAR index (fall to zero flow implies SCVAR index of 100%, no fall implies 0%). Results Clozapine (0.0625–1.0 mg/kg, s.c.) dose-dependently increased resting tail blood flow. After 1 mg/kg, the SCVAR index was 18±1%, compared with 83±2% after vehicle. SR46349B (0.01–1.0 mg/kg) and 8-OH-DPAT (0.25 mg/kg) had similar but less potent effects on cutaneous blood flow and on SCVARs. Haloperidol (0.005–0.5 mg/kg) and L741,626 (1 mg/kg) had no or little effect on these variables. SCH23390 mildly inhibited SCVARs. Conclusions Clozapine, but not haloperidol, increases resting cutaneous blood flow and decreases SCVARs. Antagonism at 5-HT_2A receptors and agonism at 5-HT_1A receptors could contribute to these actions.     

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

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

Adaptations in pre- and postsynaptic 5-HT1A receptor function and cocaine supersensitivity in serotonin transporter knockout rats

Rationale  While individual differences in vulnerability to psychostimulants have been largely attributed to dopaminergic neurotransmission, the role of serotonin is not fully understood. Objectives  To study the rewarding and motivational properties of cocaine in the serotonin transporter knockout (SERT^−/−) rat and the involvement of compensatory changes in 5-HT_1A receptor function are the objectives of the study. Materials and methods  The SERT^−/− rat was tested for cocaine-induced locomotor activity, cocaine-induced conditioned place preference, and intravenous cocaine self-administration. In addition, the function and expression of 5-HT_1A receptors was assessed using telemetry and autoradiography, respectively, and the effect of 5-HT_1A receptor ligands on cocaine’s psychomotor effects were studied. Results  Cocaine-induced hyperactivity and conditioned place preference, as well as intravenous cocaine self-administration were enhanced in SERT^−/− rats. Furthermore, SERT^−/− rats displayed a reduced hypothermic response to the 5-HT_1A receptor agonist 8-OHDPAT. S-15535, a selective somatodendritic 5-HT_1A receptor agonist, reduced stress-induced hyperthermia (SIH) in wild-type controls (SERT^+/+), while it increased SIH in SERT^−/− rats. As 5-HT_1A receptor binding was reduced in selective brain regions, these thermal responses may be indicative for desensitized 5-HT_1A receptors. We further found that both 8-OHDPAT and S-15535 pretreatment increased low-dose cocaine-induced locomotor activity in SERT^−/− rats, but not SERT^+/+ rats. At a high cocaine dose, only SERT^+/+ animals responded to 8-OHDPAT and S-15535. Conclusion  These data indicate that SERT^−/−-associated 5-HT_1A receptor adaptations facilitate low-dose cocaine effects and attenuate high-dose cocaine effects in cocaine supersensitive animals. The role of postsynaptic and somatodendritic 5-HT_1A receptors is discussed.     

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

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

Dimerization of 8-OH-DPAT increases activity at serotonin 5-HT1A receptors

[³⁵S]GTPγS binding responses can be used to measure differences between the intrinsic activity of ligands at human 5-hydroxytrypamine-_1A (h 5-HT_1A) receptors expressed in recombinant cell lines. The maximal [³⁵S]GTPγS binding response to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was lower than that to 5-HT in a recombinant C6-glial membrane preparation and dependent on the GDP concentration: it was attenuated by about 60% vs 5-HT by increasing the concentration of GDP from 0.3 to 30 and 300 μM. Whereas dimerization of 8-OH-DPAT almost did not affect its potency at h 5-HT_1A receptors (pEC₅₀: 7.45 and 7.40 for 8-OH-DPAT and its dimer at 30 μM GDP), it increased efficacy at h 5-HT_1A receptors. The maximal response to the 8-OH-DPAT dimer was systematically greater than the response to 8-OH-DPAT and identical to that to 5-HT; moreover in contrast to the 8-OH-DPAT monomer, the maximal response to the dimer was unaffected by increasing the GDP concentration. An enhanced [³⁵S]GTPγS binding response (44 to 63% vs 8-OH-DPAT) was also observed in the hippocampus, lateral septum, dorsal raphe and cingulate cortex of guinea-pig brain sections using autoradiography of 5-HT_1A receptor-activated G-proteins. Hence, the 8-OH-DPAT dimer shows increased efficacy at 5-HT_1A receptors compared to 8-OH-DPAT. The differential regulation of the maximal agonist responses by GDP suggests that the [³⁵S]GTPγS binding responses to these two ligands could be mediated by different G-protein subtypes upon activation of the 5-HT_1A receptor. Received: 23 June 1998 / Accepted: 29 July 1998     

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.