Evidence for a role of D1 dopamine receptors in d-amphetamine’s effect on timing behaviour in the free-operant psychophysical procedure

RATIONALE: Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Studies using the fixed-interval peak procedure implicated D2 dopamine receptors in these effects. Less is known about the effects of dopaminergic manipulations on temporal differentiation in other timing schedules. OBJECTIVE: To examine the effects of a D1 antagonist,8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566), and a D2 antagonist, haloperidol, on performance on the free-operant psychophysical procedure, and the ability of these antagonists to reverse the effects of the catecholamine-releasing agent, d-amphetamine on performance. The antagonists' ability to reverse d-amphetamine-induced hyperlocomotion was also examined. MATERIALS AND METHODS: Rats responded on two levers (A and B) under a free-operant psychophysical schedule, in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rate data (percent responding on B [%B] vs time [t]) in each treatment condition, and quantitative timing indices [T50 (value of t corresponding to %B=50) and Weber fraction] were compared among treatments. Effects of the treatments on locomotion were measured in a separate experiment. RESULTS: SKF-83566 (0.015, 0.03, 0.06 mg kg(-1)) did not affect timing performance. Haloperidol (0.025, 0.05 mg kg(-1)) had no effect; a higher dose (0.1 mg kg(-1)) reduced T (50). d-Amphetamine (0.4 mg kg(-1)) reduced T50; this effect was antagonised by SKF-83566 but not by haloperidol. Both antagonists reduced d-amphetamine-induced hyperlocomotion. CONCLUSIONS: The results suggest that d-amphetamine's effect on performance in the free-operant psychophysical procedure is mediated by D1 rather than D2 receptors.     

Effect of quinpirole on timing behaviour in the free-operant psychophysical procedure: evidence for the involvement of D<Subscript>2</Subscript> dopamine receptors

RATIONALE: Operant timing behaviour is sensitive to dopaminergic manipulations. It has been proposed that this effect is mediated principally by D(2)-like dopamine receptors. However, we recently found that the effect of d-amphetamine on timing in the free-operant psychophysical procedure was mediated by D(1)-like dopamine receptors. It has not been established whether stimulation of D(2)-like receptors affects timing in this schedule. OBJECTIVE: To examine the effects of a D(2)-like receptor agonist quinpirole on second-range timing and the ability of dopamine receptor antagonists to reverse quinpirole's effects. MATERIALS AND METHODS: Rats responded on two levers (A and B) under a free-operant psychophysical schedule in which reinforcement was provided intermittently for responding on A during the first half, and B during the second half, of 50-s trials. Logistic functions were fitted to the relative response rates [percent responding on B (%B) vs time (t)] under each treatment; quantitative timing indices [T (50) (value of t when %B = 50) and Weber fraction] were compared among treatments. RESULTS: Quinpirole (0.04, 0.08 mg kg(-1)) reduced T (50). This effect was attenuated by D(2)-like receptor antagonists haloperidol (0.05, 0.1 mg kg(-1)), eticlopride (0.04, 0.08 mg kg(-1)) and sulpiride (30, 60 mg kg(-1)), but not by the D(3) receptor-preferring antagonist nafadotride (0.5, 1 mg kg(-1)), the D(4) receptor antagonist L-745870 (1, 3 mg kg(-1)) or the D(1)-like receptor antagonist SKF-83566 (0.015 mg kg(-1)). CONCLUSIONS: Results suggest that quinpirole reduced T (50) via an action at D(2) receptors. D(1)-like and D(2)-like receptors may mediate behaviourally similar but pharmacologically distinct effects on timing behaviour.     

Evidence for the sensitivity of operant timing behaviour to stimulation of D1 dopamine receptors

Rationale Temporal differentiation of operant behaviour is sensitive to dopaminergic manipulations. Previous studies using the fixed-interval peak procedure implicated D₂-like dopamine receptors in these effects. However, recent findings suggest that d-amphetamine alters timing performance on the free-operant psychophysical procedure via D₁-like receptors. It is not known whether this effect of d-amphetamine is mimicked by direct D₁-like receptor stimulation. Objective The effects of a D₁-like receptor agonist 6-chloro-2,3,4,5-tetrahydro-1-phenyl-1H-3-benzazepine (SKF-81297) on performance on the free-operant psychophysical procedure and the interaction between SKF-81297 and a D₁-like receptor antagonist 8-bromo-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3-benzazepin-7-ol (SKF-83566) and a D₂-like receptor antagonist haloperidol, were examined. Materials and methods Rats were trained to respond on two levers (A and B) under a free-operant psychophysical schedule, in which sucrose reinforcement 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]) under each treatment condition, and quantitative indices of timing (T₅₀ [value of t corresponding to %B = 50] and the Weber fraction [(T₇₅-T₂₅)/2T₅₀; T₂₅ and T₇₅ are values of t corresponding to %B = 25 and %B = 75] were compared among treatments. Results SKF-81297 (0.8 mg kg⁻¹) reduced T₅₀; this effect was antagonized by SKF-83566 (0.03 mg kg⁻¹) but not by haloperidol (0.05, 0.1 mg kg⁻¹). Conclusions Stimulation of D₁-like dopamine receptors affects performance in the free-operant psychophysical procedure.     

Tolerance to the effect of 2,5-dimethoxy-4-iodoamphetamine (DOI) on free-operant timing behaviour: interaction between behavioural and pharmacological mechanisms

RATIONALE: The psychostimulant d-amphetamine, the D(2/3) dopamine receptor agonist quinpirole and the 5-HT(2) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) have similar effects on free-operant timing behaviour. There is evidence that tolerance develops to the effects of psychostimulants on timing performance during chronic treatment; this tolerance is generally attributed to behavioural adaptation rather than to pharmacological desensitisation. There have been no previous investigations of tolerance to the effect of DOI on free-operant timing behaviour. OBJECTIVE: To demonstrate tolerance to DOI's effect on timing performance and to examine the nature of this tolerance. MATERIALS AND METHODS: Rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 80-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 8-s epochs of the trials; logistic functions were fitted to the data from each rat for the derivation of timing indices (T (50) [time corresponding to %B = 50]; Weber fraction). RESULTS: In experiment 1, DOI (0.25 mg kg(-1)) reduced T (50) compared to vehicle; tolerance to this effect was seen after repeated daily treatments with DOI if the rats were exposed to behavioural training during the period of treatment but not if the repeated treatments took place during a 'holiday' from behavioural training. In experiment 2, repeated treatment with DOI resulted in tolerance to the effect of DOI on T (50) and cross-tolerance to the effect of d-amphetamine (0.4 mg kg(-1)), but no cross-tolerance was seen to the effect of quinpirole (0.08 mg kg(-1)). CONCLUSIONS: The results indicate that behavioural adaptation is involved in the development of tolerance to DOI's effect on timing. The finding of cross-tolerance to d-amphetamine but not to quinpirole suggests that the reduction of T (50) in the free-operant psychophysical procedure may be brought about by two distinct pharmacological mechanisms, one activated by DOI and d-amphetamine, and the other by quinpirole.     

Effects of 5-HT2A receptor stimulation on the discrimination of durations by rats

We recently found that rats' ability to discriminate durations of exteroceptive stimuli is disrupted by the non-selective 5-HT receptor agonist quipazine. Ketanserin reversed this effect, suggesting that the effect may be mediated by 5-HT2A receptors. Here, we report that the 5-HT2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) also disrupts temporal discrimination, and that this effect can be reversed by ketanserin and the highly selective 5-HT2A receptor antagonist (+/-)2,3-dimethoxyphenyl-1-[2-(4-piperidine)-methanol] (MDL-100907). Twenty rats were trained to discriminate durations in a discrete-trials psychophysical procedure. In each 50-s trial, a light was presented for t seconds, following which two levers (A and B) were presented. A response on A was reinforced if t < 25 s, and a response on B if t > 25 s. Logistic psychometric curves were fitted to the proportional choice of B (%B) for derivation of timing indices [T50: time corresponding to %B = 50; Weber fraction: (T75-T25)/2T50, where T75 and T25 are times corresponding to %B = 75 and 25, respectively]. DOI 0.25 mg kg (subcutaneous) significantly increased the Weber fraction and tended to increase T50. Ketanserin 2 mg kg (subcutaneous) did not alter either parameter, but completely antagonized the effects of DOI. Similarly, MDL-100907 0.5 and 1 mg kg (intraperitoneal) did not affect performance, but completely antagonized the effects of DOI. The results indicate that the mixed 5-HT2A/2C receptor agonist DOI disrupts temporal discrimination via stimulation of 5-HT2A receptors.     

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.

Generalization of clozapine as compared to other antipsychotic agents to a discriminative stimulus elicited by the serotonin (5-HT)2A antagonist,MDL100,907

Employing a two-lever, food-reinforced FR10 procedure, rats were trained to recognize a discriminative stimulus (DS) elicited by the 5-HT(2A) receptor antagonist and potential antipsychotic agent, MDL100,907 (0.16 mg/kg, i.p.). In generalization tests, by analogy to MDL100,907 itself (Effective Dose(50) (ED(50)), 0.002 mg/kg, s.c.), the 'atypical' antipsychotic, clozapine, which displays high affinity for 5-HT(2A) as compared to D(2) receptors, dose-dependently and fully generalized to MDL100,907 (ED(50), 0.2 mg/kg, s.c.). S16924 (0.05 mg/kg, s.c.), S18327 (0.09 mg/kg, s.c.), quetiapine (1.8 mg/kg, s.c.), risperidone (0.02 mg/kg, s.c.) and ziprasidone (0.01 mg/kg, s.c.), antipsychotics which possess-like clozapine-marked affinity for 5-HT(2A) versus D(2) receptors, also generalized to MDL100,907. In distinction, raclopride, an antipsychotic which selectively interacts with D(2) versus 5-HT(2A) receptors, did not display significant generalization. Interestingly, haloperidol, which shows only modest affinity for 5-HT(2A) versus D(2) sites, generalized to MDL100,907 (ED(50), 0.02 mg/kg, s.c.). In light of the antagonist properties of haloperidol, clozapine and all other antipsychotics tested (except raclopride) at alpha(1)-adrenoceptors (ARs), the selective alpha(1)-AR antagonists, prazosin and WB4101, were examined. Both dose-dependently and fully generalized to MDL100,907 (ED(50)s, 0.07 and 0.11 mg/kg, s.c., respectively). At doses showing pronounced generalization to MDL100,907, the only drugs which significantly suppressed response rates were haloperidol and, weakly, quetiapine. Raclopride also markedly decreased response rates. In conclusion, the antipsychotic agents, clozapine, ziprasidone, risperidone, S16924, S18327, quetiapine and haloperidol, all generalized to a DS elicited by MDL100,907. While D(2) receptors are not implicated in their actions, in addition to antagonist properties at 5-HT(2A) receptors, blockade of alpha(1)-ARs and other, as yet unidentified, mechanisms may be involved. These data underpin interest in MDL100,907 as a potential antipsychotic agent.     

Validation of a rat in vivo [(3)H]M100907 binding assay to determine a translatable measure of 5-HT(2A) receptor occupancy

An in vivo binding assay is characterized for [(3)H]M100907 binding to rat brain, as a measure of 5-HT(2A) receptor occupancy. Dose-response analyses were performed for various 5-HT(2A) antagonist reference agents, providing receptor occupancy ED(50) values in conjunction with plasma and brain concentration levels. Ketanserin and M100907 yielded dose-dependent increases in 5-HT(2A) receptor occupancy with ED(50)s of 0.316 mg/kg and 0.100 mg/kg, respectively. The atypical antipsychotics risperidone, olanzapine, and clozapine dose-dependently inhibited in vivo [(3)H]M100907 binding with ED(50) values of 0.051, 0.144, and 1.17 mg/kg, respectively. In contrast, the typical antipsychotic haloperidol exhibited only 20.1% receptor occupancy at 10 mg/kg despite producing dose-dependent increases in plasma and brain exposure levels. The novel psychopharmacologic agent asenapine dose-dependently occupied 5-HT(2A) receptors in rat brain with an ED(50) of 0.011 mg/kg, demonstrating higher 5-HT(2A) receptor potency compared with the other atypical antipsychotics tested. This enhanced potency was supported by a lower plasma exposure EC(50) of 0.477 ng/ml, compared with risperidone (1.57 ng/ml) and olanzapine (7.81 ng/ml) and was confirmed in time course studies. The validated [(3)H]M100907 rat in vivo binding assay allows for preclinical measurement of 5-HT(2A) receptor occupancy, providing essential data for understanding the pharmacological profile of novel antipsychotic agents. Additionally, the corresponding plasma and brain drug exposure data analyses provides a valuable data set for 5-HT(2A) reference agents by enabling direct comparison with any complementary studies performed in rats, thus providing a foundation for predictive pharmacokinetic/pharmacodynamic models and, importantly, allowing for translation to human receptor occupancy studies using [(11)C]M100907 positron emission tomography.     

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.     

Putative antipsychotics with pronounced agonism at serotonin 5-HT1A and partial agonist activity at dopamine D2 receptors disrupt basal PPI of the startle reflex in rats

INTRODUCTION: Prepulse inhibition (PPI) of the startle reflex has been extensively studied because it is disrupted in several psychiatric diseases, most notably schizophrenia. In rats, and to a lesser extent, in humans, PPI can be diminished by dopamine (DA) D(2)/D(3) and serotonin 5-HT(1A) receptor agonists. A novel class of potential antipsychotics (SSR181507, bifeprunox, and SLV313) possess partial agonist/antagonist properties at D(2) receptors and various levels of 5-HT(1A) activation. MATERIALS AND METHODS: It thus appeared warranted to assess, in Sprague-Dawley rats, the effects of these antipsychotics on basal PPI. RESULTS: SSR181507, sarizotan, and bifeprunox decreased PPI, with a near-complete abolition at 2.5-10 mg/kg; SLV313 had a significant effect at 0.16 mg/kg only. Co-treatment with the 5-HT(1A) receptor antagonist WAY100,635 (0.63 mg/kg) showed that the 5-HT(1A) agonist activity of SSR181507 was responsible for its effect. By contrast, antipsychotics with low affinity and/or efficacy at 5-HT(1A) receptors, such as aripiprazole (another DA D(2)/D(3) and 5-HT(1A) ligand), and established typical and atypical antipsychotics (haloperidol, clozapine, risperidone, olanzapine, quetiapine, and ziprasidone) had no effect on basal PPI (0.01-2.5 to 2.5-40 mg/kg). DISCUSSION: The present data demonstrate that some putative antipsychotics with pronounced 5-HT(1A) agonist activity, coupled with partial agonist activity at DA D(2) receptors, markedly diminish PPI of the startle reflex in rats. CONCLUSIONS: These data raise the issue of the influence of such compounds on sensorimotor gating in humans.     

The receptor mechanisms underlying the disruptive effects of haloperidol and clozapine on rat maternal behavior: A double dissociation between dopamine D2 and 5-HT2A/2C receptors

Many antipsychotic drugs disrupt active components of maternal behavior such as pup approach, pup retrieval and nest building at clinically relevant doses in postpartum female rats. However, the neurochemical mechanisms underlying such a disruptive effect remain to be determined. This study examined the neurochemical mechanisms that mediate the disruptive effects of haloperidol (a typical antipsychotic) and clozapine (an atypical antipsychotic) on rat maternal behavior. Postpartum rats were administered with haloperidol (0.2 mg/kg, sc) or clozapine (10.0 mg/kg, sc) together with either vehicle (saline or water), quinpirole (a selective dopamine D₂/D₃ agonist, 0.5 or 1.0 mg/kg, sc), or 2,5-dimethoxy-4-iodo-amphetamine (DOI, a selective 5-HT_2A/2C agonist, 1.0 or 2.5 mg/kg, sc), and their maternal behaviors were tested at different time points before and after drug administration. Haloperidol and clozapine treatment disrupted pup approach, pup retrieval, pup licking and nest building. Pretreatment of quinpirole, but not DOI, dose-dependently reversed the haloperidol-induced disruptions. In contrast, pretreatment of DOI, but not quinpirole, dose-dependently reversed the clozapine-induced disruptions. Quinpirole pretreatment even exacerbated the clozapine-induced disruption of pup retrieval and nest building. These findings suggest a double dissociation mechanism underlying the disruption of haloperidol and clozapine on rat maternal behavior. Specifically, haloperidol disrupts maternal behavior primarily by blocking dopamine D₂ receptors, whereas clozapine exerts its disruptive effect primarily by blocking the 5-HT_2A/2C receptors. Our findings also suggest that 5-HT receptors are involved in the mediation of rat maternal behavior.     

Effects of a 5-HT2 receptor agonist, DOI (2,5-dimethoxy-4-iodoamphetamine), and antagonist, ketanserin, on the performance of rats on a free-operant timing schedule

This experiment examined the effect of a 5-HT2 receptor agonist DOI (2,5-dimethoxy-4-iodoamphetamine), and antagonist, ketanserin, on temporal differentiation performance. Twelve rats were trained under the free-operant psychophysical procedure to press two levers (A and B) in 50-s trials in which sucrose reinforcement (0.6 mol/l, 50 microl) was provided intermittently for responding on A during the first half, and on B during the second half of the trial. Psychometric curves were derived from percent responding on B (%B), recorded in successive 5-s epochs of the trials; logistic functions were fitted to these data for the derivation of timing indices (T50 [time corresponding to %B=50%], epsilon [slope of the logistic curve], Weber fraction). Cumulative probability of switching in successive 5-s epochs was used to estimate the mean switching time, S50. DOI (0.0625, 0.125 and 0.25 mg/kg, s.c.) dose-dependently reduced T50 and S50. These effects of DOI (0.25 mg/kg) were antagonized by ketanserin (1.0 mg/kg). The results show that DOI alters temporal differentiation in the free-operant psychophysical procedure. The antagonistic effect of ketanserin indicates that the effect of DOI was probably mediated by 5-HT2A rather than 5-HT2C receptors, since ketanserin is relatively selective for 5-HT2A receptors. Comparison of these results with our previous findings with a 5-HT1A receptor agonist indicates that 5-HT1A and 5-HT2A receptors mediate qualitatively similar effects on temporal differentiation.     

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.     

Buspirone improves haloperidol-induced Parkinson disease in mice through 5-HT(1A) recaptors

BACKGROUND AND THE PURPOSE OF THE STUDY: The available literatures show that 5-HT(1A) receptors are widely distributed throughout the basal ganglia, and their activation facilitate dopamine release. Neuroleptic drugs such as haloperidol induce Parkinson-like syndrome through blocking brain D(2) receptors. This study aimed to investigate effect of buspirone, a partial agonist of 5HT(1A) receptor, on motor dysfunctions induced by haloperidol and involvement of 5HT(1A) receptors in this regard. METHODS: Study was performed on the male mice weighing 25-30 g. Animals were divided randomly to groups of 10 animals. Motor dysfunction was induced by intraperitoneal (i.p.) injection of haloperidol (1 mg/kg). Catalepsy was assayed by bar-test method 5, 60, 120 and 180 minutes after drug administration and motor imbalance was studied by rotarod test. RESULTS AND MAJOR CONCLUSION: Results showed that buspirone (20 mg/kg, i.p.) decreased significantly haloperidol-induced catalepsy and balance disorder in a dose dependent manner. Furthermore, 8-OH-DPAT (10 mg/kg, i.p.), as an agonist of 5-HT(1A) receptor, decreased haloperidol-induced catalepsy and balance disorder. The effect of buspirone (20 mg/kg, i.p.) on haloperidol-induced motor disorders was abolished by NAN-190 (10 mg/kg, i.p.), as a 5-HT(1A) receptor antagonist. From the results it may be concluded that buspirone improves haloperidol-induced catalepsy and balance disorder through activation of 5-HT(1A) receptors.     

ACP-103, a 5-HT<Subscript>2A/2C</Subscript> inverse agonist,potentiates haloperidol-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens

Rational Atypical antipsychotic drugs (APDs) such as clozapine, olanzapine, quetiapine, risperidone, and ziprasidone are serotonin (5-HT)_2A antagonists and relatively weaker dopamine (DA) D₂ antagonists, with variable 5-HT_2C antagonist properties. The ability of atypical APDs to preferentially increase DA release in the cortex compared to the limbic system is believed to be due in part to their antagonism of 5-HT_2A and D₂ receptors and believed to contribute to their beneficial effects on cognition, negative, and psychotic symptoms. Previous studies from this laboratory using microdialysis have shown that pretreatment of the 5-HT_2A antagonist M100907 with the typical APD and D₂ antagonist haloperidol produced an increase in the medial prefrontal cortex (mPFC), but not in the nucleus accumbens (NAC), DA release. However, pretreatment with the 5-HT_2A/2C receptor antagonist SR46349-B with haloperidol increased both mPFC and NAC DA release, suggesting that both 5-HT_2A and 5-HT_2C properties may be important for atypical APD effects.Objective The purpose of this study was to examine the effects of a novel putative atypical APD ACP-103 on mPFC and NAC DA release using in vivo microdialysis in freely moving rats that are awake. ACP-103 is an inverse agonist at both 5-HT_2A and 5-HT_2C receptors and has intermediate affinities for 5-HT_2C receptors relative to their affinities for 5-HT_2A receptors compared to M100907 and SR46349-B. In addition, the effects of ACP-103 were compared to M100907 and SR46349-B, and ACP-103 was also coadministered with haloperidol.Results ACP-103 10.0 mg/kg, but not 3.0 mg/kg, increased DA release in the mPFC, while neither dose increased DA release in the NAC. Like M100907, ACP-103 (3.0 mg/kg) potentiated 0.1 mg/kg haloperidol-induced DA release in the mPFC while inhibiting that in the NAC. However, ACP-103 (3.0 mg/kg), similar to SR46349-B, potentiated a high dose of haloperidol (1.0 mg/kg)-induced DA release in both regions. The potent 5-HT_2C antagonist SB242084 1.0 mg/kg significantly potentiated 0.1 mg/kg haloperidol-induced DA release in both the mPFC and NAC. However, SB242084, at 0.2 mg/kg, significantly potentiated DA release only in the NAC. Moreover, SB242084 0.2 mg/kg potentiated DA release in the NAC produced by the combination treatment of 3 mg/kg ACP-103 and 0.1 mg/kg haloperidol.Conclusion These data suggest that the relative extent of 5-HT_2A and 5-HT_2C antagonism, as well as the extent of D₂ receptor blockade, has a critical influence on DA release in the mPFC and NAC and may be a determining factor in the action of this class of atypical APDs on these two potentially clinically relevant parameters.     

脑5-HT1A/1B、α2受体及腺苷酸环化酶参与了胍丁胺的抗抑郁作用

为了在单胺受体及受体后腺苷酸环化酶(adenylate cyclase,AC)水平探讨胍丁胺(agmatine,AGM)抗抑郁作用的精细机制,采用小鼠悬尾实验和强迫游泳实验观察AGM抗抑郁行为改变。采用放射免疫方法测定大鼠前额皮层突触膜蛋白AC活性。结果表明,AGM(5～40 mg·kg^-1,ig)在小鼠悬尾实验和强迫游泳实验模型上均有显著抗抑郁活性。同时伍用β受体/5-HT_1A/1B受体阻断剂吲哚洛尔(pindolol, PIN, 20 mg·kg^-1, ip)、 α₂肾上腺素受体拮抗剂育亨宾(yohimbine, YOH, 5～10 mg·kg^-1, ip)或咪唑克生(idazoxan, IDA, 4 mg·kg^-1, ip)对AGM(40 mg·kg^-1, ig)的抗抑郁活性具有显著拮抗效应; 而β受体阻断剂普萘洛尔(propranolol, PRO, 5～20 mg·kg^-1, ip)或5-HT₃受体拮抗剂曲匹西隆(tropisetron, TRO, 5～40 mg·kg^-1, ip)对AGM(40 mg·kg^-1, ig)的抗抑郁活性无显著影响。AGM(0.1～6.4 μmol·L^-1)与大鼠前额皮层提取的突触膜共孵可剂量依赖地激活AC活性, 而PIN(1 μmol·L^-1)或YOH(0.25～1 μmol·L^-1)均显著拮抗AGM(6.4 μmol·L^-1)对AC的激活作用; 慢性给予大鼠AGM(10 mg·kg^-1, ig, bid)或氟西汀(fluoxetine, FLU, 10 mg·kg^-1, ig, bid) 2 w也显著增强大鼠前额皮层基础及Gpp(NH)p 预激活的AC活性。本研究表明, 调节脑内5-HT_1A/1B和α₂等受体功能, 并激活前额皮层AC可能是AGM抗抑郁活性的重要机制之一。     

SR46349-B, a 5-HT(2A/2C) receptor antagonist, potentiates haloperidol-induced dopamine release in rat medial prefrontal cortex and nucleus accumbens.

The combination of M100907, a putative antipsychotic drug (APD) and serotonin (5-HT)(2A) antagonist, and the typical APD haloperidol, can enhance dopamine (DA) release in rat medial prefrontal cortex (mPFC), an effect which has been postulated to be of value to improve cognition and negative symptoms. The present study demonstrated that another putative APD and 5-HT(2A/2C) antagonist, SR46349-B (10 mg/kg, but not 1-3 mg/kg) alone, but not M100907 (0.1 and 3 mg/kg) alone, increased mPFC DA release, whereas neither drug alone affected nucleus accumbens (NAC) DA release. Neither SR46349-B nor M100907 alone affected nucleus accumbens (NAC) DA release. Neither SR46349-B nor M100907 alone affected nucleus accumbens (NAC) DA release. SR46349-B (3 mg/kg) potentiated haloperidol-induced DA release in both regions, whereas M100907 (0.1 mg/kg) potentiated haloperidol (0.1 mg/kg)-induced mPFC DA release and inhibited it in the NAC. WAY100635 (0.2 mg/kg), a 5-HT(1A) antagonist, abolished the effects of haloperidol plus M100907 as well as SR46349-B on DA release in the mPFC, but did not do so in the NAC. Thus, 5-HT(2A) and 5-HT(2A/2C) antagonism together with haloperidol-induced D(2) antagonism may potentiate mPFC DA release via 5-HT(1A) agonism, whereas the combined effects of these agents on NAC DA release is not dependent upon 5-HT(1A) receptor stimulation. Interestingly, similar to the effect of SR46349-B, high dose M100907 (3 mg/kg), which might have antagonist activity at 5-HT(2C) receptors, potentiated 1 mg/kg haloperidol-induced DA release in the mPFC and NAC. These results suggest that 5-HT(2A/2C) antagonism may be more advantageous than selective 5-HT(2A) antagonism as an adjunct to D(2) antagonists to improve cognition and negative symptoms in schizophrenia.     

Effects of fenfluramine on free-operant timing behaviour: evidence for involvement of 5-HT2A 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, T50. We have examined the effect of the 5-HT releasing agent fenfluramine on temporal differentiation. OBJECTIVE: We examined whether fenfluramine's effect on temporal differentiation can be antagonised by the 5-HT1A receptor antagonist N-[2-(4-[2-methoxy-phenyl]-1-piperazinyl)ethyl]-N-2-pyridinylcyclohexane-carboxamide (WAY-100635) and the 5-HT2A 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 (T50, 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-HT1A and 5-HT2A 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 T50; this effect was attenuated by ketanserin (1.0 mg/kg) but not by WAY-100635 (100 microg/kg). Experiment 2: 8-OH-DPAT (100 microg/kg) and DOI (250 microg/kg) reduced T50 in both groups; fenfluramine reduced T50 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-HT2A receptors.     

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

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

The antidepressant effects of curcumin in the forced swimming test involve 5-HT1 and 5-HT2 receptors

Curcuma longa is a main constituent of many traditional Chinese medicines, such as Xiaoyao-san, used to manage mental disorders effectively. Curcumin is a major active component of C. longa and its antidepressant-like effect has been previously demonstrated in the forced swimming test. The purpose of this study was to explore the possible contribution of serotonin (5-HT) receptors in the behavioral effects induced by curcumin in this animal model of depression. 5-HT was depleted by the tryptophan hydroxylase inhibitor p-chlorophenylalanine (PCPA, 100 mg/kg, i.p.) prior to the administration of curcumin, and the consequent results showed that PCPA blocked the anti-immobility effect of curcumin in forced swimming test, suggesting the involvement of the serotonergic system. Moreover, pre-treatment of pindolol (10 mg/kg, i.p., a beta-adrenoceptors blocker/5-HT(1A/1B) receptor antagonist), 4-(2'-methoxy-phenyl)-1-[2'-(n-2'-pyridinyl)-p-iodobenzamino-]ethyl-piperazine (p-MPPI, 1 mg/kg, s.c., a selective 5-HT(1A) receptor antagonist), or 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane, 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist) was found to prevent the effect of curcumin (10 mg/kg) in forced swimming test. On the other hand, a sub-effective dose of curcumin (2.5 mg/kg, p.o.) produced a synergistic effect when given jointly with (+)-8-hydroxy-2-(di-n-propylamino)tetralin, (8-OH-DPAT, 1 mg/kg, i.p., a 5-HT(1A) receptor agonist), anpirtoline (0.25 mg/kg, i.p., a 5-HT(1B) receptor agonist) or ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist with higher affinity to 5-HT(2A) receptor) or R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI, 1 mg/kg, i.p., a 5-HT(2A) receptor agonist). Taken together, these results indicate that the antidepressant-like effect of curcumin in the forced swimming test is related to serotonergic system and may be mediated by, at least in part, an interaction with 5-HT(1A/1B) and 5-HT(2C) receptors.