Evidence for selective inhibition of limbic forebrain dopamine synthesis by 8-OH-DPAT in the rat

A modified adjusting delay task to assess impulsive choice between isocaloric reinforcers in non-deprived male rats: effects of 5-HT₂A/C and 5-HT₁A receptor agonists

RATIONALE

Existing animal models of impulsivity frequently use food restriction to increase subjects' motivation. In addition, behavioral tasks that assess impulsive choice typically involve the use of reinforcers with dissimilar caloric content. These factors represent energy-homeostasis limitations, which may confound the interpretation of results and limit the applicability of these models.

OBJECTIVES

This study was aimed at validating face and convergent validities of a modified adjusting delay task, which assesses impulsive choice between isocaloric reinforcers in ad libitum fed rats.

METHODS

Male Wistar rats (n = 18) were used to assess the preferredness and reinforcing efficacy of a "supersaccharin" solution (1.5% glucose/0.4% saccharin) over a 1.5% glucose solution. A separate group of rats (n = 24) was trained in a modified adjusting delay task, which involved repeated choice between the glucose solution delivered immediately and the supersaccharin solution delivered after a variable delay. To pharmacologically validate the task, the effects of the 5-HT(2A/C) receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)-DOI] and the 5-HT(1A) receptor agonist (±)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide [(±)-8-OH-DPAT] on impulsive choice were then evaluated.

RESULTS

Supersaccharin was highly reinforcing and uniformly preferred over the glucose solution by all subjects. Rats quickly learned the task, and impulsivity was a very stable and consistent trait. DOI and 8-OH-DPAT significantly and dose dependently increased impulsive choice in this modified adjusting delay task.

CONCLUSIONS

We validated a rodent task of impulsive choice, which eliminates typical energy-homeostasis limitations and, therefore, opens new avenues in the study of impulsivity in preclinical feeding and obesity research.

Contribution of the serotonin 5-HT1A receptor agonism of 8-OH-DPAT and EMD 128130 to the regulation of haloperidol-induced muscle rigidity in rats

The aim of the present study was to find out whether (+/-)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a prototypical 5-HT1A agonist, and (R)-(-)-2-[5-(4-fluorophenyl)-3-pyridylmethylaminomethyl]-chromane HCl (EMD 128130), a compound with serotonin 5-HT1A-agonist and dopamine D2-like antagonist properties, are able to attenuate the haloperidol-induced (1 mg/kg) muscle rigidity in rats. Muscle tone was examined using a combined mechano- and electromyographic (EMG) method that simultaneously measured the mechanical muscle resistance (MMG) of the rat's hind foot to passive movements in the ankle joint, and the EMG activity of two antagonist muscles. Both 8-OH-DPAT (0.125-0.5 mg/kg i.p.) and EMD 128130 (1-10 mg/kg i.p.) dose-dependently decreased the haloperidol-enhanced MMG to passive movements, as well as the tonic and the long-latency reflex EMG activities. Provided these results can be extrapolated to humans, the efficacy of EMD 128130 in relieving the haloperidol-induced muscle rigidity supports the concept that novel antipsychotics with 5-HT1A agonist and dopamine D2 antagonist activities should have a favourable extrapyramidal side-effect profile.

Additive hypothermic effects of the 5-HT1A receptor agonist 8-OH-DPAT and the dopamine D2/3 receptor agonist 7-OH-DPAT in the rat

The objective of this study was to examine possible interactions between serotonergic and dopaminergic agents lowering core temperature via stimulation of 5-HT1A and dopamine (DA) D2 receptors, respectively. The effects of the 5-HT1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT) and the DA D2/3 receptor agonist 7-OH-DPAT on core temperature was monitored in adult male Wistar rats, approximately 300 g body weight. The temperature probe was connected to a PC-assisted temperature instrument, and an automated printer device was activated when the temperature reading had stabilized (+/-0.1 degrees C) for 10 s. As expected, 7-OH-DPAT [0.5 and 2.0 micromol x kg(-1) subcutaneous (s.c.)] as well as 8-OH-DPAT (0.15-2.4 micromol x kg(-1) s.c.), produced a dose-dependent hypothermia. When combined, there were additive effects of the two compounds, although the effects of 7-OH-DPAT were attenuated by 8-OH-DPAT at the higher doses (0.6-2.4 micromol x kg(-1)), in all probability because of emerging DA D2 receptor blocking properties of the latter compound.

Temperature set-point changes induced by DA D2/3 and 5-HT1A receptor agonists in the rat

The DA D2/3 receptor agonist 7-OH-DPAT (2 micromol kg(-1)) and the 5HT1A receptor agonist 8-OH-DPAT (0.6 micromol kg(-1)) both produced a marked and similar decrease in core temperature of 3-4 degrees C at 10 and 20 degrees C ambient temperature. At 30 degrees C there were no, or weak, effects. The decrease in core temperature was accompanied by a sudden increase in tail temperature, followed by a decrease as core temperature returned to basal values. The results suggest that the hypothermia produced by the respective DA D2/3 and the 5-HT1A receptor agonists 7-OH-DPAT and 8-OH-DPAT is an active process, in all probability due to changes in a hypothalamic set-point for temperature regulation.

A9 and A10 dopamine nuclei as a site of action for effects of 8-OH-DPAT on locomotion in the rat

The 5-hydroxytryptamine (5-HT) 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was applied locally (0-5 microg bilaterally) into either the substantia nigra (A9) or the ventral tegmental area (A10) of adult male Wistar rats, and 10 min later spontaneous motor activity was observed in an open field ( approximately 0.5 m(2)) for 30 min. The rate of dopamine synthesis was estimated in neostriatal areas, the amygdala, and the prefrontal cortex, by measuring the accumulation of DOPA, following inhibition of cerebral decarboxylase by means of 3-hydroxybenzylhydrazine (NSD-1015). The A10 application of 8-OH-DPAT resulted in an increase in all aspects of spontaneous motor activity in the open field. A9 application of 8-OH-DPAT produced a stereotyped forward locomotion, characterized by a modest decrease in total horizontal activity, almost complete inhibition of rearing activity and an increase in proportion forward locomotion along the perimeter of the open-field arena. The injection of 8-OH-DPAT into the A9 was accompanied by an increased neostriatal DA rate of synthesis, whereas the A10 injection was followed by a decreased DA rate of synthesis in the amygdala and in the prefrontal cortex. It is concluded that mesencephalic dopaminergic mechanisms are involved in the stereotyped forward locomotion characteristically seen after systemic administration of the 5-HT(1A) receptor agonist 8-OH-DPAT.

R(+)-8-OH-DPAT, a selective 5-HT(1A) receptor agonist, attenuated amphetamine-induced dopamine synthesis in rat striatum, but not nucleus accumbens or medial prefrontal cortex

R(+)-8-OH-DPAT (0.05, but not 0.025, 0.1, 1 mg/kg), a 5-HT(1A) receptor agonist, decreased l-3,4-dihydroxyphenylalanine (DOPA) accumulation in rat striatum following NSD-1015, an l-aromatic amino acid decarboxylase inhibitor. Amphetamine (1 mg/kg) increased striatal DOPA accumulation, an effect attenuated by R(+)-8-OH-DPAT (0.05 mg/kg). However, both amphetamine (1 mg/kg) and R(+)-8-OH-DPAT (0.05 mg/kg) decreased cortical DOPA accumulation; there were no additional decreases from their combination. Neither amphetamine (1 mg/kg), R(+)-8-OH-DPAT (0.05 mg/kg), or the combination, significantly affected DOPA accumulation in the nucleus accumbens. The significance of and possible mechanisms for these findings are discussed.

The effect of serotonin(1A) receptor agonism on antipsychotic drug-induced dopamine release in rat striatum and nucleus accumbens

Serotonin (5-HT)(1A) receptor agonism may be of interest in regard to both the antipsychotic action and extrapyramidal symptoms (EPS) of antipsychotic drugs (APD) based, in part, on the effect of 5-HT(1A) receptor stimulation on the release of dopamine (DA) in the nucleus accumbens (NAC) and striatum (STR), respectively. We investigated the effect of R(+)-8-hydroxy-2-(di-n-propylamino)-tetralin (R(+)-8-OH-DPAT) and n-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-n-(2-pyridinyl)cyclohe xanecarboxamide trihydrochloride (WAY100635), a selective 5-HT(1A) receptor agonist and antagonist, respectively, on basal and APD-induced DA release. In both STR and NAC, R(+)-8-OH-DPAT (0.2 mg/kg) decreased basal DA release; R(+)-8-OH-DPAT (0.05 mg/kg) inhibited DA release produced by the 5-HT(2A)/D(2) receptor antagonists clozapine (20 mg/kg), low dose risperidone (0.01 and 0. 03 mg/kg) and amperozide (10 mg/kg), but not that produced by high dose risperidone (0.1 and 1.0 mg/kg) or haloperidol (0.01-1.0 mg/kg), potent D(2) receptor antagonists. This R(+)-8-OH-DPAT-induced inhibition of the effects of clozapine, risperidone and amperozide was antagonized by WAY100635 (0.05 mg/kg). WAY100635 (0.1-0.5 mg/kg) alone increased DA release in the STR but not NAC. The selective 5-HT(2A) receptor antagonist M100907 (1 mg/kg) did not alter the effect of R(+)-8-OH-DPAT or WAY100635 alone on basal DA release in either region. These results suggest that 5-HT(1A) receptor stimulation inhibits basal and some APD-induced DA release in the STR and NAC, and that this effect is unlikely to be mediated by an interaction with 5-HT(2A) receptors. The significance of these results for EPS and antipsychotic action is discussed.

Neurotransmitter system interactions revealed by drug-induced changes in motivated behavior

The present article reviews studies conducted either in collaboration with Jac Herberg, or in parallel with those studies that used consummatory behavior and responding for intracranial self-stimulation (ICSS) to investigate interactions between neurotransmitter systems. The studies reviewed include investigations of the role of dopamine in 8-OH-DPAT-induced feeding; the role of 5-HT3 receptors in the stimulant and depressant effects of nicotine on responding for ICSS; the interaction of D2 and 5-HT2 antagonists in sucrose consumption, and the differential contributions of alpha2-adrenoceptor and 5-HT2 antagonism to the rapid recovery of ICSS responding from depression produced by atypical neuroleptics. Further studies of the role of alpha2-adrenoceptor antagonism in the pattern of response decrements produced by neuroleptics on schedule-controlled responding for food confirm that the behavioral effects of monoamine interactions vary, depending on the specific receptor subtypes targeted and the behavioral paradigm employed. Consequently, the clinical relevance of findings will crucially depend on the choice of appropriate behavioral measures.

Regulation of striatal dopamine release through 5-HT1 and 5-HT2 receptors

Dopamine (DA) release in the striatum is regulated by 5-hydroxytryptamine (5-HT, serotonin) through putative heteroreceptors. However, the effect of 5-HT is controversial. The present study investigated the effects of different 5-HT receptor ligands on DA release in the rat striatum by using in vivo microdialysis in conscious and freely moving rats. Perfusion with 5-carboxamidotryptamine, anpirtoline, pindobind-5-HT1A, and isamoltane demonstrated the involvement of 5-HT1A and 5-HT1B receptors in facilitating DA release. In contrast, 5-HT2 receptors mediated inhibition of DA efflux, as shown by experiments with DOI [R-(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane] and ketanserin. A 5-HT3 agonist (1-(m-chlorophenyl)-biguanide hydrochloride) did not have any effect. None of the agonists used affected DA uptake into striatal synaptosomes. Unilateral 6-hydroxydopamine lesioning of the nigrostriatal DA pathway led to a selective decrease in 5-HT2 receptors. It is concluded that there are 5-HT2 heteroreceptors at the dopaminergic terminals that mediate inhibition of DA release. Further investigation is required to clarify the localization of the 5-HT1 receptors in the striatum.

In vivo intrinsic efficacy of the 5-HT1A receptor antagonists NAD-299, WAY-100,635 and (S)-(-)-UH-301 at rat brain monoamine receptors

The receptor-mediated control of brain monoamine synthesis was used to examine the in vivo intrinsic efficacy of the 5-HT1A receptor antagonists NAD-299, S(-)-UH-301 and WAY-100,635. The rate of monoamine synthesis was estimated by measuring the accumulation of DOPA and 5-HTP in the ventral neostriatum and the ventral hippocampus in rats pretreated with an inhibitor of cerebral aromatic L-amino acid decarboxylase. S(-)-UH-301 (2.0-32.0 micromol kg(-1)), but not WAY-100,635 (0.08-1.2 micromol kg(-1)), produced a decreased 5-HTP accumulation in the neostriatum and in the hippocampus. The administration of NAD-299 (0.75-12.0 micromol kg(-1)) resulted in a slight increase in neostriatal, but not hippocampal, 5-HTP accumulation. Neostriatal DOPA accumulation was decreased by S(-)-UH-301, whereas treatment with WAY- 100,635 resulted in an increase. NAD-299 did not affect neostriatal DOPA levels. There were no effects by any of these agents on DOPA levels in the ventral hippocampus. It is concluded that S(-)-UH-301, but not WAY-100,635 or NAD-299, displays intrinsic efficacy at brain 5-HT1A and DA D2/3 receptors, whereas WAY-100,635 behaves as a DA D2/3 receptor antagonist. By this comparison, NAD-299 appears to be the most selective and specific 5-HT1A receptor antagonist.

Is the potent 5-HT1A receptor agonist, alnespirone (S-20499), affecting dopaminergic systems in the rat brain?

The effects of the new methoxy-chroman 5-HT1A receptor agonist, alnespirone (S-20499), on the dopamine systems in the rat brain were assessed in vivo by means of electrophysiological and neurochemical techniques. Cumulative doses of alnespirone (0.032-4.1 mg kg(-1), i.v.) did not modify the spontaneous firing rate of dopamine neurons in the substantia nigra as well as in the ventral tegmental area. The local application of alnespirone (0.1-10 microM) by reverse microdialysis into the dorsal striatum did not affect the dopamine output but induced a moderate, although dose-independent, increase of 5-HT (5-hydroxytryptamine, serotonin) concentrations in the dialysate. As expected of a 5-HT1A receptor agonist, intraperitoneal (i.p.) administration of alnespirone at 2-32 mg kg(-1) markedly decreased 5-HT turnover in the striatum. Parallel measurements of dopamine turnover showed that alnespirone exerted no effect except at the highest dose (32 mg kg(-1), i.p.) for which a significant increase was observed. Interestingly, both alnespirone-induced reduction in 5-HT turnover and increase in dopamine turnover could be prevented by pretreatment with the selective 5-HT1A receptor antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclohexa ne carboxamide). Altogether, these data indicate that alnespirone does not exert any direct influence on central dopamine systems. The enhanced dopamine turnover due to alnespirone at high dose appeared to result from 5-HT1A receptor stimulation, further supporting the idea that this receptor type may play a key role in 5-HT-dopamine interactions in brain.

MKC-242, a novel 5-HT1A receptor agonist, facilitates cortical acetylcholine release by a mechanism different from that of 8-OH-DPAT in awake rats

We have previously reported that 5-¿3-[((2S)-1,4-benzodioxan-2-ylmethyl)amino]propoxy¿-1,3-be nzodioxole (MKC-242), a potent and selective serotonin (5-HT)1A receptor agonist, exerts anxiolytic- and antidepressant-like effects in animal models and that the antidepressant-like effect may be mediated by postsynaptic 5-HT1A receptors. The present study, using a microdialysis technique, was undertaken to characterize in vivo the effect of MKC-242 on cholinergic neurons. Subcutaneous injection of MKC-242 (0.5-1.0 mg/kg), like the typical 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), increased extracellular acetylcholine (ACh) levels in the rat cerebral cortex. The increase in ACh release by MKC-242 was also observed in the hippocampus. The effect of MKC-242 on cortical ACh release was attenuated by pretreatment with the 5-HT1A receptor antagonists (10 mg/kg, s.c.) propranolol and N-tert-butyl-3-(4-(2-methoxyphenyl)piperazin-1-yl)-2-phenylpropana mide. The increase in cortical ACh release by MKC-242 was blocked by lesion of serotonergic neurons with 5,7-dihydroxytryptamine, whereas that by 8-OH-DPAT was not. Lesion of noradrenergic neurons with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine did not affect the MKC-242-induced increase in ACh release. These results suggest that systemic injection of MKC-242 facilitates in vivo ACh release via an activation of somadendritic 5-HT1A autoreceptors, and that MKC-242 and 8-OH-DPAT affect cholinergic neurons in the rat cerebral cortex via different mechanisms.

Biphasic effects of 8-OH-DPAT on endurance of treadmill performance in the male rat

Administration of the 5-HT1A receptor agonist 8-OH-DPAT produced a biphasic pattern of effects on endurance performance of rats walking on top of a treadmill drum ([symbol: see text] = 166 mm, 16 rpm; approximately 8 m min-1), with enhanced performance at a low dose (0.1 mg kg-1 s.c.) followed by impairment (0.2-0.8 mg kg-1). The partial 5-HT1A receptor agonist (-)-pindolol improved the performance in the low dose range (0.5-2.0 mg kg-1 s.c.), whereas a higher dose (8 mg kg-1) was ineffective. The 5-HT1A receptor antagonist WAY-100,635 produced an impaired performance at a low dose (12.5 micrograms kg-1 s.c.), with a recovery of performance at higher doses (50-200 micrograms kg-1). It is suggested that the inhibition of central serotonergic neurotransmission produced by stimulation or blockade of 5-HT1A auto- and post-synaptic receptors, respectively, results in an improved endurance performance on the treadmill, whereas stimulation of postsynaptic 5-HT1A receptors has the opposite action. In support of this contention, the impaired performance produced by a high dose of 8-OH-DPAT (0.8 mg kg-1) was antagonized by pretreatment with (-)-pindolol (2 mg kg-1 s.c.).

Role of 5-HT1A receptors in the ability of mixed 5-HT1A receptor agonist/dopamine D2 receptor antagonists to inhibit methylphenidate-induced behaviors in rats

Behavioral effects produced by the indirect-acting dopamine receptor agonist, methylphenidate (40 mg/kg i.p.) were examined in rats after administration of the 5-HT1A receptor agonists (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and flesinoxan, the mixed 5-HT1A receptor agonist/dopamine D2 receptor antagonists buspirone and 1-[-4-fluorobenzoylamino)ethyl]-ethyl]-4-(7-methoxynaphthyl) piperazine (S 14506), the neuroleptics haloperidol and clozapine, and the sigma receptor ligand/partial 5-HT1A receptor agonist alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-butanol (BMY 14802). All of the compounds produced dose-related decreases in methylphenidate-induced stereotyped gnawing, and, as gnawing was inhibited, other methylphenidate-induced responses (i.e. sniffing, rearing and locomotion) appeared. Higher doses of haloperidol and buspirone, but none of the remaining compounds, inhibited these other responses, so that the behavior of the methyphenidate-treated animals became similar to that of normal controls. Pretreatment with the 5-HT1A receptor antagonist N-[2-4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)- cyclohexanecarboxamide (WAY-100635; 0.63 mg/kg s.c.) blocked the ability of 8-OH-DPAT, S 14506 and flesinoxan to inhibit methylphenidate-induced gnawing, demonstrating the involvement of 5-HT1A receptors in their ability to inhibit methylphenidate-induced behaviors. In contrast, pretreatment with WAY-100635 did not alter the ability of haloperidol, clozapine, buspirone, or BMY 14802 to inhibit methylphenidate-induced gnawing, or in the case of haloperidol and buspirone, to normalize behavior. The results indicate that mixed compounds with 5-HT1A receptor agonist and dopamine receptor antagonist properties can be differentiated on the basis of the ability of WAY-100635 to reverse their effects on methylphenidate-induced behaviors.

Prevention by (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin of both catalepsy and the rises in rat striatal dopamine metabolism caused by haloperidol

1. The influence of (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) on haloperidol-induced increases in the dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA), was measured in three microdissected brain regions of the rat following a quantitative assessment of catalepsy. 2. Haloperidol alone (2.66 mumol kg-1, i.p.) caused a robust cataleptic response. Given 30 min after haloperidol, 8-OH-DPAT (76 or 760 nmol kg-1, s.c.) prevented catalepsy in 30% and 100% of rats, respectively. 3. Haloperidol significantly increased the DOPAC (by 2 to 4 fold) and HVA (by 3 to 7 fold) contents of the caudate-putamen, nucleus accumbens and medial prefrontal cortex. Given alone, only the lower dose of 8-OH-DPAT caused a significant biochemical change, a doubling of cortical DOPAC. 4. In the cases where catalepsy was prevented by either dose of 8-OH-DPAT, the haloperidol-induced increases in DOPAC and HVA were consistently lower in the caudate-putamen. This pattern was true for the rise in cortical HVA but only in response to the lower dose of 8-OH-DPAT. In contrast, neither dose of 8-OH-DPAT was able to influence the haloperidol-induced rises in cortical DOPAC. In the nucleus accumbens, 8-OH-DPAT did not affect the haloperidol-induced increases in the dopamine metabolites, irrespective of the dose employed or the resulting behaviour. When catalepsy was not prevented, 8-OH-DPAT did not alter the neurochemical responses to haloperidol in any region. 5. These results suggest that part of the mechanism by which 8-OH-DPAT prevents haloperidol-induced catalepsy is reflected by a reversal of the compensatory increase in meso-striatal and/or meso-cortical dopamine neuronal activity that normally accompanies postsynaptic dopamine receptor blockade with haloperidol.

Antagonism of reserpine-induced suppression of spontaneous motor activity by stimulation of 5-HT1A receptors in rats

The 5-HT1A and the DA D2 receptor agonists 8-OH-DPAT (0.05-3.2 mg kg-1 subcutaneously, -20 min.) and quinpirole (0.08-1.25 mg kg-1 subcutaneously, -20 min.), respectively, both partially antagonized reserpine-induced (5 mg kg-1 subcutaneously, -16 hr) suppression of spontaneous motor activity in the rat. Four different aspects of the spontaneous motor activity were recorded in a photocell-equipped open-field (8 x 8 photocells, 90 mm apart, defining two horizontal planes): locomotor activity (all photocell counts at the lower level); rearing (all photocell counts at the upper level); forward locomotion (the proportion movements across the arena); peripheral activity (the proportion locomotor activity as picked up by the photocell beam closest to the wall, i.e. 25 mm). As defined by these variables, the pattern of activity produced by 8-OH-DPAT or quinpirole were indistinguishable. The effects produced by 8-OH-DPAT were fully antagonized by the 5-HT1 antagonist (-) pindolol (4 mg kg-1 subcutaneously, -30 min.), but not by the DA D2 receptor antagonist raclopride (2 mg kg-1 subcutaneously, -30 min.) nor by the 5-HT2 receptor antagonist ritanserin (2 mg kg-1 subcutaneously, -30 min.), whereas effects produced by quinpirole were fully antagonized by raclopride (2 mg kg-1 subcutaneously, -30 min.). Effects produced by quinpirole, but not 8-OH-DPAT, were potentiated by administration of the DA D1 agonist SKF-38,393 (3 mg kg-1 subcutaneously, -20 min.). It is concluded that effects by 8-OH-DPAT on spontaneous motor activity in the reserpine treated rat primarily are due to stimulation of postsynaptic 5-HT1A receptors.

Evidence for specific interactions between 5-HT1A and dopamine D2 receptor mechanisms in the mediation of extrapyramidal motor functions in the rat

Administration of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.1 mg kg-1 SC) completely antagonised the catalepsy produced by the dopamine (DA) D2 receptor antagonist raclopride (16 mg kg-1 SC). This effect by 8-OH-DPAT was in turn completely antagonised by treatment with the new 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(di-n-propylamino)tetralin [(S)-UH-301] (3.5 mg kg-1 SC), but not by the mixed 5-HT1 receptor/beta-adrenoceptor antagonist (-)pindolol (2.0 mg kg-1 SC). The failure by (-)pindolol to antagonise the effects of 8-OH-DPAT on raclopride-induced catalepsy could be due to its beta-receptor-blocking properties, since by themselves both (-)pindolol and the selective beta-adrenoceptor antagonist betaxolol (4 mg kg-1 SC) at least partially antagonised the raclopride-induced catalepsy. The present results provide further support for specific interactions between 5-HT1A and DA D2 receptor mechanisms in the mediation of extrapyramidal motor functions in the rat.

Gonadotropin and prolactin secretion in prepubertal female rats treated with 8-hydroxy-2-(di-n-propylamino) tetralin

The effect of serotoninergic activation on gonadotropin and prolactin release were analysed in 16-day-old intact female rats. In the first experiment, females were decapitated 30 min after i.p. administration of 100 mg/kg of 5-hydroxytryptophan (5-HTP) or vehicle; in the second experiment the rats were decapitated 15 and 30 min after i.p. injection of vehicle or some doses (0.1, 1 and 10 mg/kg) of 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), a selective agonist of the serotonin (5-HT)1A receptors. We found that: 1) serum follicle-stimulating hormone (FSH), luteinizing-hormone (LH) and prolactin concentrations increased after 5-HTP administration; 2) serum LH and prolactin concentrations and pituitary prolactin content increased after administration of 8-OH-DPAT. Our results indicate that in prepubertal rats, activation of serotoninergic system stimulated gonadotropin and prolactin release, and that 5-HT1A receptors are involved in this effect. In addition, the simultaneous increase in serum and pituitary prolactin content suggests that 8-OH-DPAT enhances prolactin synthesis.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of midazolam and 5-HT1A receptor agonists

An involvement of serotonergic innervation of the hippocampus (HP) and the nucleus accumbens septi (NAS) in anxiolytic activity of benzodiazepine midazolam and 5-HT1A receptor agonists was studied in two different animal models of anxiety. Injection of midazolam (10.0 and 20.0 micrograms) or 8-OH-DPAT (0.5 and 1.0 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. Buspirone given at 0.1, 0.5 and 1.0 microgram was ineffective in the Vogel test, while at 5.0 micrograms it enhanced shock-induced suppression of drinking. In the open-field test midazolam (0.01 and 0.1 microgram), 8-OH-DPAT (0.1, 0.5 and 1.0 microgram) and buspirone (2.5 and 5.0 micrograms) increased the number of entries into the central part of the open-field and the time spent in the central sector. Depletion of 5-HT had no influence on the anxiolytic-like effect in the open-field test of intrahippocampally-administered 8-OH-DPAT (0.5 microgram), but the drug tended to increase motor activity in lesioned animals. Midazolam and buspirone injected into the NAS did not have an anxiolytic effect in the Vogel test. A small increase in punished drinking was observed after 8-OH-DPAT (1.0 and 2.5 micrograms). Following intra-NAS injection, midazolam, 8-OH-DPAT and buspirone all failed to produce any marked anxiolytic-like effect in the open-field test. It appears that the hippocampus, rather than the NAS, is involved in mediating anxiolytic-like effects of 5-HT1A receptor agonists. Hippocampal postsynaptic 5-HT1A receptors may account for the anti-emotional influence of this group of drugs. The results indicate some similarities in the psychotropic profile of 5-HT1A receptor agonists and midazolam.

Effects of 5-HT1A receptor agonists on patterns of rat motor activity in relation to effects on forebrain monoamine synthesis

The effects of the 5-HT1A receptor agonists 8-OH-DPAT (0.15-2.5 mumol kg-1 subcutaneously), flesinoxan (0.6-10.0 mumol kg-1 subcutaneously) and buspirone (1.9-30.0 mumol kg-1 subcutaneously) on spontaneous motor activity in male Sprague-Dawley rats was examined in a photocell-equipped open-field arena. Following motor activity observations, the cerebral aromatic L-amino acid decarboxylase inhibitor NSD-1015 (100 mg kg-1 intraperitoneally) was administered and 30 min. later the animals were decapitated for subsequent analysis of the accumulated forebrain DOPA and 5-HTP levels, as an estimate of the rate of monoamine synthesis. 8-OH-DPAT and flesinoxan produced a similar and characteristic pattern of changes of the spontaneous motor activity in normal animals i.e. a moderate decrease in locomotor activity, a marked suppression of rearing and an increase in the relative amount of forward locomotion and of activity in the periphery of the open-field arena. This behavioural profile was closely related to a decrease in forebrain 5-HTP accumulation, indicating 5-HT receptor stimulation. In agreement with these observations buspirone also produced an increase in peripheral activity and a suppression of rearing. In contrast to effects by 8-OH-DPAT and flesinoxan, however, buspirone produced a further reduction of locomotor activity and reduced the forward locomotion. This difference in behavioural profile between buspirone and the other two compounds is probably explained by its DA receptor blocking properties, as indicated by an increased DOPA accumulation in the neostriatum. At least partially, 8-OH-DPAT, flesinoxan and buspirone, all antagonized reserpine-induced (5 mg kg-1 subcutaneously--16 hr) suppression of locomotor activity. This stimulation of locomotor activity in reserpine-treated rats is in all probability related to 5-HT1A receptor stimulation since concomitant DA D2 receptor blockade, in the case of buspirone, did not markedly affect this behavioural response.

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Brain monoaminergic neurotransmission in the mediation of lordosis behavior in the female rat

The present paper reviews recent pharmacological evidence indicating an important role for central monoaminergic neurotransmission in the mediation of lordosis behavior in the female rat. Following the initial observations by Meyerson and his colleagues in the early 1960s, demonstrating an inhibitory serotonergic mechanism, continued studies have identified 5-hydroxytryptamine (5-HT) 5-HT1A receptors to be of particular importance. Stimulation of central 5-HT2 receptors appear to have an opposite role, i.e. facilitating the elicitation of the lordosis response. As regards brain dopaminergic neurotransmission, it appears that brain dopamine (DA) D2 receptors mediate an inhibitory effect on the lordosis behavior, whereas no distinct role has been ascribed to DA D1 receptors. The serotonergic and the dopaminergic mechanisms can be manipulated independently, perhaps indicating the operation of parallel mechanisms. Most of the evidence presented above has been obtained in experiments on ovariectomized female rats primed with subthreshold doses of estrogen. It appears, however, that at least the inhibitory effects mediated by serotonergic neurotransmission are not dependent on the presence of progesterone.

Lack of involvement of haloperidol-sensitive sigma binding sites in modulation of dopamine neuronal activity and induction of dystonias by antipsychotic drugs

The present studies evaluated previous suggestions that haloperidol-sensitive sigma binding sites are involved in the modulation of dopamine (DA) neuronal activity and in the induction of the dystonic effects of antipsychotic drugs. These issues were addressed by evaluating the effects of compounds that have differing affinities for sigma binding sites, on the firing activity of DA neurons in the substantia nigra in chloral hydrate-anesthetized rats and on the ability to induce extrapyramidal motor dysfunction in squirrel monkeys sensitized to the dystonic effects of haloperidol. The agents studied included haloperidol, DTG (1,3-di-o-tolylguanidine), (+)-pentazocine, (+)-SKF 10,047, BMY 14802, 8-OH-DPAT and sulpiride. There was no relationship between affinity for sigma binding sites and the ability to either alter DA neuronal activity or to induce extrapyramidal motor dysfunction.

Behavioral and biochemical effects of the 5-HT1A receptor agonists flesinoxan and 8-OH-DPAT in the rat

The 5-HT1A receptor agonists flesinoxan (0.2-3.2 mg kg-1 s.c.) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.025-0.4 mg kg-1 s.c.) produced (1) a dose-dependent facilitation of male rat ejaculatory behavior and (2) characteristic, dose-dependent effects on spontaneous motor activity. Thus, total locomotor activity and rearing activity were decreased. However, forward locomotion and peripheral locomotion were increased relative to the total horizontal activity. Furthermore, (3) 5-HTP accumulation, after inhibition of cerebral decarboxylase, was dose dependently decreased by both compounds in the ventral striatum and in the prefrontal cortex. There was a statistically significant decrease in DOPA accumulation in the ventral striatum after administration of a high dose of flesinoxan (3.2 mg kg-1), and a tendency for 8-OH-DPAT to produce the same effect. The efficacy of the compounds to affect male rat sexual behavior, spontaneous motor activity in the open-field and forebrain 5-HT synthesis was approximately the same, whereas flesinoxan was about an order of magnitude less potent than 8-OH-DPAT.

Region-selective inhibition of male rat sexual behavior and motor performance by localized forebrain 5-HT injections: a comparison with effects produced by 8-OH-DPAT

The local application of 5-HT (0-40 micrograms side-1) into the nucleus accumbens was found to inhibit male rat sexual behavior, as evidenced by an increase in number of mounts and intromissions preceding ejaculation and in time to ejaculation. There were no effects on male rat sexual behavior after similar 5-HT injections into other striatal areas, including the dorsolateral, the ventromedial and the posterior neostriatum, as well as the olfactory tubercle. The same groups of animals were also scored for motor activity and body posture after the injection of 5-HT, and only animals injected into the nucleus accumbens showed a statistically significant decrease in motor activity and an increase in the display of a flat body posture. 8-OH-DPAT (0-5 micrograms side-1), injected into the nucleus accumbens, produced a facilitation of the male rat sexual behavior, as evidenced by a decrease in number of mounts and intromissions to ejaculation, as well as in the postejaculatory interval. 8-OH-DPAT injections into the nucleus accumbens produced a decrease in motor activity and an increase in the per cent animals with a flat body posture. Injections into the olfactory tubercle had no effects on the sexual behavior or on the motor activity, whereas the per cent flat body posture was increased. Local application of 8-OH-DPAT (0-5 micrograms) into the median raphe nucleus, facilitated male rat sexual behavior, as evidenced by a decrease in number of intromissions preceding ejaculation and in time to ejaculation. The same doses of 8-OH-DPAT injected into the dorsal raphe had no effects on the sexual behavior. In an additional experiment, 3 groups of animals were injected with 5-HT (40 micrograms) or 8-OH-DPAT (5 micrograms) into the nucleus accumbens, the dorsal and the median raphe nuclei and thereafter observed for treadmill performance. No statistically significant effects were found after injections in any of these brain areas. The present results strongly suggest an inhibitory role of ventral forebrain 5-HT in the mediation of male rat sexual behavior. The facilitation produced by 8-OH-DPAT is possibly due to a blockade of 5-HT2 receptors. Facilitation by 8-OH-DPAT of the male rat copulatory performance after median raphe injections is probably due to stimulation of 5-HT1A autoreceptors in this brainstem region. In contrast to their opposite effects on sexual behavior, both compounds produced a decrease in motor activity and an increased display of flat body posture after accumbens injections.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential behavioural activation following intra-raphe infusion of 5-HT1A receptor agonists

Microinfusion of the selective 5-HT1A receptor agonist, 8-hydroxy-(di-N-propylamino)tetralin (8-OHDPAT), into the dorsal raphe nucleus (DRN) produced a marked behavioural hypoactivity and flat body posture. Injections of similar doses into the median raphe nucleus (MRN) elicited hyperactivity but no postural change. Reductions in rearing and grooming were also observed after DRN and MRN infusions of 8-OHDPAT. The behavioural profiles of other 5-HT1A selective compounds, gepirone and BMY7378 were found to be similar to 8-OHDPAT. The contrasting behavioural profiles of the 5-HT1A agents observed after DRN or MRN microinfusions are probably related to the differential innervation of forebrain structures by each raphe nucleus. Thus, the present data confirms and extends previous results illustrating the influence of 5-HT systems on motor behaviour in the rat and identifies unique behavioural profiles following activation of the DRN and MRN.

5-HT1A agonists and dopamine: the effects of 8-OH-DPAT and buspirone on brain-stimulation reward

Two specific 5-HT1A agonists, 8-OH-DPAT (0-300 micrograms/kg), and buspirone (0-3.0 mg/kg), were tested on variable-interval, threshold-current self-stimulation of rat lateral hypothalamus. Buspirone produced a prolonged monotonic depression of responding, whereas the effects of 8-OH-DPAT were biphasic: 3.0 micrograms/kg produced a sustained enhancement of responding while higher doses (100-300 micrograms/kg) produced a relatively short-lasting depression. This biphasic pattern parallels previously reported effects of 8-OH-DPAT on food intake and on various other behaviours. Threshold-current self-stimulation is highly sensitive to alterations in dopaminergic transmission but relatively insensitive to changes in 5-HT. Thus the facilitatory effect of low-dose 8-OH-DPAT seems most plausibly interpreted in terms of enhanced dopaminergic transmission. This could be brought about by 5HT1A autoreceptor-mediated inhibiton of 5-HT release and consequent disinhibition of dopaminergic transmission. Depression of self-stimulation by higher doses of 8-OH-DPAT may reflect the activity of 8-OH-DPAT at postsynaptic 5-HT receptors, with consequent inhibition of DA transmission. Suppression of responding after buspirone at all doses tested may reflect the action of this compound as a partial agonist at postsynaptic 5-HT receptors, and/or its effects on other systems.

Antipsychotic-like profile of combined treatment with raclopride and 8-OH-DPAT in the rat: enhancement of antipsychotic-like effects without catalepsy

The administration of the 5-HT1A agonist 8-OH-DPAT, 0.1 mg kg-1 sc-20 min, produced a moderate suppression of conditioned avoidance behavior (60% of controls) in the rat. This effect, however, was not seen after administration of higher doses, 0.4 and 1.6 mg kg-1 sc. The number of intertrial crosses were not affected by the lower dose but significantly increased by administration of the two higher doses of 8-OH-DPAT. The dopamine D2 receptor blocking agent raclopride, 0.05 mg kg-1, by itself did not suppress the avoidance behavior, but in combination with 8-OH-DPAT produced suppression of avoidance behavior (30% of controls) as well as intertrial crosses. Open field locomotor activity was suppressed by raclopride, 0.1 mg kg-1 sc, or by 8-OH-DPAT, 0.1 mg kg-1 sc. The combined treatment produced a further suppression of locomotor activity and a marked increase in "immobility" (stationary movements). Treadmill locomotion, however, was not affected by either compound by itself, whereas the combined treatment impaired treadmill performance. Suppression of treadmill performance by a higher dose of raclopride, 0.4 mg kg-1 sc, was not altered by the additional treatment with 8-OH-DPAT, 0.1 mg kg-1. In contrast to the additive effects of 8-OH-DPAT and raclopride on conditioned avoidance behavior, open field locomotion and treadmill performance, the catalepsy produced by raclopride, 16 mg kg-1 was completely antagonised by treatment with 8-OH-DPAT 0.1 mg kg-1. Taken together, the present findings demonstrate strong interactions between a 5-HT agonist and a DA D2 antagonist on some critical tests for antipsychotic-like actions and extrapyramidal motor effects in rats, and suggest new possibilities in the search for new antipsychotic drugs with higher clinical efficacy and less extrapyramidal side effects.

Dorsal raphe microinjection of 5-HT and indirect 5-HT agonists induces feeding in rats

Previous work has shown that 5-hydroxytryptamine (5-HT) receptor agonists such as 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) reduce 5-HT neurotransmission and induce feeding in rats. The effects of 8-OH-DPAT appear to be mediated in part in the dorsal raphe nucleus by serotonergic somatodendritic autoreceptors which normally regulate impulse flow in 5-HT dorsal raphe neurons. The present experiments sought to examine whether suppression of dorsal raphe serotonergic neural activity induced by exogenously applied, or endogenously released 5-HT would increase feeding. Free-feeding rats were microinjected in the dorsal raphe with 5-HT, the 5-HT releasing compound d-fenfluramine, the 5-HT re-uptake inhibitor zimelidine, or the type-A monoamine oxidase inhibitor brofaromine. Dose dependent increases in food intake over a 1 h period were found following treatment with 5-HT and the three indirectly acting compounds. Thus, increased serotonergic activity within the dorsal raphe increases feeding, presumably by inhibiting the activity of dorsal raphe 5-HT neurons. In addition the effects of 5-HT were blocked by pretreatment with haloperidol, indicating the involvement of a dopaminergic mechanism in mediating the effects of feeding of a suppression in dorsal raphe 5-HT neural activity. The results are discussed in terms of the general role which serotonergic neurons arising from the dorsal raphe may play in behavioural inhibition.

Effects of local application of 5-HT and 8-OH-DPAT into the dorsal and median raphe nuclei on motor activity in the rat

The spontaneous motor activity of male Wistar rats was recorded after the local application of 5-HT (10 or 40 micrograms) or 8-OH-DPAT (1 or 5 micrograms) into the dorsal raphe (DR) or median raphe (MR) nuclei. Motor activity was monitored for 30 min, beginning 10 min after injections. The injection volume was 0.5 microliters and injections were made by means of 31-ga needles at a rate of 0.33 microliters min-1. The raphe nuclei were approached at 30 degrees in order to avoid penetration of the cerebral aqueduct and also to avoid passage of the DR with injections aimed at the MR. The application of 5-HT into the DR produced a marked suppression of locomotor activity and rearing, and no or a slight increase in these variables were noted after MR injections. 8-OH-DPAT injections into the DR resulted in a decrease in locomotor activity and rearing after the 5- but not the 1-microgram dose, whereas a marked locomotor stimulation was found after injections into the MR. In the latter case 1 microgram of 8-OH-DPAT was more effective than 5 micrograms, possibly due to overstimulation by the higher dose. The results demonstrate different roles for serotonergic mechanisms in the DR and MR in the mediation of spontaneous motor activity in the rat, and also a possible specific role for 5-HT1 receptors. The involvement of 5-HT1 receptors was further supported by the ability of (-)pindolol, 2 mg kg-1 SC, to antagonise 8-OH-DPAT-induced hyperactivity after application into the MR.

Effects of 5-HT and 8-OH-DPAT on forebrain monoamine synthesis after local application into the median and dorsal raphe nuclei of the rat

5-HT (10 and 40 micrograms) and 8-OH-DPAT (1 and 5 micrograms) were locally applied into the dorsal or median raphe nuclei in awake, unrestrained, rats. All animals were also treated with the 5-HTP and DOPA decarboxylase inhibitor NSD-1015, 100 mg kg-1 SC, 30 min before decapitation. 5-HT or 8-OH-DPAT were administered 5 min before NSD-1015. The regional brain in vivo rate of tyrosine and tryptophan hydroxylase activity was estimated by measuring the accumulation of DOPA and 5-HTP. The following brain regions were sampled: neocortex, hippocampus, dorso-lateral neostriatum, ventro-medial neostriatum, nucleus accumbens, olfactory tubercle, globus pallidus, septum and the amygdala. Compared to normal controls, there were small and inconsistent effects on forebrain 5-HTP accumulation by saline injections into the dorsal or the median raphe (an increase in 3 out of 36 experiments), whereas strong effects by the injection procedure were noted on forebrain DOPA accumulation (an increase in 15 out of 36 experiments). Injections of 5-HT (same effect by 10 or 40 micrograms) into the dorsal raphe, produced a decrease in 5-HTP accumulation in all forebrain areas except for the hippocampus and the septum, whereas no effects were seen in any area after median raphe injections. In contrast, 8-OH-DPAT preferentially produced a decrease in forebrain 5-HTP accumulation after median raphe injections and less, but statistically significant effects by dorsal raphe injections. The 8-OH-DPAT injection into the median raphe primarily affected limbic forebrain areas (hippocampus, nucleus accumbens, ventro-medial neostriatum, amygdala and the septum). This dissociation of the effects of 5-HT and 8-OH-DPAT on forebrain 5-HT synthesis after local application into the dorsal or the median raphe strongly supports the contention of heterogeniety in the brain 5-HT receptor population in terms of receptor subtypes and/or receptor regulation.

Increased dopamine turnover in the ventral striatum by 8-OH-DPAT administration in the rat

The administration of the 5-HT1A agonist 8-OH-DPAT (0.8 mumols kg-1 s.c.-40 min) produced an increase in dopamine (DA) turnover, estimated by the quotient (DOPAC + HVA) DA-1, in the ventral striatum of the rat. No statistically significant effects were obtained in the dorsal striatum. The accumulation of 3-MT in pargyline-treated animals (375 mumols kg-1 s.c.-60 min) was not affected by 8-OH-DPAT treatment (0.15-2.4 mumols kg-1 s.c.-30 min). These findings indicate that 8-OH-DPAT has weak antagonist properties at striatal DA receptors in normal rats. Both the 5-HT1A agonist flesinoxan (0.06-17.8 mumos kg-1 s.c. -50 min) and the mixed 5-HT1 and 5-HT2 agonist 5-MeODMT (1.6-26.0 mumols kg-1 s.c.-50 min) produced a decrease in forebrain 5-HTP accumulation (striatum and neocortex), following decarboxylase inhibition by means of NSD-1015 in reserpine treated rats, indicating stimulation of central 5-HT receptors by these two compounds. At the same time, the DOPA accumulation by the ventral striatum was decreased by flesinoxan and increased by 5-MeODMT treatment. These observations show that, under these conditions, the decrease in DA synthesis is not directly coupled to the decreased 5-HT synthesis produced by flesinoxan, as previously demonstrated for 8-OH-DPAT. Taken together with previous observations, the present results suggest that 8-OH-DPAT, depending on the experimental conditions, is an agonist or antagonist at striatal DA receptors, in all probability due to partial DA receptor agonist properties of the compound.