Behavioural assessment in rats of the antipsychotic potential of the potent dopamine D2 receptor antagonist, (-)eticlopride

Caffeine increases alcohol self-administration, an effect that is independent of dopamine D2 receptor function

The rising popularity of alcohol mixed with energy drinks (AmEDs) has become a significant public health concern, with AmED users reporting higher levels of alcohol intake than non-AmED users. One mechanism proposed to explain this heightened level of alcohol intake in AmED users is that the high levels of caffeine found in energy drinks may increase the positive reinforcing properties of alcohol, an effect that may be dependent on interactions between adenosine receptor signaling pathways and the dopamine D₂ receptor. Therefore, the purpose of the current study was to confirm whether caffeine does increase the positive reinforcing effects of alcohol using both fixed ratio (FR) and progressive ratio (PR) designs, and to investigate a potential role of the dopamine D₂ receptor to caffeine-induced increases in alcohol self-administration. Male Long-Evans rats were trained to self-administer a sweetened alcohol solution (10% v/v alcohol + 2% w/v sucrose) on an FR2 schedule of reinforcement, and the effects of caffeine (0, 5, 10, and 20 mg/kg, i. p. [intraperitoneally]) on the maintenance of alcohol self-administration and alcohol break point were examined. Parallel experiments in rats trained to self-administer sucrose (0.8% w/v) were conducted to determine whether caffeine's reinforcement-enhancing effects extended to a non-drug reinforcer. Caffeine pretreatment (5-10 mg/kg) significantly increased sweetened alcohol self-administration and motivation for a sweetened alcohol reinforcer. However, similar increases in self-administration of a non-drug reinforcer were not observed. Contrary to our hypothesis, the D₂ receptor antagonist eticlopride did not block a caffeine-induced increase in sweetened alcohol self-administration, nor did it alter caffeine-induced increases in motivation for a sweetened alcohol reinforcer. Taken together, these results support the hypothesis that caffeine increases the positive reinforcing effects of alcohol, which may explain caffeine-induced increases in alcohol intake. However, the reinforcement-enhancing effects of caffeine appear to be independent of D₂ receptor function.

Dopamine D1 Receptor Within Basolateral Amygdala Is Involved in Propofol Relapse Behavior Induced by Cues

Propofol has been proven to be potentially abused by humans and laboratory animals; however, studies that have examined propofol relapse behavior are limited, and its underlying mechanism remains unclear. In this study, we examined whether basolateral amygdala-specific or systematic administration of the dopamine receptor antagonist alters cue-induced propofol-seeking behaviors in a rat model. Male Sprague-Dawley rats first received 14 days of propofol self-administration training, where active nose poke resulted in the delivery of propofol infusion paired with a tone and light cues. After 1-30 days of forced abstinence, the cue-induced propofol-seeking behaviors were tested in the operant chamber. We demonstrated, for the first time, after a few days of withdrawal from intravenous bolus administration of propofol, propofol-related cues could induce robust reinstatement of drug-seeking behavior. Systematic administration of dopamine D₁ receptor antagonist (SCH-23390) or dopamine D₂ receptor antagonist (spiperone) inhibited propofol relapse behavior induced by drug-related cues. Furthermore, we show that microinfusion of SCH-23390 into basolateral amygdala dose-dependently attenuated cue-induced propofol drug-seeking behavior, whereas infusion of spiperone had no effect on the propofol relapse behavior. Our results reveal the involvement of dopamine receptors within the basolateral amygdala in the cue-induced propofol relapse behavior in rats.

The role of dopaminergic transmission through D1-like and D2-like receptors in amphetamine-induced rat ultrasonic vocalizations

RATIONALE

Systemic amphetamine (AMPH) administration increases the rate of 50-kHz ultrasonic vocalizations (USVs) in adult rats and preferentially enhances the 'trill' subtype; these effects of AMPH critically depend on noradrenergic transmission, but the possible contributions of dopamine are unclear.

OBJECTIVE

To assess the role of dopamine in 50-kHz USVs emitted drug-free and following systemic AMPH administration.

METHODS

Adult male Long-Evans rats pre-selected for high AMPH-induced calling rates were tested with AMPH (1 mg/kg, intraperitoneal (IP)) and saline following pretreatment with the following dopamine receptor antagonists: SCH 23390 (0.005-0.02 mg/kg, subcutaneous (SC)), SCH 39166 (0.03-0.3 mg/kg, SC), haloperidol (0.1, 0.2 mg/kg, IP), sulpiride (20-80 mg/kg, SC), raclopride (0.1-0.5 mg/kg, SC), clozapine (4 mg/kg, SC), risperidone (0.5 mg/kg, SC), and pimozide (1 mg/kg, IP). The dopamine and noradrenaline reuptake inhibitors (GBR 12909 and nisoxetine, respectively) were also tested, alone and in combination.

RESULTS

SCH 23390, SCH 39166, haloperidol, and raclopride dose-dependently inhibited vocalizations under AMPH and suppressed the proportion of trill calls. Sulpiride, however, had no discernable effect on call rate or profile, even at a high dose that reduced locomotor activity. Single doses of clozapine, risperidone, and pimozide all markedly decreased calling under saline and AMPH. Finally, GBR 12909 and nisoxetine failed to promote 50-kHz USVs detectably or alter the subtype profile, when tested alone or in combination.

CONCLUSIONS

The rate of 50-kHz USVs and the call subtype profile following systemic AMPH administration depends on dopaminergic neurotransmission through D1-like and D2-like receptors. However, inhibiting dopamine and/or noradrenaline reuptake appears insufficient to induce calling.

Anterior hypothalamic dopamine D2 receptors modulate adolescent anabolic/androgenic steroid-induced offensive aggression in the Syrian hamster

In the Syrian hamster, treatment with anabolic/androgenic steroids (AAS) throughout adolescence increases dopamine and D2 receptor expression in the anterior hypothalamus (AH), a brain region implicated in the control of aggression. D2 receptor antagonists have reduced aggression in various species and animal models. However, these studies used systemic administration of drugs and reported concomitant changes in mobility. These data complicate the question of whether pharmacology targeting D2 receptors is specific to aggression or whether these drugs exert their antiaggressive effects through nonspecific mechanisms. To resolve this discrepancy, the current studies investigate whether administration of the D2 receptor antagonist eticlopride (0.01-10.0 microg in a final volume of 0.5 microl) into the AH modulates AAS-induced aggression. Antagonism of AH D2 receptors effectively suppressed AAS-induced aggression beginning at the 0.1 microg dose, with higher doses producing a floor effect, when compared with AAS-treated animals injected with saline into the AH. Importantly, these reductions in aggressive responding occurred in the absence of changes in locomotor behavior. Our findings identify a neuroanatomical locus where D2 receptor antagonism suppresses adolescent AAS-induced aggression in the absence of alterations to general mobility.

Effects of dopamine antagonists on drug cue-induced reinstatement of nicotine-seeking behavior in rats

Dopaminergic neurotransmission has been implicated in associative learning processes related to drugs of abuse. However, it is not clear whether blockade of activation of dopamine receptors alters conditioned incentive properties of nicotine-associated cues. Using a response-reinstatement procedure, this study examined the effects of antagonists selective for the D1 and the D2 subtypes of dopamine receptors on cue-induced reinstatement of nicotine-seeking behavior. Male Sprague-Dawley rats were trained in 30 daily 1 h sessions to intravenously self-administer nicotine (0.03 mg/kg/infusion) on a fixed ratio 5 schedule and associate a conditioned stimulus (cue) with each nicotine delivery. After extinction of responding by withholding nicotine (saline substitution) and its cue, the reinstatement tests were conducted following subcutaneous administration of a D1 antagonist SCH23390 (0, 5, 10, 30 microg/kg) or a D2 antagonist eticlopride (0, 5, 10, 30 microg/kg) in different groups of animals. Both SCH23390 and eticlopride significantly attenuated the magnitude of cue-elicited reinstatement of nicotine-seeking responding. These results indicate that activation of dopaminergic D1 and D2 receptors may play a role in mediating the conditioned motivational effects of nicotine-associated cues as measured in the response-reinstatement procedure. These findings suggest that manipulation of dopaminergic neurotransmission at D1 and/or D2 receptors may prove to be a potential target for the development of pharmacotherapy for prevention of environmental nicotine cue-triggered smoking relapse.

Renewal of sucrose-seeking behavior in rats: Role of D(2) dopamine receptors

The present study characterized renewal of sucrose-seeking behavior in rats (Experiment 1). The role of the dopamine subtype-2 (D(2)) receptors in mediating renewal of sucrose-seeking behavior also was examined (Experiment 2). Rats were trained to respond for sucrose pellets (45mg each) on a fixed-ratio 25 (Experiments 1 and 2) schedule of reinforcement in Context A. Following acquisition, rats underwent extinction and 4 renewal tests in Contexts B and A, respectively. In Experiment 2, rats were pretreated with vehicle or the D(2) dopamine receptor antagonist eticlopride (5, 10, 20, or 40microg/kg) 30min prior to the first renewal test session. A follow-up experiment (Experiment 3) examined the effect of a high eticlopride dose (40microg/kg) on locomotor activity. Renewal of sucrose-seeking behavior persisted for 3 sessions. Eticlopride dose-dependently blocked renewal of sucrose-seeking behavior without suppressing locomotor activity, implicating a role of D(2) dopamine receptors in mediating renewal of sucrose-seeking behavior.

Adolescent exposure to anabolic/androgenic steroids and the neurobiology of offensive aggression: a hypothalamic neural model based on findings in pubertal Syrian hamsters

Considerable public attention has been focused on the issue of youth violence, particularly that associated with drug use. It is documented that anabolic steroid use by teenagers is associated with a higher incidence of aggressive behavior and serious violence, yet little is known about how these drugs produce the aggressive phenotype. Here we discuss work from our laboratory on the relationship between the development and activity of select neurotransmitter systems in the anterior hypothalamus and anabolic steroid-induced offensive aggression using pubertal male Syrian hamsters (Mesocricetus auratus) as an adolescent animal model, with the express goal of synthesizing these data into an cogent neural model of the developmental adaptations that may underlie anabolic steroid-induced aggressive behavior. Notably, alterations in each of the neural systems identified as important components of the anabolic steroid-induced aggressive response occurred in a sub-division of the anterior hypothalamic brain region we identified as the hamster equivalent of the latero-anterior hypothalamus, indicating that this sub-region of the hypothalamus is an important site of convergence for anabolic steroid-induced neural adaptations that precipitate offensive aggression. Based on these findings we present in this review a neural model to explain the neurochemical regulation of anabolic steroid-induced offensive aggression showing the hypothetical interaction between the arginine vasopressin, serotonin, dopamine, gamma-aminobutyric acid, and glutamate neural systems in the anterior hypothalamic brain region.

Alterations in the anterior hypothalamic dopamine system in aggressive adolescent AAS-treated hamsters

Anabolic androgenic steroid (AAS) treatment throughout adolescence facilitates offensive aggression in male Syrian hamsters (Mesocricetus auratus). The present study was conducted to investigate the role of the dopaminergic system in the modulation of AAS-induced aggressive behavior. Hamsters were administered AAS during adolescence, scored for offensive aggression using the resident-intruder paradigm, and then examined for alterations in DA immunoreactivity in brain regions implicated in the aggressive phenotype, including the anterior hypothalamus (AH), the bed nucleus of the stria terminalis (BNST), the medial and central amygdala (MeA and CeA), the lateral septum (LS) and the ventrolateral hypothalamus (VLH). When compared with non-aggressive sesame-oil-treated controls, aggressive AAS-treated animals showed increased tyrosine hydroxylase immunoreactivity in anterior hypothalamic subnuclei, namely the nucleus circularis (NC) and medial supraoptic nucleus (mSON). In addition, AAS-treated animals showed altered D(2) receptor expression in the AH and the VLH, as measured by D(2)-immunoreactivity. Together these results suggest that alterations in DA synthesis and function together with modifications in D(2) receptor expression in the AH may underlie neuroplastic events which facilitate AAS-induced aggression.

A review of the discovery, pharmacological characterization, and behavioral effects of the dopamine D2-like receptor antagonist eticlopride

Eticlopride is a substituted benzamide analog with high affinity and selectivity for dopamine (DA) D2-like receptors that was initially developed as a potential antipsychotic agent. A great deal of research has utilized this drug to better understand central DA receptor function, the role of D2-like receptors in behavior, and the influence of blockade of these receptors on several preclinical animal models. This review highlights research utilizing this drug and compares it to typical and atypical antipsychotics used clinically. First, we describe structure-activity relationships as it relates to binding at DA receptors and the consequences on behavior. This is followed by a discussion of several imaging strategies including the use of eticlopride for in vivo, in vitro, and ex vivo examination of DA D2-like receptor densities and function. Finally, we discuss the use of eticlopride in several behavioral models predictive of antipsychotic activity, extrapyramidal side effects (EPS), and learning and memory. While eticlopride is not used clinically, it remains a viable research tool for understanding DA receptor function and behavior.

Differential effects of μ-opioid, δ-opioid and κ-opioid receptor agonists on dopamine receptor agonist-induced climbing behavior in mice

Interactions between the dopaminergic system and opioids have not been adequately clarified. The present study was designed to investigate the effects of micro-opioid (morphine), delta-opioid (SNC80) and kappa-opioid (U50 488H) receptor agonists on dopamine receptor agonist-induced climbing behavior in mice. Apomorphine (dopamine-receptor agonist) increased stereotyped climbing behavior, unlike methamphetamine, morphine, U-50 488H and (+/-)7-hydroxy-N,N-di-n-propyl-2-aminotetralin hydrobromide (D2-like receptor agonist). Furthermore, SKF81297 (D1 receptor agonist) and SNC80 caused climbing behavior. In addition, while morphine (20 mg/kg), but not U50 488H or SNC80, significantly attenuated high-dose apomorphine (2.0 mg/kg)-induced climbing behavior, it significantly potentiated low-dose apomorphine (0.5 mg/kg)-induced climbing behavior. These results suggest that morphine may have dual effects on the behavioral effects induced by apomorphine. Furthermore, we interestingly showed that the combination of apomorphine or SKF81297 and SNC80 enhanced frequent nonstereotypic climbing behavior, suggesting that delta/D1 interactions may play a prominent role in the expression of certain types of behavior in mice. Thus, micro-opioid, delta-opioid and kappa-opioid receptor agonists induce possible differential effects on the dopaminergic system in mice.

Effects of raclopride on aggression and stress in diversely selected chicken lines

Genetic selection for chickens of high (HGPS) and low (LGPS) group productivity and survivability, resulted in two distinct genetic lines characterized by differences in cannibalism, flightiness, and immunocompetence. Additionally, birds exhibited differences in behaviour and social stress coping strategy. HGPS birds have a superior stress coping strategy compared with birds of LGPS or Dekalb XL (DXL), a commercial strain. Line differences in stress response and behaviour could be due to selection-induced differences in expression of the dopaminergic system. The dopamine (D2) receptor, an integral part of the dopaminergic system, was hypothesized to be a key contributory factor of the stress response. We tested this hypothesis by injecting either a D2 antagonist (raclopride) or saline in the dominant individual in pair-housed birds for 10 days and examining stress coping ability. Results showed that dominant birds of all strains showed a reduced frequency of aggressive pecks on subordinates following raclopride injection. In contrast, subordinates paired with raclopride-injected birds increased pecking frequency. Two days after stopping injections, LGPS and DXL birds returned to pre-injection levels of aggressive threats, while HGPS birds maintained depressed frequency of threats. Strain differences in aggressive responsiveness coincided with increased epinephrine levels in raclopride treated LGPS birds relative to control LGPS birds, but not by HGPS and DXL birds. Our findings suggest a functional linkage between the genetic basis of stress coping ability and the dopamine regulation of aggressive responsiveness. The data further indicate that the sympathetic-adreno-medullary axis is directly involved in regulating both stress coping strategy and aggressiveness.

Alterations in D1/D2 synergism may account for enhanced stereotypy and reduced climbing in mice lacking dopamine D2L receptor

Concurrent activation of dopamine D1 and D2 receptors (D1 and D2) is required for the expression of certain dopamine (DA)-mediated responses, such as climbing and stereotyped behaviors. Such interactions between D1 and D2 (i.e. D1/D2 synergism) represent an important aspect of dopaminergic function and plasticity. The D2 receptor exists in two isoforms: D2L and D2S. We have generated mice that selectively lack D2L (D2L-/-). Here we showed that treatment with the indirect DA agonist amphetamine, the direct DA agonist apomorphine, or combination of D1 and D2 agonists elicited intense climbing in wild type mice (which express predominantly D2L in the striatum), but this behavior was absent or reduced in D2L-/- mice. On the other hand, apomorphine, the D2 agonist quinpirole, or combination of quinpirole and the D1 agonist SKF 81297 induced more stereotyped behavior such as biting or head movements in D2L-/- mice (which express only D2S) than in wild type mice. The D1 receptor functioned normally in D2L-/- mice. Taken together, these results suggest that D2L and D1 interactions may play a greater role in DA agonist-induced climbing, whereas D2S and D1 interactions may have a larger impact on DA agonist-induced stereotypy (and possibly psychosis). DA agonists, which are clinically used to treat Parkinson's disease and attention-deficit hyperactivity disorder (ADHD), are known to induce psychotic side effects. Thus, our findings may provide novel insights for designing anti-parkinsonian, anti-ADHD and antipsychotic drugs with greater therapeutic efficacy and fewer side effects.

D(3) and D(2) dopamine receptor agonists differentially modulate isolation-induced social-emotional reactivity in mice

Following isolation housing, mice typically exhibit heightened emotional reactivity to mild social stimulation. Aggression, social avoidance and a variety of defensive behaviors that differ in terms of motor activation (e.g. freezing, escape) can be observed depending on strain. Previous studies suggested that D(2)-like dopamine (DA) receptors play an important, albeit strain specific, role in the mediation of particular forms of defensive behavior. D(3) receptors are subtypes of D(2)-like receptors that are highly expressed in limbic areas of the brain and, therefore, they have been hypothesized to mediate emotional behavior. This study examined the effects of the putative D(3) receptor agonists 7-OH-DPAT and PD128907 on social-emotional behavior in isolated C57BL/6J and A/J mice. These effects were compared with those of the selective D(2) receptor agonist PNU91356A. All three DA agonists increased non-locomotor forms of defensive behavior (e.g. freezing, upright defensive posture). These effects were observed at low doses in C57BL/6J and at higher doses in A/J mice. Only the D(3) receptor agonists were effective in increasing locomotor forms of defensive behavior (i.e. escape, jump) at higher doses. These effects were more pronounced in C57BL/6J mice than A/J mice. The increases in stationary and locomotor defensive behavior were accompanied by marked reduction in social investigation in both the strains. Aggressive behavior was also abolished in the aggressive C57BL/6J strain. These results support previous findings and suggest that DA agonists potentiate defensive behavior and/or social fearfulness. They also suggest that D(3) and D(2) DA receptors differentially modulate the expression of social-emotional reactivity and indicate the importance of strain in examining the effects of DA ligands on emotional behavior.

Chiral analysis of selected dopamine receptor antagonists in serum using capillary electrophoresis with cyclodextrin additives

Dopamine D-2 receptor antagonists' eticlopride and sulpiride were determined in serum using capillary electrophoresis with cyclodextrin additives. Chiral resolution of S(-) and R(+) sulpiride and eticlopride were achieved using 2% sulfated-beta-cyclodextrin (S-beta-CD) in 20 mM citrate run buffer (pH 2.90). A 72-cm fused silica capillary operated in the reversed polarity mode voltage of 20 kV was used for the analysis. The analytes of interest were isolated from serum using a solid phase extraction procedure with recoveries in excess of 85% for all four enantiomers. The D-2 receptor antagonist (-) butaclamol was used as internal standard. The limits of detection were 0.3 and 0.1 microg/ml for S(-) and R(+) eticlopride and for S(-) and R(+) sulpiride, respectively, in 1 ml of serum. The limits of quantitation were 2 and 1 microg/ml for S(-) and R(+) eticlopride, and for S(-) and R(+) sulpiride, respectively. Calibration curves were linear over the 2-20 micro/ml range for eticlopride and 1-20 microg/ml range for sulpiride. The coefficients of determination were greater than 0.99 (n = 12 for eticlopride and n = 15 for sulpiride). Precision and accuracy of the method were 0.27-6.38 and 0.20-3.60% for S(-) eticlopride, 2.33-4.28 and 0.80-5.73%, for R(+) eticlopride, 3.46-6.84 and 0.80-4.26%, for S(-) sulpiride; and 4.71 -6.47 and 2.00-6.67%, for R(+)-sulpiride, respectively.

Effects of the putative dopamine D3 receptor antagonist PNU 99194A on motor behavior and emotional reactivity in C57BL/6J mice

Due to the regional expression of D3 dopamine receptors in limbic areas of the brain, there has been considerable interest in the potential role of this receptor subtype in mediating emotional behavior. Previous studies in habituated rats have shown that the putative dopamine D3 receptor antagonist 5,6-dimethoxy-2-(di-n-propylamino)indan (PNU 99194A) increased locomotor behavior. The present study examined the effects PNU 99194A on motor and emotional behaviors in C57BL/6J mice. Motor behavior was assessed in both habituated and nonhabituated mice. Emotional behavior was assessed using the elevated plus-maze and a social context involving an isolated C57BL/6J mouse and a nonaggressive conspecific. In mice habituated to the activity chamber prior to drug administration, PNU 99194A increased locomotion and rearing at lower doses (5, 10 mg/kg) whereas higher doses (20, 30 mg/kg) reduced these behaviors early in the test session. Thigmotaxis was increased independently of the effects on motor behavior. In mice exposed to the activity chamber for the first time, PNU 99194A produced a weak motor activation at lower doses and an initial decrease in motor behavior at higher doses that was followed by an increase in locomotion later in the test session. PNU 99194A had no systematic effects on activity in the elevated plus-maze, but dose-dependently increased flight reactivity in the social reactivity paradigm. These and previous findings raise questions about the role of dopamine D3 receptors in mediating motor behavior and emotional reactivity as well as the pharmacology of this putative dopamine D3 receptor antagonist.

Involvement of dopamine D2 receptors in the effect of cocaine on sexual behaviour and stretching-yawning of male rats

The effect of cocaine (7.5, 15 and 30 mg/kg) administered in acute or subchronic mode, on the mating behaviour of sexually active male rats varied in a dose- and mode-dependent manner. Regardless of mode of treatment, 30 mg/kg markedly impaired the rats copulatory ability and impairment continued for a week after suspension of subchronic treatment. An acute dose of 15 mg/kg reduced intromission frequency, while in subchronic mode it also reduced ejaculation latency. Mount frequency was increased by 7.5 and 15 mg/kg, but only on first injection. In the case of sexually-naive male rats, acute administration of cocaine (3-30 mg/kg) stimulated penile erections at 7.5 mg/kg and motor hyperactivity at all doses. (-) Eticlopride (0.025 and 0.05 mg/kg), a DA D2 antagonist, counteracted cocaine-induced motor hyperactivity but not penile erection, which it enhanced. (-) Eticlopride at the same doses also antagonized cocaine potentiation of lisuride (0.2 mg/kg)-induced behavioural effects. When male rats treated with subchronic cocaine (15 mg/kg) were injected with the DA D2 agonist SND 919 (0.1 mg/kg), they displayed a more marked stretching-yawning behaviour than control animals receiving SND 919 at the same dose. The involvement of DA D2 receptors in cocaine-induced effects is suggested.

Involvement of dopamine receptors in the antipsychotic profile of (-) eticlopride

The present study was performed to assess the effects exerted by the dopamine (DA) D2/D3 antagonist (-) eticlopride on rodent behavioral models considered to be predictive of antipsychotic activity, namely, antagonism toward DA agonist-induced stereotyped behavior (SB), and ketamine-and cocaine-induced hypermotility. (-) Eticlopride (10-50 micrograms/kg) dose-dependently inhibited SB elicited by SND 919 (10 mg/kg), CQP 201-403 (0.5 mg/kg), and 7-OH-DPAT (5 mg/kg); moreover, it significantly counteracted the hypermotility induced in rats and mice by ketamine (5 and 10 mg/kg). When (-) eticlopride was injected before cocaine (15 mg/kg) either acutely or subchronically administered in male rats, it also potently antagonized the hypermotility typically induced by the drug. These results are discussed in the light of putative D2/D3 receptor involvement, and are considered as predictive of antipsychotic potential.

Influence of eticlopride on cocaine- and DA D2 agonist-induced behavioral effects in rats

The influence of the DA D2 antagonist (-) eticlopride on cocaine- and DA D2 agonist-induced behavioral effects was investigated by means of two series of experiments, in rats. In the first 10-day series, coadministration of (-) eticlopride (10 and 50 micrograms/kg, SC) always potently inhibited cocaine (15 mg/kg, IP)-induced hypermotility but did not modify the penile erection (PE)-enhancement produced by the drug at the first injection; it actually counteracted the inhibitory effect of subchronic cocaine on PE. In the second series, (-) eticlopride, at the same doses, antagonized PE elicited by various DA D2 agonists at nonstereotyping doses; when, along with PE, stereotyped behavior was induced, only the latter was inhibited by (-) eticlopride, which even increased PE.

Behavioural effects of the dopamine D3 receptor agonist 7-OH-DPAT in rats

The putative selective dopamine (DA) D3 receptor agonist, 7-OH-DPAT (25-4000 micrograms kg-1), enhanced stretching-yawning and penile erection in male rats, besides respectively increasing and decreasing sedation at low (25-200 micrograms kg-1) and high (1600 and 4000 micrograms kg-1) doses and inducing stereotypy from 800 micrograms kg-1 upwards. The DA D2 antagonist, (-) eticlopride (10 and 20 micrograms kg-1), antagonized stretching-yawning and penile erection induced by a low dose of 7-OH-DPAT (50 micrograms kg-1) but not those produced by high doses (1600 and 4000 micrograms kg-1), when stereotyped behaviour, on the other hand, was potently inhibited. Comparative experiments performed with the DA agonist SND 919 gave similar results.