Analysis of D2 and D3 receptor-selective ligands in rats trained to discriminate cocaine from saline.

This study examined the role of dopamine D3 receptors in the stimulus generalization produced by 7-OH-DPAT and PD 128907 in rats trained to discriminate cocaine from saline. Twelve male Sprague-Dawley rats were trained to discriminate cocaine (10 mg/kg) from saline in a two-choice operant procedure using a FR20 schedule of water reinforcement. Stimulus generalization tests were administered with the D3-preferring agonists (+/-)-7-OH-DPAT (0.01-0.3 mg/kg), (+)-7-OH-DPAT (0.01-0.3 mg/kg), and PD 128907 (0.01-0.3 mg/kg), and the selective D2 agonist PNU-39156 (0.01-0.3 mg/kg). Complete generalization to cocaine was observed with (+/-)-7-OH-DPAT at doses that markedly suppressed response rate. Only partial stimulus generalization was observed with (+)-7-OH-DPAT and PD 128907 when these compounds were administered intraperitoneally, although subcutaneous injections of these compounds produced complete substitution. Response rate was also significantly reduced by these compounds. The selective D2 agonist, PNU-91356 also fully substituted for the cocaine cue and suppressed response rate in a dose-dependent manner. To ascertain the importance of D3 receptor actions in the stimulus generalization produced by (+/-)-7-OH-DPAT (0.1 mg/kg) and PD-128907 (0.3 mg/kg), the fairly selective D3 antagonist, PNU-99194A (2.5-20 mg/kg) was also tested in combination with these compounds. Although PNU-99194A partially attenuated the stimulus generalization produced by (+/-)-7-OH-DPAT, it failed to block PD-128907 substitution for cocaine. These results indicate at least some involvement of D3 receptors in the stimulus effects of (+/-)-7-OH-DPAT, although further investigations are clearly warranted. The present results also suggest that the cue properties of cocaine may be dissociated from the locomotor activating effects of this drug, because D3/D2 receptor agonists suppress locomotor activity but produce stimulus generalization to cocaine.     

Antagonism of the discriminative stimulus effects of (+)-7-OH-DPAT by remoxipride but not PNU-99194A

The dopamine (DA) agonist 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin (7-OH-DPAT) has been used extensively as a tool to investigate the role of DA D(3) receptors in the reinforcing and discriminative stimulus properties of psychostimulant drugs. The present study examined the relative importance of D(3) vs. D(2) receptor actions in the discriminative stimulus effects of (+)-7-OH-DPAT (0.03 mg/kg, sc) in 16 male Sprague-Dawley rats trained to discriminate this compound from saline in a two-lever, water-reinforced operant procedure under a FR 20 schedule. Stimulus generalization and antagonism tests were conducted with cocaine and with various selective D(2) and D(3) receptor ligands. In contrast to previous findings that (+)-7-OH-DPAT substitutes for cocaine, the present results demonstrated that cocaine does not produce stimulus generalization in animals trained to discriminate (+)-7-OH-DPAT. Although two D(3)-preferring agonists, PD-128907 and pramipexole, produced complete stimulus generalization to the training drug, two highly selective D(3) antagonists (PNU-99194A, PD 152255) failed to block the discriminative stimulus effects of (+)-7-OH-DPAT. However, the D(2) antagonist remoxipride (3.0 mg/kg) produced a rightward shift in the (+)-7-OH-DPAT dose-response curve. These findings suggest that D(2) receptors are critically involved in mediating the cue properties of (+)-7-OH-DPAT. However, alternative interpretations that PNU-99194A is not entirely D(3) receptor selective should also be considered.     

Antagonism of the discriminative stimulus effects of cocaine at two training doses by dopamine D2-like receptor antagonists

RATIONALE: The relative contributions of different dopamine receptor subtypes to the discriminative stimulus effects of cocaine may be influenced by the training dose of cocaine. Substitution tests with dopamine receptor agonists have suggested that the role of dopamine D2-like receptors is diminished relative to that of D1-like receptors at a training dose of 3 mg/kg cocaine compared with a training dose of 10 mg/kg. OBJECTIVES: To test whether dopamine D2-like receptor antagonists were differentially effective at attenuating cocaine's discriminative stimulus effects at different training doses, and to test for the first time an antagonist that is selective for the dopamine D2 receptor within the D2-like receptor subfamily. METHODS: Rats were trained to press one lever after receiving cocaine and another after receiving saline (maintaining >95% drug-appropriate responding). Three dopamine D2-like receptor antagonists (haloperidol, raclopride and L-741,626) were tested in rats trained at 3 mg/kg or 10 mg/kg cocaine. At the lower training dose, the D1-like receptor antagonist SCH 39166 was also tested. RESULTS: The antagonists were not differentially effective between training groups: they all produced parallel, rightward shifts in cocaine's dose-effect function, indicating surmountable antagonism. CONCLUSIONS: The results demonstrate that D2-like receptor antagonists with different affinities for the various D2-like receptors can antagonise the discriminative stimulus effects of cocaine at two training doses. Importantly, antagonism by L-741,626 implies that stimulation of D2 receptors alone (not D3 or D4 receptors) is sufficient to mediate cocaine's discriminative stimulus effects. Finally, the claim that D1-like receptors are preferentially involved at low training doses of cocaine is only consistent with the current findings if indirect stimulation of D2 receptors by low doses of cocaine remains necessary for the expression of the D1-like receptor-mediated effect.     

Assessment of D3 versus D2 receptor modulation of the discriminative stimulus effects of (+)-7-OH-DPAT in rats

Although there are presently no highly selective agonists for the D3 dopamine receptor, a number of compounds reported to bind with moderate selectivity to D3 receptors are currently employed to investigate the importance of D3 receptors in the behavioral effects of psychostimulant drugs. For example, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) has been used extensively to investigate the role of D3 receptors in the reinforcing and discriminative stimulus properties of cocaine and d-amphetamine. However, recent investigations with a relatively selective D3 antagonist, PNU-99194A, have led us to question the importance of D3 receptors in the discriminative stimulus effects of 7-OH-DPAT. In the present study, 16 male Sprague-Dawley rats were trained to discriminate (+)-7-OH-DPAT (0.03 mg/kg, subcutaneously (s.c.)) from saline in a two-choice operant procedure using a fixed-ratio 20 schedule of water reinforcement. Consistent with previous findings, PNU-99194A appeared to attenuate only partially (+)-7-OH-DPAT discrimination at a dose that disrupted responding in most subjects. Moreover, a highly selective D2 agonist, PNU-91356A, substituted completely and in a dose-dependent manner for (+)-7-OH-DPAT, while d-amphetamine produced only partial substitution for the training drug. These data indicate that D2 receptor actions appear to be more important than D3 receptor actions in exerting the discriminative stimulus effects of (+)-7-OH-DPAT. Continued efforts to determine the relative importance of D2 vs D3 receptor actions in the modulation of the discriminative stimulus effects of (+)-7-OH-DPAT are discussed.     

Dopamine D2/D3 receptors modulate cocaine's reinforcing and discriminative stimulus effects in rhesus monkeys

Numerous studies have suggested that dopamine (DA) D2 and D3 receptors are involved in the behavioral effects of cocaine. The present experiments evaluated the reinforcing and cocaine-like discriminative stimulus effects of several D2/D3 agonists in rhesus monkeys. In the first experiment, animals (n = 4) were trained to self-administer 0.03 mg/kg/inj cocaine under a fixed-interval (FI) 5-min schedule. When substituted for cocaine, the D2/D3 agonist quinpirole (0.003-0.03 mg/kg/inj) functioned as a reinforcer in all monkeys. In two cocaine-naive monkeys trained to respond under an FI 3-min schedule of food presentation, quinpirole maintained low rates of responding in one subject, while at the highest dose (0.03 mg/kg/inj) it functioned as a reinforcer in the second monkey. In this animal, increased activity was observed at this dose, which may have contributed to the overall rate of responding. In the second experiment, monkeys (n = 4) were trained to discriminate cocaine from saline using a two-lever, food-reinforced, drug discrimination procedure. The D2/D3 agonists quinpirole, (+/-)-7-OH-DPAT, and R-( + )-7-OH-DPAT fully substituted for cocaine. However, the time-course of substitution differed between quinpirole, which substituted for cocaine 10 min after administration, and (+/-)- and R-(+)-7-OH-DPAT, which required 60-min pretreatments. The behavioral potencies, as determined from ED50, values, correlated with previously reported in vitro binding affinity and functional activity at the D3 receptor [R-(+ )-7-OH-DPAT > (+/-)-7-OH-DPAT > quinpirole]. These results further indicate that direct-acting D2/D3 agonists can function as reinforcers and produce cocaine-like discriminative stimulus effects, and support the idea that D3 receptors should continue to be a valuable target for future behavioral studies evaluating cocaine's mechanisms of action.     

Discriminative stimulus effects of putative D3 dopamine receptor agonists in rats

Three separate groups of rats were trained to discriminate the putative D3 dopamine receptor agonists (+/-)-7-hydroxydipropylaminotetralin (7-OH-DPAT) (0.03 mg/kg), PD 128,907 (1.0 mg/kg) and quinpirole (0.03 mg/kg) from saline. Food was presented after each 10 (7-OH-DPAT and PD 128,907) or 20 (quinpirole) consecutive responses on one lever after administration of the training drug, and the other lever after the administration of saline. Once stable performances were obtained, the effects of various doses of several dopaminergic agonists were assessed during test sessions in which responses on either lever were reinforced. The substitution tests were conducted to determine if differences in potencies would be obtained, which would be suggestive of differences in the mechanisms underlying the discriminative effects of the training drugs. Non-selective agonists with activity at both D2 and D3 dopamine receptors (D2-like agonists) substituted for each of the three training drugs. In addition, the selective D2 dopamine receptor agonist U91356A also generalized to both 7-OH-DPAT and PD 128,907. The potencies of the D2-like agonists in substituting for each training drug were highly correlated with potencies in substituting for the others. SKF 82958 and SKF 81297, agonists with selectivity for D1 and D5 dopamine receptors (D1-like agonists), partially substituted for 7-OH-DPAT but not PD 128,907. The D1-like partial agonist SKF 38393 did not substitute for any of the training drugs for which it was tested. Cocaine produced intermediate substitution in 7-OH-DPAT- and PD 128,907-trained subjects and did not substitute at all in quinpirole-trained subjects. The dopamine D1-like antagonist SCH 39166 (0.001-0.03 mg/kg) did not alter the discriminative stimulus effects of PD 128,907, whereas the D2-like dopamine antagonist spiperone (0.001-0.1 mg/kg) produced at the highest dose an insurmountable antagonism of the discriminative effects of PD 128,907. In contrast, there was no appreciable antagonism of the effects of PD 128,907 on response rates. The data collected are consistent with a distinction between the effects of each of these training drugs and the indirectly acting agonist cocaine. Further, these data indicate that there are differences in the mechanisms underlying the discriminative effects of PD 128,907 and its effects on response rates. Moreover, these data indicate that each of the training drugs is distinct from drugs acting through D1 dopaminergic mechanisms. However, there were no data that clearly distinguished these training drugs from each other or from drugs acting through D2 dopaminergic mechanisms.     

Effects of the putative dopamine D3 receptor agonist 7-OH-DPAT in rhesus monkeys trained to discriminate cocaine from saline

These studies were designed to evaluate the effects of the putative dopamine D₃ receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT), alone and in combination with cocaine, in four rhesus monkeys trained to discriminate cocaine (0.4 mg/kg, IM) from saline under a fixed-ratio 30 schedule of food presentation. Under these conditions, cumulative doses of cocaine (0.013–1.3 mg/kg) produced a dose-dependent and complete generalization to the training dose of cocaine in all monkeys, while producing only minimal effects on response rates. The discriminative stimulus effects of cocaine were antagonized by the non-selective dopamine receptor antagonist flupenthixol (0.018 mg/kg, IM) in all four monkeys. The effects of 7-OH-DPAT (0.01–1.8 mg/kg) were inconsistent across monkeys. In two of the four monkeys (monkeys L990 and L958), 7-OH-DPAT consistently and completely generalized to cocaine and decreased response rates in a dose-dependent manner. Both the cocaine-like discriminative stimulus effects and rate-decreasing effects of 7-OH-DPAT were antagonized by flupenthixol in these two monkeys. Pretreatment with low doses of 7-OH-DPAT (0.01–0.032 mg/kg) had no effect on the cocaine dose-effect curve in monkeys L990 and L958; however, higher doses of 7-OH-DPAT (0.032–0.32 mg/kg) shifted the cocaine dose-effect curve to the left. In the other two monkeys (monkeys 150F and 89B036), 7-OH-DPAT produced a dose-dependent decrease in response rates but did not consistently generalize to cocaine. Flupenthixol did not antagonize the rate-decreasing effects of 7-OH-DPAT in these two monkeys, and pretreatment with 7-OH-DPAT (0.1–0.32 mg/kg) produced a decrease in response rates but had no effect on the cocaine dose-effect curve. Time-course experiments revealed that 7-OH-DPAT (0.32 mg/kg) displayed a slower onset and a longer duration of effect than the training dose of cocaine. Finally, the D₃/D₂ dopamine agonist quinpirole completely generalized to cocaine in three monkeys, and partially in the fourth monkey. Quinpirole showed the highest potency in those monkeys in which 7-OH-DPAT consistently generalized to cocaine. The results of the present study suggest that, in rhesus monkeys, 7-OH-DPAT produces cocaine-like effects and may modulate the discriminative stimulus effects of cocaine in some monkeys.This paper is dedicated to the memory of Xavier Lamas, who died on 26 August 1995 on Mount Everest     

Reevaluation of PNU-99194A discriminative stimulus effects: potentiation by both a D2 antagonist and a D3/D2 agonist

This study evaluated the relative importance of D(3) receptor antagonism in the discriminative stimulus effects of the putative D(3) receptor antagonist PNU-99194A. Eight male Sprague-Dawley rats were trained to discriminate PNU-99194A (10 mg/kg sc) from vehicle in a two-choice drug discrimination procedure under a FR 20 schedule of food reinforcement. The selective D(3) antagonists PD 152255 and S14297 were examined for stimulus generalization. The D(2) antagonist haloperidol and the D(2)/D(3) receptor agonist (+)-7-OH-DPAT were also assessed for antagonism of PNU-99194A discrimination. PD 152255 (1.0-3.0 mg/kg) engendered no generalization to PNU-99194A. Due to its markedly rate-suppressive effects, PD 152255 could not be tested at higher doses. S-14297 produced partial substitution (66%) for PNU-99194A at both 3.0 and 8.0 mg/kg. Neither haloperidol nor (+)-7-OH-DPAT blocked the discrimination of PNU-99194A and, surprisingly, actually appeared to potentiate its effects. These data, along with other recent findings, suggest that the discriminative stimulus effects of PNU-99194A appear to involve complex pharmacological actions and are not solely mediated by D(3) receptor antagonism.     

Effects of 5-HT1B receptor ligands microinjected into the ventral tegmental area on cocaine discrimination in rats

Some recent data indicate a significant interaction between serotonin (5-hydroxytryptamine; 5-HT) and dopamine in mesolimbic brain structures (e.g. the ventral tegmental area) which modulate the behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in the ventral tegmental area in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, intraperitoneally (i.p.)) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, the rats were stereotaxically implanted with bilateral cannulae in the ventral tegmental area and were then microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist, 3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR 55562; 0.1-1 microg/side), did not evoke cocaine-lever responding, whereas the 5-HT(1B) receptor agonist, 1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrolo[3,2-b]pyridin-5-one (CP 93129; 0.3-1 microg/side), induced partial substitution for cocaine. Intra-tegmental microinjections with the 5-HT(1B) receptor antagonist, GR 55562 (0.1-1 microg/side), before cocaine (5 mg/kg), which alone produced 98% cocaine-lever responses, decreased in a dose-dependent manner the discriminative stimulus effects of the psychostimulant. On the other hand, combination tests using a fixed dose of CP 93129 (0.3 or 1 microg/side), given into the ventral tegmental area prior to low systemic doses of cocaine (1.25-2.5 mg/kg), increased cocaine discrimination. These results seem to indicate that tegmental 5-HT(1B) receptors are necessary to express the discriminative stimulus effects of cocaine.     

7-Hydroxy-N,N'-di-n-propyl-2-aminotetraline,a preferential dopamine D3 agonist,induces c-fos mRNA expression in the rat cerebellum

The effects of a preferential dopamine D3 receptor agonist 7-hydroxy-N,N'-di-n-propyl-2-aminotetralin (7-OH-DPAT) on c-fos mRNA expression in the rat cerebellum were studied by Northern blot analysis. 7-OH-DPAT (0.003-10 mg/kg, s.c.) markedly increased c-fos mRNA expression in the cerebellum, while its effects in the striatum, nucleus accumbens, and frontal cortex were negligible. The effect of 7-OH-DPAT on cerebellar c-fos mRNA expression was dose-dependent and statistically significant at doses of 0.3 mg/kg or more. A preferential dopamine D2 agonist, bromocriptine (0.01-3 mg/kg, s.c.), failed to increase c-fos mRNA expression in the cerebellum. The effect of 7-OH-DPAT was blocked by two dopamine D2-type-receptor antagonists, haloperidol and perospirone, but not the D1-type-receptor antagonist SCH23390. Furthermore, dopaminergic denervation by 6-hydroxydopamine did not inhibit but rather potentiated the 7-OH-DPAT-induced c-fos mRNA expression in the cerebellum. These findings suggest that 7-OH-DPAT increases c-fos mRNA expression in the rat cerebellum, probably through postsynaptic dopamine D3 receptor activation.     

The D3R partial agonist, BP 897, attenuates the discriminative stimulus effects of cocaine and D-amphetamine and is not self-administered

Growing attention has been directed towards the potential involvement of the dopamine D3 receptor (D3R) in modulating effects of psychomotor stimulants. BP 897 (N-[4-[4-(2-methoxyphenyl)-1-piperazinyl]butyl]-2-naphthylcarboxamide; aka BP 4.897 and DO897) is amongst the most selective partial agonists for the D3R receptor thus far reported. BP 897 was tested for its ability to support self-administration in rhesus monkeys (0.3-30 microg/kg) and for its ability to produce cocaine- and D-amphetamine-like discriminative stimulus effects in mice (0.01-17 mg/kg i.p.). BP 897 was not self-administered above vehicle and saline levels in any of the four monkeys tested, and produced less than 30% generalization from either the cocaine or D-amphetamine stimulus. When BP 897 was administered before administrations of cocaine or D-amphetamine, percent drug-lever selections were reduced. These results suggest that BP 897 has a profile of activity suitable for consideration as a potential treatment for cocaine dependency disorders.     

Differential involvement of dopamine receptors in conditioned suppression induced by cocaine

Cocaine-paired stimuli can suppress food-reinforced operant behavior in rats, providing an animal model of conditioned drug effects. To study the neuropharmacological basis of this phenomenon, we examined the effects of various dopamine receptor antagonists on the acquisition and expression of cocaine-induced conditioned suppression in rats. Superimposed on an ongoing baseline of food-reinforced operant responding, a stimulus was paired with response-independent cocaine (3.0 mg/kg, i.v.) during each of 8 training sessions. To study acquisition, independent groups of rats were given saline, the dopamine D(1)-like receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390) (0.001-0.03 mg/kg, i.p.), or the dopamine D(2)-like receptor antagonist eticlopride (0.001-0.03 mg/kg, i.p.) prior to each training session. To study expression, independent groups of rats were trained first, then given saline, SCH 23390, eticlopride, or N-[4-(4-(2-methoxyphenyl)piperazinyl)butyl]-2-naphthamide (BP 897) (a dopamine D(3) partial receptor agonist; 0.1-1.0 mg/kg, i.p.) before test sessions in which the stimulus was presented without cocaine. Pre-treatment with either SCH 23390 or eticlopride during acquisition reduced the direct suppressant effects of cocaine, but conditioning was blocked only in rats that were treated with SCH 23390 during acquisition training. Expression of conditioning was attenuated only by eticlopride. Thus, dopamine at least partially mediates both the acquisition and expression of cocaine-induced conditioned suppression, with activation of dopamine D(1)- and D(2)-like receptors underlying these respective processes.     

Biphasic effects of 7-OH-DPAT on the acquisition of responding for conditioned reward in rats.

Previous studies have shown that dopamine (DA) receptor subtype-specific agonists differentially affect responding for conditioned reward D1-like agonists impair, whereas D2-like agonists enhance responding. The present study compared the effects of the D2-like agonists bromocriptine and 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). Food-deprived rats (N=159) were preexposed to a chamber with two levers, one producing a tone (3 s) and the other turning the house lights off (3 s), for five 40-min sessions. In four subsequent 65-min conditioning sessions with the levers removed, the lights-off stimulus was paired with food (80 presentations per session). During two 40-min test sessions, the lights-off (CR) and tone (NCR) levers were replaced and responses at each lever were recorded. Confirming previous results, bromocriptine (0.50-5.0 mg/kg) dose-dependently enhanced responding on the lever producing conditioned reward. In contrast, 7-OH-DPAT had a biphasic effect on responding for conditioned reward. Low doses (0.10-0.25 mg/kg) reduced CR lever responding, whereas a higher dose of 1.0 mg/kg enhanced such responding. An intermediate dose of 0.50 mg/kg neither impaired nor enhanced CR lever responding. The biphasic profile of 7-OH-DPAT may arise through differential actions at D3 vs. D2 receptors or presynaptic vs. postsynaptic DA receptors at low and high doses, respectively.     

Multiple dopamine receptors mediate the discriminative stimulus effects of dihydrexidine in rats

The dopamine D(1) receptor agonist dihydrexidine (DHX) [(±)-trans-10,11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a] phenanthridine hydrochloride] has shown efficacy in animal models of Parkinson's disease and improved cerebral blood flow and working memory of schizophrenic patients. Although the discriminative stimulus effects of DHX, an in-vivo predictor of human subjective effect profile, have only been characterized with respect to activity at D(1) receptors, DHX also has significant affinity for D(2) receptors. This study was designed to characterize the role of D(1) and D(2)/D(3) receptors in mediating the discriminative stimulus effects of DHX. Rats were trained to discriminate DHX [3 mg/kg, intraperitoneally (i.p.)] from the vehicle. The selective dopamine D(1) receptor partial agonist SKF 38393 was fully substituted for DHX. The D(1) receptor antagonist SCH 23390 (0.1 mg/kg, s.c.) and the D(3)-selective antagonist U99194 (10 mg/kg, i.p.) significantly attenuated the discriminative stimulus effects of the training dose of DHX by 80 and 60%, respectively, suggesting that both D(1) and D(3) receptors mediate the discriminative stimulus effects of DHX. In contrast, raclopride (1 mg/kg, i.p.) did not significantly alter the discriminative stimulus effects of DHX, indicating a lack of D(2)-mediated effects. The D(2)/D(3) receptor preferring agonists, quinpirole and (+)-PD 128907 were fully substituted, whereas (+)-7-OH-DPAT was partially substituted for DHX. The DHX bound to D(2) receptors with a Ki of 4.3+0.7 nmol/l was compared with 33.7+4.6 nmol/l at D(3) receptors. Determinations of activity at second messenger systems revealed that DHX functioned as a full agonist at D(3) receptors and a partial agonist at D(2) receptors in vitro. These activities at D(2)/D(3) receptors have shown effects in some preclinical models and clinical disease states. Therefore, the prominent in-vivo agonist activity of DHX at both D(1) receptors and D(2)/D(3) receptors should be considered while making predictions of effects in humans.     

Effects of serotonin (5-HT)(1B) receptor ligands,microinjected into accumbens subregions,on cocaine discrimination in rats

Recent data indicate a significant input of serotonin (5-HT) on mesoaccumbens dopamine-dependent behavioral effects of cocaine in rats. The present study investigated the role of 5-HT(1B) receptors in nucleus accumbens subregions (the shell and the core) and the effect of stimulation of those receptors in the discriminative stimulus effects of cocaine in rats. Male Wistar rats were trained to discriminate cocaine (10 mg/kg, i.p.) from saline (i.p.) in a two-choice, water-reinforced fixed-ratio 20 procedure. After reaching the cocaine-saline discrimination criterion, rats were stereotaxically implanted with bilateral cannulae in the accumbens shell or core, and then were microinjected with selective 5-HT(1B) receptor ligands. In substitution studies, microinjections of the 5-HT(1B) receptor antagonist GR 55562 (0.1-10 microg/side) or the 5-HT(1B) receptor agonist CP 93129 (0.1-10 microg/side) into accumbens subregions did not evoke cocaine-lever responding. Pretreatment with the 5-HT(1B) receptor antagonist GR 55562 (0.1-10 microg/side) in the accumbens shell or core failed to modulate the discriminative stimulus effects of cocaine (5 mg/kg). Combination tests using a fixed dose of CP 93129 (1-10 microg/side) into the accumbens shell prior to cocaine administration (0.6-5.0 mg/kg) did not affect cocaine discrimination. CP 93129 (1 microg/side, but not 0.1 microg/side) microinjected in the accumbens core, and low doses of systemic cocaine (0.6-2.5 mg/kg) produced a leftward shift in the cocaine dose-response curve and a decrease in its ED(50) value. GR 55562 (1 microg/side) significantly attenuated the enhancement of cocaine discrimination evoked by a combination of CP 93129 (1 microg/side) and cocaine (1.25 mg/kg or 2.5 mg/kg). These results seem to exclude a major role for the accumbens shell and core 5-HT(1B) receptors in controlling the discriminative stimulus effects of cocaine. However, they do suggest that the stimulation of 5-HT(1B) receptors in the accumbens core, but not in the shell, enhances cocaine discrimination in rats.     

Differential involvement of dopamine in mediating the discriminative stimulus effects of low and high doses of caffeine in rats

The hypothesis that low and high doses of caffeine produce effects that are differentially mediated by dopamine (DA) receptor mechanisms was investigated in rats trained to discriminate either 10 or 56 mg/kg of caffeine from saline. Rats trained to discriminate 56 mg/kg of caffeine acquired the discrimination in an average of 74 sessions, whereas rats trained to discriminate 10 mg/kg of caffeine required an average of 108 sessions. The DA D1 receptor agonist SKF 81297 and the DA D2 receptor agonist R(-)-propylnorapomorphine (NPA) generalized partially (50-75%) in rats trained to discriminate 10 mg/kg of caffeine, but produced predominantly saline-appropriate responding (< 40%) in rats trained to discriminate 56 mg/kg of caffeine. When SKF 81297 and NPA were combined, stimulus generalization was no greater than it was when either agonist was tested alone. The DA uptake inhibitors cocaine and GBR 12909 produced predominantly saline-appropriate responding in both groups of rats. Neither the DA D1 receptors antagonists SCH 23390 and SCH 31966, nor the DA D2 receptor antagonists eticlopride and sulpiride, generalized in rats trained to discriminate 10 or 56 mg/kg of caffeine. When administered in combination with caffeine, both the DA D1 and DA D2 antagonists antagonized completely the discriminative stimulus effects of the low training dose of caffeine, but did not alter the discriminative stimulus effects of the high training dose. These results suggest that the discriminative stimulus effects of 10 mg/kg of caffeine, but not 56 mg/kg of caffeine, are dependent on, but not limited to, DA D1 and D2 receptor mechanisms.     

The broad-spectrum anti-emetic activity of AS-8112, a novel dopamine D2, D3 and 5-HT3 receptors antagonist

The anti-emetic and pharmacological profile of AS-8112 ((R)-5-bromo-N-(1-ethyl-4-methylhexahydro-1H-1,4-diazepin-6-yl)-2-methoxy-6-methylamino-3-pyridinecarboxamide.2 fumarate), a novel and potent dopamine D2, D3 and 5-hydroxytryptamine-3 (5-HT3) receptors ligand, was investigated in the present study. In guinea-pig isolated colon, AS-8112 produced a rightward shift of the concentration-response curves of 2-methyl-5HT, a 5-HT3 receptor agonist (pA2 value of 7.04). Other 5-HT3 receptor antagonists also produced such a shift in the following antagonistic-potency order: granisetron> ondansetron=AS-8112>metoclopramide. In mice, AS-8112 (1.0 - 3.0 mg kg(-1) s.c.) potently inhibited hypothermia induced by the dopamine D3 receptor agonist; R(+)-7-OH-DPAT (R(+)-7-hydroxy-2-(N,N-di-n-propylamino)tetraline) (0.3 mg kg(-1) s.c.). Domperidone and haloperidol, which have affinity for dopamine D3 receptor, also inhibited R(+)-7-OH-DPAT-induced hypothermia. In ferrets or dogs, AS-8112 dose-dependently inhibited emesis induced by R(+)-7-OH-DPAT, apomorphine, morphine or cisplatin with ID50 values of 2.22 microg kg(-1) s.c., 10.5 microg kg(-1) s.c., 14.2 microg kg(-1) i.v. and 17.6 microg kg(-1) i.v., respectively. Moreover, oral administration of AS-8112 significantly inhibited emesis induced by these emetogens. AS-8112 (0.3 mg kg(-1) i.v.) significantly inhibited emesis induced by cyclophosphamide and doxorubicin. In conclusion, AS-8112 is a potent dopamine D2, D3 and 5-HT3 receptors antagonist, and a novel anti-emetic agent with a broad-spectrum of anti-emetic activity. These results suggest that this compound is worthy of clinical investigation.     

Pharmacological characterization of the discriminative stimulus properties of the dopamine D1 agonist, SKF 81297

A range of selective dopamine D1 and D2 receptor agonists and antagonists was used to characterize to the discriminative stimuli produced by d-amphetamine (0.5mg/kg) and the D1 agonists SKF 81297 (0.1mg/kg). In rats trained to discriminate d-amphetamine (0.5mg/kg) from saline, d-amphetamine produced a dose-related increase in per cent drug lever responding, and SKF 81297 did not show any d-amphetamine-like discriminative stimulus effects; neither did SKF 81297 potentiate nor antagonize the d-amphetamine discriminative stimulus. In rats trained to discriminate SKF 81297 (0.1mg/kg) from saline, SKF 81297 produced a dose-related increase in per cent drug lever responding, and SKF 38393 and SKF 83565 elicited full SKF 81297-like effects despite the fact that these compounds have widely differing efficacies for stimulating adenylate cyclase. SKF 81297 had a 25-fold greater potency than SKF 38393 in this assay. The D2 agonists, PHNO and ropinirole, did not display any SKF 81297-like discriminative stimulus effects. The SKF 81297 discriminative stimulus was completely blocked by the D1 antagonist SCH 23390 but was not blocked by the D2 antagonist BRL 34778.     

The role of D2-like dopamine receptors in the locomotor stimulant effects of cocaine in mice

RATIONALE: Previous studies indicate antagonism of cocaine-stimulated locomotor activity by dopamine D2-like receptor antagonists, but only at doses of the antagonists, that by themselves attenuate locomotor activity, raising questions of the specificity of the interaction and whether it might be due solely to a summation of opposing effects. OBJECTIVES: The interactions of cocaine and several D2-like dopamine antagonists and non-dopamine "physiological antagonists" were compared across a full range of doses in order to fully characterize the interaction and assess the specificity of the effects of dopamine antagonists and cocaine. METHODS: Swiss-Webster mice were treated with either vehicle, a D2-like antagonist (haloperidol, spiperone, raclopride, spiperone, (+) or (-) eticlopride), or a "physiological" antagonist (chlordiazepoxide, clonidine, or R(-) N6-(2-phenylisopropyl)adenosine) and cocaine (5-80 mg/kg) prior to a 30-min locomotor activity test. RESULTS: All test drugs decreased locomotor activity when given alone. All test drugs attenuated cocaine-induced locomotion and decreased peak responding to cocaine. In general, the D2-like antagonists also decreased maximal responding to cocaine and decreased the slope of the ascending limb of the cocaine dose-effect curve, effects not obtained with physiological antagonists. CONCLUSIONS: Blockade of D2-like receptors resulted in an interaction with cocaine that was fundamentally different from that produced through non-dopaminergic mechanisms and appears to be more than a summation of opposing effects. The present data suggest that D2-like receptors are involved in the mechanisms underlying the induction of locomotor activity by cocaine.     

Effects of nitric oxide synthase inhibitors on the discriminative stimulus effects of cocaine in rats

RATIONALE: Nitric oxide synthase (NOS) inhibitors may modulate the discriminative stimulus effects of cocaine because they alter dopamine (DA) release. OBJECTIVES: The effects of the NOS inhibitors NG-nitro-L-arginine methyl ester (L-NAME) and 7-nitro-indazole (7-NI) were examined in experiments designed to better understand the mechanisms that may underlie the interactions between NOS inhibitors and cocaine. METHODS: Rats were trained to discriminate 10 mg/kg cocaine from saline, and then substitution and pretreatment tests with L-NAME and 7-NI were conducted. To determine if the combined effects of NOS inhibitors and cocaine might be related to DA mechanisms and/or to N-methyl-D-aspartate (NMDA) receptor mechanisms, substitution tests with other indirect DA agonists and NMDA antagonists were carried out in the presence and absence of L-NAME. In addition, the roles of the D1 and D2 families of DA receptors in mediating the cocaine-altering effects of L-NAME and 7-NI were examined in antagonism tests using SCH 23390 and haloperidol, respectively. RESULTS: The results demonstrated that neither NOS inhibitor alone substituted for the 10 mg/kg cocaine training dose, but when given as a pretreatment, 100 mg/kg L-NAME as well as 10 mg/kg 7-NI enhanced the discriminative stimulus and rate-decreasing effects of cocaine. L-NAME pretreatment also enhanced the potency of (+)-amphetamine and GBR 12909, but not MK-801, phencyclidine, or NPC 17742, for producing discriminative stimulus and rate-decreasing effects in substitution tests. Further testing showed that the cocaine-enhancing effects of L-NAME and 7-NI were attenuated by doses of haloperidol and SCH 23390 that minimally altered the effects of cocaine alone. CONCLUSIONS: These findings suggest that L-NAME and 7-NI may increase the potency of cocaine and other indirect DA agonists through a central mechanism whereby DA neurotransmission is directly enhanced by NOS inhibition.