Evidence for an involvement of D1 and D2 dopamine receptors in mediating nicotine-induced hyperactivity in rats

Previous studies have suggested that repeated exposure of rats to the drug or to the experimental environment is necessary to observe nicotine-induced locomotor stimulation. In the present study the role of habituation to the experimental environment on the stimulant effect of nicotine in rats was examined. In addition, the role of dopamine receptors in mediating nicotine-induced locomotor stimulation was investigated by examining the effects of selective D1 and D2 dopamine receptor antagonists on activity induced by nicotine. Locomotor activity was assessed in male Sprague-Dawley rats tested in photocell cages. Nicotine (1.0 mg/kg) caused a significant increase in locomotor activity in rats that were habituated to the test environment, but had only a weak and delayed stimulant action in rats that were unfamiliar with the test environment. The stimulant action of nicotine was blocked by the central nicotinic antagonist mecamylamine but not by the peripheral nicotinic blocker hexamethonium, indicating that the response is probably mediated by central nicotinic receptors. Nicotine-induced hyperactivity was blocked by the selective D1 antagonist SCH 23390, the selective D2 antagonist raclopride and the D1/D2 antagonist fluphenazine. Pretreatment with the D2 agonist PHNO enhanced nicotine-induced hyperactivity, whereas the D1 agonist SKF 38393 had no effect. The results indicate that acute nicotine injection induces a pronounced hyperactivity in rats habituated to the test environment. The effect appears to be mediated by central nicotine receptors, possibly located on dopaminergic neurons, and also requires the activation of both D1 and D2 dopamine receptors.     

D1 and D2 dopamine binding site up-regulation and apomorphine-induced stereotypy

Treatments with drugs to up-regulate specific receptors is a strategy often employed in mechanism of action studies. In this type of experiment, changes in the numbers of receptors and concomitant changes in an animal's sensitivity to the drug have been used as evidence for the participation of the binding site in the behavior. In these studies, to test for the role of D1 and D2 receptors in apomorphine-induced stereotypy (AIS), dopamine binding sites were up-regulated by appropriate pre-treatments and the ability of these pre-treatments to alter AIS was subsequently investigated. In the first experiment, 19 days of pre-treatment with SCH 23390 or haloperidol selectively increased by 35 and 40% the numbers of striatal D1 and D2 binding sites, respectively, without affecting their affinities. However, when challenged with apomorphine, only the animals pre-treated with the D2 antagonist showed behavioral supersensitivity. In the second experiment, reserpine pre-treatment (30 mg/kg IP, 24-hr pre-test) increased the numbers of D1 binding sites by 18%, but did not significantly alter the numbers of striatal D2 binding sites. Behaviorally, these rats were supersensitive to apomorphine's stereotypy-inducing effects; however, they also showed an increased sensitivity to the ability of either haloperidol or SCH 23390 to block AIS. Moreover, this blockade was only attenuated by a D2 (but not a D1) agonist. Collectively, these data suggest that AIS is mediated by both D1 and D2 binding sites, but that D2 binding sites have a more important role.     

Possible involvement of differing classes of dopamine D-2 receptors in yawning and stereotypy in rats

The present experiments were performed to investigate differences in the properties of the dopamine D-2 receptors related to yawning and stereotypy. Subcutaneous injection of talipexole (B-HT 920) (10–250 µg/kg) or SND 919 ((S)-2-amino-4,5,6,7-tetrahydro-6-propylamino-benzothiazole) (25–500 µg/kg) evoked yawning behavior with bell-shaped responses. However, SK&F 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (0.5–10 mg/kg SC) did not elicit yawning and decreased yawning responses to low doses of talipexole (25 µg/kg SC) or SND 919 (100 µg/kg SC). These low but effective doses for inducing yawning of talipexole or SND 919 in combination with SK&F 38393 (0.5–10 mg/kg SC) did not elicit stereotypy. In contrast, yawning behavior was not produced after very high doses of talipexole (500 µg/kg SC) or SND 919 (1000 µg/kg SC) given alone or in combination with SK&F 38393 (0.5–10 mg/kg SC). These extremely high doses of talipexole or SND 919 evoked slight stereotypy, which was enhanced by the combined treatment with SK&F 38393. The present results suggest that the dopamine D-2 receptors related to yawning are more sensitive to dopamine receptor agonists than those related to stereotypy, and that concurrent stimulation of postsynaptic dopamine D-1 receptors with D-2 receptors reduces the incidence of yawning but enhances that of stereotypy.     

Excitatory neuronal responses to dopamine in the cerebral cortex: involvement of D2 but not D1 dopamine receptors.

The technique of microelectrophoresis was used to evaluate the relative contribution of D1 and D2 dopamine receptors towards the mediation of the excitatory response of single neurones to dopamine in the somatosensory cortex of the rat. The selective D1 dopamine receptor agonist, SKF 38393, failed to excite any of the cells to which it was applied. In contrast, the selective D2 dopamine receptor agonist, LY 171555, excited the majority of cells tested. The apparent potency of LY 171555 was significantly lower than that of dopamine. When the mobilities of SKF 38393 and LY 171555 were assessed by an in vitro method, they were found to be at least as great as those of dopamine and phenylephrine, suggesting that the lack of effect of SKF 38393 and the lower apparent potency of LY 171555 compared to dopamine reflect genuine biological phenomena. The alpha 1-adrenoceptor antagonist, prazosin, discriminated between excitatory responses to the alpha 1-adrenoceptor agonist, phenylephrine, and LY 171555: responses to phenylephrine were more susceptible to antagonism than were those to LY 171555. The dopamine receptor antagonist, haloperidol, produced the reverse discrimination: responses to LY 171555 were more affected than were those to phenylephrine. Neither antagonist reduced the response to the control agonist, acetylcholine. When applied continuously with low ejecting currents, LY 171555 antagonized the excitatory response to dopamine while the response to phenylephrine was relatively preserved. The response to acetylcholine was unaffected. When similarly applied, SKF 38393 had no selective action on the response to dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential involvement of D1 and D2 dopamine receptors in the expression of morphine withdrawal signs in rats

The role of dopamine (DA) receptors in the expression of opioid dependence was examined by use of an unbiased conditioned place preference paradigm. Male Sprague-Dawley rats were implanted s.c. with two pellets containing placebo or 75mg morphine. Animals received one conditioning session with saline and one with the DA D1 receptor antagonist SCH23390 (0.01-0.05mg, s.c.) or the DA D2 receptor antagonist raclopride (0.25-1.0mg/kg, s.c.). Conditioning sessions were conducted 4 days after pellet implantation. During each of these sessions, physical signs of withdrawal were quantified. In morphine-pelleted animals, the D2 receptor antagonist raclopride produced conditioned place aversions, with a minimum effective dose of 0.5mg/kg. Administration of a higher dose also resulted in wet-dog shakes, ptosis and diarrhea in morphine-pelleted animals. This effect was not observed in response to lower doses of raclopride or in placebo-pelleted animals. The D1 receptor antagonist SCH23390 failed to produce conditioned place aversions in either morphine- or placebo-pelleted animals after single-trial conditioning. This antagonist was also ineffective in producing physical withdrawal signs. After two conditioning sessions with SCH23390, both the morphine- and placebo-pelleted animals exhibited a marked aversion for the SCH23390-paired place. However, there was no difference between groups in the magnitude of this effect. These data demonstrate that the acute blockade of D2 receptors produces aversive effects in opioid-dependent animals and that this effect occurs in the presence of few, if any, prototypic physical withdrawal signs. Furthermore, the inability of a selective D1 receptor antagonist to produce conditioned aversive effects or physical signs of withdrawal suggests an important role of D2 as compared to D1 receptors in the expression of morphine withdrawal signs.     

Post-training minaprine enhances memory storage in mice: involvement of D1 and D2 dopamine receptors

Post-training administration of minaprine (2.5, 5 and 10 mg/kg) dose-dependently improved retention of an inhibitory avoidance response in mice. Animals receiving nine daily injections of 5 mg/kg and administered a challenge dose post-training showed an improvement in memory consolidation similar to that produced by acute injection of 10 mg/kg. The effects on retention performance induced by the drug appear to be due to an effect on memory consolidation. They were observed when drugs were given at short, but not long, periods of time after training, i.e. when the memory trace was susceptible to modulation. Moreover, these effects are not to be ascribed to an aversive or a rewarding or non-specific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during training were not affected by post-training drug administration. The effects of an acutely injected dose (10 mg/kg) of minaprine as well as those of a challenge dose (5 mg/kg) of the drug administered to repeatedly treated animals were reversed by pretreatment with either selective D₁ or D₂ dopamine receptor antagonists SCH 23390 and (-)-sulpiride administered at per se non-effective doses (0.025 and 6 mg/kg, respectively), thus suggesting that D₁ and D₂ receptor types are similarly involved in the effects of minaprine on memory consolidation. These results show that minaprine improves memory consolidation and that repeated drug administration leads to potentiation of this effect. Moreover, the effects of minaprine on memory consolidation are related to its dopaminergic action.     

Evidence for involvement of both D1 and D2 receptors in maintaining cocaine self-administration.

Rats trained to self-administer cocaine (0.75 mg/kg/infusion) on an FR-5 schedule were treated with selective D1 or D2 antagonists. A69045, a D1 antagonist with no appreciable affinity for 5-HT receptors increased cocaine self-administration to 147, 172 and 167% of baseline at doses of 2.5, 5.0 or 10.0 mumol/kg, SC respectively. SCH-23390 (0.007, 0.015 and 0.030 mumol/kg, SC) increased self-administration to 116, 147 and 165% of baseline, respectively. Both D1 antagonists decreased responding in some animals at the highest dose tested. The D2 antagonist YM-09151-2 showed a similar profile, increasing cocaine self-administration at 0.01 and 0.016 mumol/kg, SC and suppressing responding by most animals at the dose of 0.03 mumol/kg, SC. These data give further support to the hypothesis that both D1 and D2 receptors are involved in maintaining cocaine self-administration.     

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

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

Relative dopamine D1 and D2 receptor affinity and efficacy determine whether dopamine agonists induce hyperactivity or oral stereotypy in rats

The effects of a range of dopamine (DA) agonists on stereotyped behaviour in rats were analysed and compared both with the affinity of the compounds for D1 and D2 receptor binding sites in vitro and their ability to stimulate the adenylate cyclase activity in rat striatal homogenates. Full and partial agonists at the D1 receptor coupled to adenylate cyclase do not induce sterotypies when given alone, whereas full D2 agonists (e.g. quinpirole) induce hyperactivity but not oral sterotypies. Partial D2 agonists (e.g. (-)-3-PPP) only induce sedation. Mixed D1/D2 agonists (e.g. apomorphine) induce both hyperactivity and oral stereotypies. Maximum stereotypies were induced by combination of SK & F 38393 and a series of D2 agonists, including full agonists and the partial D2 agonist B-HT 920, whereas partial agonists with low intrinsic activity (e.g. (-)-3-PPP, EMD 23448) did not induce stereotypies when given together with SK & F 38393. However, these partial agonists reduced the maximum effect of apomorphine, whereas the full agonists (e.g. quinpirole, (-)-NPA) and B-HT 920 had no apomorphine antagonistic activity. The mixed D1/D2 agonists apomorphine and N,N-dipropyl-5,6-ADTN were only weakly influenced by SK & F 38393, or not at all. D1 agonists with central effects, including SK & F 38393, SK & F 81297 (with relatively high efficacies), and the partial agonist SK & F 75670 with low efficacy, changed the hyperactivity induced by quinpirole into maximum oral stereotypy, whereas the peripheral D1 agonist fenoldopam had no such effect. Inhibition of DA and NA synthesis with alpha-methyl-p-tyrosine depleted striatal DA levels by 72 per cent and antagonized the hyperactivity induced by the D2 agonists quinpirole and (-)-NPA, but not that of apomorphine. Combination of SK & F 38393 and quinpirole induced maximum stereotypy in DA-depleted animals. These results suggest that D1 receptor tonus is a necessary prerequisite for the expression of a DA agonist's effect. The hyperactivity induced by full D2 agonists appears to be mediated by D1 tonus provided by endogenous DA activity, but stronger D1 stimulation is necessary to induce oral stereotypy. A high degree of D1 receptor activation increases the ability of partial D2 agonists to induce hyperactivity or oral stereotypies since treatment with both SK & F 38393 and B-HT 920 had marked effects while B-HT 920 was ineffective.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence that the enhancement of dopamine function by repeated electroconvulsive shock requires concomitant activation of D1-like and D2-like dopamine receptors

In this study, the behavioural response to dopamine D₁-like receptor agonists (SKF 38393, SKF 81297 and SKF 77434) and D₂-like receptor agonists (quinpirole and RU 24213), administered alone and in combination to rats treated repeatedly with electroconvulsive shock (five ECS over 10 days) or sham, was tested. Agonist-induced behaviour was monitored by automated activity meters and direct observation using a checklist scoring method. Repeated ECS (compared to sham controls) had no significant effect on the behavioural response to SKF 38393 (7.5 mg/kg SC), SKF 81297 (0.2 mg/kg SC), SKF 77434 (0.1 mg/kg SC), quinpirole (0.1 and 0.25 mg/kg SC) or RU 24213 (0.3 mg/kg SC), when administered alone. In contrast, repeated ECS markedly increased locomotion (activity counts and scores) induced by the non-selective dopamine agonist apomorphine (0.5 mg/kg SC) and by co-administration of a D₁-like agonist plus a D₂-like agonist [SKF 38393 (7.5 mg/kg SC) plus quinpirole (0.25 mg/kg SC), SKF 81297 (0.2 mg/kg SC) plus quinpirole (0.1 mg/kg SC), and SKF 77434 (0.1 mg/ kg SC) plus RU 24213 (0.3 mg/kg SC)]. This ECS-induced enhancement of dopamine-mediated behaviour was observed for up to 3 weeks after cessation of ECS treatment. In addition, ECS also enhanced the locomotor response to intra-accumbens SKF 38393 plus quinpirole (0.4 and 1.0 μg/side, respectively). These results provide evidence that the enhancement of dopamine function by repeated ECS requires concomitant stimulation of both D₁-like and D₂-like receptors, and that this effect is long-lasting. Received: 24 January 1997 /Final version: 5 March 1997     

The involvement of dopamine D1 and D2 receptors in the locomotor stimulation produced by (+)-amphetamine in naive and dopamine-depleted mice

The interaction between (+)-amphetamine and dopamine (DA) D1 and D2 receptors was investigated. In na?ve mice, i.e., mice with intact stores of DA, both the selective D1 antagonist SCH23390 and the selective D2 antagonist spiperone blocked the locomoter stimulation produced by (+)-amphetamine. The selective D1 agonist SKF38393 (6 mg/kg intraperitoneally) did not produce a consistent dose-dependent effect on the response to (+)-amphetamine in na?ve mice. In mice depleted of DA with reserpine 24 hr before a challenge with (+)-amphetamine, neither SCH23390 nor spiperone were completely effective in blocking (+)-amphetamine. A combination of spiperone plus SCH23390 was, however, more effective than either drug alone, although significant activity remained even after the combination. In mice pretreated with reserpine and various doses of alpha methyl-p-tyrosine (alpha MPT, intraperitoneally), the degree of stimulation produced by (+)-amphetamine was dependent on the amount and frequency of alpha MPT dosage - the higher and more frequent the dose, the more effective the blockade. In these animals, both SKF38393 and the selective D2 agonist quinpirole potentiated the stimulation induced by (+)-amphetamine when the dose of alpha MPT was not maximal. However, in those animals pretreated with reserpine plus two doses each of 400 mg/kg alpha MPT, neither SKF38393 nor quinpirole were effective in potentiating (+)-amphetamine. Nevertheless, when SKF38393 and quinpirole were administered simultaneously to these mice, marked locomotor stimulation occurred implying that the pretreatment itself had not rendered the mice incapable of locomotion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential effects of olanzapine at dopamine D1 and D2 receptors in dopamine depleted animals

We investigated the degree of striatal dopamine-2 (D₂) receptor occupancy in six schizophrenic patients receiving clinically effective antipsychotic treatment with olanzapine 10–25 mg/day in comparison to patients treated with clozapine 300–600 mg/day (n = 6) or haloperidol 5–20 mg/day (n = 10). ¹²³I Iodobenzamide (IBZM) and single photon emission computerized tomography (SPECT) were used for the visualization of striatal D₂ receptors. For the quantification of striatal D₂ receptor occupancy, striatal IBZM binding in patients treated with antipsychotics was compared to that in untreated healthy controls (n = 8) reported earlier. Olanzapine led to a mean striatal D₂ receptor occupancy rate of 75% (range 63–85). Haloperidol-treated patients showed dose-dependently (Pearson r = 0.64; P < 0.05) a significantly higher (P < 0.05) mean occupancy rate of 84% (range 67–94). During clozapine treatment, the mean D₂ receptor occupancy of 33% (range < 20–49) was significantly lower than with olanzapine (P < 0.005). The higher striatal D₂ receptor occupancy of haloperidol was correlated with the incidence and severity of extrapyramidal motor side-effects (EPS). No clinical relevant EPS occurred during treatment with olanzapine or clozapine. There was no correlation between the degree of striatal D₂ receptor occupancy and clinical improvement. Received: 18 March 1998/Final version: 10 June 1998     

Evidence for dopamine D(1) receptor involvement in the stimulus selection task: overshadowing in the rat

RATIONALE: A number of lines of evidence suggest that dopamine might play a role in stimulus selection, the process whereby specific cues are selected to guide action. OBJECTIVES: In order further to define the potential role for dopamine in stimulus selection, the present series of studies examined whether dopaminergic drugs modulate overshadowing, a paradigm that involves stimulus selection in rats. Overshadowing is where preferential learning occurs to one (usually the more salient) element of a stimulus compound. METHODS: Overshadowing was measured in rats using a thirst motivated conditioned emotional response paradigm (CER). Two simultaneously presented stimuli (light and tone) were paired with an aversive unconditioned stimulus (mild footshock); overshadowing is observed when learning to the less salient stimulus is weaker than learning to the same stimulus when it is conditioned alone. RESULTS: d-Amphetamine sulphate (1 mg/kg, IP) was found selectively to disrupt overshadowing, without affecting the CER in control animals. The dopamine (DA) D(2) receptor antagonists, haloperidol (0.2 mg/kg, IP) or raclopride (0.5 mg/kg, IP), failed to reverse amphetamine-induced disruption of overshadowing. In contrast, the selective DA D(1) antagonist SCH 23390 (0.05 mg/kg, IP) reversed amphetamine-induced disruption of overshadowing. The partial DA D(1) agonist SKF 38393 (5 mg/kg, IP) was found to abolish overshadowing when given alone. CONCLUSION: These data indicate a modulatory role for the DA D(1) receptor in the expression of stimulus selection and suggest that the DA D(1) receptor might play a role in salience allocation aspects of learning.     

Preferential involvement of D3 versus D2 dopamine receptors in the effects of dopamine receptor ligands on oral ethanol self-administration in rats

There is evidence that dopamine transmission is involved in reinforcement processes and the present study investigated the relative involvement of D₃ versus D₂ dopamine receptors in the effects of dopamine ligands on the reinforcing action of ethanol. Rats were trained to self-administer ethanol (10% v/v) orally in a free-choice two-lever operant task using a saccharin-fading procedure. When preference in responding for ethanol over water had developed the rats were tested with several dopamine agonists and antagonists. Pretreatment with the non-selective dopamine agonist, apomorphine (0.01–0.1 mg/kg), the preferential D₂ agonist, bromocriptine (1–10 mg/kg) and the selective D₃ agonists, 7-OH-DPAT (0.003–0.1 mg/kg), PD 128907 (0.1–3 mg/kg), (+)3PPP (0.3–3 mg/kg), quinelorane (0.0001–0.003 mg/kg) and quinpirole (0.003–0.03 mg/kg), resulted in dose-dependent decreases in responding for ethanol. The relative potencies of the dopamine agonists to decrease ethanol self-administration were highly correlated with their published potencies to produce in vitro functional D₃ but not D₂ responses. Active doses could be considered as those selectively stimulating receptors involved in the control of dopamine release, suggesting that reduction of dopamine transmission was associated with a decrease in ethanol-reinforced responding. This conclusion was further supported by the finding that pretreatment with the D₂/D₃ dopamine antagonists, haloperidol (0.1–0.4 mg/kg) and tiapride (10–60 mg/ kg), decreased responding for ethanol at doses which have been shown previously to block dopamine transmission. Received: 25 January 1998/Final version: 24 April 1998     

Rotigotine is a potent agonist at dopamine D1 receptors as well as at dopamine D2 and D3 receptors

Background and Purpose

Rotigotine acts as a dopamine receptor agonist with high affinity for the dopamine D₂, D₃, D₄ and D₅ receptors but with a low affinity for the dopamine D₁ receptor. We have investigated this further in radioligand binding and functional studies and compared the profile of rotigotine with that of other drugs used in the treatment of Parkinson''s disease (PD).

Experimental Approach

The binding of rotigotine to human dopamine D₁, D₂, D₃, D₄ and D₅ receptors was determined in radioligand binding studies using [³H]rotigotine and compared with that of standard antagonist radioligands. Functional interactions of rotigotine with human dopamine receptors was also determined.

Key Results

[³H]rotigotine can be used as an agonist radioligand to label all dopamine receptor subtypes and this can be important to derive agonist affinity estimates. Rotigotine maintains this high affinity in functional studies at all dopamine receptors especially D₁, D₂ and D₃ receptors and, to a lesser extent, D₄ and D₅ receptors. Rotigotine, like apomorphine but unlike ropinirole and pramipexole, was a potent agonist at all dopamine receptors.

Conclusions and Implications

Rotigotine is a high-potency agonist at human dopamine D₁, D₂ and D₃ receptors with a lower potency at D₄ and D₅ receptors. These studies differentiate rotigotine from conventional dopamine D₂ agonists, used in the treatment of PD, such as ropinirole and pramipexole which lack activity at the D₁ and D₅ receptors, but resembles that of apomorphine which has greater efficacy in PD than other dopamine agonists but has suboptimal pharmacokinetic properties.