The putative dopamine D3 agonist, 7-OH-DPAT, reduces dopamine release in the nucleus accumbens and electrical self-stimulation to the ventral tegmentum

Enhancement of memory consolidation by an avoidance conditioned stimulus: Modulation by the D3 receptor

Conditioned stimuli (CS) paired with foot-shock can enhance memory consolidation. Because the dopamine D3R has been implicated in mediating various responses to CSs, the current study explored its potential role in modulation of memory consolidation by an avoidance CS. Male Sprague-Dawley rats trained to avoid foot-shocks in a two-way signalled active avoidance task (8 sessions, 30 trials per session, 0.8 mA foot-shock) were pre-treated with the D3R antagonist NGB-2904 (Vehicle, 0.1 or 5 mg/kg) and exposed to the CS immediately after the sample phase of an object recognition memory task. Discrimination ratios were assessed 72 h later. Immediate, but not delayed (6 h), post-sample exposure to the CS enhanced object recognition memory and this effect was dose-dependently blocked by NGB-2904. Control experiments with the beta-noradrenergic receptor antagonist propranolol (10 or 20 mg/kg) and D2R antagonist pimozide (0.2 or 0.6 mg/kg) indicated that NGB-2904 targeted post-training memory consolidation. Exploring the pharmacological selectivity of the D3R effect, it was found that: 1) 5 mg/kg NGB-2904 blocked conditioned memory modulation produced by post-sample exposure to a "weak" CS (one day of avoidance training) and concurrent stimulation of catecholamine activity by 10 mg/kg bupropion; 2) post-sample exposure to a "weak" CS and concurrent administration of the D3R agonist 7-OH-DPAT (1 mg/kg) enhanced consolidation of object memory. Finally, because 5 mg/kg NGB-2904 had no effect on modulation by avoidance training in the presence of foot-shocks, the findings herein support the hypothesis that the D3R plays an important role in modulation of memory consolidation by CSs.

Attenuated dopamine receptor signaling in nucleus accumbens core in a rat model of chemically-induced neuropathy

Neuropathy is major source of chronic pain that can be caused by mechanically or chemically induced nerve injury. Intraplantar formalin injection produces local necrosis over a two-week period and has been used to model neuropathy in rats. To determine whether neuropathy alters dopamine (DA) receptor responsiveness in mesolimbic brain regions, we examined dopamine D₁-like and D₂-like receptor (D_1/2R) signaling and expression in male rats 14 days after bilateral intraplantar formalin injections into both rear paws. D₂R-mediated G-protein activation and expression of the D₂R long, but not short, isoform were reduced in nucleus accumbens (NAc) core, but not in NAc shell, caudate-putamen or ventral tegmental area of formalin- compared to saline-treated rats. In addition, D₁R-stimulated adenylyl cyclase activity was also reduced in NAc core, but not in NAc shell or prefrontal cortex, of formalin-treated rats, whereas D₁R expression was unaffected. Other proteins involved in dopamine neurotransmission, including dopamine uptake transporter and tyrosine hydroxylase, were unaffected by formalin treatment. In behavioral tests, the potency of a D₂R agonist to suppress intracranial self-stimulation (ICSS) was decreased in formalin-treated rats, whereas D₁R agonist effects were not altered. The combination of reduced D₂R expression and signaling in NAc core with reduced suppression of ICSS responding by a D₂R agonist suggest a reduction in D₂ autoreceptor function. Altogether, these results indicate that intraplantar formalin produces attenuation of highly specific DA receptor signaling processes in NAc core of male rats and suggest the development of a neuropathy-induced allostatic state in both pre- and post-synaptic DA receptor function.

Self-regulation therapy to reproduce drug effects: a suggestion technique to change personality and the DRD3 gene expression

This study proposes a strategy, based on self-regulation therapy, to change personality and its biological substrate, the DRD3 gene expression. It has been demonstrated that acute doses of stimulating drugs, like methylphenidate, are able to change personality and the expression of certain genes in the short term. On the other hand, self-regulation therapy has been proven to reproduce the effects of drugs. Thus, it is feasible to hope that self-regulation therapy is equally effective as methylphenidate in changing personality and the gene expression. This is a preliminary study with a single-case experimental design with replication in which 2 subjects participated. The results and potential implications for research and psychotherapy are discussed.

The dopamine D3 receptor partial agonist CJB090 and antagonist PG01037 decrease progressive ratio responding for methamphetamine in rats with extended-access

Previous work suggests a role for dopamine D3-like receptors in psychostimulant reinforcement. The development of new compounds acting selectively at dopamine D3 receptors has opened new possibilities to explore the role of these receptors in animal models of psychostimulant dependence. Here we investigated whether the dopamine D3 partial agonist CJB090 (1-10 mg/kg, i.v) and the D3 antagonist PG01037 (8-32 mg/kg, s.c.) modified methamphetamine (0.05 mg/kg/injection) intravenous self-administration under fixed- (FR) and progressive- (PR) ratio schedules in rats allowed limited (short access, ShA; 1-hour sessions 3 days/week) or extended access (long access, LgA; 6 hour sessions 6 days/week). Under a FR1 schedule, the highest dose of the D3 partial agonist CJB090 selectively reduced methamphetamine self-administration in LgA but not in ShA rats, whereas the full D3 antagonist PG01037 produced no effect in either group. Under a PR schedule of reinforcement, the D3 partial agonist CJB090 reduced the maximum number of responses performed ('breakpoint') for methamphetamine in LgA rats at the doses of 5 and 10 mg/kg, and also it produced a significant reduction in the ShA group at the highest dose. However, the D3 full antagonist PG01037 only reduced PR methamphetamine self-administration in LgA rats at the highest dose of 32 mg/kg with no effect in the ShA group. The results suggest that rats might be more sensitive to pharmacological modulation of dopamine D3 receptors following extended access to methamphetamine self-administration, opening the possibility that D3 receptors play a role in excessive methamphetamine intake.

Discrimination: from behaviour to brain

Discrimination is a skill needed by many organisms for survival: decisions about food, shelter, and mate selection all require the ability to distinguish among stimuli. This article reviews the how and why of discrimination and how researchers may exploit this natural skill in the laboratory to learn more about what features of stimuli animals use to discriminate. The paper then discusses the possible neurophysiological basis of discrimination and proposes a model, based on one of stimulus-association put forth by Beninger and Gerdjikov (2004) [Beninger, R.J., Gerdjikov, T.V., 2004. The role of signaling molecules in reward-related incentive learning. Neurotox. Res., 6, 91-104], to account for the role of dopamine in how an animal learns to discriminate rewarded from non-rewarded stimuli.

7-OH-DPAT effects on latent inhibition: low dose facilitation but high dose blockade: Implications for dopamine receptor involvement in attentional processes

7-OH-DPAT is a dopamine D2/D3 agonist, which at low doses acts preferentially on D3 receptors but at high doses it acts on D2 and D3 receptors. The present study investigated the contribution of D3 and D2 receptors on latent inhibition (LI) by using two dose levels of 7-OH-DPAT: a low dose, 0.1 mg/kg (D3 receptor activation) and a high dose, 1.0 mg/kg, (D2/D3 receptor activation) in a conditioned emotional response (CER) paradigm. The LI Protocols included CS pre-exposure (10 or 40 CS alone trials), CER induction and a non-drug CER test phase. Additionally, the drug effects upon CER acquisition without LI were assessed using the same treatments and test environment pre-exposure protocols but without the tone CS. The effects of 7-OH-DPAT on crossing, rearing and grooming were also measured in an open field 1 day after the CER test phase. The results showed that the low dose 7-OH-DPAT treatment potentiated LI at 10 but not at 40 CS pre-exposures. The high dose 7-OH-DPAT treatment blocked LI at both the 10 and 40 stimulus pre-exposures; and it also induced hyperactivity. Thus, D3 stimulation induced by a low dose of 7-OH-DPAT can facilitate LI but these effects are contingent upon and are specific to the number of stimulus presentations. Altogether, these findings indicate that D3 stimulation can enhance attentional processes, but D2 stimulation can impair attentional processes.

Effects of 7-OH-DPAT and U 99194 on the behavioral response to hot plate test, in rats

Aim of present study was to investigate in male Wistar rats, whether behavioral response to hot plate test application could be influenced by systemic administration of 7-OH-DPAT, a dopaminergic (DA) D3 versus D2 receptor agonist, or U 99194, a DA D3 versus D2 receptor antagonist. Each trial lasted no more than 10 s and the whole experimental session lasted 120 min. Animal behavior was recorded by means of a digital videocamera and later, frame by frame examined using a professional videorecorder. Latency of each behavioral pattern, characterizing the response, was analysed, showing significant changes only with U 99194. A multivariate cluster analysis indicated the presence of three main behavioral clusters (exploratory, primary responses to pain, escape) which, also, resulted significantly modified by both drugs. In addition, diagrams of preferential direction, obtained through multivariate stochastic analysis, evidenced switching probabilities differences among different patterns and clusters. Results demonstrate that the behavioral response to hot plate test application is altered following 7-OH-DPAT or U 99194 administration. These findings are discussed in terms of a) drugs influence on behavioral switching and learning processes; b) a likely activity on DA D3 versus D2 receptors.

Dopaminergic control of food choice: Contrasting effects of SKF 38393 and quinpirole on high-palatability food preference in the rat

The aim of the present study was to determine the behavioural effects of the selective dopamine D1 receptor agonist, SKF 38393, and of the selective dopamine D2/D3 receptor agonist, quinpirole, on the feeding performance of food-deprived rats in a model of food-preference behaviour. The animals were familiarised with a choice between a high-palatability, high-fat, high-sugar food (chocolate biscuits/cookies) and their regular maintenance diet. Following administration of either SKF 38393 (1.0-10.0 mg/kg, s.c.) or quinpirole (0.03-0.3 mg/kg, s.c.), the animals were observed throughout a 15-min test period, and their feeding behaviour was carefully monitored. Other behavioural categories were also observed. The resulting data were subject to a microstructural analysis to determine the loci of the behavioural effects. The results indicated that SKF 38393 and quinpirole had contrasting effects on the preference for the high-palatability chocolate food. SKF 38393 enhanced the preference, whereas quinpirole eliminated it. These data reinforce the view that forebrain dopamine mechanisms are closely involved in responses to high-palatability energy-dense food constituents, including chocolate. The data also indicate that pharmacological characterization is important, such that dopamine receptor subtypes appear to mediate contrasting effects on food preference for a high-fat, high-sugar food. Hence, brain dopamine appears to be involved in potentially complex ways in determining food preferences, and this may carry implications in the growing evidence for a link between brain dopamine and human obesity.

Facilitation of brain stimulation reward by MK-801 (dizocilpine) may be independent of D2-like dopamine receptor stimulation in rats

RATIONALE

Dopamine (DA) and glutamate (Glu) interactions in the mesocorticolimbic pathway may regulate motivation and reward and contribute to schizophrenia and drug abuse. We have recently demonstrated synergistic effects of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor blockade and D(2/3) DA receptor stimulation in brain stimulation reward (BSR).

OBJECTIVES

This study was conducted to explore interactions between DA and Glu systems in BSR using the NMDA receptor antagonist MK-801 and the DA receptor agonists 7-OH-DPAT and apomorphine.

METHODS

Systemic effects of these compounds were measured in male Sprague-Dawley rats using rate-frequency threshold analysis of ventral tegmental area (VTA) BSR (n=27). Effects of bilateral applications of MK-801 and 7-OH-DPAT into the nucleus accumbens (NAS) shell subregion were also investigated (n=10).

RESULTS

MK-801 (0.03 or 0.13 mg kg(-1) i.p. or 0.66 mug intra-NAS) reduced reward thresholds while 7-OH-DPAT (0.03 mg kg(-1) s.c. or 5.0 microg intra-NAS) or apomorphine (0.05 mg kg(-1), s.c.) increased this measure. MK-801 combined with apomorphine or with 7-OH-DPAT, systemically or in the NAS shell, induced additive effects.

CONCLUSIONS

Lack of interaction between DA agonists and MK-801 in this study contrasts with our previous work showing synergistic reward-decreased effects of AMPA/kainate receptor blockade and D(2/3) DA receptor stimulation in the NAS shell, and indicates possible independence of DA and N-methyl-D-aspartate (NMDA) receptor effects in VTA electrical self-stimulation.

Simultaneous AMPA/kainate receptor blockade and dopamine D2/3 receptor stimulation in the nucleus accumbens decreases brain stimulation reward in rats

Interactions between dopamine (DA) and glutamate (GLU) in the mesocorticolimbic pathway of the brain may influence motivation and reward. Previous work from this laboratory has demonstrated that alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate receptor blockade may potentiate decreases in exploratory motor activity induced by the DA D(2/3) receptor agonist 7-OH-DPAT in the nucleus accumbens septi (NAS). This study investigated the interaction of AMPA/kainate receptor antagonists CNQX or NBQX with 7-OH-DPAT on ventral tegmental area (VTA) brain stimulation reward (BSR). Effects of these compounds, alone and combined, were measured in male Sprague-Dawley rats stereotaxically implanted with a unilateral VTA electrode and bilateral guide cannulae in the NAS core or shell subregions. Rate-frequency analysis was used to assess BSR frequency thresholds and maximum response rates of rats trained to lever-press for reinforcing electrical stimulation. When given alone, CNQX (0.5 microg), NBQX (0.5 microg), or 7-OH-DPAT (5.0 microg) did not affect BSR frequency thresholds. Co-administration of CNQX or NBQX with 7-OH-DPAT synergistically increased BSR frequency thresholds, indicative of decreased reward. These data indicate that simultaneous AMPA/kainate receptor blockade and DA D(2/3) receptor stimulation in the NAS may act synergistically to inhibit motivated behaviours such as electrical brain self-stimulation.

Neuroadaptations to hyperdopaminergia in dopamine D3 receptor-deficient mice

The dopamine D3 receptor (D3R) has been implicated in schizophrenia, drug addiction, depression and Parkinson's disease. The D3R is localized post-synaptically on nucleus accumbens neurons, but is also an autoreceptor on dopaminergic neurons in the mesencephalon. Its functional role as autoreceptor is highly debated, but supported by the elevated basal extracellular dopamine levels found in D3R-deficient mice. To investigate the functional role of the D3R in vivo, we used mice with a targeted disruption of the D3R gene. We found a higher basal level of grooming in D3R-deficient mice, compared to their wild-type littermates. This behavior, which is under the control of D1R stimulation, may be related to an increased dopaminergic tone, since no changes in the gene expression of dopamine D1 and D2 receptors were noticed in the striatum of these mice. D3R-deficient mice displayed other neuroadaptive changes, including decreased tyrosine hydroxylase, increased dopamine transporter mRNAs and increased dopamine reuptake in striatum. The level of tyrosine hydroxylase protein was unchanged in the striatum, as preprodynorphin and preproenkephalin gene expressions. All the changes identified in D3R-deficient mice cannot explain hyperdopaminergia, but, on the contrary, tend to attenuate this phenotype. These results support a distinct role for D2R and D3R as autoreceptors: the D2R is the release-regulating and firing rate-regulating autoreceptor, whereas the D3R may control basal dopamine levels in the striatum, by an unknown mechanism, which does not involve regulation of dopamine transporters or tyrosine hydroxylase. This hyperdopaminergia phenotype of D3R-deficient mice may explain their hyperactivity to drug-paired environmental cues.

Modulation of the locomotor activating effects of the noncompetitive NMDA receptor antagonist MK801 by dopamine D2/3 receptor agonists in mice

The noncompetitive NMDA receptor antagonist MK801 (dizocilpine) produces behavioral stimulation mediated, in part, through indirect activation of the dopamine (DA) system. Previous reports indicate that D2/3 agonists inhibit MK801-induced stereotypies; however, it is unclear if these agonists also attenuate MK801-induced locomotion. As such, the ability of the D2/3 agonists, quinelorane and quinpirole, and the partial D3 agonist, BP897, to attenuate the locomotor activating effects of MK801 was examined in mice. MK801 (0.1-1.0 mg/kg) produced a biphasic effect on total distance traveled with the intermediate dose of 0.3 mg/kg producing the greatest stimulation. The increase in MK801-induced total distance traveled was attenuated by the coadministration of quinelorane and quinpirole at doses that alone had no effect on activity. Similarly, the partial D3 agonist, BP897, blocked the effects of MK801. The D3-preferring antagonist, nafadotride, reversed the attenuation of quinelorane and partially reversed the attenuation of quinpirole. The D2-preferring antagonist, eticlopride, reversed the attenuating effects of quinelorane, but was not effective against quinpirole. Nafadotride and eticlopride were ineffective against the attenuating effects of BP897 on MK801-induced locomotion. Because BP897 is a partial agonist it was tested against quinelorane/MK801 and quinpirole/MK801 combinations. BP897 reversed the attenuating effects of quinelorane, but not those of quinpirole on MK801's effects. These results demonstrate that the DA system, through D2/3 receptor activation, modulates the locomotor activating effects produced by noncompetitive NMDA receptor blockade.

Morphine-induced alterations of immune status are blocked by the dopamine D2-like receptor agonist 7-OH-DPAT

Morphine administration produces profound effects on the immune system, including reductions in natural killer cell activity, mitogen-induced lymphocyte proliferation, and cytokine production. Although it has been established that the activation of central nervous system (CNS) micro-opioid receptors by morphine induces immunomodulation, little is known about the neural mechanisms underlying such processes. Interestingly, it has been shown that the dopamine (DA) D2-like receptor agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) blocks the effect of morphine on a number of behaviors that are mediated by central dopamine pathways. The present study examined whether dopamine is involved in the immunomodulatory effects of morphine. In separate experiments, 7-OH-DPAT was administered either systemically (subcutaneous, s.c.) or centrally (intracerebroventricularly, i.c.v.) prior to morphine treatment in male Lewis rats. The results demonstrate that both systemic and central administration of 7-OH-DPAT attenuate the suppressive effect of morphine on several measures of immune status. Overall, these findings provide the first evidence that CNS dopaminergic mechanisms are directly involved in morphine-induced immunomodulation.

Modulation of the discriminative stimulus effects of mu opioid agonists in rats: II. Effects of dopamine D2/3 agonists

Dopamine (DA) D2/3 receptor agonists have been shown to attenuate the behavioral effects of mu opioid agonists. This study was designed to examine the modulatory actions of the D2/3 agonists quinelorane, quinpirole and (+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide (7-OH-DPAT) on the discriminative stimulus effects of the higher-efficacy mu agonists heroin, methadone and morphine, as well as the lower-efficacy agonist nalbuphine, in rats trained to discriminate heroin from water. All three D2/3 agonists attenuated the heroin-like discriminative stimulus effects of morphine, methadone and nalbuphine, whereas quinpirole and 7-OH-DPAT, but not quinelorane, effectively attenuated the discriminative stimulus effects of heroin. Each D2/3 agonist administered alone occasioned water-appropriate responding and decreased rates of responding. These results extend previous findings, which demonstrated that activation of D2/3 receptors attenuates the antinociceptive effects of mu agonists, to now include their discriminative stimulus effects as well. The exact nature of this modulation of opioid effects by dopamine agonists is unclear, and may include neurochemical interactions as well as psychological mechanisms such as perceptual masking.

The effects of 7-OH-DPAT, quinpirole and raclopride on licking for sucrose solutions in the non-deprived rat

Pharmacological manipulations that alter dopamine (DA) at DA receptor subtypes produce reductions in feeding behaviour. What remains uncertain is the exact way in which these reductions in feeding are achieved as a consequence of differing drug actions at separate receptor subtypes. In this study our aim was to compare the anorectic effects of the preferential D3/D2 agonists 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT) and quinpirole and the non-selective D2/D3 antagonist raclopride on the microstructure of licking responses in non-deprived rats. In a 20-min test, trained adult, male hooded rats had access to one of three solutions: 1%, 3% or 10% sucrose. 7-OH-DPAT (0.1-1.0 mg/kg, i.p.), quinpirole (0.03-0.3 mg/kg, s.c.), raclopride (0.03-0.3 mg/kg, i.p.) or vehicle were injected 20 min prior to the start of the licking test. A lickometer recorded the timing of each lick, from which the microstructural parameters of bout frequency and bout duration were also computed. All compounds reduced the mean bout duration, while 7-OH-DPAT and raclopride also brought about a compensatory increase in bout number. Analysis of the licking rates over the test session showed that 7-OH-DPAT, quinpirole and raclopride decreased the initial rate, without affecting the rate of decline of licking. Changes in licking microstructure (i.e. initial rate of licking and mean bout duration) after the administration of 7-OH-DPAT, quinpirole and raclopride, are consistent with an action of these dopaminergic compounds to reduce palatability.

Cocaine-seeking behavior in response to drug-associated stimuli in rats: involvement of D3 and D2 dopamine receptors

Previous studies employed a second-order schedule paradigm maintained by cocaine reinforcement to show that BP897, a dopamine D(3) partial agonist, selectively modulated drug-seeking behavior. We investigated its effect on drug-seeking behavior induced by presentation of stimuli associated with and predictive of cocaine availability after a period of extinction and in the absence of any further cocaine. Male rats were trained to associate discriminative stimuli (S(D)) with the availability of intravenous (i.v.) 0.25 mg/0.1 ml/infusion cocaine (S(D+)) or no-reward (S(D-)) saline solution. Each infusion of cocaine or saline was followed by a response-cue signaling 20-s time-out (TO). After meeting the self-administration training criterion rats were placed on extinction conditions during which i.v. solutions and S(D)s were withheld. Every other 3 days on which rats met the extinction criterion, reinstatement tests were conducted, presenting the S(D+) or S(D-) noncontingently together with a contingent presentation of cocaine- or saline-cues signaling 20-s TO. Regardless of the order of presentation or the nature of the stimuli (auditory or visual), cocaine-associated but not saline-associated stimuli reinstated responding on the previously active lever. Presentation of cocaine-associated stimuli induced lasting drug-seeking behavior for at least eight test sessions. BP897 (1.0 mg/kg i.p.) significantly attenuated this behavior. Since it has been reported that BP897 can interact with a panel of different receptors with high affinity, we evaluated the effects of 7-OH-DPAT, an agonist to D(3) receptors, raclopride, a preferential antagonist to D(2) receptors, and WAY 100,635, an antagonist at 5-HT(1A) receptors, on drug-seeking behavior. 7-OH-DPAT (0.1-3.0 mg/kg i.p.) had biphasic effects on reinstatement induced by the cocaine-associated cues, low dosages reducing and high dosages increasing the impact of cocaine-associated stimuli on rats' behavior. Raclopride (0.1, 0.3 mg/kg s.c.) completely prevented drug-seeking behavior induced by the reintroduction of cocaine-associated stimuli. WAY 100,635 (0.1-1.0 mg/kg s.c.) had no effect on this behavior. These results, while confirming that the partial agonist at the D(3) receptors, BP897, might be a useful medication, also suggest a role of D(2) receptors in cue-induced cocaine-seeking behavior.

Dopamine D3 receptor antagonism inhibits cocaine-seeking and cocaine-enhanced brain reward in rats

dopamine D3 receptor is preferentially localized to the mesocorticolimbic dopaminergic system and has been hypothesized to play a role in cocaine addiction. To study the involvement of the D3 receptor in brain mechanisms and behaviors commonly assumed to be involved in the addicting properties of cocaine, the potent and selective D3 receptor antagonist trans-N-[4-[2-(6-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl] cyclohexyl]-4-quinolininecarboxamide (SB-277011-A) was administered to laboratory rats, and the following measures were assessed: (1) cocaine-enhanced electrical brain-stimulation reward, (2) cocaine-induced conditioned place preference, and (3) cocaine-triggered reinstatement of cocaine seeking behavior. Systemic injections of SB-277011-A were found to (1) block enhancement of electrical brain stimulation reward by cocaine, (2) dose-dependently attenuate cocaine-induced conditioned place preference, and (3) dose-dependently attenuate cocaine-triggered reinstatement of cocaine seeking behavior. Thus, D3 receptor blockade attenuates both the rewarding effects of cocaine and cocaine-induced drug-seeking behavior. These data suggest an important role for D3 receptors in mediating the addictive properties of cocaine and suggest that blockade of dopamine D3 receptors may constitute a new and useful target for prospective pharmacotherapies for cocaine addiction.

Up-regulation of D3 dopaminergic receptor mRNA in the core of the nucleus accumbens accompanies the development of seizures in a genetic model of absence-epilepsy in the rat

The basal ganglia system is thought to play a key role in the control of absence-seizures and there is ample evidence that epileptic seizures modify brain dopamine function. We recently reported that local injections of dopamine D1 or D2 agonists in the core of the nucleus accumbens suppressed absence-seizures in a spontaneous, genetic rodent model of absence-epilepsy whereas injections of D1 or D2 antagonists had aggravating effects. These findings raised the possibility that the dopaminergic system may be altered in absence-epilepsy prone rats. Therefore, we studied by in situ hybridization histochemistry the expression of pre- and postsynaptic components of the dopaminergic system in this strain of rats. When compared to non-epileptic control rats, epileptic rats displayed no change in the expression of mRNAs coding for the neuronal dopaminergic markers (tyrosine hydroxylase, membraneous and vesicular dopamine transporters). In addition, there was no difference between the two strains concerning the expression of the dopamine receptor transcripts D1, D2 and D5. In adult absence-epilepsy prone rat with an overt epileptic phenotype, however, an elevated level of D3 mRNA expression was observed in neurons of the core of the nucleus accumbens (+23% increase in silver grain density compared to non-epileptic control rats). D3 transcripts were not increased in juvenile epileptic rats without seizures. These findings suggests that up-regulation of D3 receptor mRNA is part of the epileptic phenotype in absence-epilepsy prone rats. Its localization in the core of the nucleus accumbens bears close resemblance to the dopamine-sensitive antiepileptic sites in ventral striatum and further support the involvement of ventral structures of the basal ganglia system in the control of absence-seizures.

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.

Differential effects of 7-OH-DPAT on the development of behavioral sensitization to apomorphine and cocaine

The primary objective of this study was to determine whether concurrent treatments with a low dose of the dopamine D(3)-preferring receptor agonist 7-OH-DPAT would attenuate the development of behavioral sensitization to the indirect dopamine receptor agonist, cocaine, or the direct dopamine receptor agonist, apomorphine. In two experiments, male Wistar rats (250-350 g) were given seven daily injections of 7-OH-DPAT (0.05 mg/kg sc) or vehicle in combination with either cocaine (15 mg/kg ip), apomorphine (1.0 mg/kg sc), or vehicle. After the injections, the rats were tested for activity in photocell arenas for 40 min, and three measures of motor behavior (distance traveled, rearing, and stereotypy) were recorded at 10-min intervals. A total of 24 h after the last preexposure session, all rats were given a challenge injection of either cocaine (10.0 mg/kg ip, Experiment 1) or apomorphine (1.0 mg/kg sc, Experiment 2) and tested for activity. Major findings were as follows: (a) 7-OH-DPAT treatments alone suppressed all measures of locomotor activity and did not affect subsequent behavioral sensitivity to either cocaine or apomorphine; (b) cocaine treatments acutely increased all measures of activity, and repeated treatments produced behavioral sensitization to the horizontal locomotor-activating effects of cocaine; (c) apomorphine treatments alone increased horizontal activity and stereotypy but completely abolished rearing behavior; (d) like cocaine, repeated treatments with apomorphine induced behavioral sensitization; (e) concurrent treatments of 7-OH-DPAT with cocaine acutely attenuated cocaine-induced increases in motor behavior but enhanced the development of behavioral sensitization to cocaine; and (f) concurrent 7-OH-DPAT treatments did not significantly affect either the acute or chronic effects of apomorphine. It is evident from these results that concurrent treatment with 7-OH-DPAT does not block the development of behavioral sensitization to either cocaine or apomorphine. Moreover, the differential acute and chronic effects of 7-OH-DPAT on cocaine- and apomorphine-induced hyperactivity appear to be mediated by dopamine autoreceptor stimulation.

Stimulus properties of 7-OH-DPAT versus auto- and postsynaptic receptor-specific doses of quinpirole

The five types of dopamine (DA) receptor subtypes have been grouped into two families, the D(1)-like (D(1) and D(5) receptors) and D(2)-like (D(2), D(3), and D(4) receptors). Experimental evidence indicates that D(2)-like receptors can be located either presynaptically, where they modulate the synthesis and release of DA, or postsynaptically. Controversy exists, however, over the precise location and role of the D(3) subtype of DA receptor. To investigate this issue, rats were trained using standard operant drug discrimination procedures to discriminate 0.10 mg/kg of the putatively D(3) receptor-preferring agonist R(+)-7-hydroxy-N,N,-di-n-propyl-2-aminotetralin (7-OH-DPAT) from saline. Patterns of generalization to D-amphetamine, AMPT, and SCH 23390 indicated a presynaptic action of 7-OH-DPAT, while apomorphine generalization patterns suggested a postsynaptic action; quinpirole generalization suggested both a pre- and postsynaptic action of 7-OH-DPAT. The ability of spiperone, eticlopride, SCH 23390, and UH 232 to partially antagonize the 7-OH-DPAT stimulus attests to its lack of receptor subtype specificity. These results suggest both pre- and postsynaptic actions of 7-OH-DPAT along with a lack of specificity of the various pharmacological compounds for the D(3) receptor.

Signal transmission, rather than reception, is the underlying neurochemical abnormality in schizophrenia

OBJECTIVE

This review aims to summarise the outcome of studies on changes in the molecular architecture of the brain of subjects with schizophrenia and formulate a hypothesis on mechanisms involved in the pathology of the illness.

METHOD

The outcomes from key studies using neuroimaging techniques and tissue obtained post-mortem that have been directed toward identifying abnormalities in the molecular architecture of the brain in subjects with schizophrenia were summarised. Using the results from these studies hypotheses were formulated on the underlying pathological process that precipitate schizophrenia.

RESULTS

Studies using neuroimaging techniques or tissue obtained post-mortem have revealed changes in the dopaminergic, serotoninergic, glutamatergic, GABAergic and cholinergic systems of the brain in schizophrenia. Some of these studies have identified abnormalities in presynaptic proteins or functioning that may be central to the pathology of schizophrenia.

CONCLUSIONS

There appears to be diverse changes in the molecular cytoarchitecture of the brains from subjects with schizophrenia. It could be that it is by affecting these multiple systems that the atypical antipsychotic drugs produce their improved clinical outcomes. Abnormal functioning of presynaptic processes could be central to the pathology of schizophrenia. If the 'presynaptic' hypothesis is proven, future antipsychotic drug design should be directed away from post-synaptic receptor antagonism toward the modulating the functions of presynaptic neurones.

Native human neocortex release-regulating dopamine D2 type autoreceptors are dopamine D2 subtype

Dopamine (DA) autoreceptors expressed at DA nerve terminals regulate DA release. Considerable evidence has indicated that, in rodents, these autoreceptors belong to the D2 type of the DA receptor family, which, in turn, comprises the D2, D3 and D4 subtypes. We investigated here, for the first time, the subclassification of native human DA autoreceptors by studying the release of [3H]DA evoked by electrical stimulation in fresh human neocortical slices. The results have been compared with those obtained in three animal systems: rat neocortical and striatal slices and rat mesencephalic neuronal cultures. In human neocortical slices, the D2/D3 receptor agonist quinpirole (1 nM-10 microM) inhibited tritium release with a calculated EC50 of 17 nM and a maximal inhibition of approximately 75% reached at 1 microM. In the presence of the D2/D3 receptor antagonist (-)-sulpiride (0.1 and 1 microM), the concentration-response curve of quinpirole was shifted to the right, and the apparent pA2 mean value was 8.5 (8.14-8.77); on the other hand, the inhibitory effects of quinpirole were not affected by the D3 receptor-selective antagonist [7-N,N-dipropylamino-5,6,7, 8-tetrahydro-naphtho(2,3b) dihydro,2,3-furane] (S 14297) and the D4 receptor-selective antagonist 3-(4-[4-chlorophenyl]piperazin-1-yl)-methyl-1H-pyrrolo [2,3-b]pyridine (L-745,870) (0.01-1 microM in each case). Superimposable results have been obtained when the release was elicited from rat striatal slices or dopamine mesencephalic neurons in culture, whereas quantitative differences emerged in the case of rat cortical slices. It is concluded that in human brain, as well as in rat brain, the release of DA in the terminal region of midbrain dopaminergic neurons is regulated through autoreceptors of the D2 subtype.

A dopamine D1 agonist elevates self-stimulation thresholds: comparison to other dopamine-selective drugs

The effects of the high-efficacy D1 receptor agonist SKF 81297 and the D2/3 receptor agonist 7-OH-DPAT on brain stimulation reward thresholds and on response latencies in responding for the stimulation, were compared to the effects of subtype-selective receptor antagonists and a dopamine uptake blocker. SKF 81297 produced dose-dependent elevations in reward thresholds but did not alter response latencies. In contrast, 7-OH-DPAT produced inconsistent reward threshold elevations, yet dose dependently increased response latencies. Both the dopamine D1 receptor antagonist SCH 23390 and the D2 antagonist raclopride elevated reward thresholds, but only raclopride significantly increased response latencies. The dopamine uptake inhibitor GBR 12909 lowered reward thresholds and did not influence response latencies. The present results provide a clear demonstration that a selective, high-efficacy D1 receptor agonist elevates brain stimulation reward thresholds without producing performance deficits. Furthermore, it was observed that the effects upon reward measures of D1-selective compounds, but not D2/D3-selective compounds, are dissociable from their effects upon response latency in this task. These results are discussed with regard to a distinction between the effects of indirect and direct dopamine agonists on reward thresholds, a distinction that does not depend upon the subtype-selectivity of the direct agonists tested.

Dopamine D3 receptor mutant and wild-type mice exhibit identical responses to putative D3 receptor-selective agonists and antagonists

Previous studies using a variety of drugs with different affinities for the dopamine (DA) D3 receptor suggested that this receptor is involved in regulating motor activity and hypothermia. However, the in vivo selectivity of many of these compounds has been repeatedly questioned. To examine the precise roles of the DA D3 receptor in motor activity and hypothermic responses, we used mutant mice lacking the DA D3 receptor to evaluate the in vivo effects of several putative D3 receptor-selective agonists and antagonists. Using automated photocell activity chambers, we observed that the decreases in locomotor activity produced by putative D3 receptor-selective agonists as well as increases in locomotor activity produced by putative D3 receptor antagonists are identical in D3 receptor mutant and wild-type mice. In addition, the hypothermia produced by the putative D3 receptor-selective agonist PD 128907 is identical in both groups of mice. Based on these findings, we propose that D3 receptors are unlikely to be involved in these effects and we caution that the putative D3 ligands that have been used to reach conclusions regarding the functional roles of D3 receptors lack the necessary in vivo selectivity to support such conclusions.

Sensitivity to ageing of the limbic dopaminergic system: a review

The limbic system includes the complex of brain centres, nuclei and connections that provide the anatomical substrate for emotions. Although the presence of small amounts of dopamine (DA) in several limbic structures has been recognized for a long time, for many years it was thought that limbic DA represented a precursor of noradrenaline in the biosynthetic pathway of catecholamines. More recent evidence has shown that limbic centres and nuclei are supplied with a dopaminergic innervation arising from the ventral tegmental area (field A10) and in smaller amounts from the mesencephalic A9 field. The dopaminergic limbic system is sensitive to ageing. Parameters of dopaminergic neurotransmission (DA levels, biosynthetic and catabolic markers and DA receptors) undergo age-related changes which depend on the structure and species investigated and are characterized mainly by a decline of different parameters examined. In this paper, the influence of ageing on DA biosynthesis, levels, metabolism and receptors are reviewed in laboratory rodents, monkeys and humans as well as in cases of Alzheimer's disease and Parkinson's disease. The possibility that changes of dopaminergic neurotransmission markers in the limbic system are associated with cognitive impairment and psychotic symptoms affecting the elderly is discussed. Better knowledge of dopaminergic neurotransmission mechanisms in the so-called physiological ageing and in senile dementia may provide new insights in the treatment of behavioural alterations frequently occurring in old age.

[3H](+)-7-OH-DPAT and [3H]pramipexole binding in the striatum and nucleus accumbens of Sprague-Dawley and Fischer-344 rats

The binding of the D2-like agonists, (+)-7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin (7-OH-DPAT) and [3H]pramipexole (2-amino-4,5,6-tetrahydro-6-propylaminobenzthiazole; MIRAPEX) were determined in membranes from adult male Sprague-Dawley and Fischer-344 rats. Saturation analysis, which optimized binding to D3 receptors, revealed 3-6 fold differences in Bmax values between the two radioligands with no change in affinity. [3H](+)7-OH-DPAT labeled 41.4+/-4.1 to 61.8+/-3.0 fmol/mg protein in nucleus accumbens and striatal homogenates, yet [3H]pramipexole labeled only 7.0+/-1.2 to 18.9+/-5.3 fmol/mg protein. Regional differences with both radioligands were observed in Fischer-344 rats; the striatum exhibited a 52%-69% greater density of sites in comparison to the nucleus accumbens. These data suggest that D3 receptor density can vary significantly between animal strains depending on the radioligand used, and [3H]pramipexole identifies a different ratio of sites in the striatum and nucleus accumbens compared to [3H](+)7-OH-DPAT.

Alterations in dopamine release but not dopamine autoreceptor function in dopamine D3 receptor mutant mice

Dopamine (DA) autoreceptors expressed along the somatodendritic extent of midbrain DA neurons modulate impulse activity, whereas those expressed at DA nerve terminals regulate both DA synthesis and release. Considerable evidence has indicated that these DA autoreceptors are of the D2 subtype of DA receptors. However, many pharmacological studies have suggested an autoreceptor role for the DA D3 receptor. This possibility was tested with mice lacking the D3 receptor as a result of gene targeting. The basal firing rates of DA neurons within both the substantia nigra and ventral tegmental area were not different in D3 receptor mutant and wild-type mice. The putative D3 receptor-selective agonist R(+)-trans-3,4,4a, 10b-tetrahydro-4-propyl-2H,5H-(1)benzopyrano(4,3-b)-1,4-oxazin+ ++-9-ol (PD 128907) was equipotent at inhibiting the activity of both populations of midbrain DA neurons in the two groups of mice. In the gamma-butyrolactone (GBL) model of DA autoreceptor function, mutant and wild-type mice were identical with respect to striatal DA synthesis and its suppression by PD 128907. In vivo microdialysis studies of DA release in ventral striatum revealed higher basal levels of extracellular DA in mutant mice but similar inhibitory effects of PD 128907 in mutant and wild-type mice. These results suggest that the effects of PD 128907 on dopamine cell function reflect stimulation of D2 as opposed to D3 receptors. Although D3 receptors do not seem to be significantly involved in DA autoreceptor function, they may participate in postsynaptically activated short-loop feedback modulation of DA release.

A circuitry model of the expression of behavioral sensitization to amphetamine-like psychostimulants

Repeated exposure to psychostimulants such as cocaine and amphetamine produces behavioral sensitization, which is characterized by an augmented locomotor response to a subsequent psychostimulant challenge injection. Experimentation focused on the neural underpinnings of behavioral sensitization has progressed from a singular focus on dopamine transmission in the nucleus accumbens and striatum to the study of cellular and molecular mechanisms that occur throughout the neural circuitry in which the mesocorticolimbic dopamine projections are embedded. This research effort has yielded a conglomerate of data that has resisted simple interpretations, primarily because no single neuronal effect is likely to be responsible for the expression of behavioral sensitization. The present review examines the literature and critically evaluates the extent to which the neural consequences of repeated psychostimulant administration are associated with the expression of behavioral sensitization. The neural alterations found to contribute to the long-term expression of behavioral sensitization are centered in a collection of interconnected limbic nuclei, which are termed the 'motive' circuit. This neural circuit is used as a template to organize the relevant biochemical and molecular findings into a model of the expression of behavioral sensitization.

The involvement of dopamine D2 receptors, but not D3 or D4 receptors, in the rewarding effect of brain stimulation in the rat

To identify the subtype of dopamine receptors critically involved in the rewarding effect of brain stimulation, four dopamine antagonists were intracranially injected in 25 rats. The importance of dopamine D1 receptors had been demonstrated previously by using SCH 23390, a highly selective D1 antagonist. Rats were implanted with electrodes into the medial forebrain bundle and cannulae into either one of the following structures: the nucleus accumbens, the vicinity of the islands of Calleja, or the ventral tegmental area, all ipsilateral to the electrodes. The animals were trained to press a bar for electrical stimulation, and the frequency-response functions were plotted before and after injection of each dopamine antagonist through the cannulae. Raclopride and haloperidol, which have high affinities for D2 receptors, reduced the rewarding effect after injection into any one of the three cannula sites. Neither (+)-UH232, a selective D3 antagonist, nor clozapine, a D4 antagonist, influenced the rewarding effect. The results suggest that dopamine D2, but not D3 or D4, receptors are critically involved in producing the rewarding effect of brain stimulation.

Upregulation of (+)-7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin binding following intracerebroventricular administration of a nitric oxide generator

Nitric oxide modulation of dopamine D2 and D3 receptor binding was examined using [125I]epidepride (D2) and (+)7-hydroxy-N,N-di-n-[3H]propyl-2-aminotetralin ([3H](+)-7-OH-DPAT, D3). Nitric oxide, generated by i.c.v. injection of S-nitroso-N-acetyl-penicillamine (SNAP; 5 microg or 10 microg), significantly increased the density of [3H](+)-7-OH-DPAT binding sites (39% and 134%, respectively) in the striatum 24 hours post-injection in the absence of Gpp(NH)p, representing an upregulation of either D3 receptors or high affinity D2 receptors. In the presence of 10 microM Gpp(NH)p, D3 receptor upregulation was maintained in both the 5 microg (increased 35%) and 10 microg SNAP (increased 44%) groups. [3H](+)-7-OH-DPAT binding was reduced in both striatum and nucleus accumbens in the presence of 10 microM Gpp(NH)p compared to binding in the absence of Gpp(NH)p, suggesting an upregulation of D3 receptors. Administration of SNAP did not alter total specific [125I]epidepride binding in either brain region. These data suggest that; 1) D3 receptor density is modified following nitric oxide generation, and 2) the density of high affinity D2 receptors identified by [3H](+)-7-OH-DPAT increases in the striatum, but decreases in the nucleus accumbens.

Ultrastructural immunocytochemical localization of the dopamine D2 receptor within GABAergic neurons of the rat striatum

Classical antipsychotics, which block dopamine (DA) D2 receptors, showing intrastriatal variation in their effectiveness in modulating GABAergic function. To determine the cellular basis for such differences, we examined the electron microscopic immunocytochemical labeling of D2 receptors and GABA in the dorsolateral caudate-putamen (CPn) and the nucleus accumbens (Acb) shell. In both regions, peroxidase reaction product and gold-silver deposits representing D2 receptor immunoreactivity (D2-IR) and GABA immunoreactivity (GABA-IR), respectively, were detected in dendrites and perikarya having characteristics of either spiny projection neurons or aspiny interneurons. Some perikarya in both regions are dually labeled with D2-IR and GABA-IR. Neurons axon terminals in each region also contained one or both markers. However, there were notable regional differences in the immunolabeling patterns. In the CPn, D2-IR was more commonly seen in dendrites/spines than in axon terminals, and proportionally more dendrites were dually labeled than in the Acb. In the Acb shell, D2-IR was detected with similar frequency in terminals and dendrites/spines, but more terminals co-localized D2-IR and GABA-IR in this region compared with the CPn. These results provide the first ultrastructural evidence for direct D2-mediated effects of DA on striatal GABAergic neurons. They further suggest that modulation of GABAergic neurons by DA acting at D2 receptors may be relatively more postsynaptic in the CPn, but more presynaptic in the Acb shell.

Dopamine D3 receptor density elevation in aged Fischer-344 x Brown-Norway (F1) rats

The density of dopamine D3 receptors was determined in young (4-month-old) and aged (37-month-old) Fischer-344 x Brown-Norway (F1) male rats using the putative D3 receptor-preferring agonist, [3H](+)-7.hydroxy-2-(N,N-di-n-propylamino)tetralin ([3H](+)-7-OH-DPAT). In the presence of the non-hydrolyzable GTP analog, 5'-guanylylimidodiphosphate (Gpp(NH)p), the density of dopamine D3 receptors in the striatum and nucleus accumbens was significantly increased (29-102%, respectively) in aged Fischer-344 x Brown-Norway (F1) rats compared to young adults. These findings suggest that dopaminergic activity in aged rats is compromised by increased D3 receptor density, resulting in altered striatal/nucleus accumbens function via presynaptic or postsynaptic modifications.

Behavioural effects in the rat of the putative dopamine D3 receptor agonist 7-OH-DPAT: comparison with quinpirole and apomorphine

This study assessed the effects of IP injections of (+/-) 7-hydroxy-2(N,N-di-n-propylamino)tetralin (7-OH-DPAT), a dopamine agonist that has been reported to have preferential affinity for the dopamine D3 sub-type of receptor, on four behavioural procedures in the rat: 1) spontaneous locomotion, 2) electrical self-stimulation of the ventral tegmental area (VTA), using the curve-shift procedure 3) operant responding for food under a progressive-ratio (PR) schedule and 4) induction of stereotypies. The effects of (+/-) 7-OH-DPAT were compared to the effects of apomorphine, a non-specific DA agonist, and quinpirole, a selective D2/D3 agonist. All three dopamine agonists decreased locomotor activity at low doses (0.01-0.3 mg/kg), and only apomorphine had clear locomotor stimulant effects at the highest dose tested (3 mg/kg). The three drugs dose-dependently depressed VTA self-stimulation in a similar way, with low doses inducing a fairly parallel rightward shift of the frequency/rate curves and higher doses flattening the curves. In contrast, responding for food under the PR schedule appeared to be differentially affected by the three agonists: 7-OH-DPAT induced a biphasic effect, with a maximal decrease in lever-pressing at 0.1 mg/kg, followed by a return to baseline levels with increasing doses (0.3-3 mg/kg); quinpirole showed a tendency to decrease responding over the whole dose-range tested with a maximal effect of about 50% of baseline between 0.25 and 1 mg/kg, and apomorphine dose-dependently decreased responding, with rats ceasing to respond at 0.3 mg/kg. All three DA agonists induced stereotypies, but there was a difference in the maximal stereotypy score induced by each of the ligands: 7-OH-DPAT produced a lower maximal effect than quinpirole or apomorphine. This indicates that each of the three dopamine agonists preferentially induced different types of stereotypies. Together, these data suggest that the putative dopamine D3 agonist 7-OH-DPAT, at low doses, has depressant effects similar to those induced by low doses of the other two DA agonists. Differences in the behavioural effects of higher doses were, however, mostly observed in two procedures, PR responding and induction of stereotypies.

Motor actions of 7-OH-DPAT in normal and reserpine-treated mice suggest involvement of both dopamine D2 and D3 receptors

In non-habituated mice, 7-hydroxy-N,N-di-n-propylaminotetralin (7-OH-DPAT, 0.04-10 mg/kg s.c.) potently and rapidly suppressed species-typical behaviours and induced frozen postures, with only occasional evidence of weak behavioural stimulation occurring at 5-10 mg/kg. This inhibitory effect was reversed by the dopamine D1 receptor agonist 2,3,4,5-tetrahydro-7,8-di-hydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393, 10 mg/kg i.p.). 7-OH-DPAT (3-10 mg/kg) did not reinstate locomotion in 4 h habituated mice, either when administered alone or in conjunction with a threshold dose of SKF 38393 (3 mg/kg). By contrast, 7-OH-DPAT (0.2-10 mg/kg) dose-dependently reversed the akinesia of 24 h reserpine-treated mice. This response was blocked by the dopamine D2 receptor antagonist raclopride (10 mg/kg i.p.), but not by the dopamine D1 receptor antagonist (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1H-3- benzazepine-7-ol hemimaleate (SCH 23390, 0.05 mg/kg i.p.), and was potentiated synergistically by coinjection of SKF 38393 (3 mg/kg). These and earlier data suggest the motor inhibitory effects of 7-OH-DPAT (low doses) in normal animals are mediated by dopamine autoreceptors (D2 and/or D3), whilst its motor stimulant actions in normal (high doses) and in dopamine-depleted, supersensitive animals, are mediated by dopamine D2 receptors.