Effects of dopamine D1 antagonists SCH23390 and SK&F83566 on locomotor activities in rats

Effort-based decision making in response to high-dose androgens: role of dopamine receptors

INTRODUCTION

Anabolic-androgenic steroids (AAS) are performance-enhancing drugs used by both world-class and rank-and-file athletes. AAS abuse has been linked with risky decision-making, ranging from drunk driving to abusing multiple drugs. Our lab uses operant behavior in rats to test the effects of AAS (testosterone) on decision making. In our previous study, testosterone caused rats to work harder for food reward during an effort discounting (ED) task. ED is sensitive to dopamine in the nucleus accumbens, and AAS alter accumbens dopamine receptor expression. Accordingly, we determined if testosterone increases response to dopamine receptor antagonists during ED.

METHODS

Rats were treated chronically with high-dose testosterone (7.5 mg/kg; n = 9) or vehicle (n = 9). We measured baseline preference for the large reward in an ED task, where rats choose between a small easy reward (one lever press for one sugar pellet) and a large difficult reward (2, 5, 10, or 15 presses for three pellets). Preference for the large reward was measured after administration of D1-like (SCH23390, 0.01 mg/kg) or D2-like (eticlopride, 0.06 mg/kg) receptor antagonists.

RESULTS

At baseline, testosterone- and vehicle-treated rats showed similar preference for the large reward lever (FR5, testosterone: 68.6 ± 9.7% and vehicle: 85.7 ± 2.5%). SCH23390 reduced large reward preference significantly in both groups (FR5, testosterone: 41.3 ± 9.2%; vehicle: 49.1 ± 8.2%; F(1,16) = 17.7; P < 0.05). Eticlopride decreased large reward preference in both groups, but more strongly in testosterone-treated rats (FR5: testosterone: 37.0 ± 9.7%; vehicle: 56.3 ± 7.8%; F(1,16) = 35.3; P < 0.05).

CONCLUSION

Testosterone increases response to dopamine D2-like receptor blockade, and this contributes to previously observed changes in decision-making behaviors.

The Arousal-motor Hypothesis of Dopamine Function: Evidence that Dopamine Facilitates Reward Seeking in Part by Maintaining Arousal

Dopamine facilitates approach to reward via its actions on dopamine receptors in the nucleus accumbens. For example, blocking either D1 or D2 dopamine receptors in the accumbens reduces the proportion of reward-predictive cues to which rats respond with cued approach. Recent evidence indicates that accumbens dopamine also promotes wakefulness and arousal, but the relationship between dopamine's roles in arousal and reward seeking remains unexplored. Here, we show that the ability of systemic or intra-accumbens injections of the D1 antagonist SCH23390 to reduce cued approach to reward depends on the animal's state of arousal. Handling the animal, a manipulation known to increase arousal, was sufficient to reverse the behavioral effects of the antagonist. In addition, SCH23390 reduced spontaneous locomotion and increased time spent in sleep postures, both consistent with reduced arousal, but also increased time spent immobile in postures inconsistent with sleep. In contrast, the ability of the D2 antagonist haloperidol to reduce cued approach was not reversible by handling. Haloperidol reduced spontaneous locomotion but did not increase sleep postures, instead increasing immobility in non-sleep postures. We place these results in the context of the extensive literature on dopamine's contributions to behavior, and propose the arousal-motor hypothesis. This novel synthesis, which proposes that two main functions of dopamine are to promote arousal and facilitate motor behavior, accounts both for our findings and many previous behavioral observations that have led to disparate and conflicting conclusions.

Inhibition of striatal dopamine D5 receptor attenuates levodopa-induced dyskinesia in a rat model of Parkinson's disease

Levodopa-induced dyskinesia (LID) is experienced by most patients of Parkinson's disease (PD) upon the long-term use of the dopamine precursor levodopa. Striatal dopaminergic signaling plays a critical role in the pathogenesis of LID through its interactions with dopamine receptors. The specific roles of striatal dopaminergic D5 receptors in the pathophysiological process of LID are still poorly established. In the study, we investigated the role of striatal dopamine D5 receptor in LID by using PD rats with or without dyskinetic symptoms after chronic levodopa administration. The experimental results showed that the expression level of D5 receptors in the sensorimotor striatum of dyskinetic rats is significantly higher than that of the non-dyskinetic controls. The administration of levodopa increased c-Fos expression in a subpopulation of sensorimotor striatum neurons of dyskinetic rats, but not in non-dyskinetic rats. The majority of the c-Fos+ neurons activated by levodopa in the striatum are positive for D5 receptor staining. Intrastriatal injection of D1-like (D1 and D5) dopamine receptor antagonist, SCH-23390, significantly inhibited dyskinetic behavior in dyskinetic rats after the injection of levodopa, meanwhile, intrastriatal administration of SKF-83959, a partial D5 receptor agonist, yielded significant dyskinetic movements in dyskinetic rats without levodopa. In contrast, intrastriatal perfusion of small interfering RNA directed against DRD5 downregulated D5 receptors expression and moderately inhibited dyskinetic behavior of dyskinetic animals. Our data suggested that the striatal dopamine D5 receptor might play a novel role in the pathophysiology of LID.

The Appetite Suppressant D-norpseudoephedrine (Cathine) Acts via D1/D2-Like Dopamine Receptors in the Nucleus Accumbens Shell

D-norpseudoephedrine (NPE), also known as cathine, is found naturally in the shrub Catha edulis “Khat.” NPE has been widely used as an appetite suppressant for the treatment of obesity. Although it is known that NPE acts on α1-adrenergic receptors, there is little information about the role of dopamine receptors on NPE’s induced anorectic and weight loss effects. Equally untouched is the question of how NPE modulates neuronal activity in the nucleus accumbens shell (NAcSh), a brain reward center, and a pharmacological target for many appetite suppressants. To do this, in rats, we characterized the pharmacological effects induced by NPE on weight loss, food intake, and locomotion. We also determined the involvement of dopamine D1- and D2-like receptors using systemic and intra-NAcSh antagonists, and finally, we recorded single-unit activity in the NAcSh in freely moving rats. We found that NPE decreased 24-h food intake, induced weight loss, and as side effects increased locomotor activity and wakefulness. Also, intraperitoneal and intra-NAcSh administration of D1 and D2 dopamine antagonists partially reversed NPE’s induced weight loss and food intake suppression. Furthermore, the D1 antagonist, SCH-23390, eliminated NPE-induced locomotion, whereas the D2 antagonist, raclopride, only delayed its onset. We also found that NPE evoked a net activation imbalance in NAcSh that propelled the population activity trajectories into a dynamic pharmacological brain state, which correlated with the onset of NPE-induced wakefulness. Together, our data demonstrate that NPE modulates NAcSh spiking activity and that both dopamine D1 and D2 receptors are necessary for NPE’s induced food intake suppression and weight loss.

5-HT2A receptor activation is necessary for CO2-induced arousal

Hypercapnia-induced arousal from sleep is an important protective mechanism pertinent to a number of diseases. Most notably among these are the sudden infant death syndrome, obstructive sleep apnea and sudden unexpected death in epilepsy. Serotonin (5-HT) plays a significant role in hypercapnia-induced arousal. The mechanism of 5-HT's role in this protective response is unknown. Here we sought to identify the specific 5-HT receptor subtype(s) involved in this response. Wild-type mice were pretreated with antagonists against 5-HT receptor subtypes, as well as antagonists against adrenergic, cholinergic, histaminergic, dopaminergic, and orexinergic receptors before challenge with inspired CO2 or hypoxia. Antagonists of 5-HT(2A) receptors dose-dependently blocked CO2-induced arousal. The 5-HT(2C) receptor antagonist, RS-102221, and the 5-HT1A receptor agonist, 8-OH-DPAT, attenuated but did not completely block CO2-induced arousal. Blockade of non-5-HT receptors did not affect CO2-induced arousal. None of these drugs had any effect on hypoxia-induced arousal. 5-HT2 receptor agonists were given to mice in which 5-HT neurons had been genetically eliminated during embryonic life (Lmx1b(f/f/p)) and which are known to lack CO2-induced arousal. Application of agonists to 5-HT(2A), but not 5-HT(2C), receptors, dose-dependently restored CO2-induced arousal in these mice. These data identify the 5-HT(2A) receptor as an important mediator of CO2-induced arousal and suggest that, while 5-HT neurons can be independently activated to drive CO2-induced arousal, in the absence of 5-HT neurons and endogenous 5-HT, 5-HT receptor activation can act in a permissive fashion to facilitate CO2-induced arousal via another as yet unidentified chemosensor system.

Medial prefrontal cortex lesions impair decision-making on a rodent gambling task: reversal by D1 receptor antagonist administration

Decision-making is a complex cognitive process that is impaired in a number of psychiatric disorders. In the laboratory, decision-making is frequently assessed using "gambling" tasks that are designed to simulate real-life decisions in terms of uncertainty, reward and punishment. Here, we investigate whether lesions of the medial prefrontal cortex (PFC) cause impairments in decision-making using a rodent gambling task (rGT). In this task, rats have to decide between 1 of 4 possible options: 2 options are considered "advantageous" and lead to greater net rewards (food pellets) than the other 2 "disadvantageous" options. Once rats attained stable levels of performance on the rGT they underwent sham or excitoxic lesions of the medial PFC and were allowed to recover for 1 week. Following recovery, rats were retrained for 5 days and then the effects of a dopamine D1-like receptor antagonist (SCH23390) or a D2-like receptor antagonist (haloperidol) on performance were assessed. Lesioned rats exhibited impaired decision-making: they made fewer advantageous choices and chose the most optimal choice less frequently than did sham-operated rats. Administration of SCH23390 (0.03 mg/kg), but not haloperidol (0.015-0.03 mg/kg) attenuated the lesion-induced decision-making deficit. These results indicate that the medial PFC is important for decision-making and that excessive signaling at D1 receptors may contribute to decision-making impairments.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effect of dopamine and serotonin receptor antagonists on fencamfamine-induced abolition of latent inhibition

The purpose of this investigation was to verify the role of dopamine and serotonin receptors in the effect of fencamfamine (FCF) on latent inhibition. FCF is a psychomotor stimulant with an indirect dopaminergic action. Latent inhibition is a model of attention. Latent inhibition is blocked by dopaminergic agents and facilitated by dopamine receptor agonists. FCF has been shown to abolish latent inhibition. The serotonergic system may also participate in the neurochemical mediation of latent inhibition. The selective dopamine D(1) receptor antagonist SCH 23390 (7-chloro-3-methyl-1-phenyl-1,2,4,5-tetrahydro-3-benzazepin-8-ol), D(2) receptor antagonists pimozide (PIM) and methoclopramide (METH), and serotonin 5-HT(2A/C) receptor antagonist ritanserin (RIT) were used in the present study. Latent inhibition was evaluated using a conditioned emotional response procedure. Male Wistar rats that were water-restricted were subjected to a three-phase procedure: preexposure to a tone, tone-shock conditioning, and a test of the effect of the tone on licking frequency. All of the drugs were administered before the preexposure and conditioning phases. The results showed that FCF abolished latent inhibition, and this effect was clearly antagonized by PIM and METH and moderately attenuated by SCH 23390. At the doses used in the present study, RIT pretreatment did not affect latent inhibition and did not eliminate the effect of FCF, suggesting that the FCF-induced abolition of latent inhibition is not mediated by serotonin 5-HT(2A/C) receptors. These results suggest that the effect of FCF on latent inhibition is predominantly related to dopamine D(2) receptors and that dopamine D(2) receptors participate in attention processes.

Prenatal ethanol exposure alters synaptic plasticity in the dorsolateral striatum of rat offspring via changing the reactivity of dopamine receptor

Prenatal exposure to high-level ethanol (EtOH) has been reported to produce hyperlocomotion in offspring. Previous studies have demonstrated synaptic plasticity in cortical afferent to the dorsolateral (DL) striatum is involved in the pathogensis of hyperlocomotion. Here, prenatal EtOH-exposed rat offspring were used to investigate whether maternal EtOH exposure affected synaptic plasticity in the DL striatum. We found high-frequency stimulation (HFS) induced a weaker long-term potentiation (LTP) in EtOH rats than that in control rats at postnatal day (PD) 15. The same protocol of HFS induced long-term depression (LTD) in control group but still LTP in EtOH group at PD 30 or PD 40. Furthermore, enhancement of basal synaptic transmission accompanied by the decrease of pair-pulse facilitation (PPF) was observed in PD 30 EtOH offspring. The perfusion with D1-type receptors (D1R) antagonist SCH23390 recovered synaptic transmission and blocked the induction of abnormal LTP in PD 30 EtOH offspring. The perfusion with D2-type receptors (D2R) agonist quinpirole reversed EtOH-induced LTP into D1R- and metabotropic glutamate receptor-dependent LTD. The data provide the functional evidence that prenatal ethanol exposure led to the persistent abnormal synaptic plasticity in the DL striatum via disturbing the balance between D1R and D2R.

Dopamine receptors modulate ethanol's locomotor-activating effects in preweanling rats

Near the end of the second postnatal week motor activity is increased soon after ethanol administration (2.5 g/kg) while sedation-like effects prevail when blood ethanol levels reach peak values. This time course coincides with biphasic reinforcement (appetitive and aversive) effects of ethanol determined at the same age. The present experiments tested the hypothesis that ethanol-induced activity during early development in the rat depends on the dopamine system, which is functional in modulating motor activity early in ontogeny. Experiments 1a and 1b tested ethanol-induced activity (0 or 2.5 g/kg) after a D1-like (SCH23390; 0, .015, .030, or .060 mg/kg) or a D2-like (sulpiride; 0, 5, 10, or 20 mg/kg) receptor antagonist, respectively. Ethanol-induced stimulation was suppressed by SCH23390 or sulpiride. The dopaminergic antagonists had no effect on blood ethanol concentration (Experiments 2a and 2b). In Experiment 3, 2.5 g/kg ethanol increased dopamine concentration in striatal tissue as well as locomotor activity in infant Wistar rats. Adding to our previous results showing a reduction in ethanol induced activity by a GABA B agonist or a nonspecific opioid antagonist, the present experiments implicate both D1-like and D2-like dopamine receptors in ethanol-induced locomotor stimulation during early development. According to these results, the same mechanisms that modulate ethanol-mediated locomotor stimulation in adult rodents seem to regulate this particular ethanol effect in the infant rat.

The effect of a full agonist/antagonist of the D1 receptor on locomotor activity, sensorimotor gating and cognitive function in dizocilpine-treated rats

Cognitive impairment has been found across all subtypes of schizophrenia. The location and function of dopamine-1 receptors (D1Rs) make them attractive targets for the treatment of cognitive impairment in schizophrenia. Here we investigate the systemic effect of a D1R agonist (A77636) and antagonist (SCH 23390) on hyperlocomotor activity and cognitive deficit induced by an NMDA receptor antagonist (MK-801). Wistar rats (250-300 g) received A77636 (0.1, 0.5 or 1 mg/kg) or SCH 23390 (0.02 or 0.05 mg/kg) with MK-801 (0.1 mg/kg) or saline for 4 d. On day 4 we assessed the prepulse inhibition of the acoustic startle response, locomotor activity in a novel arena and active allothetic place avoidance (spatial memory task) 15 min after the last injection. Systematic administration of the D1R agonist at 0.1 mg/kg ameliorates cognitive dysfunction in our model of schizophrenia, but increases stereotypy and locomotor activity (model of psychotic symptoms) at higher doses (0.5 or 1 mg/kg). Administration of the D1R antagonist had no effect on cognitive function, but decreased hyperlocomotion induced by MK-801. Thus, based on our results, over-activation of D1Rs may exacerbate psychotic symptoms in patients with schizophrenia.

Exposure to nicotine and sensitization of nicotine-induced behaviors

Evidence for an important link between sensitization of midbrain dopamine (DA) neuron reactivity and enhanced self-administration of amphetamine and cocaine has been reported. To the extent that exposure to nicotine also sensitizes nucleus accumbens DA reactivity, it is likely that it will also impact subsequent drug taking. It is thus necessary to gain an understanding of the long-term effects of exposure to nicotine on nicotinic acetylcholine receptors (nAChRs), neuronal excitability and behavior. A review of the literature is presented in which different regimens of nicotine exposure are assessed for their effects on upregulation of nAChRs, induction of LTP in interconnected midbrain nuclei and development of long-lasting locomotor and DA sensitization. Exposure to nicotine upregulates nAChRs and nAChR currents and produces LTP of excitatory inputs to midbrain DA neurons. These effects appear in the hours to days following exposure. Exposure to nicotine also leads to long-lasting sensitization of nicotine's nucleus accumbens DA and locomotor activating effects. These effects appear days to weeks after drug exposure. A model is proposed in which nicotine exposure regimens that produce transient nAChR upregulation and LTP consequently produce long-lasting sensitization of midbrain DA neuron reactivity and nicotine-induced behaviors. These neuroadaptations are proposed to constitute critical components of the mechanisms underlying the initiation, maintenance and escalation of drug use.

Morris water maze learning in Long-Evans rats is differentially affected by blockade of D1-like and D2-like dopamine receptors

Dopaminergic neurotransmission is involved in several brain functions including spatial cognition. In the present study we examine the effects of systemic administration of D1-like receptor antagonist SCH23390 and D2-like receptor antagonist sulpiride on the acquisition of the Morris water maze task. We used visible versus hidden platform versions of the MWM in order to distinguish between the effects of the drugs on the procedural versus cognitive aspects of the task. SCH23390 was found to prolong escape latencies to the visible platform at a higher dose (0.05mg/kg), whilst the lower dose (0.02mg/kg) left both procedural and cognitive functions almost unchanged. SCH23390 was also found to reduce swimming speed. Sulpiride did not affect the visible platform learning at any of three doses studied (30, 60 and 100mg/kg); the highest dose of sulpiride (100mg/kg) impaired place navigation to the hidden platform, without affecting the swim speed. The results of the present study show a difference in the involvement of D1-like and D2-like receptors in the MWM acquisition.

Further study of the effects of dopaminergic D1 drugs on place avoidance behavior using pretraining: some negative evidence

Dopaminergic neurotransmission is considered to modulate cognitive processes, including spatial memory. The aim of this study was to further evaluate the role of the D1 receptor system in an active allothetic place avoidance (AAPA) task using pretrained rats. Our previous results showed enhanced AAPA learning after systemic injections of low doses of D1 agonist A77636, and the impairment of AAPA acquisition by D1 blocker SCH23390 [Stuchlik A, Vales K. Effect of dopamine D1 receptor antagonist SCH23390 and agonist A77636 on active allothetic place avoidance, a spatial cognition task. Behav Brain Res 2006;172(2):250-255]. In the present study, we used the intact-pretraining paradigm, in which animals were trained to the task prior to the injections and subsequently retrieval and reacquisition of AAPA while under the effects of the drugs was tested. Results showed that the intact pretraining partly eliminated the effects of A77636 and SCH23390 on AAPA performance, but a higher dose of SCH23390 caused a motor deficit in the retrieval session. We conclude that in the previous study, D1-active drugs may have had influence upon the non-spatial aspects of the AAPA.

A developmental role for catecholamines in Drosophila behavior

Tyrosine hydroxylase (TH), the enzyme which catalyzes the conversion of tyrosine to L-DOPA and is the rate limiting step in catecholamine biosynthesis, is genetically expressed during development in Drosophila. Null mutant alleles of the single copy gene which codes for this enzyme are developmentally lethal as is a conditional TH mutant at its restrictive temperature. In adult flies, inhibition of TH by alpha-methyl-p-tyrosine (alphaMT) decreases locomotor activity in a dose-dependent manner. This behavioral effect is accompanied by reductions in brain levels of dopamine, the primary CNS catecholamine in Drosophila, and can be prevented by the coadministration of L-DOPA. Similar effects are found with reserpine and at the restrictive temperature in flies with a temperature conditional mutation for TH. In agreement with published studies in mammals, inhibition of TH by alphaMT during Drosophila development results in enhanced expression of this enzyme in the progeny of surviving adults. This biochemical outcome is accompanied behaviorally by increased sensitivity to the locomotor effects of both alphaMT and reserpine, drugs which act via depletion of brain catecholamines. Since TH is the rate limiting enzyme responsible for the conversion of tyrosine to L-DOPA and L-DOPA is converted to dopamine by aromatic amino acid decarboxylase (AAAD), the results indicate that depletion of catecholamine levels in the fly embryo results in increased dopamine biosynthesis in the next generation accompanied by alterations in behavior.

Locomotor hyperactivity and alterations in dopamine neurotransmission are associated with overexpression of A53T mutant human alpha-synuclein in mice

Genetic and biochemical abnormalities associated with alpha-synuclein are implicated in the etiology of Parkinson's disease (PD). In this study, altered locomotor behavior linked to the expression of mutant or wildtype human alpha-synuclein was investigated. A53T alpha-synuclein transgenic (A53T-tg) mice exhibited normal activity at 5 months of age; however, by 7 months, they developed marked hyperactivity that remained evident until 19 months. By contrast, mice expressing human wildtype or A30P mutant alpha-synuclein showed no locomotor alterations. Hyperactivity in A53T-tg mice was reversed by the D1 receptor antagonist SCH 23390. Furthermore, A53T-tg mice were supersensitive to the D1 receptor agonist SKF 81297 but not to the serotonin1B receptor agonist RU 24969. Hyperactivity in A53T-tg mice was also associated with increased D1 receptor expression in the substantia nigra and decreased dopamine transporter expression in the nucleus accumbens and striatum. Finally, striatal dopamine uptake measured by high-speed chronoamperometry was reduced by 40% in A53T-tg mice. Thus, expression of A53T mutant human alpha-synuclein in mice results in adult-onset hyperactivity associated with D1 receptor and dopamine transporter-mediated alterations in dopamine neurotransmission.

Identification of human dopamine D1-like receptor agonist using a cell-based functional assay

AIM

To establish a cell-based assay to screen human dopamine D1 and D5 receptor agonists against compounds from a natural product compound library.

METHODS

Synthetic responsive elements 6 cAMP response elements (CRE) and a mini promoter containing a TATA box were inserted into the pGL3 basic vector to generate the reporter gene construct pCRE/TA/Luci. CHO cells were co-transfected with the reporter gene construct and human D1 or D5 receptor cDNA in mammalian expression vectors. Stable cell lines were established for agonist screening. A natural product compound library from over 300 herbs has been established. The extracts from these herbs were used for human D1 and D5 receptor agonist screenings.

RESULTS

A number of extracts were identified that activated both D1 and D5 receptors. One of the herb extracts, SBG492, demonstrated distinct pharmacological characteristics with human D1 and D5 receptors. The EC(50) values of SBG492 were 342.7 microg/mL for the D1 receptor and 31.7 microg/mL for the D5 receptor.

CONCLUSION

We have established a cell-based assay for high-throughput drug screening to identify D1-like receptor agonists from natural products. Several extracts that can active D1-like receptors were discovered. These compounds could be useful tools for studies on the functions of these receptors in the brain and could potentially be developed into therapeutic drugs for the treatment of central nervous system diseases.

Effects of galanin-like peptide (GALP) on locomotion, reproduction, and body weight in female and male mice

Galanin-like peptide (GALP) has been implicated in the neuroendocrine regulation of both feeding and reproduction. In male rodents and primates, intracerebroventricular (icv) infusions of GALP stimulate luteinizing hormone (LH) release, induce Fos expression in brain areas implicated in feeding and reproduction, and affect food intake and body weight in rodents. In gonad-intact and castrated male rats, icv administration of GALP also stimulates male sexual behavior. While the effects of GALP on male physiology and behavior are well documented, no studies have addressed such a role of GALP in females. We tested the effects of icv GALP infusions on LH release, locomotor activity, motor control, and body weight regulation in adult ovariectomized female mice hormonally primed with estradiol benzoate and progesterone. In addition, sexually-experienced male and female mice were treated with GALP and tested for sexual behavior. In females, GALP reduced open-field locomotor activity, the ability to maintain grip on an accelerating rotarod, and 24-h body weight in a dose-dependent manner. GALP also increased LH secretion in female mice, an effect that was blocked by pre-treatment with Antide, a gonadotropin-releasing hormone (GnRH) type-1 receptor antagonist. GALP infusions slightly decreased the occurrence of lordosis behavior in female mice and significantly increased the latencies with which females displayed receptivity. Unlike previous reports in male rats, GALP inhibited male sexual behavior in mice. Our data indicate that in female mice, GALP stimulates LH release via GnRH, and decreases body weight, motor control, and locomotor activity via GnRH-independent pathways. Furthermore, our sexual behavior and locomotor findings suggest species-specific differences in the mechanism and/or location of GALP action in the brains of rats and mice.

Effect of SCH 23390 on (±)-3,4-methylenedioxymethamphetamine hyperactivity and self-administration in rats

Recently, we demonstrated that (+/-)-3,4-methylenedioxymethamphetamine (MDMA; ecstasy) was reliably and dose-dependently self-administered by previously drug-naïve laboratory rats. The neurochemical basis of MDMA self-administration has not, however, been extensively studied. The present study investigated the role of dopamine in MDMA self-administration and hyperactivity. Pretreatment with the D1-like antagonist, SCH 23390 (0.01-0.08 mg/kg) produced a dose-dependent attenuation of MDMA (20.0 mg/kg)-produced hyperactivity. In self-administration tests, the baseline rate of responding maintained by intravenous infusions varied inversely with MDMA dose; as the dose available was changed, responding also changed so that about 10.0 mg/kg MDMA was self-administered during each daily 2-h session. Pretreatment with SCH 23390 (0.02 mg/kg) produced a rightward shift in the MDMA dose-response curve. These findings suggest that MDMA self-administration, like self-administration of other drugs of abuse, is dependent on the activation of dopaminergic substrates.

Behavioral, neuroendocrine and thermoregulatory actions of apelin-13

As the distribution of apelinergic neurons in the brain suggests an important role of apelin-13 in the regulation of neuroendocrine processes, in the present experiments the effects of this recently identified neuropeptide on the open-field activity, the hypothalamo-pituitary-adrenal (HPA) system and the body temperature were investigated. I.c.v. administration of apelin-13 (1-10 microg) to rats caused significant increases in square crossing, rearing, plasma corticosterone release and core temperature, whereas it did not influence the spontaneous motor activity during telemetric observation. To determine the mediation of the actions of apelin, a corticotropin-releasing hormone (CRH) antagonist, the nonselective dopamine antagonist haloperidol, the selective dopamine D1 receptor antagonist SCH-23390 and the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME) were administered to the rats. The apelin-evoked HPA activation was diminished by preadministration of the CRH antagonist, while the dopamine antagonist and L-NAME attenuated only the square crossing and rearing induced by apelin-13. To characterize the transmission of the thermoregulatory action of apelin, animals were pretreated either with L-NAME, the CRH antagonist or with the cyclooxygenase inhibitor noraminophenazone. L-NAME and the CRH antagonist did not cause significant inhibition of the apelin-evoked increase in core temperature, while the cyclooxygenase inhibitor, applied 30 min before peptide treatment, did not prove effective in preventing the apelin-evoked thermoregulatory response, whereas when it was administered 2 h after the peptide treatment, it transiently and significantly reduced the hyperthermic response. The present data suggest that apelin-13 plays an important role in the regulation of behavioral, endocrine and homeostatic responses in the CNS, and dopamine, nitric oxide and prostaglandins seem to take part in the mediation of its effects. Since the corticosterone response could be blocked by the CRH antagonist, it is likely to be mediated through the activation of the CRH neurons.

Distinct roles of D1 and D5 dopamine receptors in motor activity and striatal synaptic plasticity

Stimulation of dopamine (DA) receptors in the striatum is essential for voluntary motor activity and for the generation of plasticity at corticostriatal synapses. In the present study, mice lacking DA D1 receptors have been used to investigate the involvement of the D1-like class (D1 and D5) of DA receptors in locomotion and corticostriatal long-term depression (LTD) and long-term potentiation (LTP). Our results suggest that D1 and D5 receptors exert distinct actions on both activity-dependent synaptic plasticity and spontaneous motor activity. Accordingly, the ablation of D1 receptors disrupted corticostriatal LTP, whereas pharmacological blockade of D5 receptors prevented LTD. On the other side, genetic ablation of D1 receptors increased locomotor activity, whereas the D1/D5 receptor antagonist SCH 23390 decreased motor activity in both control mice and mice lacking D1 receptors. Endogenous DA stimulated D1 and D5 receptors in distinct subtypes of striatal neurons to induce, respectively, LTP and LTD. In control mice, in fact, LTP was blocked by inhibiting the D1-protein kinase A pathway in the recorded spiny neuron, whereas the striatal nitric oxide-producing interneuron was presumably the neuronal subtype stimulated by D5 receptors during the induction phase of LTD. Understanding the role of DA receptors in striatal function is essential to gain insights into the neural bases of critical brain functions and of dramatic pathological conditions such as Parkinson's disease, schizophrenia, and drug addiction.

Behavioral response profiles following drug challenge with dopamine receptor subtype agonists and antagonists in developing rat

As part of an investigation into the effects of gestational ethanol (ETOH) exposure on the developing dopamine (DA) system, pregnant Sprague-Dawley rats were exposed to one of three conditions: ETOH, pair-fed (PF) to the ETOH group, or ad libitum lab chow controls (LC). In this paper we report behavioral drug challenge effects for offspring of the two control groups (PF and LC). Male and female pups between postnatal days (PNDs) 21 and 23 in age were exposed to one of three intraperitoneal/subcutaneous doses of one of eight drugs chosen to assess the functional status of the DA D(1), D(2), and D(3) receptor subtype, or a saline control. Agonists were SKF 38393, apomorphine (APO), quinpirole (QUIN), and 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin [7-OH-DPAT (DPAT)]; antagonists were spiperone (SPIP), SCH 23390, and two recently developed D(3) antagonists nafadotride (NAF) and PD 152255. Immediately following drug injection, pups were placed in observation cages, where eight behaviors (square entries, grooming, circling, rearing, sniffing, head and oral movements, and yawning) were scored at 3-min intervals for 30 min. Classic behavioral profiles were generally obtained for the high-dose mixed agonists APO, DPAT, and QUIN, which potently increased square entries, rearing, and sniffing, while reducing grooming and head movements. However, low-dose APO had no effect on behavior. The D(1) agonist, SKF 38393, had a strikingly different behavioral profile; it had no effect on square entries at any dose, while increasing grooming and sniffing at the medium dose. The D(1) antagonist, SCH 23390, profoundly decreased all behaviors except oral and head movements, especially at high doses. In contrast, the effects of the D(2) antagonist, SPIP, were limited to increasing sniffing at the medium dose. The two putative D(3) antagonists, NAF and PD 152255, presented strikingly different profiles. NAF induced a pattern of behavioral suppression that resembled the profile of high-dose SCH, while high-dose PD 152255 stimulated behavior. The failure of low-dose APO to have any effect on behavior suggests that the D(2) autoreceptor is not functional in preweanling rats. This hypothesis is further supported by the lack of behavioral suppression seen with low-dose QUIN and DPAT. Failure of NAF to produce behavioral activation at low doses and the stimulatory effects seen with PD 152255 suggests that either the D(3) autoreceptor, the postsynaptic D(3) receptor, or both are not fully functional at this age as well.

Locomotor behavior of dopamine D1 receptor transgenic/D2 receptor deficient hybrid mice

Mice that incorporate the dopamine D1 receptor transgene controlled by the D1 receptor promoter exhibit a marked increase of D1 binding in several extra-striatal brain regions and show a paradoxical hypokinetic response to D1 agonist [Exp. Neurol. 157 (1999) 169]. The agonist-induced locomotor behavior of D1 receptor transgenic mice is similar to baseline locomotor activity manifested by D2 receptor deficient mice [J. Neurosci. 18 (1998) 3470]. The similarity between these two behavioral phenotypes raised the possibility that stimulation of the over-expressed D1 receptors in the transgenic mice could cause a suppression of D2 receptor responses that manifest in hypokinesia. Alternatively, the similar phenotypes could result from altered D1/D2 receptor balance in both animal models. Two different approaches were undertaken to test these alternative hypotheses. (1) The effects of pharmacological blockade of D2 receptors on D1 agonist-stimulated hypokinesia of the D1 over-expressing animals were investigated. (2) The behavioral phenotype of hybrid D1 receptor over-expressing/D2 receptor deficient mice generated by crossbreeding the D2 knockout mice and the D1 transgenic animals was studied. The results of these studies suggested that the hypomotor response of the D1 transgenic mice was not a result of an interaction of the over-expressed D1 receptors with the native D2 receptors and that over-expressed D1 receptors likely mediate hypokinesia in the D1 transgenic animals. Considering the significance of the D1 dopamine receptor as a therapeutic target for Parkinson's disease, this D1 receptor over-expressing model provides an important experimental system to probe the basis for altered behavioral responses following stimulation of transgenetically up-regulated receptors.

Studies on the involvement of the dopaminergic system in the 5-HT2 agonist (DOI)-induced premature responding in a five-choice serial reaction time task

The present experiments investigated whether the enhanced premature (impulsive) responding induced by DOI, [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], a 5-HT2A/2C receptor agonist, is mediated by activation of the dopaminergic system and if this effect of DOI occurs in the nucleus accumbens. Therefore, the effects of a dopamine (D1/2) receptor antagonist given alone or combined with DOI were examined on the performance of rats in a five-choice serial reaction time (5-CSRT) task. Secondly, the effects of DOI in nucleus accumbens core and shell were studied, in order to find the target brain area for DOI-induced premature responding. The results indicate that DOI (0.1 mg/kg, subcutaneously) increases the number of premature responses, as found previously. alpha-Flupenthixol (0.03 mg/kg), a D1/2 dopamine receptor antagonist, and raclopride (0.015 mg/kg), a D2 receptor antagonist, attenuated the DOI-induced enhancement in premature responding. SCH 23390 (0.005 mg/kg), a selective D1 receptor antagonist with little affinity to 5-HT2 receptors totally blocked the effect of DOI. Those doses of DA antagonists did not significantly decrease premature responding when given alone. On the other hand, higher doses of all of these dopamine antagonists increased the number of omissions and decreased the number of ITI hole responses. In contrast to subcutaneous administration, direct injections of DOI (1, 3, and 10 microg bilaterally) to the nucleus accumbens shell or core had no effect on premature responding. These results suggest that the activation of the dopamine system mediates, at least in part, the effect of a 5-HT2 agonist on premature responding, but the nucleus accumbens is not the primary site for this action.

Effect of the dopamine D(1/5) antagonist SCH 23390 on the acquisition of conditioned fear

The authors previously reported that typical and atypical antipsychotic drugs inhibited the acquisition but not expression of conditioned fear. The present study examined the effects of the selective dopamine D(1/5) agonist (SKF 38393) and antagonist (SCH 23390) on the acquisition and expression of conditioned fear. Drugs were administered subcutaneously to male Sprague-Dawley rats 30 min before foot shock (2.5 mA for 5 min). Twenty-four hours after foot shock, rats were again placed and observed in the shock chamber without shocks (conditioned fear). Freezing behavior induced by conditioned fear, an index of anxiety or fear, was recorded using a time-sampling procedure. SCH 23390 (0.1-1 mg/kg) inhibited the acquisition of conditioned freezing. The administration of SCH 23390 at a dose of 0.1 mg/kg 30 min after foot shock did not affect conditioned freezing. Taken together, it is concluded that D(1/5) antagonism inhibits the acquisition of conditioned fear. SKF 38393 (3-20 mg/kg) failed to change the acquisition of conditioned fear. SCH 23390 or SKF 38393 administered prior to testing did not reduce the expression of conditioned fear. These results suggest that D(1/5) receptors may play a role in the development of fear or anxiety.

Structure and function of dopamine receptors

Dopamine (DA) is the most abundant catecholamine in the brain. The involvement and importance of DA as a neurotransmitter in the regulation of different physiological functions in the central nervous system (CNS) is well known. Deregulation of the dopaminergic system has been linked with Parkinson's disease, Tourette's syndrome, schizophrenia, attention deficit hyperactive disorder (ADHD) and generation of pituitary tumours. This review focuses on the pharmacological and biochemical features shared by the dopamine receptors. We address their coupling to secondary messenger pathways and their physiological function based upon studies using pharmacological tools, specific brain lesions and, more recently, genetically modified animal models.

Paradoxical locomotor behavior of dopamine D1 receptor transgenic mice

The behavioral effects of augmenting dopamine D1 receptor expression in the brain were investigated in mice incorporating additional copies of the mouse D1 receptor gene. Two transgenic lines showed increases in brain D1 receptor binding sites, which were greatest in extrastriatal regions. The full D1 agonist SKF 81297, when administered systemically to control animals, stimulated a dose-dependent increase in locomotor activity. In contrast, in D1 receptor overexpressing transgenic mice, this drug caused a marked suppression of locomotion due to a decrease in the frequency of movement initiation. Amphetamine and cocaine induced comparable locomotor activation in both transgenic animals and their control littermates. In the transgenic animals, D1 agonist-induced rearing and climbing behaviors were suppressed. However, on rotarod testing, the agonist-treated transgenic and control mice performed comparably, indicating that sensorimotor coordination was unaffected. These studies demonstrate that altering the levels of D1 receptor expression reverses the effects of D1 agonism on locomotor initiation and rearing.

The expression of neuropeptide-induced excessive grooming behavior in dopamine D1 and D2 receptor-deficient mice

Grooming behavior in rodents has long been related to dopamine receptors in the brain. However, the relative contribution of dopamine D1-like receptors (D1 and D5) and D2-like receptors (D2, D3 and D4) in this behavior has not been established yet. Spontaneous novelty-induced grooming (as assessed with a 30-min sampling test) was reduced in knockout mice lacking the dopamine D1, receptor. Furthermore, the intracerebroventricular (i.c.v.) injection of small quantities of oxytocin, prolactin or the adrenocorticotrophic hormone 1-24 fragment, ACTH-(1-24) was followed by a diminished level of novelty-induced excessive grooming. These neuropeptides caused a sustained increase in grooming level of control animals (wild type). Interestingly, the i.c.v. injection of beta-endorphin enhanced novelty-induced grooming to a level similar in control and knockout mice. The systemic administration of the dopamine D2 receptor antagonist, sulpiride did not suppress the residual grooming activity shown by animals injected with oxytocin, prolactin or ACTH-(1-24), and did not change the behavioral expression of those injected with beta-endorphin. In contrast, the systemic administration of the opioid receptor antagonist, naloxone, totally suppressed the residual grooming activity of oxytocin-, prolactin- or ACTH-(1-24)-injected mice and of those treated with beta-endorphin. In contrast with the behavioral deficit observed in dopamine D1 receptor-deficient mice, dopamine D2 receptor-null animals showed a normal expression of spontaneous novelty-induced grooming and a high level of grooming activity induced by i.c.v. injection of oxytocin, prolactin, ACTH-(1-24) or beta-endorphin. Again, the peripheral injection of naloxone was followed by a suppression of neuropeptide-induced excessive grooming in these animals. These data suggest that dopamine D1 receptors are involved in the expression of novelty-induced grooming in mice. In contrast, dopamine D2 receptors seem not to be important for the expression of this behavior. Furthermore, neuropeptide-enhanced grooming involves dopamine D1, but not dopamine D2 receptors. However, neurotransmitters other than dopamine (e.g., endorphins) may play a supplementary role in neuropeptide-enhanced grooming in mice.

Dopamine D1 and D2 receptor-mediated acute and long-lasting behavioral effects of glial cell line-derived neurotrophic factor administered into the striatum

To determine the differences in behavioral effects between intrastriatal and intracerebroventricular glial cell-derived neurotrophic factor (GDNF) administration, spontaneous locomotor activity was measured after intrastriatal or intracerebroventricular injection of GDNF (10 microg) in normal adult rats with implanted guide cannulae. In addition, the distribution of GDNF after intracerebral injection was studied immunohistochemically. Intrastriatal administration of GDNF significantly increased rearing behavior 3-4 h after injection. Increases in all three aspects of locomotor activity (motility, locomotion, and rearing) were most pronounced 3 days after intrastriatal injection, and they lasted for several days. This hyperactivity was blocked by the selective dopamine D1 receptor antagonist SCH22390 and by the selective D2 receptor antagonist raclopride at doses of the dopamine receptor antagonists, which by themselves did not affect spontaneous locomotor activity. These results suggest that GDNF has both acute and long-lasting pharmacological effects on dopamine neurons in adult animals and stimulates locomotor activity by activating both dopamine D1 and D2 receptors. On the other hand, intracerebroventricular administration of the same dose of GDNF failed to increase locomotor activity at any time during the test period (12 days). The immunohistochemical study demonstrated widespread distribution of GDNF in the entire body of the striatum within 24 h after intrastriatal injection. It also revealed deep penetration of GDNF from the ventricular space into the brain parenchyma after intracerebroventricular injection. GDNF-immunoreactive neuronal cell bodies were seen in the ipsilateral substantia nigra pars compacta most frequently 6 h after intrastriatal injection. The number of such cell bodies after intracerebroventricular administration, on the other hand, was much lower than that seen after intrastriatal administration. Taken together, these data suggest that intrastriatal administration of GDNF is an effective approach for affecting DA transmission. Long-lasting behavior effects are mediated via dopamine D1 and D2 receptors. Higher doses of GDNF would probably be needed using the intracerebroventricular route as compared to intraparenchymal delivery to exert effects on the nigrostriatal system in Parkinson's disease patients.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

D1 dopamine receptor-deficient mouse: cocaine-induced regulation of immediate-early gene and substance P expression in the striatum

Psychomotor stimulants such as cocaine alter gene expression in neurons of the striatum. Whereas many of these effects are mediated by D1 dopamine receptors, the involvement of other dopamine receptor subtypes or neurotransmitters is likely. To distinguish between these possibilities, regulation by cocaine of immediate-early genes and genes encoding neuropeptides was analysed in mice that lack functional D1 receptors. Gene expression was examined with in situ hybridization histochemistry. In these animals, cocaine failed to induce the immediate-early genes c-fos and zif 268. In contrast, substance P expression was abnormally increased by this drug. These results demonstrate that some of the effects of cocaine on gene regulation are mediated via D1 receptor-dependent mechanisms, as evidenced by the absence of immediate-early gene induction in D1-deficient mice, whereas others also involve additional, non-D1 receptor mechanisms, as shown for substance P expression.

Altered striatal function in a mutant mouse lacking D1A dopamine receptors

Of the five known dopamine receptors, D1A and D2 represent the major subtypes expressed in the striatum of the adult brain. Within the striatum, these two subtypes are differentially distributed in the two main neuronal populations that provide direct and indirect pathways between the striatum and the output nuclei of the basal ganglia. Movement disorders, including Parkinson disease and various dystonias, are thought to result from imbalanced activity in these pathways. Dopamine regulates movement through its differential effects on D1A receptors expressed by direct output neurons and D2 receptors expressed by indirect output neurons. To further examine the interaction of D1A and D2 neuronal pathways in the striatum, we used homologous recombination to generate mutant mice lacking functional D1A receptors (D1A-/-). D1A-/- mutants are growth retarded and die shortly after weaning age unless their diet is supplemented with hydrated food. With such treatment the mice gain weight and survive to adulthood. Neurologically, D1A-/- mice exhibit normal coordination and locomotion, although they display a significant decrease in rearing behavior. Examination of the striatum revealed changes associated with the altered phenotype of these mutants. D1A receptor binding was absent in striatal sections from D1A-/- mice. Striatal neurons normally expressing functional D1A receptors are formed and persist in adult homozygous mutants. Moreover, substance P mRNA, which is colocalized specifically in striatal neurons with D1A receptors, is expressed at a reduced level. In contrast, levels of enkephalin mRNA, which is expressed in striatal neurons with D2 receptors, are unaffected. These findings show that D1A-/- mice exhibit selective functional alterations in the striatal neurons giving rise to the direct striatal output pathway.

Lack of evidence for an involvement of nucleus accumbens dopamine D1 receptors in the initiation of heroin self-administration in the rat

The involvement of dopamine D1 receptor systems in the reinforcing properties of opiate reward was studied by examining the effect of the dopamine D1 antagonist SCH23390 on the initiation of heroin self-administration in rats. The D1 antagonist was administered daily systemically or locally in the nucleus accumbens (NAC), after which the animals were allowed to self-administer heroin (IV) in a 3-h session for 5 consecutive days. Systemic treatment with SCH23390 (0.17 and 0.5 mg.kg-1) significantly decreased heroin intake during initiation of heroin self-administration, while a dose of 0.06 mg.kg-1 was not effective. Local administration of SCH23390 (0.5 and 2.5 micrograms/site) in the NAC did not affect heroin intake. Both systemic and intra-accumbal administration of SCH23390 dose dependently decreased motor behavior measured in a small open field. The attenuation of heroin intake during initiation of heroin self-administration by blockade of dopamine D1 receptor systems may be due to a decrease in the reinforcing effects of heroin or more likely to a reduction in non-reinforcement-related behavior. The dopamine D1 receptors present in the NAC are probably not involved in opiate reward.

Dopamine D1 receptor family agonists, SK&F38393, SK&F77434, and SK&F82958, differentially affect locomotor activities in rats

Dopamine D1 receptor family agonists, 2,3,4,5,-tetrahydro-7,8-dihydroxy-1phenyl-1H-3-benzazepine (SK&F38393), 3-allyl-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine (SK&F77434), and 3-allyl-6-chloro-2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazep ine (SK&F82958), were compared for their behavioral effects on horizontal movement time, rearing time, stereotypy time, and thigmotaxis time. All agonists resulted in biphasic effects with attenuation followed by potentiation (0.01-10.0 mg/kg, SC). While SK&F38393 did not potentiate horizontal movement and rearing times, and had minor effects on thigmotaxis, SK&F77434 and SK&F82958 potentiated horizontal movement and rearing behaviors and attenuated thigmotaxis. The results were discussed in terms of the binding characteristics and current receptor theory.

Effects of intraaccumbens dopamine agonist SK&F38393 and antagonist SCH23390 on locomotor activities in rats

The present study examined the effects of the dopamine D1 and D5 subtype receptors agonist, R(+)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SK&F38393), and antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-ben zaz epine (SCH23390), on locomotor activities after bilateral microinjection into the nucleus accumbens (Acb). SK&F38393 (0.1-10.0 micrograms) significantly potentiated and SCH23390 (0.01-1.0 microgram) significantly attenuated locomotor activity as measured by horizontal distance in cm. The data were supportive of the hypothesis that dose-related locomotor activities induced by microinjections of SK&F38393 into the Acb are independently mediated by D1 and D5 subtype receptors.