Dopamine D-2 receptor agonist-induced behavioural depression: critical dependence upon postsynaptic dopamine D-1 function. A behavioural and biochemical study

Restless legs syndrome, neuroleptic-induced akathisia, and opioid-withdrawal restlessness: shared neuronal mechanisms?

Restlessness is a core symptom underlying restless legs syndrome (RLS), neuroleptic-induced akathisia, and opioid withdrawal. These three conditions also share other clinical components suggesting some overlap in their pathophysiology. Recent prospective studies demonstrate the frequent incidence of RLS-like symptoms during opioid withdrawal and supervised prescription opioid tapering. Based on the therapeutic role of µ-opioid receptor (MOR) agonists in the three clinical conditions and recent preclinical experimental data in rodents, we provide a coherent and unifying neurobiological basis for the restlessness observed in these three clinical syndromes and propose a heuristic hypothesis of a key role of the specific striatal neurons that express MORs in akathisia/restlessness.

Effect of Motor Impairment on Analgesic Efficacy of Dopamine D2/3 Receptors in a Rat Model of Neuropathy

Testing the clinical efficacy of drugs that also have important side effects on locomotion needs to be properly designed in order to avoid erroneous identification of positive effects when the evaluation depends on motor-related tests. One such example is the evaluation of analgesic role of drugs that act on dopaminergic receptors, since the pain perception tests used in animal models are based on motor responses that can also be compromised by the same substances. The apparent analgesic effect obtained by modulation of the dopaminergic system is still a highly disputed topic. There is a lack of acceptance of this effect in both preclinical and clinical settings, despite several studies showing that D2/3 agonists induce antinociception. Some authors raised the hypothesis that this antinociceptive effect is enhanced by dopamine-related changes in voluntary initiation of movement. However, the extent to which D2/3 modulation changes locomotion at analgesic effective doses is still an unresolved question. In the present work, we performed a detailed dose-dependent analysis of the changes that D2/3 systemic modulation have on voluntary locomotor activity and response to four separate tests of both thermal and mechanical pain sensitivity in adult rats. Using systemic administration of the dopamine D2/3 receptor agonist quinpirole, and of the D2/3 antagonist raclopride, we found that modulation of D2/3 receptors impairs locomotion and exploratory activity in a dose-dependent manner across the entire range of tested dosages. None of the drugs were able to consistently diminish either thermal or mechanical pain perception when administered at lower concentrations; on the other hand, the larger concentrations of raclopride (0.5–1.0 mg/kg) strongly abolished pain responses, and also caused severe motor impairment. Our results show that administration of both agonists and antagonists of dopaminergic D2/3 receptors affects sensorimotor behaviors, with the effect over locomotion and exploratory activity being stronger than the observed effect over pain responses.

Elevated dopamine D2 receptor in prefrontal cortex of CUMS rats is associated with downregulated cAMP-independent signaling pathway

Because depression is associated with significant morbidity and functional disability, it is important to reveal the mechanism of action. A variety of studies have suggested the involvement of dopaminergic receptors in the pathophysiological mechanism of non-stress-associated depression-like behavior in rodents. Nevertheless, controversy exists about whether chronic stress acts on dopaminergic receptors in the prefrontal cortex. Thus, we investigated the level of dopamine D2 receptors (DRD2) and the possible mechanisms involved in a chronic unpredictable mild stress (CUMS) rat model of depression. The results showed CUMS-induced, depression-like symptoms in the rat, characterized by reduced sucrose consumption and body mass, and increased duration of immobility in a forced swimming test. Moreover, chronic stress upregulated the expression of DRD2 but downregulated protein kinase A (PKA), transcription factor cAMP response element binding protein (CREB), and phospho-CREB (p-CREB) in the prefrontal cortex, as demonstrated by Western blot. Notably, in the rat model of depression, decreased cyclic adenine monophosphate (cAMP) levels and PKA activity were present at the same time, which is consistent with clinical findings in depressed patients. Our findings suggested that dopaminergic system dysfunction could play a central role in stress-related disorders such as depression.

Involvement of the brain histaminergic system in addiction and addiction-related behaviors: a comprehensive review with emphasis on the potential therapeutic use of histaminergic compounds in drug dependence

Neurons that produce histamine are exclusively located in the tuberomamillary nucleus of the posterior hypothalamus and send widespread projections to almost all brain areas. Neuronal histamine is involved in many physiological and behavioral functions such as arousal, feeding behavior and learning. Although conflicting data have been published, several studies have also demonstrated a role of histamine in the psychomotor and rewarding effects of addictive drugs. Pharmacological and brain lesion experiments initially led to the proposition that the histaminergic system exerts an inhibitory influence on drug reward processes, opposed to that of the dopaminergic system. The purpose of this review is to summarize the relevant literature on this topic and to discuss whether the inhibitory function of histamine on drug reward is supported by current evidence from published results. Research conducted during the past decade demonstrated that the ability of many antihistaminic drugs to potentiate addiction-related behaviors essentially results from non-specific effects and does not constitute a valid argument in support of an inhibitory function of histamine on reward processes. The reviewed findings also indicate that histamine can either stimulate or inhibit the dopamine mesolimbic system through distinct neuronal mechanisms involving different histamine receptors. Finally, the hypothesis that the histaminergic system plays an inhibitory role on drug reward appears to be essentially supported by place conditioning studies that focused on morphine reward. The present review suggests that the development of drugs capable of activating the histaminergic system may offer promising therapeutic tools for the treatment of opioid dependence.

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.

Differential behavioral responses to dopaminergic stimulation of nucleus accumbens subregions in the rat

The following experiments investigated the behavioral response to local microinfusion of dopamine (DA) and selective DA agonists into the core and shell subregions of the nucleus accumbens. Rats were implanted with chronic indwelling cannulae aimed at these subregions. Two experiments were conducted. In experiment 1, the response to DA (0, 2, 5, 10 microg/0.5 microl/side), the D-1 agonist SKF-82598 (0, 0.1, 1.0 microg), the D-2/3 agonist quinpirole (0, 1, 5, 15 microg) and the D-3 preferring agonist pramipexole (0.1, 1.0, 10.0 microg) was examined in photocell activity cages. Locomotor (horizontal) and rearing (vertical) activities were measured. DA and SKF-82958 induced relatively greater increases in activity following stimulation of the shell as compared with the core. Quinpirole induced a dose-dependent suppression of activity after infusion into both sites, although the core was more sensitive to the suppressive effect than the shell. Pramipexole induced time-dependent, biphasic effects that were small in magnitude and did not differentiate between site. In experiment 2, an observation procedure was used to record behaviors (locomotion, rearing, feeding, drinking). Dopamine (0, 2, 10 microg) elicited greater increases in rearing and feeding behavior in the shell than in the core. SKF-82958 (0, 0.75 microg) enhanced locomotion and rearing to a similar extent in both subregions in this test, whereas a mixture of a low dose (0.25 microg) of the D-1 and D-2 agonists selectively induced behavioral activation in the shell. In contrast to the results in the activity cage test, quinpirole (0, 1, 5 microg) increased motor activity at the lower dose when infused into the shell but not into the core. No alterations in feeding were observed following infusion of selective agonists, and no changes in drinking were found with any of the treatments. In summary, the shell appears to be relatively more sensitive to the motor activating effects of DA agonists than the core. Moreover, circuits associated with shell may be preferentially involved in feeding.

Morphine- and cocaine-induced conditioned place preference: effects of quinpirole and preclamol

The role of dopamine in opioid reward is unresolved. Furthermore, the issue is somewhat unclear regarding cocaine and the place preference paradigm. In the present study we investigated whether the drugs activating dopamine autoreceptors affect cocaine- and morphine-induced place preference in rats. Neither the dopamine D2/D3 receptor agonist, quinpirole (0.05 mg/kg, SC), nor the partial dopamine autoreceptor agonist, preclamol (2 or 8 mg/kg, SC), induced place conditioning by itself. Quinpirole had no significant influence on the place preference induced either by morphine (3 mg/kg, SC) or cocaine (5 mg/kg, IP). Preclamol, when given at the dose of 8 mg/kg SC, significantly attenuated the effect of cocaine but failed to modify the effect of morphine. Our results suggest that the rewarding properties of morphine involve DA-independent mechanisms whereas in the cocaine-induced reward the role of brain DA is critical. Furthermore, as regards place conditioning, we propose that the activation of DA autoreceptors is not sufficient to reliably modify the rewarding effect of cocaine.

Neurochemical bases of locomotion and ethanol stimulant effects

The locomotor stimulant effect produced by alcohol (ethanol) is one of a large number of measurable ethanol effects. Ethanol-induced euphoria in humans and locomotor stimulation in rodents, a potential animal model of human euphoria, have long been recognized and the latter has been extensively characterized. Since the euphoria produced by ethanol may influence the development of uncontrolled or excessive alcohol use, a solid understanding of the neurochemical substrates underlying such effects is important. Such an understanding for spontaneous locomotion and for ethanol's stimulant effects is beginning to emerge. Herein we review what is known about three neurochemical substrates of locomotion and of ethanol's locomotor stimulant effects. Several lines of research have implicated dopaminergic, GABAergic, and glutamatergic neurotransmitter systems in determining these behaviors. A large collection of work is cited, which strongly implicates the above-mentioned neurotransmitter substances in the control of spontaneous locomotion. A smaller, but persuasive, body of evidence suggests that central nervous system processes utilizing these transmitters are involved in determining the effects of ethanol on locomotion. Particular emphasis has been placed on the mesolimbic ventral tegmental area to nucleus accumbens dopaminergic pathway, and on the ventral pallidum/substantia innominata, where GABA and glutamate have been found to play a role in altering the activity of this dopaminergic pathway. Research on ethanol and drug locomotor sensitization, increased responsiveness to the substance with repeated administration, is also reviewed as a process that may be important in the development of drug addiction.

Dopamine agonists facilitate footshock-elicited locomotion in rats, and suppress level-press responding for food

Several dopamine agonists (apomorphine, quinpirole, 7-OH-DPAT, and U-91356A) suppressed locomotor activities of rats exploring a Y-maze, presumably through activation of dopamine autoreceptors. If brief electric shocks were applied to the grid floor during exploration, locomotion was unchanged in control rats, but the locomotor suppression from the dopamine agonists was converted to a profound stimulation. This locomotor stimulation was completely antagonized by pretreatment with sulpiride. SKF 38393 and clonidine produced no locomotor stimulation in the shock environment. To test whether the locomotor stimulant effect from dopamine agonists generalized to a food-reinforced behavior, rats were trained to lever-press for food according to a multiple (VI-10", VI-40") schedule. The above compounds only suppressed responding with no stimulation, and the suppressant effect on food-reinforced behavior was also blocked by sulpiride. It is concluded that the behavioral inhibitory effect from dopamine autoreceptor activation can be readily overcome by exteroceptive stimulation, which uncovers a powerful motor stimulant effect. This stimulant effect, however, did not generalize to lever-press responding for food.

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.

The role of serotonin in schizophrenia

The hypothesis that the LSD psychosis and by inference schizophrenic psychoses are related to dysfunctions in central serotonergic systems, formulated by Woolley and Shaw in the early 1950s was the first testable theory of modern biological psychiatry. Initially, it did not get the scientific attention it deserved. First, because LSD fell into disrepute and was to all intents and purposes banned from human experimentation. Secondly, the antipsychotics were discovered in the same period, and it became clear that these compounds block dopaminergic transmission and hence for many years thereafter the dopaminergic system occupied center stage in biological schizophrenia research. Presently, interest in the relation between serotonin and schizophrenia has been revived, due to the development of serotonin-blocking agents that appear to exert therapeutic effects in schizophrenia. In this paper the evidence for and against a link between serotonergic defects and schizophrenia psychopathology is critically discussed. The conclusion to be reached is threefold. (1) Interruption of certain serotonergic circuits represents an antipsychotic principle. (2) Tentative evidence suggests the involvement of serotonergic dysfunctions in the pathogenesis of schizophrenic psychoses. (3) It is not yet known whether serotonergic lesions contribute directly to the occurrence of schizophrenic psychopathology or via alterations in the dopaminergic system.

Effects of D1 and D2 agonists on spontaneous motor activity in MPTP treated mice

Two experiment were performed to study the effect of combining bromocriptine with SKF 38393 (SKF), or vice/versa, upon parameters of spontaneous motor activity in MPTP treated and saline (control) treated mice. Treatment with MPTP (2 x 40 mg/kg, subcutaneously) induced a hypoactive condition compared with saline treated mice. Bromocriptine (10 mg/kg, subcutaneously), administered to MPTP mice 2 hr, but not 1 or 4 hr, after SKF (6 mg/kg, subcutaneously) caused a marked increase in locomotion and rearing behaviour. The administration of bromocriptine (10 mg/kg, subcutaneously) 4 hr before SKF (6 mg/kg, subcutaneously) elevated all three parameters of spontaneous activity in the MPTP treated mice, independent of the injection of SKF. Bromocriptine injection 1 or 2 hr before SKF decreased locomotion in both MPTP and control mice. Neurochemical analysis confirmed the dopamine depletion in the MPTP treated mice. These results are discussed in terms of the reliability of the MPTP model of parkinsonism in mice and the dopamine D1/D2 receptor hypersensitivity following denervation with the neurotoxin.

Dopamine receptor antagonists block amphetamine and phencyclidine-induced motor stimulation in rats

d-Amphetamine (DEX) and phencyclidine (PCP) increased motor activity in rats as measured in automated activity cages. Analysis of the stimulation indicated that both drugs increased horizontal activity (total activity), locomotion, and peripheral activity. However, DEX increased while PCP decreased the incidence of rearing. The ability of different drugs to antagonise DEX- and PCP-induced increases in total activity (called stimulation) was measured. Dopamine (DA) D1 receptor antagonists (SCH23390, NNC-01-0112) were 7-8 times more potent in blocking DEX than PCP. DA D2 receptor antagonists (raclopride, remoxipride, haloperidol) were only 1-2 times more potent against DEX-induced stimulation. Nonselective DA receptor antagonists were also tested. Chlorpromazine was more potent against DEX than against PCP. Buspirone and sertindole were slightly more potent in blocking PCP than DEX. Ritanserin (5-HT2 receptor antagonist) was inactive against both stimulants. 8-OH-DPAT (5-HT1A receptor agonist) potentiated the stimulant effects of DEX and PCP. Prazosin (alpha 1-adrenergic receptor antagonist) partially blocked both DEX and PCP. Most drugs tested depressed spontaneous motor activity. Remoxipride and sertindole, however, caused very little depression even at doses several times higher than those needed to block DEX or PCP. The data show clear pharmacological differences between DEX- and PCP-induced stimulation.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Dopaminergic control of locomotion, mouthing, snout contact, and grooming: opposing roles of D1 and D2 receptors

The study compares the behavioral profiles induced in rats (N = 118) by the D2-dopaminergic receptor agonist quinpirole (0.03 and 0.5 mg/kg), and the D1-agonist SKF38393 (1.25-40 mg/kg), and both agonists administered together. Locomotion and snout contact frequency were reduced by the low but increased by the high dose of quinpirole; SKF38393 also reduced these behaviors and attenuated the effect of the high quinpirole dose. Only the high dose of quinpirole increased the duration of snout contact bouts and the frequency of mouthing; SKF38393 had no effect but in combination with the high dose of quinpirole, it enhanced the performance of these behaviors greatly. The duration of mouthing bouts was not affected by either agonist but was greatly extended when SKF38393 was administered together with the high dose of quinpirole. Grooming was inhibited by both the low and the high dose of quinpirole, and stimulated by the injection of SKF38393 or its addition to the low dose of quinpirole. These findings suggest that snout contact is controlled by modulating the frequency of episodes whereas mouthing is controlled by modulating the duration of episodes. Moreover, although they do not disprove the prevailing notion of D1-D2 receptor synergism, the present data are consistent also with an oppositional model of D1-D2 receptor interaction in the regulation of locomotion, snout contact, mouthing, and grooming in intact animals.

Locomotor-activating effects of the D2 agonist bromocriptine show environment-specific sensitization following repeated injections

Biphasic effects of bromocriptine (2.0, 5.0, 10.0, and 20.0 mg/kg IP) on locomotion were quantified in photocell activity boxes in rats. Following early suppression of activity, bromocriptine produced a clear, dose-dependent increase in locomotion that lasted several hours. When a low dose of bromocriptine (5.0 mg/kg) was administered daily over a 3-week period, the locomotor-activating effects of the drug showed progressive enhancement over days. The sensitization was environment specific; rats administered bromocriptine six times in the home cage showed no sign of a sensitized response to bromocriptine when subsequently tested in the activity box. Thus, selective stimulation of D2 receptors stimulates locomotion and sensitizes animals to subsequent injections, just as do the indirect-acting dopamine agonists cocaine and amphetamine.

In vivo assessment of release and metabolism of dopamine in the ventrolateral striatum of awake rats following administration of dopamine D1 and D2 receptor agonists and antagonists

The ability of specific dopamine (DA) receptor agonists and antagonists to modify the release and metabolism of DA in the ventrolateral striatum of awake rats was assessed using in vivo microdialysis. The specific DA D2 receptor antagonist, raclopride (0.1, 0.5 and 2.0 mg/kg, i.p.), dose-dependently increased release of DA and levels of the metabolites DOPAC and HVA, while the D2 receptor agonist, quinpirole (0.03, 0.1 and 0.3 mg/kg), decreased levels of DA, DOPAC and HVA. The DA D1 receptor antagonist, SCH23390 [(R + (+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl- 1H-3-benzazepin-7-ol) (0.01, 0.05 and 0.25 mg/kg), produced an increase in DA, DOPAC and HVA but of a lesser magnitude than raclopride. The D1 agonist SKF38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol) (1.0, 3.0 and 10.0 mg/kg) failed to affect the release of metabolism of DA at any dose. These results support previous findings that activation of D2 receptors has greater control over in vivo DA function, than drugs specifically affecting D1 receptors.

A scaling approach to find order parameters quantifying the effects of dopaminergic agents on unconditioned motor activity in rats

1. Three experiments were conducted in the Behavioral Pattern Monitor (BPM) to assess the effects of the D1 agonist SKF-38393, the D2 agonist quinpirole, and the interaction of the D2 antagonists haloperidol with amphetamine or cocaine on the amount, the structure, and the unpredictability of micro-events of rat exploratory behavior. 2. SKF-38393 (0.3, 1.0, 3.0, and 10.0 mg/kg) did not change the amount of motor behavior indicated by the temporal scaling exponent alpha, a descriptor of the local degree of acting, during a 60 min exposure in the BPM. However, SKF-38393 (3.0, and 10.0 mg/kg) significantly increased the spatial scaling exponent d, indicating an increased component of local circumscribed movements. 3. Quinpirole (0.03, 0.1, 0.3, and 1.0 mg/kg) produced a biphasic dose response with respect to the amount of motor behavior. Low doses (0.03, 0.1) significantly decreased the local degree of acting, whereas alpha returned to control group levels for higher doses (0.3, 1.0 mg/kg). The change in activity was accompanied by a significant increase of local movements, i.e. d was increased for the lower doses. 4. Haloperidol (15.0 micrograms/kg) reduced a slightly increased d measure for amphetamine (1.0 mg/kg) treated animals and increased a significantly reduced d for cocaine (20.0 mg/kg) treated animals, without affecting the increases of motor activity induced by both treatments. 5. It is concluded that the structure of motor activity provides an important measure of unconditioned motor behavior, which can be affected independently of the typically measured amount of motor activity.

Anticonvulsant action of SCH 23390 in the striatum of the rat

This study investigates the role of forebrain D1 receptors in the motor expression of seizures induced by pilocarpine. Conscious rats receiving bilateral intracaudate injections of saline, just failed to convulse to 200 mg/kg pilocarpine, but responded vigorously to 600 mg/kg of the cholinomimetic. LY 171555 significantly protected rats against 600 mg/kg pilocarpine, when delivered into the anterior striatum, as also did SCH 23390, from all rostrocaudal levels of the striatum. Intrastriatal SKF 38393 or CY 208-243 neither facilitated nor ameliorated pilocarpine-induced convulsions. SCH 23390 was also anticonvulsant from the nucleus accumbens, while intra-accumbens CY 208-243 was without effect. It is concluded that SCH 23390 affords protection against pilocarpine-induced limbic motor seizures by blocking the effects of endogenous dopamine released tonically onto D1 receptors in the corpus striatum and nucleus accumbens. The inability of additional D1 receptor stimulation to intensify such seizures, could indicate that forebrain D1 receptors are already maximally stimulated by the endogenous transmitter.

Lack of a dopamine autoreceptor selective profile of B-HT 920 in functional in vitro model systems of D2 receptors in rat striatum

Based on the results of in vivo studies, the thiazoloazepine derivative B-HT 920 has been proposed to be a selective agonist of dopamine autoreceptors. In the present study, we investigated the effects of B-HT 920 in two functional in vitro model systems of D2 receptors and compared these effects with the effects of the classical D2 agonist LY 171555. B-HT 920 and LY 171555 concentration dependently inhibited the electrically evoked release of radiolabeled dopamine and acetylcholine and the forskolin-induced stimulation of adenylate cyclase activity in rat striatal tissue slices with comparable efficacies. In striatal tissue slices prepared after 6-hydroxydopamine-induced destruction of dopaminergic terminals, both drugs were still able to inhibit forskolin-stimulated adenylate cyclase activity with a efficacy similar to that in tissue obtained from unlesioned rats. It is concluded that, in vitro, B-HT 920 is an agonist at both presynaptic and 'normosensitive' postsynaptic D2 receptors showing relatively high intrinsic activity.

Behavioural, biochemical and electrophysiological studies on the motor depressant and stimulant effects of bromocriptine

Bromocriptine (BRC) produced a biphasic behavioural effect in mice; an early depressant phase which lasted for about 1 h and a later stimulant phase which lasted from about 1 to 5 h. The stimulation was blocked with SCH23390. Both phases of activity were accompanied by marked striatal DA autoreceptor effects as indicated by reductions in dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels and by a reduction in the accumulation of DOPA (after inhibition of nigrostriatal DA nerve firing and DOPA decarboxylase). However, while the autoreceptor effects were still evident during the behavioural stimulant phase, there was a gradual rise in DOPAC and HVA from 1 to 4 h after injection, indicating a gradually increasing DA turnover. We were unable, using a variety of behavioural and biochemical paradigms, to demonstrate any change in DA autoreceptor sensitivity after one dose of BRC. In electrophysiological studies, however, it was found that prior exposure of rats to one dose of BRC rendered them subsensitive to the rate-inhibiting effects of a second dose of BRC, as measured in anaesthetized animals using extracellular single cell recordings of identified DA neurons in the substantia nigra pars compacta. It is concluded firstly, that the stimulant phase of BRC in mice occurs despite continued occupation of the DA autoreceptors by BRC because adequate endogenous DA is available to provide the required D1 receptor stimulation and secondly, that the terminal autoreceptors in the striatum (as assessed in mice using biochemical techniques) may be regulated differently to the somatodendritic autoreceptors (as assessed electrophysiologically in rats).

Seizure promotion and protection by D-1 and D-2 dopaminergic drugs in the mouse

Mice injected with pilocarpine (100-400 mg/kg plus 1 mg/kg methylscopolamine), picrotoxin (0.75-6 mg/kg) or strychnine (0.75-6 mg/kg) exhibited clonic or clonic/tonic convulsions. Pretreatment with the D-1 agonist CY 208-243 (0.375-1.5 mg/kg) dose-dependently potentiated the convulsions elicited by 100 mg/kg pilocarpine, but had neither a convulsant nor anticonvulsant effect in mice receiving picrotoxin (3 or 6 mg/kg) or strychnine (0.75 or 1.5 mg/kg). This facilitatory effect of CY 208-243 was abolished by the D-1 antagonist SCH 23390 (0.2 mg/kg). SCH 23390 by itself (0.05-0.8 mg/kg) dose-dependently protected mice against pilocarpine (400 mg/kg) seizures. Stimulating D-2 receptors with LY 171555 (0.167-4.5 mg/kg) dose-dependently protected mice against seizure activity induced by pilocarpine, but neither protected nor sensitised mice given picrotoxin or strychnine. The neuroleptics haloperidol (1-4 mg/kg), sulpiride (10-50 mg/kg), metoclopramide (1.25-6.25 mg/kg), thioridazine (0.5-2 mg/kg) and clozapine (0.5-2 mg/kg) had no effect on the seizure threshold to 100 mg/kg pilocarpine by themselves, although 10 mg/kg thioridazine and clozapine caused 100% convulsions, possibly through a toxic action. When administered in conjunction with a minimally effective quantity of CY 208-243 (0.375 mg/kg), however, all five neuroleptics interacted synergistically with the D-1 agonist to promote convulsions to pilocarpine (100 mg/kg). No such interaction occurred between submaximally protective doses of the D-1 blocker SCH 23390 (0.05 and 0.2 mg/kg) and a wide range of doses of the D-2 stimulant LY 171555 (0.167-4.5 mg/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

D-2 dopamine autoreceptor selective drugs: do they really exist?

The catecholamine dopamine plays an important role as a neurotransmitter or neurohormone in the brain and pituitary gland. Dopamine exerts its effects through activation of two types of receptors called D-1 and D-2. These receptors are distinguished by their different pharmacological characteristics and signal transduction mechanism(s). Release of dopamine inhibits the activity of dopaminergic neurons through activation of so-called dopamine autoreceptors which are of the D-2 type. In general, these receptors occur both in the soma-dendritic region of the dopaminergic neuron, where they are involved in the inhibition of the firing rate and on the dopaminergic terminals where they mediate the inhibition of dopamine synthesis and release. D-2 receptors occur also on the target cells of dopaminergic neurons both in the brain (postsynaptic D-2 receptors) and pituitary gland. On the basis of data gathered from in vivo (behavioral- as well as electrophysiological) studies it has been concluded that D-2 agonists are much more potent at dopamine autoreceptors as compared to postsynaptic D-2 receptors, indicating the possibility of a pharmacological distinction between these differentially located D-2 receptors. This concept led to the introduction of a whole group of drugs allegedly displaying a selective agonist profile at the dopamine autoreceptor. In contrast, biochemical (in vitro) studies with brain tissue as well as the pituitary gland, did not reveal any significant difference between the pharmacological profiles of autoreceptors and postsynaptic D-2 receptors. In the present minireview a balanced discussion is presented of these in vivo and in vitro findings and it is concluded that both autoreceptors as well as postsynaptic D-2 receptors are similar if not identical entities.

Motor depression: a new role for D1 receptors?

The aims of this study were two-fold. Firstly, to characterize the behavioral properties of a potential new dopamine D1 receptor agonist, (-)-4,6,6a,7,8,12b-hexahydro-7-methyl-indolo[4,3-ab]phenanth ridine (CY 208-243), to determine its suitability as a tool for investigating D1 receptor function in vivo. Secondly, to investigate how the behavioural properties of D1 agonists are modified in the presence of D2 receptor blocking drugs. For this purpose, using mice, we employed CY 208-243 and 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride (SKF 38393) as reference D1 agonists, and the substituted benzamides metoclopramide and sulpiride as selective D2 antagonists. CY 208-243 (0.25-10 mg/kg) caused only a modest increase in grooming in non-habituated mice, but stimulated locomotion, rearing, grooming and orofacial activities in habituated animals. These responses were inhibited by a D1 antagonist, but not by D2 antagonists, suggesting CY 208-243 behaves as a selective agonist of D1 receptors in vivo. In non-habituated mice, doses of metoclopramide and sulpiride which had little or no effect on motor behaviour by themselves, interacted synergistically with CY 208-243 (4 mg/kg) and SKF 38393 (30 mg/kg) to cause extended periods of immobility. Other species-typical behaviours were not affected in this way. For example, grooming was decreased by metoclopramide and increased by sulpiride, indicating that an increase in behavioural competition from this parameter was not the cause of the hypokinesia. To explain the apparent ability of D1 receptor stimulation to increase exploratory activity in earlier experiments and to decrease it here, it is proposed that this behaviour is regulated by D1 receptors coupled to two functionally opposite postsynaptic D2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)