Effects of D(1) receptor agonist SKF 38393 on male rat sexual behavior and postcopulatory departure in the goal compartment-runway paradigm

Male rats were tested in an apparatus in which the goal compartment of the runway was connected with the start compartment by a one-way door. In this apparatus, the male spontaneously left the goal compartment containing the estrous female after a mount bout (the cluster of one or more copulatory events) and a new run started. This effect (the postcopulatory departure) seems to be due to the competition between the incentive value of the stimulus female and the runway. The dopamine D(1) receptor agonist SKF 38393 at the doses of 1-5 mg/kg sc significantly prolonged the time spent by the male in the goal compartment, while both run latency and run duration remained unaffected. Further analysis showed that the prolongation of the time in goal compartment results from increased duration of copulatory behavior accompanied by increased number of copulatory events. Although SKF 38393 did not influence the latency of postcopulatory departures, incomplete departures occurred in five out of eight males. In the free access to female conditions, the SKF 38393 did not influence copulatory performance, except for a reduction in the number of intromissions at the dose of 1 mg/kg sc. On the other hand, a significant reduction of postejaculatory ultrasonic vocalization was observed.

Striatal dopaminergic markers in dementia with Lewy bodies, Alzheimer's and Parkinson's diseases: rostrocaudal distribution

Dementia with Lewy bodies (DLB) is a neuropsychiatric disease associated with extrapyramidal features which differ from those of Parkinson's disease, including reduced effectiveness of L-dopa and severe sensitivity reactions to neuroleptic drugs. Distinguishing Alzheimer's disease from DLB is clinically relevant in terms of prognosis and appropriate treatment. Dopaminergic activities have been investigated at coronal levels along the rostrocaudal striatal axis from a post-mortem series of 25 DLB, 14 Parkinson's disease and 17 Alzheimer's disease patients and 20 elderly controls. [(3)H]Mazindol binding to the dopamine uptake site was significantly reduced in the caudal putamen in DLB compared with controls (57%), but not as extensively as in Parkinson's disease (75%), and was unchanged in Alzheimer's disease. Among three dopamine receptors measured (D1, D2 and D3), the most striking changes were apparent in relation to D2. In DLB, [(3)H]raclopride binding to D2 receptors was significantly reduced in the caudal putamen (17%) compared with controls, and was significantly lower than in Parkinson's disease at all levels. D2 binding was significantly elevated at all coronal levels in Parkinson's disease compared with controls, most extensively in the rostral putamen (71%). There was no change from the normal pattern of D2 binding in Alzheimer's disease. The only significant alteration in D1 binding ([(3)H]SCH23390) in the groups examined was an elevation (30%) in the caudal striatum in Parkinson's disease. There were no differences in D3 binding, measured using [(3)H]7-OH-DPAT, in DLB compared with controls. A slight, significant decrease in D3 binding in the caudal striatum of Parkinson's disease (13%) patients and an increase in Alzheimer's disease (20%) in the dorsal striatum at the level of the nucleus accumbens were found. The concentration and distribution of dopamine were disrupted in both DLB and Parkinson's disease, although in the caudate nucleus the loss of dopamine in DLB was uniform whereas in Parkinson's disease the loss was greater caudally. In the caudal putamen, dopamine was reduced by 72% in DLB and by 90% in Parkinson's disease. The homovanillic acid : dopamine ratio, a metabolic index, indicated compensatory increased turnover in Parkinson's disease, which was absent in DLB despite the loss of substantia nigra neurons (49%), dopamine and uptake sites. These differences between DLB, Parkinson's disease and Alzheimer's disease may explain some characteristics of the extrapyramidal features of DLB and its limited response to L-dopa and severe neuroleptic sensitivity. The distinct changes in the rostrocaudal pattern of expression of dopaminergic parameters are relevant to the interpretation of the in vivo imaging and diagnosis of DLB.

Dopaminergic activities in the human striatum: rostrocaudal gradients of uptake sites and of D1 and D2 but not of D3 receptor binding or dopamine

The human striatum, which receives dopaminergic innervation from the substantia nigra and ventral tegmental area (cell groups A8, A9 and A10), has structural and functional subdivisions both rostrocaudally and dorsoventrally. These relate to motor and non-motor origins of cortical projections and the specific areas of the substantia nigra and ventral tegmental area providing dopaminergic innervation. In the present study, we have evaluated the distribution of a number of dopaminergic parameters in the caudate, putamen and nucleus accumbens at separate coronal levels in a post mortem study in a series of elderly normal individuals aged 55-94 years, with analysis of the effect of post mortem variables. Dopamine D1 receptor density displayed a rostrocaudally declining gradient in the putamen but not in the caudate, such that at levels posterior to the anterior commissure, there was significantly lower D1 binding in the putamen compared to the caudate. The density of dopamine D2 receptors was similar in the putamen and caudate, increasing rostrocaudally. The density of dopamine uptake sites exhibited an increasing rostrocaudal gradient in the caudate, especially ventrally, but not in the putamen, where binding was more constant. The dopamine D3 receptor was concentrated in the ventral striatum, particularly the nucleus accumbens, although there was no evidence of a rostrocaudal gradient. With respect to striosome-matrix compartmentalization, there was no complete segregation, although D1 and D3 receptors were concentrated in striosomes, whereas D2 receptors and uptake sites showed higher density in the matrix. Levels of dopamine were similar in the caudate and putamen, and were significantly elevated at levels including the nucleus accumbens and the anterior commissure. Homovanillic acid and the metabolic index (homovanillic acid/dopamine ratio) were significantly higher in the putamen compared to the caudate, especially at levels from and caudal to the anterior commissure. These distributions of dopamine receptors and metabolic indicators, reflecting the different functional domains of the striatum, are relevant to the interpretation of current in vivo imaging of the dopamine transporter and receptors in neurological and psychiatric disorders. They provide information to assist in the detection of perturbations in expression, in specific diseases, at particular points on rostrocaudal, lateromedial and dorsoventral axes, a level of resolution beyond current neuroimaging capability.

Timing: A critical determinant of the functional consequences of neonatal 6-OHDA lesions

Previous data have indicated that intrastriatal (IS) lesions of the dopamine (DA) system early in development result in a selective effect on D1 receptor expression and sensitivity, which is not seen with adult lesions or lesions made later in development. The purpose of the present study was to test the hypothesis that the timing of the lesion is a critical determinant of the consequences of DA depletion during development. Rats received IS injections of 6-hydroxydopamine (6-OHDA) on day of birth/postnatal day 1 (P0/1) or P7, which resulted in similar decreases in the number of DA uptake sites (> or =70% loss), a measure of DA terminal density. As adults, lesioned rats were challenged with DA receptor agonists to examine the functional sensitivity of D1 and D2 receptors. In adulthood, P0/1-lesioned rats exhibited increases in oral dyskinesias and rearing behavior following treatment with the partial D1 receptor agonists, SKF38393 and SKF77434, whereas rats lesioned on P7 exhibited increases in grooming. P7-lesioned rats also exhibited increases in gnawing, explosive jumping, and self-biting behavior following treatment with the full D1 receptor agonist SKF82958, which were not observed in the other groups. The results support the hypothesis that the timing of DA denervation is of paramount importance for governing the functional consequences of neonatal lesions, as measured by the incidence of DA agonist-induced behaviors in adulthood.

Effects of SCH-23390 on dopamine D1 receptor occupancy and locomotion produced by intraaccumbens cocaine infusion

This study examined the effects of both systemic and intraaccumbens administration of SCH-23390 in rats on dopamine D1 receptor occupancy and on locomotor activity produced by intraaccumbens infusion of cocaine. In experiment 1, rats received SCH-23390 (0-1 mg/kg, i.p.) 15 minutes prior to intraaccumbens infusion of cocaine (0 or 100 microg/side). In experiment 2, rats received coinfusion of SCH-23390 (0-1 microg/side) and cocaine (0 or 100 microg/side) into the nucleus accumbens (NAc). After behavioral testing, receptors occupied by SCH-23390 were quantified by injecting animals with their respective dose of SCH-23390, followed by a systemic injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Receptors occupied by SCH-23390, and therefore protected from EEDQ-induced inactivation, were quantified from autoradiograms of sections labeled with 3H-SCH-23390. Systemic administration of SCH-23390 dose-dependently (0.1-1.0 mg/kg) reversed cocaine-induced locomotion and occupied 72-100% of D1-like receptors in the anterior NAc. D1 receptor occupancy following systemic administration of SCH-23390 was evident as an inverted U-shaped, dose-dependent change, with the greatest occupancy observed at the intermediate dose of 0.3 mg/kg. Intraaccumbens infusion of SCH-23390 did not alter cocaine-induced locomotor activity despite occupying 40-60% of D1-like receptors in the anterior NAc core and shell. The findings that systemic, but not intraaccumbens, administration of SCH-23390 potently reversed locomotion produced by intraaccumbens cocaine infusion suggest that stimulation of D1 receptors in regions other than the NAc is involved in locomotion produced by intraaccumbens infusion of cocaine, and that stimulation of D1 receptors in the NAc is not necessary for this behavior.

D1/D2 dopamine receptor interaction in membrane abolished by 6-hydroxydopamine lesion

D1/D2 interaction in rat striatum was investigated by examining the effect of the D2 antagonist spiperone on the binding of [3H]SCH23390 to D1 dopamine (DA) receptors. In the presence of endogenous DA, spiperone blocked D2 receptors, then caused the increase of the binding of [3H]SCH23390 in rat striatal homogenate. After the 6-hydroxydopamine (6-OHDA) lesion, the increase was not found even if in the addition of exogenous DA. The results suggest that the D2 antagonist can modify the interaction between endogenous DA and D1 receptors labeled with [3H]SCH23390, while 6-OHDA lesion may change the state of D1/D2 interaction operating at the receptor level.

Dopamine receptors: from structure to function

The diverse physiological actions of dopamine are mediated by at least five distinct G protein-coupled receptor subtypes. Two D1-like receptor subtypes (D1 and D5) couple to the G protein Gs and activate adenylyl cyclase. The other receptor subtypes belong to the D2-like subfamily (D2, D3, and D4) and are prototypic of G protein-coupled receptors that inhibit adenylyl cyclase and activate K+ channels. The genes for the D1 and D5 receptors are intronless, but pseudogenes of the D5 exist. The D2 and D3 receptors vary in certain tissues and species as a result of alternative splicing, and the human D4 receptor gene exhibits extensive polymorphic variation. In the central nervous system, dopamine receptors are widely expressed because they are involved in the control of locomotion, cognition, emotion, and affect as well as neuroendocrine secretion. In the periphery, dopamine receptors are present more prominently in kidney, vasculature, and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. Numerous genetic linkage analysis studies have failed so far to reveal unequivocal evidence for the involvement of one of these receptors in the etiology of various central nervous system disorders. However, targeted deletion of several of these dopamine receptor genes in mice should provide valuable information about their physiological functions.

Antagonistic effects of isofloxythepin on dopamine D1 and D2 receptors and behaviors in rats

1. In vitro, isofloxythepin competed for the binding of [3H]SCH 23390 to striatal D1 receptors and for the binding of [3H]spiperone to striatal D2 receptors with IC50 values of 6.1 (+/- 1.2) x 10(-10)M and 8.4 (+/- 2.6) x 10(-10)M, respectively. Isofloxythepin further inhibited the D1 receptor-mediated, dopamine-stimulated adenylate cyclase in the striatal tissue. 2. Fifteen hours after rats were injected with a single dose of isofloxythepin (1 mg/kg, SC), the ex vivo binding curve of [3H]spiperone to striatal D2 receptors was markedly inhibited, whereas the binding curve of [3H]SCH 23390 to striatal D1 receptors was unaffected. 3. Fifteen hours after isofloxythepin pretreatment, D1 agonist SKF 38393 (15 mg/kg, IP)-stimulated grooming behavior was not affected, whereas the D2 agonist quinpirole (3 mg/kg, IP)-stimulated stereotyped behavior was completely abolished. 4. On the basis of the findings from in vivo studies, we conclude that, although isofloxythepin is found to have high affinity for both D1 and D2 receptors in vitro, it lacks D1 antagonistic potency in vivo.

Compartmentally specific effects of quinpirole on the striatal Fos expression induced by stimulation of D1-dopamine receptors in intact rats

Injections of the full D1-agonist A-77636 (1.45 mg/kg) were found to induce clear Fos-like immunoreactivity (FLI) in the striatum of neurologically intact rats. Pretreatment with the D2-like agonist quinpirole (3 mg/kg) potentiated staining in the lateral striatum, but actually decreased the number of immunoreactive cells observed in the medial portion of the rostral striatum. Comparison with adjacent sections processed for the calcium binding protein calbindin, indicated that quinpirole pretreatment specifically suppressed staining in the matrix compartment of the striatum while tending to potentiate it in the striosomes, resulting in an extremely patchy pattern of labeling. These results suggest that exogenous stimulation of D2-receptors, although not essential for the induction of FLI, may play an important role in the compartmental patterning of neuronal activity within the striatum.

Comparison of the effects of prototypical behavioral stimulants on locomotor activity and rotational behavior in rats

The present study was performed to characterize on rotational behavior the dose- and time-effect relationship of four prototypical behavioral stimulants that interact with dopamine systems via different mechanisms of action. Drug effects on rotational behavior was compared with effects on locomotor activity. The drugs examined were apomorphine (0.03-1.0 mg/kg), d-amphetamine (0.1-3.0 mg/kg), cocaine (3.0-56 mg/kg), and caffeine (10-100 mg/kg). SKF-38393 (0.3-10 mg/kg), a dopamine receptor agonist that has only modest effects on locomotor activity, was tested as a comparison. In rats with unilateral 6-hydroxydopamine (6-OHDA)-induced lesions of the nigrostriatal tract, d-amphetamine and cocaine dose dependently increased both the duration and the maximum number of turns/10 min, whereas apomorphine and caffeine increased only the duration of turning. There was a significant correlation of the effects of the four drugs on rotational behavior with effects on locomotor activity, but effects across drugs were not identical. Dose-response curves revealed potency differences among drugs in their effects on the two behaviors (e.g., apomorphine stimulated rotational behavior at a lower dose than it stimulated locomotor activity, whereas the converse was true with caffeine). Different mechanisms of action of these drugs might account for the differences in their effects on these behaviors.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

Behavioural pharmacology of 'D-1-like' dopamine receptors: further subtyping, new pharmacological probes and interactions with 'D-2-like' receptors

1. D-1 receptors are now recognised to play a critical psychopharmacological role in the regulation of unconditioned motor and numerous other aspects of behaviour. 2. There appears to exist a broad family of 'D-1-like' receptors in terms both of differential coupling to distinct messenger/transduction mechanisms and of gene cloning, whose behavioural roles remain to be clarified. 3. The adenylyl cyclase-inhibiting benzazepine SK&F 83959 induces behavioural responses in rats that are similar to those induced by the full efficacy cyclase-stimulating isochroman A 68930 but not to those induced by its high efficacy partial agonist benzazepine congener R-6-Br-APB; these data indicate roles for individual 'D-1-like' receptors in mediating distinct elements of dopaminergic behaviour. 4. The putative D-1 autoreceptor agonist B-HT 920 and the putative D-3 agonist 7-OH-DPAT demonstrate different behavioural profiles when given both alone and in combination with the selective 'D-1-like' antagonist BW 737C; D-3 receptors may participate in cooperative/synergistic but not in oppositional 'D-1-like': 'D-2-like' interactions. 5. Such interactions apparent at the level of behaviour are complemented by evidence for similar interactions at numerous alternative levels of function, though these may differ between rodent and primate species. 6. A broader range of more selective agonists and antagonists, able to distinguish between individual members of the 'D-1-like' and of the 'D-2-like' receptor families are needed to clarify these issues.

Glutamate/dopamine D1/D2 balance in the basal ganglia and its relevance to Parkinson's disease

The recent availability of selective ligands for NMDA and AMPA receptors has enabled neuroscientists to test the hypothesis that Parkinson's disease is a glutamate hyperactivity disorder and hence treatable with glutamate antagonists. This review takes a critical look at the motor characteristics of this new class of drugs in rodent and primate models of parkinsonism and assesses the clinical potential and pitfalls of this radical new approach. Monotherapy of Parkinson's disease with glutamate antagonists appears impractical at the present time, due to their low efficacy and unacceptable side effects, but polypharmacy with L-DOPA and a glutamate antagonist as adjuvant is a more realistic prospect. This review will focus on the ways in which glutamate receptor blockade facilitates motor recovery with L-DOPA and will examine whether the basis for this beneficial effect can be traced to a specific interaction with dopamine at D1 or D2 receptors, and therefore to discrete motor pathways within the basal ganglia.

Differential effects of low versus high doses of apomorphine on retinal dopamine metabolism in light- and dark-adapted rabbits

Previous electrophysiologic results from this laboratory indicate that apomorphine exerts a differential dose-related effect on rabbit electroretinograms, with low doses increasing the b-wave and higher doses decreasing this parameter. Results were interpreted as reflecting apomorphine's agonistic properties at two different receptors: 1.0 mg/kg acting at the postsynaptic site, and the lower dose, 0.01 mg/kg, preferentially stimulating inhibitory autoreceptors. The purpose of this experiment was to investigate further this hypothesis by determining retinal levels of dopamine, dihydroxyphenylacetic acid, and homovanillic acid in retinas of light- or dark-adapted rabbits treated with saline, 1.0, 0.1, or 0.01 mg/kg apomorphine intravenously. Results indicate that in dark-adapted rabbits only the highest dose tested, 1.0 mg/kg, decreased dopamine concentrations. In animals exposed to light, the lowest dose tested, 0.01 mg/kg, significantly reduced dopamine and metabolite levels, whereas the highest dose unexpectedly increased retinal dopamine turnover. Results are discussed in terms of receptor sites and the influence of lighting conditions.

Potentiation of dopamine-dependent locomotion by clonidine in reserpine-treated mice is restricted to D2 agonists

Mice treated with reserpine (5 mg/kg IP), 24 h beforehand, were completely akinetic. Fluent locomotion was reinstated with the D1-selective agonist SKF 38393 (3-30 mg/kg IP), the D2-selective agonist RU 24213 (0.5-5 mg/kg SC) and the mixed D1/D2 agonist apomorphine (0.025-0.5 mg/kg SC). Clonidine (0.03125-1 mg/kg IP) caused a dose-dependent sedation in dopamine-intact mice, but had no effect by itself on the locomotor activity of monoamine-depleted mice. In drug interaction experiments, clonidine did not modify the motor stimulant action of SKF 38393, but greatly enhanced the motor responses to RU 24213 and apomorphine. These results support the hypothesis that alpha-adrenoceptor agonists facilitate dopamine D2 but not dopamine D1 motor responding in the reserpine-treated mouse model of Parkinson's disease.

Long-term treatment with low doses of the D1 antagonist NNC 756 and the D2 antagonist raclopride in monkeys previously exposed to dopamine antagonists

Eight Cebus apella monkeys previously exposed to D1 and D2 antagonists were treated subcutaneously for 8 weeks with the D1 antagonist NNC 756 (0.01 mg/kg), followed by a wash-out period of 4 weeks and treatment with the D2 antagonist raclopride for 8 weeks (end doses 0.01 mg/kg). NNC 756 induced no dystonia, while marked dystonia was induced by raclopride. Mild tolerance to the dystonia-inducing effect of raclopride slowly developed. Both drugs induced significant sedation and mild bradykinesia. Sedation induced by NNC 756 was stronger than that of raclopride, while no differences were found regarding bradykinesia. The sedative effect of both NNC 756 and raclopride increased over time during chronic treatment. No changes in bradykinesia developed. No significant dyskinesia was induced by NNC 756, while raclopride significantly induced both acute and tardive oral dyskinesia. Furthermore, raclopride-induced acute dyskinesia worsened during chronic treatment. Concomitant treatment with NNC 756 tended to reduce the D1 agonist SKF 81297-induced dyskinesia and grooming, while concomitant treatment with raclopride increased SKF 81297-induced dyskinesia and tended to decrease SKF 81297-induced grooming. Chronic treatment with raclopride induced supersensitivity to both the D2/D3 agonist LY 171555 and SKF 81297, while chronic NNC 756 treatment only induced supersensitivity to SKF 81297. The findings indicate that D1 antagonists may induce less dystonia and oral dyskinesia as compared with D2 antagonists and support the hypothesis of both a permissive and an inhibitory interaction between D1 and D2 receptor systems.

Functional interaction between dopamine D1 and D2 receptors in 'MPTP' monkeys

We have studied the motor response induced by independent administration of 4 different doses of a dopamine D2 [(+)-PHNO] and a dopamine D1 (CY 208-243) receptor agonist in 5 MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) monkeys. Both drugs had similar antiparkinsonian effects and both elicited choreic dyskinesias. Simultaneous administration of (+)-PHNO [(+)-4-propyl-9-hydroxynaphthoxazine] and CY 208-243 [(-)4,6,6a,7,8,12b-hexahydro-7-methylindolo[4,3a-b]phenan thyxidine] did not result in modification of the dose-response curve induced by each dopamine receptor agonist given alone. Pretreatment with the dopamine D1 receptor antagonist SCH 23390 (0.8 mg/kg) and the dopamine D2 receptor antagonist sulpiride (60 mg/kg) reduced the magnitude and the duration of the motor response induced by (+)-PHNO and CY 208-243, respectively, but did not modify the intensity and characteristics of choreic dyskinesias. These results demonstrate that the motor effects and the dyskinesias cannot be dissociated by selective dopamine D1 and D2 receptor stimulation. It appears that stimulation of dopamine D1 and D2 receptors by endogenous dopamine is required to obtain the full motor response induced by selective dopamine receptor agonists as demonstrated by the reduction of the motor improvement found after pretreatment with SCH 23390 and sulpiride.

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.

Evidence for D1 dopamine receptor-mediated modulation of the synaptic transmission from motor axon collaterals to Renshaw cells in the rat spinal cord

The possible modulatory role of D1 dopamine receptors on the excitability of lumbar spinal Renshaw cells was studied in anesthetized rats spinalized at T4 level. Burst responses elicited by single electrical shocks to ipsilateral ventral roots L6 (frequency 0.5 Hz, stimulus width 0.1 ms) and spontaneous activity were recorded extracellularly using conventional 3 M KCl filled glass micropipettes. The specific D1 agonist SKF 38393 (0.5-1 mg/kg i.v.) enhanced Renshaw cell burst responses by 20-60% (n = 7) and increased their spontaneous discharge rate (n = 3). This effect was clearly antagonized by the specific D1 antagonist SCH 23390 (1 mg/kg i.v.) although SCH 23390 proved ineffective per se. We conclude that SKF 38393 induced facilitation was due to activation of the specific D1 receptors which could be the functional counterpart of the presynaptic D2 receptors described earlier by us in the same synapse.

Cocaine: a microstructural analysis of its effects on feeding and associated behaviour in the rat

Cocaine (5.6-30 mg/kg, i.p.) was administered to nondeprived male rats trained to eat a palatable sweetened mash. Over a 60-min period, their behaviour was observed and recorded for a microstructural analysis. Cocaine suppressed feeding in a dose-dependent manner (significantly at 10 mg/kg and greater), and this was due in the main to a reduction in the frequency of eating bouts. In contrast, the mean duration of eating bouts was unaffected, except at the highest dose, 30 mg/kg. In addition, the rate of eating was not significantly affected by cocaine at any dose. Time-course data revealed that cocaine, at anorectic doses (10-30 mg/kg), initially suppressed feeding completely, and the duration of this suppression was proportional to the dose. In effect, cocaine delayed the initiation of feeding, thus bringing about the reduction in the number of eating bouts. Cocaine caused some stimulation of locomotor activity and rearing to the side of the observation tank, but did not affect rearing away from the centre, or immobility. Grooming proved to be very sensitive to cocaine's suppressant effect, with substantial inhibition occurring at 5.6 mg/kg (a sub-anorectic dose). These data are compared with previously published work with D-amphetamine and are contrasted with results for selective D1 and D2 dopamine receptor agonists.

New insights into dopamine receptors in the central nervous system

Whereas biochemical and pharmacological studies indicated that there were two subclasses of dopamine receptor (D1, D2) the application of molecular biology techniques has defined at least six dopamine receptor isoforms. These may be divided into D1-like (D1, D5) and D2-like (D2(short), D2(long), D3, D4) subfamilies on the basis of their structural and pharmacological properties. In this commentary the common properties of these dopamine receptor species are described, including the predicted structures of seven transmembrane alpha-helices, amino acid homologies and conserved amino acids that may play important structural and functional roles. The D1-like and D2-like receptor isoforms have individual properties and these are described in terms of their structures, pharmacological and biochemical properties and localizations in different brain regions. The existence of multiple dopamine receptor isoforms is important for understanding how certain drugs achieve their therapeutic effects and how unwanted side effects arise. This is considered for the anti-parkinsonian and anti-schizophrenic drugs. The localization of D1 and D2 dopamine receptors to particular cell types in the neostriatum allows new insights to be made into the normal mode of action of dopamine to control motor function and how this is disturbed in disease stages e.g. Parkinson's disease, Huntington's disease. The detailed mode of action of anti-parkinsonian drugs can also be better understood from this. The availability, from molecular biology studies, of the amino acid sequences of the receptor isoforms allows predictions to be made of the structures of these species. In particular it is possible to produce speculative models of the three dimensional structures of the ligand-binding sites of these receptors. These speculations can be complemented by chemical modification, pH dependency and mutagenesis studies which provide information on the amino acid residues at the ligand binding site that actually interact with the ligand. In time it should be possible to understand in some detail the mechanism of receptor-ligand interaction and this will be important for the design of drugs targeted at specific isoforms.

Relative roles of ventral striatal D1 and D2 dopamine receptors in responding with conditioned reinforcement

Several experiments investigated the involvement of D1 and D2 dopamine receptors in the ventral striatum in the control over behaviour by a conditioned reinforcer using an acquisition of new response procedure. Intra-accumbens infusion of either the D1 receptor antagonist, SCH 23390, or the D2 receptor antagonist, raclopride, completely blocked the potentiative effects of intra-accumbens d-amphetamine on responding with conditioned reinforcement and reduced responding to control levels. SCH 23390 was more potent than raclopride. At higher doses in the absence of d-amphetamine, both antagonists also blocked the preference for responding on the lever producing the conditioned reinforcer. Intra-accumbens infusions of either the D1 receptor agonist, SKF 38393, or the D2/3 receptor agonist, LY 171555 (quinpirole), selectively potentiated responding on the lever producing the conditioned reinforcer. Various combined infusions of the D1 and D2 agonists in specific low doses had additive, but not synergistic, effects on responding with conditioned reinforcement. None of the drugs affected the drinking of water in deprived subjects when infused intra-accumbens. These results suggest that both D1 and D2 receptors in the nucleus accumbens are involved in mediating the effects of dopamine in potentiating the control over behaviour by conditioned reinforcers.

Behavioural evidence for "D-1-like" dopamine receptor subtypes in rat brain using the new isochroman agonist A 68930 and isoquinoline antagonist BW 737C

The full efficacy, high potency isochroman D-1 agonist A 68930 demonstrated greater than 220-fold selectivity for D-1 over D-2 receptors. A 68930 (0.06 and 0.25 mg/kg) readily induced intense grooming, together with vacuous chewing; these responses became less evident following higher doses (1.0 and 4.0 mg/kg) and sniffing became prominent. Intense grooming was blocked by three D-1 antagonists, the benzazepines SCH 23390 (0.01-1.0 mg/kg) and NNC-756 (0.01-1.0 mg/kg), and the isoquinoline BW 737C (0.2-5.0 mg/kg); however, vacuous chewing was not antagonised by SCH 23390 and NNC-756, but was blocked by BW 737C. Intense grooming was attenuated by the D-2 antagonist YM 09151 (0.005-0.5 mg/kg) while vacuous chewing was enhanced. These data suggest that intense grooming is mediated by a "D-1 like" receptor that recognises all known chemical classes of D-1-selective compounds, while vacuous chewing may be mediated by a pharmacologically distinct subtype of "D-1-like" receptor that recognises preferentially the isochromans and isoquinolines.

Studies on the structure of the ligand-binding site of the brain D1 dopamine receptor

A series of group-specific modifying reagents were tested for their effects on [3H]SCH23390 binding to brain D1 dopamine receptors in order to identify amino acid residues at the ligand binding site of the D1 dopamine receptor that are critical for ligand binding. The dependence of ligand binding on the pH of the incubation medium was also examined. The histidine-selective reagent, diethylpyrocarbonate did affect ligand binding but this is probably not due to an effect at the ligand binding site. Experiments with N-acetylimidazole and ethylacetimidate indicated that modification of tyrosine and amino residues did not exert major influences at the ligand binding site. The use of the thiol dithiothreitol indicated that breakage of a disulphide bond altered ligand binding, probably by affecting the receptor conformation, and the use of the sulphydryl reagent 5,5'-dithio-bis-nitrobenzoic acid showed that modification of a sulphydryl group on the receptor inhibited ligand binding. The carboxyl reagent N,N'-dicyclohexyl carbodiimide (DCCD) potently inhibited ligand binding and the effect could be prevented by occupancy of the receptor site by an agonist or antagonist so that there is an important carboxyl group at the receptor binding site. The total number of D1 receptors was reduced after the modification by DCCD and 70% of the residual receptors showed a reduced affinity for binding [3H]SCH23390, the remainder having the same affinity as untreated receptors. [3H]SCH23390 binding is also reduced by a decrease of pH and this effect seems to depend on the protonation of a group of pKa 6.9. Saturation analysis of [3H]SCH23390 binding performed at pH 7.5 shows a single class of high affinity sites whereas at pH 6.0, two classes of sites with higher and lower affinities are seen. These studies suggested a model whereby [3H]SCH23390 binding is to two receptor isoforms with different pH dependencies for [3H]SCH23390 binding.

Synthesis and characterization of biotinylated and photoactivatable neuroleptics. Novel bifunctional probes for dopamine receptors

We have synthesized and characterized a series of novel derivatives of established antagonists of the neurotransmitter dopamine, i.e. butyrophenones, hexahydrocarbolines and phenothiazines. All derivatives were biotinylated, some of them carried an additional (photoactivatable) azido group. In the case of butyrophenones, the structural modifications were introduced at the aliphatic keto group and/or the heterocyclic ring system, both modifications resulting in significant decreases in binding affinity to dopamine D2 and dopamine D1 receptor subtypes. Biotinylation of hexahydrocarbolines significantly increased their binding affinity to D1 receptors, with the affinity for D2 receptors increasing only slightly, or remaining approximately the same, as compared to the parent compound. As a consequence, the derivatized hexahydrocarbolines behaved as nonselective antagonists of dopamine. Biotinylation of phenothiazines increased their binding affinity to both main subtypes of dopamine receptors by at least one order of magnitude, resulting in binding affinities in the nM range. These derivatives bound to both D1 and D2 receptor subtypes. In three of the biotinylated derivatives the photoactivatable azido group was introduced. These compounds bound to synaptosomal membranes from bovine caudate nuclei with similar affinity and subtype specificity as the biotinylated derivatives, and photoaffinity labelling was shown to proceed under mild conditions and selectively. These novel bifunctional ligands may become useful tools in the purification and characterization of dopamine receptors including their visualization and localization in the central nervous system and in tissue culture.

Synergistic behavioural effects of dopamine D1 and D2 receptor agonists are determined by circadian rhythms

The effects of continuous subcutaneous infusions of rats for 336 h with vehicle, SKF 38393 (a dopamine D1 receptor agonist), (+)-4-propyl-9-hydroxynaphthoxazine (PHNO, a dopamine D2 receptor agonist) or both D1 and D2 receptor agonists, on locomotor activity were investigated. Rats were maintained under constant lighting conditions, either continuous dark (dark:dark) or continuous light (light:light), before and during drug treatments in order to determine the influence of free-running circadian rhythms on drug responses. The D2 receptor agonist initially increased locomotion in rats kept under dark:dark during both subjective night (period of maximum locomotion) and day (period of minimum locomotion), but had no effect in rats maintained in light:light throughout the 336 h of treatment. The motor stimulant effects of the D2 receptor agonist on rats kept in dark:dark increase during the course of treatment during subjective night (sensitization), but decreased during the rats' subjective day (tolerance). The D1 receptor agonist, SKF 38393, had no effect on its own regardless of the lighting conditions and the duration of treatment. However, the D1 receptor agonist interacted synergistically with the D2 receptor agonist in rats maintained under light:light, depending on the duration of treatment. Synergistic effects were also observed on initiation of treatment in rats under dark:dark but only during subjective day. Tolerance to the synergistic effects of the receptor agonists occurred as a function of treatment duration, but only during subjective day. The D1 receptor agonist blocked the effects of the D2 receptor agonist during the rats' subjective night after 100 h of treatment, but not after 25 or 325 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Two directions of dopamine D1/D2 receptor interaction in studies of behavioural regulation: a finding generic to four new, selective dopamine D1 receptor antagonists

A range of new, chemically distinct D1 dopamine receptor antagonists, SCH 39166, NO 756, A-69024 and BW 737C, were studied for their effects on behavioural responses to the selective D2 agonist RU 24213. Each D1 antagonist not only blocked typical sniffing and locomotor responses to RU 24213 but also released atypical myoclonic jerking behaviour, while the selective D2 antagonist YM 09151 blocked these typical responses but did not release jerking. The rank order of effectiveness of these D1 antagonists to release such D2 agonist-induced jerking was similar to that of their selectivities as D1 antagonists; also, the action of BW 737C showed complete enantioselectivity, the inactivity of its R-antipode BW 736C paralleling enantioselective blockade of D1 but not D2 receptors. It appears that while tonic activity through D1 receptors is necessary for the expression of typical D2-stimulated behaviour, via well-known cooperative/synergistic D1:D2 interactions, D1 tone also normally inhibits, via oppositional D1:D2 interactions, the expression of atypical D2-stimulated behaviours such as jerking. Oppositional D1:D2 interactions are evident using all of the classes of selective D1 antagonist currently known, and appear to constitute another general mode of dopaminergic regulation.

New classes of selective D-1 dopamine receptor antagonist provide further evidence for two directions of D-1:D-2 interaction

Initial notions of cooperative/synergistic interactions between D-1 and D-2 dopamine receptors have been followed by recent evidence suggesting more complex forms of D-1:D-2 interaction. Each of the new, putative selective D-1 antagonists SCH 39166, NO 756 and A-69024 antagonised typical behavioural responses to the selective D-2 agonist RU 24213 and concurrently released atypical behaviour. These data extend new notions of both cooperative/synergistic and oppositional D-1:D-2 interactions in the regulation of typical and atypical behaviours, which may involve further subtypes of D-1 and of D-2 receptor.

D-1 dopamine receptors and the topography of unconditioned motor behaviour: studies with the selective, 'full efficacy' benzazepine D-1 agonist SKF 83189

Approaches to studying the role of the D-1 receptor in the regulation of unconditioned motor behaviour, and the current status of results derived therefrom, are reviewed; the desirability of utilizing drug tools other than the benzazepine partial D-1 agonist SKF 38393 is emphasized, particularly the need for studies with full D-1 agonists. Behavioural responses to the benzazepine putative full D-1 agonist SKF 83189 were compared with those to its high potency but only partial agonist counterpart SKF 77434. Both agents produced qualitatively and generally quantitatively similar behavioural responses, particularly intense grooming in the absence of vacuous chewing; thus, their behavioural properties appeared entirely unrelated to their efficacies to stimulate adenylate cyclase, the classical definition of a D-1 agonist. There may be complex interactions between selectivity, intrinsic activity and CNS penetrability, or a high D-1 receptor reserve ('spare' receptors); however, these results point towards the notion of behaviourally relevant subtypes of D-1 receptor, possibly utilizing transduction mechanisms other than, or additional to, adenylyl cyclase, for which there is emerging evidence.

Parametric and pharmacological analyses of the enhanced grooming response elicited by the D1 dopamine receptor agonist SKF 38393 in the rat

The present report investigated several parametric and pharmacological aspects of the enhanced self-grooming behavior of rats following systemic administration of the selective D1 dopamine (DA) receptor agonist SKF 38393. The amount of time that rats spent grooming themselves was measured continuously for 30 min following drug administration to provide a quantitative measure of the drug-induced behavior. SKF 38393 increased the amount of grooming in a dose-dependent manner (0.5-16 mg/kg, SC). The onset of this effect required at least 5 min and it persisted for at least 60 min. The ability of SKF 38393 to enhance grooming was shared by R-SKF 38393, but not S-SKF 38393, consistent with the affinities of these enantiomers for the D1 DA receptor. Unlike SKF 38393, the peripheral D1 agonist fenoldopam (SKF82526) failed to cause an increased grooming response, suggesting a central site of action for elicitation of this behavior. The SKF 38393-induced increase in grooming was competitively antagonized by the D1 selective antagonist SCH 23390 (0.5 mg/kg, SC). Although the D2 DA receptor-selective antagonist eticlopride reduced SKF 38393-elicited grooming, this antagonism appeared to be of a physiological rather than pharmacological nature. When eticlopride was coadministered with the non-selective (mixed) D1/D2 agonist apomorphine, an increase in grooming behavior similar to that produced by SKF 38393 was observed. Inactivation of D1 and D2 DA receptors produced by pretreatment with the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), at a dose which reduces D1 and D2 receptor density by > or = 50% (8.0 mg/kg, IP), reduced SKF 38393-induced grooming by approximately 50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Motor activity following the administration of selective D-1 and D-2 dopaminergic drugs to MPTP-treated common marmosets

The ability of selective D-1 agonist and antagonist drugs to alter motor deficits and locomotor activity was studied in MPTP-treated common marmosets. Both the D-2 agonist quinpirole and the mixed D-1/D-2 agonist apomorphine reversed the motor impairments and induced locomotor activity. The D-1 antagonist SCH 23390 and the D-2 antagonist raclopride given alone further reduced motor function in MPTP-treated animals. The actions of quinpirole were potently and completely inhibited by raclopride but only partially and inconsistently by SCH 23390. In contrast, the effects of apomorphine were markedly but incompletely inhibited by both raclopride and SCH 23390. The D-1 agonist SKF 38393 alone caused a dose related reduction in motor activity. SKF 38393 weakly and partially inhibited the improvements in motor function produced by quinpirole but had a more pronounced effect on apomorphine induced motor activity. The induction of motor activity in MPTP treated common marmosets may separately involve both D-1 and D-2 receptors. Comparison with our previous data on the effect of the same drugs in normal common marmosets provides some evidence for a breakdown of linkage between D-1 and D-2 systems following MPTP treatment. The actions of SKF 38393 in MPTP-treated common marmosets contrasts with its ability to induce behavioural activation and a facilitation of D-2 mediated behaviour in rodents. SKF 38393 may not be the compound with which to delineate the role of D-1 receptors in primates.

Age-related changes in the regulation of behavior by D-1:D-2 dopamine receptor interactions

Functional interactions between D-1 and D-2 dopamine receptor systems appear important in the regulation of psychomotor behavior, and may alter with aging. Male Sprague-Dawley rats of 5 and 20-24 months were challenged with the selective D-2 agonist LY 163502 alone or following pretreatment with the selective D-1 antagonist SCH 23390. Typical sniffing and locomotor responses to LY 163502 alone were significantly reduced in aged animals. Pretreatment with SCH 23390 blocked these typical responses in both young and aged animals, consistent with their regulation by cooperative D-1:D-2 interactions; however, SCH 23390 released a significant excess of atypical limb/body jerking to LY 163502 in aged animals, a response which appears to have its basis in oppositional D-1:D-2 interactions. These results suggest that the net effect of aging on dopaminergic transmission is to reduce tonic activity through D-1 receptors to a greater extent than that occurring through D-2 receptors. As the present aged animals showed a selective loss of striatal D-2 but not of D-1 receptors in radioligand binding studies, such a reduction of D-1-mediated transmission with aging would seem to involve loss of presynaptic function or of postsynaptic mechanisms beyond the D-1 recognition site.

Neurochemical changes associated with the persistence of spontaneous oral dyskinesia in rats following chronic reserpine treatment

Rats treated chronically with reserpine develop spontaneous oral dyskinesia. The present study examined the development of the oral dyskinesia during the course of reserpine treatment, and its persistence after termination of treatment. Rats were injected with either reserpine (1 mg/kg, s.c.) or vehicle once daily for 4 days and then every other day for 6 weeks. Oral dyskinesia developed rapidly, reaching a maximal level after 3 days. It persisted at a maximal level for up to 20 days after termination of reserpine treatment, and continued to persist above control level for at least 60 days. The reserpine-treated rats also exhibited stereotypy in response to acute injection of the D1-selective agonist SKF-38393 (10 mg/kg), which was not observed in control rats. In contrast to the oral dyskinesia, this altered sensitivity to SKF-38393 returned to normal within 20 days after terminating the reserpine treatment, suggesting that these two behavioral responses involve different neural mechanisms. Quantitative autoradiographic measurement of dopamine receptor subtypes revealed that both D1 and D2 receptors were increased in the caudate-putamen (Cpu) and nucleus accumbens. Only the increase in D2 receptor density in the CPu correlated with the persistence of the oral dyskinesia; both changes persisted following termination of the reserpine treatment, and their magnitude was less at 60 days than at 1 and 20 days post-treatment. These results may have important implications for tardive dyskinesia.

Supersensitized D1 receptors mediate enhanced oral activity after neonatal 6-OHDA

Enhanced oral responses have been observed in rats that are treated shortly after birth with 6-hydroxydopamine (6-OHDA). A series of studies was conducted to characterize this effect. A dose-response curve demonstrated that the dopamine D1 receptor agonist, SKF 38393, produced a maximal response in 6-OHDA-treated rats at a dose of 0.10 mg/kg (IP). With the D2 receptor antagonist, spiperone, a bell-shaped dose-response curve was seen, with a maximal effect in the 6-OHDA group occurring at 80 micrograms/kg. There were only slight increases in oral activity with different SKF 38393 or spiperone doses in the saline group, indicating that there was an overt supersensitization of D1 receptors in the 6-OHDA-treated rats. The D1 antagonist SCH 23390 (0.30 mg/kg, IP) attenuated the response to both SKF 38393 and spiperone. The oral response to the D2 agonist, quinpirole (0.10 mg/kg, IP) was not preferentially increased in the 6-OHDA group of rats. These findings indicate that the enhanced oral response in neonatal 6-OHDA-treated rats is mediated by supersensitive dopamine D1 receptors. The persistence of the enhanced oral response in 6-OHDA-treated rats at 8 months demonstrates that this sensitization of D1 receptors is a long-lived phenomenon.

Relationship between D1 dopamine receptors, adenylate cyclase, and the electrophysiological responses of rat nucleus accumbens neurons

The electrophysiological effects of three selective D1 dopamine (DA) receptor agonists, which exhibit different potencies and efficacies for stimulation of adenylate cyclase, were compared in the rat nucleus accumbens (NAc) using single unit recording and microiontophoretic techniques. The partial agonists SKF75670 and SKF38393, and the full agonist SKF81297 produced nearly identical current-response curves for the inhibition of firing of NAc neurons. In rats acutely depleted of DA by alpha-methyl-p-tyrosine (AMPT) pretreatment, all three D1 agonists enabled the inhibition of firing produced by the selective D2 receptor agonist quinpirole, with SKF38393 exerting the greatest efficacy, followed by SKF81297 and SKF75670. Thus, no apparent relationship was found between the previously reported ability of these compounds to stimulate cyclic adenosine monophosphate (cAMP) production and their ability either to inhibit the firing of NAc neurons or to enable quinpirole-mediated inhibition of firing in DA-depleted rats. In addition, the membrane-permeable cAMP analog 8-bromo-cAMP also caused a current-dependent inhibition of the firing of NAc neurons, but failed to enable quinpirole-mediated inhibition in AMPT-pretreated animals. These results suggest either that only a small percentage of D1 receptors need to be stimulated to produce these electrophysiological effects, or that D1 receptors exist within the rat NAc which are linked to transduction mechanisms other than, or in addition to, adenylate cyclase.

Overview of radiopharmaceuticals for diagnosis of central nervous disorders

There are two types of imaging instruments, single-photon emission computed tomography (SPECT) and positron emission tomography (PET) that use radiopharmaceuticals for the diagnosis of brain disorders. Brain perfusion imaging agents, labeled either with 123I or 99mTc, are useful in detecting various cerebral vascular abnormalities, such as stroke and transient ischemia with SPECT. The management of other neurological disorders (i.e., in Alzheimer's, epilepsy, schizophrenia, and head trauma patients) may also be benefitted by these agents. The exact trapping mechanisms and their relationships with potential clinical applications still remain to be elucidated. Imaging studies using 18F fluorodeoxyglucose with PET is currently the most promising diagnostic tool for the evaluation of local glucose metabolism related to various disease states, such as Alzheimer's disease, brain tumor, and epilepsy. In the past few years significant progress has been made in the design and characterization of new CNS neuronal and postsynaptic receptor imaging agents for PET and SPECT. The new diagnostic agents are aimed at measurements of localization and changes of neuronal function. It is likely that these types of agents have potential for clinical application, especially in the diagnosis of psychiatric disorders that do not involve morphological changes.

Motor activity following the administration of selective D-1 and D-2 dopaminergic drugs to normal common marmosets

In normal common marmosets administration of the D-1/D-2 agonist apomorphine or the selective D-2 agonist quinpirole caused a dose-dependent increase in motor activity and induced stereotyped behaviour. Both the selective D-2 antagonist raclopride and the selective D-1 antagonist SCH 23390 inhibited normal locomotor activity and induced catalepsy. Quinpirole- and apomorphine-induced motor activity were potently inhibited by pretreatment with raclopride. The effects of quinpirole, but not apomorphine, were weakly inhibited by SCH 23390. The selective D-1 partial agonist SKF 38393 decreased motor activity and did not induce grooming, oral movements or other behaviours. SKF 38393 inhibited motor activity induced by the administration of quinpirole but did not alter apomorphine-induced motor behaviour. Locomotor activity in normal common marmosets appears to be mediated mainly via D-2 systems. In contrast to rodents, administration of SKF 38393 does not induce behavioural activation and there does not appear to be a facilitating effect of D-1 systems on D-2 function in the normal common marmoset. However, the ability of both SKF 38393 and SCH 23390 to inhibit quinpirole locomotor activity suggests some interaction between D-1 and D-2 systems to occur in this species.

Neonatal dopamine lesion in the rat results in enhanced adenylate cyclase activity without altering dopamine receptor binding or dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein (DARPP-32) immunoreactivity

Newborn male Sprague-Dawley rats were treated neonatally with an intracisternal injection of 75 micrograms 6-hydroxydopamine (6-OHDA) following desipramine pretreatment in order to induce a permanent selective dopamine (DA) lesion. At 60-70 days of age a massive loss of tyrosine hydroxylase (TH) immunoreactive (IR) cells was seen in substantia nigra. The TH-IR terminal density was reduced by 92% in striatum, 77% in nucleus accumbens and by 72% in tuberculum olfactorium. Quantitative autoradiography using 3H-SCH-23390 and 3H-spiperone did not reveal any alteration of DA D1 and D2 receptor binding in the denervated regions studied. Furthermore, no change in the Bmax or Kd of 3H-SCH-23390 or 3H-spiperone in vitro binding was observed in membrane preparations of striatum following the neonatal DA lesion. Basal and DA-stimulated accumulation of cAMP was increased in striatal membrane preparations of the neonatally DA-lesioned rats. No alteration of the immunoreactivity of the D1 receptor associated phosphoprotein dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein (DARPP-32), was observed as visualized using quantitative immunohistochemistry. Thus, neonatal DA lesions seem to induce a selective functional supersensitivity reflected by an enhanced activity of D1 receptor-coupled adenylate cyclase, without any alteration in the number of affinity of D1 and D2 receptor sites. Furthermore, the appearance of DARPP-32 seems to be independent of intact DA input during development.

Unilateral injections of a D2 but not D1 agonist into the frontal cortex of rats produce a contralateral directional bias

Unilateral manipulations of frontocortical dopamine have been found in previous studies to produce a directional bias in the circling behaviour of rats. Agonists produced contralateral circling and antagonists produced ipsilateral turning. To examine the role of dopamine receptor subtypes, the present studies investigated the ability of unilateral intrafrontal cortical microinjections of the D1 agonist, SKF 38393 or the D2 agonist, quinpirole to produce contralateral circling in rats. The antagonist, cis-flupenthixol was also tested and was expected to produce ipsilateral circling. In 3 separate experiments, rats received 7 50-min sessions in a circular arena separated by at least 48 hr. The first and final sessions were preceded by no injection, the second and sixth by saline [or the inactive trans isomer (2.5 micrograms) in the flupenthixol experiment] and the middle 3 sessions by doses of cis-flupenthixol (1, 10, 25 micrograms in 0.5 microliter), quinpirole (3, 6, 12 micrograms) or SKF 38393 (2, 4, 8 micrograms), the order being counterbalanced across rats. cis-Flupenthixol and quinpirole produced dose-dependent ipsi- and contralateral circling, respectively, whereas SKF 38393 was without significant effect. No reliable directional bias was seen in any no-injection, saline or trans-flupenthixol sessions. Results suggested that the D2 receptor may mediate the motor effects of frontal cortical dopamine.

Selective localization of striatal D1 receptors to striatonigral neurons

A new technique for producing anatomically selective lesions within the brain was used to investigate the cellular localization of the D1 and D2 receptor. The cytotoxic lectin, volkensin, is taken up by nerve terminals and retrogradely transported, killing those neurons projecting to the site of injection. Comparison of D1 and D2 binding following a unilateral volkensin injection into the substantia nigra has demonstrated that striatal D1 binding sites are selectively localized to striatonigral projection neurons.

Current and future radiopharmaceuticals for brain imaging with single photon emission computed tomography

Development of radiopharmaceuticals for functional brain imaging has progressed rapidly in recent years. Measurement of regional cerebral blood flow in humans can be achieved by using [123I]-iodoamphetamine or [99mTc]-HMPAO. Several other lipid-soluble [99mTc]-technetium complexes are currently undergoing clinical trials. New 123I-labeled agents designed to measure central nervous system receptors, including D1 and D2 dopamine, serotonin, muscarinic, and benzodiazepine receptors, have been developed. In conjunction with single photon emission computed tomography, they may provide useful tools to evaluate brain function related to changes in receptor concentration.

The D2 agonist quinpirole potentiates the discriminative stimulus effects of the D1 agonist SKF 38393

Although there are two dopamine (DA) receptors (D1 and D2) in the brain, the functional role, particularly of D1 receptors, has remained unclear. Recent research has suggested that D1 and D2 receptors interact synergistically in the generation of certain D2 agonist-induced motor responses. On the other hand, an antagonistic interaction between the receptors has been reported for D1 agonist-induced perioral movements. The purpose of the present experiment was to characterize further the interaction between D1 and D2 receptors using a drug discrimination paradigm, a behavioral paradigm that is sensitive and selective for D1 and D2 agonist and antagonist activity. Rats (N = 8) were trained to discriminate the D1 agonist SKF 38393 (SKF; 10 mg/kg, IP, 30 minutes presession) from saline (1.0 ml/kg, IP, 30 minutes presession) in a 2-lever, food-reinforced-drug discrimination paradigm, SKF (0.2-12.8 mg/kg) produced a dose-related increase in SKF-appropriate responding (maximum 87.5% at 12.8 mg/kg). The D2 agonist quinpirole (QUIN; 0.012-0.1 mg/kg, IP, 10 minutes presession) given alone did not substitute for SKF (maximum 37% SKF-appropriate responding at 0.05 mg/kg). QUIN (0.012 or 0.025 mg/kg) in combination with SKF significantly (p less than 0.05) shifted the SKF dose-response function to the left, suggesting that stimulation of D2 receptors can potentiate a behavioral effect mediated by D1 receptors. Furthermore, when taken together with previous findings that SKF failed to potentiate the discriminative stimulus effects of QUIN, the present results suggest that the nature of D1/D2 receptor interactions depends not only upon the behavior under investigation but also upon the receptor action that the behavior reflects.