The dopamine D3-receptor: a postsynaptic receptor inhibitory on rat locomotor activity

Modification of dopamine D(1) receptor knockout phenotype in mice lacking both dopamine D(1) and D(3) receptors

Experimental evidence suggests that dopamine D(1) and D(3) receptors may interact in an opposing or synergistic fashion. To investigate interactions between both receptors in behaviour, we have used dopamine D(1) and D(3) receptor knockout mice to generate mice lacking both receptors. D(1)(-/-)D(3)(-/-) mice were viable, fertile and showed no gross morphological abnormalities. In an open field, they exhibited lower activity than wild-type, D(1)(-/-) and D(3)(-/-) mice. D(1)(-/-)D(3)(-/-) mice performed equally poorly in the rotarod and Morris water maze tasks as their D(1)(-/-) littermates. Basal locomotor activity and anxiety-like behaviour were normal in D(1)(-/-)D(3)(-/-) mice. Combined deletion of both receptors abolished the exploratory hyperactivity and anxiolytic-like behaviour of dopamine D(3) receptor mutant phenotype and further attenuated the low exploratory phenotype of D(1)(-/-) mice. These results imply an interaction of both receptors in the expression of exploratory behaviour in a novel environment, and the need for the presence of intact dopamine D(1) receptor for the expression of certain behaviours manifested in dopamine D(3) receptor mutant phenotype. In addition, dopamine D(1) receptor, but not dopamine D(3) receptor, is involved in the ability to perform on the rotarod and spatial learning.

The dopamine D3 receptor antagonist nafadotride inhibits development of locomotor sensitization to amphetamine

Behavioral sensitization is a well-studied model of behavioral plasticity mediated at least in part by dopaminergic systems believed to play an important role in several psychiatric conditions. In the rodent, locomotion is regulated by the opposing balance of D3 and D2 receptors, with D2 activation increasing and D3 stimulation inhibiting locomotion. However, receptor occupancy of D3 dopamine receptors is far greater than D2 or D1 occupancy at typical post-stimulant dopamine concentrations. We therefore hypothesized that tolerance of D3 receptor inhibition of locomotion contributes to the development of sensitization. To test this hypothesis, we examined the effect of the D3 receptor antagonist nafadotride on sensitization. As predicted, nafadotride inhibits augmentation of the locomotion response to repetitive amphetamine. This finding is consistent with the proposed model of adaptive down-regulation of D3 dopamine receptor function contributing to the development of behavioral sensitization.

PD 158771, a potential antipsychotic agent with D2/D3 partial agonist and 5-HT(1A) agonist actions. II. Preclinical behavioral effects

PD 158771 has been described in receptor binding and biochemical tests as a partial agonist at dopamine (DA) D2 and D3 receptors as well as an agonist at serotonin (5-HT)(1A) receptors. The present studies describe the profile of PD 158771 in rodent and primate behavioral tests. PD 158771 reduced spontaneous locomotor activity in mice (ED(50)=0.38 mg/kg, i.p.) and rats (ED(50) = 1.2 mg/kg, i.p. and 0.16 mg/kg, s.c.), and reduced amphetamine-stimulated locomotion in mice (ED(50) = 0.13 mg/kg, i.p.). At relatively higher doses up to 3 mg/kg, s.c. in rats, PD 158771 did not produce locomotor stimulation or induce stereotypy, indicating a lack of postsynaptic DA agonist activity. PD 158771 reduced apomorphine stimulated locomotion in rats at a dose 4.6-fold greater than those that reduced spontaneous locomotor activity, indicating weak postsynaptic DA antagonist actions; results consistent with a partial agonist profile. PD 158771 produced anxiolytic-like effects in the water-lick (Vogel) conflict test, effects possibly due to the 5-HT(1A) activity. However, PD 158771 was inactive in the water wheel behavioral despair model in rats, indicating lack of antidepressant properties. Similar to known antipsychotics, PD 158771 produced a potent and long-lasting inhibition of conditioned avoidance responding in squirrel monkeys. In contrast to standard antipsychotics, and similar to clozapine, PD 158771 did not cause catalepsy in rats at a dose 20-fold higher than the ED(50) dose for locomotor inhibition. PD 158771 also had a somewhat lower liability than haloperidol to produce extrapyramidal dysfunction in squirrel and cebus monkeys sensitized to the dystonic effects of haloperidol. The data indicate that PD 158771 is a DA partial agonist with weak intrinsic activity that selectively activates brain DA autoreceptors. PD 158771 produced behavioral effects consistent with potential antipsychotic and anxiolytic efficacy, and has an improved profile in the extrapyramidal side effect model when compared to certain currently available antipsychotic agents.

PD 158771, a potential antipsychotic agent with D(2)/D(3) partial agonist and 5-HT(1A) agonist actions. I. Neurochemical effects

The neurochemical effects of a novel dopamine (DA) D(2)-like and serotonin (5-HT) 5-HT(1A) agonist, PD 158771, are described. PD 158771 exhibited affinities for human D(2L), D(3) and D(4.2) receptors expressed in Chinese hamster ovary (CHO)-K1 cells with K(i) (nM) values of 5.2, 13.7 and 34.8 respectively. PD 158771 showed high affinity for cloned human 5-HT(1A) (K(i) = 2.6 nM) and rat hippocampal 5-HT(1A) receptors (K(i) = 3.5 nM). Weaker affinities were observed at alpha 1-adrenergic (K(i) = 43 nM), histamine H(1) (IC(50) = 30 nM), 5-HT(2A) (K(i) = 24.5 nM) and sigma (sigma) -1 binding sites (K(i) = 24.5 nM). In measures of in vitro functional activity, PD 158771 stimulated [(3)H]thymidine uptake in CHO p-5 cells transfected with hD(3) receptors with a maximal effect of 23% relative to quinpirole. In hD(2)L, the corresponding value was 60% with an EC(50) of 29 nM, again indicating partial DA agonist action of PD 158771. In vivo, PD 158771 produced a dose-related decrease in DA synthesis in the striatum and mesolimbic regions of rat brain treated with gamma-butyrolactone (GBL), indicating a DA autoreceptor agonist action. In animals not treated with GBL, PD 158771 produced a dose-related decrease in DA synthesis and extracellular DA. A decrease in 5-HT synthesis in several brain areas was observed consistent with an agonist response. Further support for DA autoreceptor agonist action is that PD 158771 produced a partial inhibition of the firing of substantia nigra zona compacta DA neurons, an effect reversed by haloperidol. In conclusion, PD 158771 exhibited affinities for DA and 5-HT receptors, appears to possess DA and 5-HT agonist actions; and it could provide improved antipsychotic profile with minimal side effects.

Postglomerular vasoconstriction induced by dopamine D(3) receptor activation in anesthetized rats

In the present study we investigated the renal hemodynamic effects of dopamine D(3) receptor activation by R(+)-7-hydroxy-dipropylaminotetraline (7-OH-DPAT) in thiopental-anesthetized Sprague-Dawley rats. In clearance experiments infusion of 7-OH-DPAT (0.01-1.0 microg. kg(-1). min(-1)) dose-dependently elevated glomerular filtration rate (GFR) without affecting mean arterial blood pressure (MAP). In renal blood flow experiments 7-OH-DPAT infusion (1.0 microg. kg(-1). min(-1)) increased GFR by 16 +/- 2%, associated with an unexpected fall in renal blood flow by 20 +/- 3% and a significant elevation of renal vascular resistance by 18 +/- 3%. The renal hemodynamic changes were not influenced by pretreatment with the D(2)-receptor antagonist S(-)-sulpiride but were completely abolished during D(3) receptor inhibition by 5,6-dimethoxy-2-(di-n-propylamino)indane (U-99194A). In micropuncture experiments 7-OH-DPAT (1.0 microg. kg(-1). min(-1)) significantly elevated stop-flow pressure measured in the early proximal tubules and reduced hydrostatic pressure at the first branching point of the efferent arteriole without altering MAP. We conclude from these data that pharmacological activation of dopamine D(3) receptors affects renal hemodynamics in anesthetized rats by preferential postglomerular vasoconstriction.

Activation of dopamine D(3) receptors induces c-fos expression in primary cultures of rat striatal neurons

A modulation of the expression of immediate-early genes (IEGs) such as c-fos is likely involved in the long-term influence of dopaminergic ligands on the activity of basal ganglia neurons. The roles of individual dopamine receptor types in this regard remain unclear, and the present study employed primary cultures of rat striatal neurons to examine the potential modulation of c-fos expression by D(3) receptors. Neurons were treated with the preferential D(3) receptor agonists, (+/-)-7-OH-DPAT and PD 128,907, which showed, respectively, 16-fold and 56-fold selectivity for recombinant rat D(3) vs. D(2) receptors (K(i) values, rD(2)/rD(3) = 59.5/3.7 nM and 251/4.5 nM, respectively). At concentrations of 3 and 30 nM, respectively, (+/-)-7-OH-DPAT and PD 128,907 significantly increased the expression of c-fos mRNA. The action of (+/-)-7-OH-DPAT was expressed stereospecifically; its (+)-isomer (K(i) values, D(3)/D(2) = 1.6/56.7 elicited a 26% +/- 7.6% increase in c-fos expression whereas its (-)-isomer (K(i) values, D(3)/D(2) = 59/1,060 nM) was ineffective. Further, stimulation of c-fos mRNA expression by PD 128,907 (20 nM) was markedly and significantly (P < 0.05) attenuated (-91.8% +/- 5.3%) by 30 nM of the selective D(3) receptor antagonist, (+)-S 14297 (K(i) values, D(3)/D(2) = 11/401 nM). In contrast, the action of PD 128,907 was significantly less affected (-24.5% +/- 7.8%) by 30 nM of its less active distomer, (-)-S 17777 (K(i) values, D(3)/D(2) = 294/3,191 nM). An examination of the concentration-response relationship revealed that (+/-)-7-OH-DPAT and PD 128,907 both produced bell-shaped dose-response curves for c-fos induction. The sequential activation of D(2) receptors-which inhibit striatal c-fos expression (Simpson and Morris [1995] Neuroscience 68:97-106)-by higher concentrations of (+/-)-7-OH-DPAT and PD 128,907 is presumably involved in the inflexion at higher doses. In conclusion, the present data demonstrate that activation of D(3) receptors results in a selective induction of c-fos mRNA expression in cultured striatal neurons. These data show that neuronal D(3) receptors control the expression of IEGs and suggest that D(3) receptors may mediate long-term adapative changes in the activity of neurons in the basal ganglia.

Discriminative stimulus properties of the dopamine D3 receptor agonists, PD128,907 and 7-OH-DPAT: a comparative characterization with novel ligands at D3 versus D2 receptors

Rats were trained to recognize a discriminative stimulus (DS) elicited by the preferential dopamine D3 receptor agonists, PD128,907 (0.16 mg/kg, i.p.) and 7-OH-DPAT (0.16 mg/kg, i.p.). PD128,907 and 7-OH-DPAT showed "full" (> or = 80%) and mutual generalization. Chemically-diverse, preferential D3 versus D2 agonists, quinelorane, CGS15855A, pramipexole, ropinirole and piribedil, generalized to PD128,907 (and 7-OH-DPAT) in this order of potency, which correlated more strongly with affinity/activity at cloned human (h)D3 (r=0.68/0.81, n=7) than hD2 (0.27/0.64, n=7) receptors. Further, generalization potency strongly correlated with potency for suppression of response rates (0.86), induction of hypothermia (0.92), reduction of striatal dopamine turnover (0.92) and diminution of immobility in a forced-swim procedure (0.97). Nafadotride, UH232 and AJ76, which show a mild preference for D3 versus D2 sites, blocked the PD128,907 DS, and the modestly-selective D3 antagonist, U99194A, was partially effective. Both nafadotride and U99194A blocked the 7-OH-DPAT DS. However, antagonist potency (n=4) versus PD128,907 correlated better with affinity at D2 (0.89) versus D3 (0.27) sites. Further, whereas the preferential D2 versus D3 antagonist, L741,626, antagonized the PD128,907 DS, the selective D3 antagonists, S11566, S14297 (its eutomer) and GR218,231 were ineffective against PD128907 and 7-OH-DPAT DS. S11566 and GR218,231 likewise did not generalize to PD128,907. In conclusion, under the present conditions, D2 receptors are principally implicated in the DS properties of PD128,907 and 7-OH-DPAT.

Dopamine D(3) receptors in the rat kidney: role in physiology and pathophysiology

It is well accepted that dopamine receptors play an important role in the regulation of cardiovascular and kidney function. Most of the knowledge on the renal actions of dopamine has been accumulated focussing on the prototypes of the two known dopamine receptor subfamilies, i.e. D1 and D2. The dopamine D3 receptor is a member of the D2-like subfamily and has been intensively studied in the neurosciences. Recently, the peripheral actions of this receptor subtype have also raised considerable interest as well because its effects on kidney function appear to be different from that of the other dopamine receptors. This short overview will summarize the data reported and add new results on the role of D3 receptors in the regulation of renal function as well as their potential pathophysiological implications.

The D3 antagonist PNU-99194A potentiates the discriminative cue produced by the D3 agonist 7-OH-DPAT

Based on correlations between potencies of various dopamine D2/D3 agonists to substitute for the 7-OH-DPAT discriminative cue and their in vitro (mitogenesis test) potencies, it has been suggested that the 7-OH-DPAT cue is mediated by activity at the D3 subtype. We sought to verify that the 7-OH-DPAT cue could be blocked by PNU-99194A, a commercially available preferential D3 antagonist. Rats were trained (FR10 two-lever, food-reinforced schedule) to press one lever following 7-OH-DPAT (0.1 mg/kg i.p.) and the other lever following saline. Rats were then tested with various doses of 7-OH-DPAT alone or in combination with PNU-99194A. 7-OH-DPAT (0.003 to 0.3 mg/kg) engendered dose-dependent substitution; PNU-99194A (1 to 10 mg/kg) failed to antagonize the cue induced by 0.1 mg/kg of 7-OH-DPAT and, at 10 mg/kg, given in combination with 0.003 to 0.1 mg/kg of 7-OH-DPAT, PNU-99194A markedly shifted the 7-OH-DPAT dose-effect curve to the left, i.e., potentiated the 7-OH-DPAT cue. If PNU-99194A is a preferential D3 antagonist, the present data do not confirm the previous hypothesis that the 7-OH-DPAT cue is mediated by the D3 subtype.

Locomotor activity and accumbens Fos expression driven by ventral hippocampal stimulation require D1 and D2 receptors

Numerous studies have suggested that excitatory projections from the ventral hippocampus to the nucleus accumbens modulate locomotor activity in rats. Furthermore, the ability of ventral hippocampal neurons to alter locomotor activity may involve the dense dopaminergic innervation found in the nucleus accumbens. The purpose of this study was to: (i) more fully characterize the locomotor effects of acute alterations in ventral hippocampal activity; (ii) ascertain the influence of dopamine agonists and antagonists on locomotor changes produced by altered ventral hippocampal activity; and (iii) use immediate early gene induction to determine whether dopamine antagonists alter the response of nucleus accumbens neurons to ventral hippocampal stimulation. By comparing a variety of excitatory amino acid agonists, it was found that ventral hippocampal infusion of N-methyl-D-aspartate elevated locomotor activity in a subconvulsive manner, while other excitatory amino acid receptor agonists did not. Inactivation of the ventral hippocampus achieved by lidocaine infusion did not suppress ongoing locomotor activity, nor did it affect amphetamine-induced increases in locomotor activity. Increases in locomotor activity induced by ventral hippocampal N-methyl-D-aspartate infusion were blocked by systemic administration of haloperidol (a D2 receptor antagonist), SCH-23390 (a D1 receptor antagonist) or reserpine. Cellular expression of the protein product of the immediate early gene, c-fos, was dramatically increased in the nucleus accumbens shell after ventral hippocampal N-methyl-D-aspartate infusion, and haloperidol, SCH-23390 and reserpine attenuated this effect. These results suggest that the increases, but not decreases, in ventral hippocampal activity have a measurable effect on ongoing rates of locomotion, and that this effect requires both D1 and D2 receptors. Moreover, the studies of Fos expression suggest that dopamine receptor antagonists attenuate neuronal responses to ventral hippocampal stimulation within the nucleus accumbens, a brain region important in the generation and maintenance of locomotor activity.

Discriminative stimulus properties of antipsychotics

Drug discrimination methodology has been used in a number of ways to analyze the actions of novel and putative novel antipsychotics in vivo. Recent studies suggest (a) in contrast to earlier theorizing, antagonism of the low-dose d-amphetamine stimulus in rats may not be an effective screen for novel antipsychotics; (b) dopamine D2-like agonists and antagonists, some of which are putative antipsychotics, can be studied in vivo as discriminative cues, although there is a pressing need for more selective drugs that differentiate the various members of the D2 family. (c) antagonism of the cue induced by the noncompetitive NMDA antagonist MK-801, which has been proposed as a possible screen for clozapine-like compounds, may be an unreliable assay; and (d) the clozapine stimulus is probably a compound cue (a drug "mixture"), which can be used to screen for novel clozapine-like antipsychotics, although the precise receptor mechanisms involved in mediating the clozapine stimulus, and its direct relevance to the antipsychotic action of clozapine remains to be proven conclusively.

Dopamine D2 receptor knock-out mice are insensitive to the hypolocomotor and hypothermic effects of dopamine D2/D3 receptor agonists

The dopamine (DA) D2-like family of receptors is comprised of three subtypes, the D2, D3, and D4 receptors. It has been suggested that the potency of DA receptor agonists to produce hypothermia and hypolocomotion in rodents correlates more strongly with the in vitro affinity for, or potency (mitogenesis test) at the D3 than at the D2 subtype. However, it has recently been reported that when tested in DA D3 receptor knock-out mice, several DA D2/D3 receptor agonists (7-OH-DPAT, PD 128907 and quinelorane) induced levels of hypothermia and decreases of locomotor activity similar to those obtained in control (wild-type) mice. These results do not argue in favour of an implication of DA D3 receptors in these in vivo effects. In order to investigate whether the DA D2 receptor is the subtype that mediates hypothermia and hypolocomotion produced by DA D2/D3 receptor agonists, we tested the effects of ip administration of the DA D2/D3 receptor agonists 7-OH-DPAT and PD 128907, on core temperature and locomotor activity in DA D2 receptor knock-out mice (homozygotes: D2(-/-) and heterozygotes: D2(+/-)), and in wild-type (D2(+/+)) mice. 7-OH-DPAT (0.1-3 mg/kg) and PD 128907 (1-10 mg/kg) induced hypothermia and decreased locomotion in D2(+/+) mice, but had no effects in D2(-/-) mice; the magnitude of the hypothermic and locomotor-reducing effects of these two agonists in D2(+/+) mutants was approximately half that of D2(+/+) mice. During the first 10 min in the activity chambers, the level of spontaneous locomotor activity of D2(-/-) individuals was almost 50% below that of D2(+/+) mice; basal locomotor activity of D2(+/-) mice was between that of D2(-/-) and D2(+/+) individuals. Neither type of mutant showed spontaneous catalepsy or deficits in forelimb muscle strength (grip-strength test). These results show that the presence of DA D2 receptors is necessary for the expression of the locomotor- and core temperature-decreasing effects of DA D2/D3 receptor agonists such as 7-OH-DPAT and PD 128907.

Pharmacological characterization of extracellular acidification rate responses in human D2(long), D3 and D4.4 receptors expressed in Chinese hamster ovary cells

This study characterized pharmacologically the functional responses to agonists at human dopamine D2(long) (hD2), D3 (hD3) and D4.4 (hD4) receptors separately expressed in cloned cells using the cytosensor microphysiometer. Dopaminergic receptor agonists caused increases in extracellular acidification rate in adherent Chinese hamster ovary (CHO) clones expressing hD2, hD3 or hD4 receptors. Acidification rate responses to agonists in other cell lines expressing these receptors were smaller than those in adherent CHO cells. The time courses and maximum increases in acidification rate of the agonist responses in adherent CHO cells were different between the three dopamine receptor clones. Responses were blocked by pretreatment of cells with pertussis toxin or amiloride analogues. Most agonists had full intrinsic activity at each of the dopamine receptor subtypes, as compared to quinpirole, however both enantiomers of UH-232 and (-)3-PPP were partial agonists in this assay system. The functional potency of full agonists at each of the three receptors expressed in CHO cells was either higher than, or similar to, the apparent inhibition constants (Ki) determined in [125I]-iodosulpride competition binding studies. Functional selectivities of the agonists were less than radioligand binding selectivities. The rank orders of agonist potencies and selectivities were similar, but not identical, to the rank orders of radioligand binding affinities and selectivities. The dopamine receptor antagonists, iodosulpride and clozapine, had no effect on basal acidification rates but inhibited acidification responses in CHO cells to quinpirole in an apparently competitive manner. Antagonist potencies closely matched their radioligand binding affinities in these cells.

Neonatal ventral hippocampal lesions attenuate the nucleus accumbens dopamine response to stress: an electrochemical study in the adult rat

Neonatal damage to the ventral hippocampus (VH) can lead, during adulthood, to behaviours that are believed to reflect enhanced mesocorticolimbic dopamine (DA) transmission. In the present study, the effects of neonatal excitotoxic lesions to the VH on spontaneous locomotor activity and stress-elicited increases in extracellular nucleus accumbens (NAcc) DA levels were examined in adult rats. Male pups received, on postnatal day 7, bilateral injections of either an ibotenic acid solution (lesioned) or vehicle (sham-lesioned) into the VH. At 3-4 months of age, animals were assessed during five daily sessions for changes in spontaneous locomotor activity associated with habituation to a novel environment. Voltammetry was used in separate groups of sham- and VH-lesioned animals to monitor the NAcc DA response to each of five once-daily exposures to tail-pinch stress. The results indicate that while VH-lesioned animals seem to habituate to novelty, they remain hyperactive relative to sham-lesioned controls. In contrast, however, stress consistently elicited in VH-lesioned animals smaller and shorter-lasting increases in NAcc DA than in sham-lesioned controls. These data suggest that neonatal excitotoxic damage to VH leads to changes in DA function that persist into adulthood. The blunted response to stress seen in VH-lesioned animals indicates that one consequence of such damage is a functional hyporeactivity in meso-NAcc DA neurons. The fact that these animals are spontaneously more active suggests compensatory changes in DA function that are efferent to DA terminals in NAcc.

Neuroprotection by pramipexole against dopamine- and levodopa-induced cytotoxicity

Pramipexole, a novel non-ergoline dopamine (DA) agonist, has been applied successfully for treatment of Parkinson's disease (PD). We report here that pramipexole can protect dopaminergic cell line Mes23.5 against dopamine- and levodopa-induced cytotoxicity possibly through a mechanism related to antioxidant activity. In the MES 23.5 cultures, DA and L-DOPA induce a dose- and time-dependent cytotoxicity, as determined by tetrazolium salt and trypan blue assays. Furthermore, an in situ terminal deoxynucleotidyl transferase assay demonstrates that DA-induced cell death is apoptotic. Pretreatment with pramipexole in a concentration range (4-100 microM) significantly attenuates DA- or L-DOPA-induced cytotoxicity and apoptosis, an action which is not blocked by D3 antagonist U-99194 A or D2 antagonist raclopride. Pramipexole also protects MES 23.5 cells from hydrogen peroxide-induced cytotoxicity in a dose-dependent manner. In cell-free system, pramipexole can effectively inhibit the formation of melanin, an end product resulting from DA or L-DOPA oxidation. These results indicate that pramipexole exerts its neuroprotective effect possibly through a mechanism, which is independent of DA receptors but related to antioxidation or scavenging of free radicals (e.g. hydrogen peroxide). As a direct DA agonist and potentially neuroprotective agent, pramipexole remains attractive in the treatment of PD.

Both dopamine and the putative dopamine D3 receptor antagonist PNU-99194A induce a biphasic inhibition of phorbol ester-stimulated arachidonic acid release from CHO cells transfected with the dopamine D3 receptor

In Chinese hamster ovary (CHO) cells transfected with the cDNA for the dopamine D3 receptor, low concentrations of dopamine (IC50: 0.5 nM) counteracted the release of arachidonic acid (AA) induced by the protein kinase C activator TPA (maximal inhibition: 15% at 10 - 30 nM). The effect of dopamine -- which was antagonized by pretreatment with pertussis toxin (PTX) or by the dopamine receptor antagonist haloperidol -- was biphasic; thus, at increasing concentrations of dopamine (100 nM - 1 microM), AA levels approached baseline. The preferential dopamine D3 receptor ligand PNU-99194A displayed an effect similar to that of dopamine; thus, whereas low concentrations of PNU-99194A (IC50: 1.9 nM) reduced TPA-induced AA release (maximal inhibition: 15% at 30 - 100 nM), higher concentrations (> or =1 microM) were ineffective. When dopamine and PNU-99194A were administered together at concentrations yielding maximal inhibition of AA release, no additive effect was observed; moreover, a high concentration of dopamine counteracted the AA-reducing effect of a low concentration of PNU-99194A and vice versa. It is suggested that D3 receptors in transfected CHO cells may exert mainly an inhibitory, but also a stimulatory influence on TPA-induced AA release, and that PNU-99194A acts as an agonist in this system.

Dopamine D3 receptor antisense administration reduces basal c-fos and NGFI-B mRNA levels in the rat forebrain

The physiological role of the dopamine D3 receptor is still unclear. The absence of selective pharmacological tools that can discriminate D3 over D2 receptor subtype activity is a major drawback in the elucidation of D3-mediated functions. In order to study D3 receptor actions in rat brain, we have developed an antisense strategy, using oligodeoxynucleotide (ODN) directed against the mRNA of the D3 receptor. Dopamine D2-like agents induce a cascade of events that affect numerous genes in the CNS. Transcription factors are among the most dramatically affected. Using the antisense strategy, we explored the involvement of the D3 receptor on the expression of two classes of transcription factors, the c-fos and NGFI-B. Intracerebroventricular injections of ODNs were made into the lateral ventricle (8 microg/hour, for 5 days). The effect of antisense administration on dopamine D1, D2, and D3 receptor binding was measured by means of receptor autoradiography, whereas transcription factor mRNA levels (c-fos and NGFI-B) were evaluated by in situ hybridization using specific complementary RNA probes. Dopamine D3 receptor levels were significantly decreased in the shell of nucleus accumbens of rats that received the D3 antisense ODN, whereas dopamine D1 and D2 receptor levels were not affected. Basal c-fos mRNA levels were concomitantly reduced in both cingulate and medial prefrontal cortices. Basal NGFI-B mRNA levels were also reduced in the cingulate cortex, shell of nucleus accumbens, and in the dorsomedial striatum, whereas the core of nucleus accumbens and the dorsolateral striatum were not affected after D3 antisense ODN treatments. Our results suggest that D3 receptors may tonically regulate transcription factor expression in rat forebrain. This supports the hypothesis of a constitutive activity of the D3 receptor in vivo.

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.

Lewis and Fischer rats: a comparison of dopamine transporter and receptors levels

Previous reports have shown that the inbred strains of rat, Lewis (LEW) and Fischer 344 (F344), differ in several behavioural and biochemical indices of mesolimbic dopamine (DA) function. Specifically, these two strains differ in their behavioural and neurochemical response to novel environments, and acute amphetamine or cocaine challenge as well as in their susceptibility to addiction. To investigate if differences in DA D1-like, D2-like, D3 receptors and DA transporter could be correlated with these behavioural differences between strains, a comparative autoradiographic study of DA receptors and transporter within the striatal and accumbal regions was undertaken. We observed strain and region specific differences in binding levels for DA D2-like and D3 receptors and for the DA transporter. Namely, DA transporter levels in the striatum, nucleus accumbens and olfactory tubercle of LEW rats were significantly lower than in F344 rats. DA D3 densities in the shell of the nucleus accumbens and olfactory tubercle of LEW rats were lower than the levels found in the F344 rats. Finally, LEW rats have a lower levels of D2-like receptors in the striatum and the core of the nucleus accumbens compared to F344 rats. These data suggest that differences in DA transporter and DA receptors may in part contribute to differences in DA related behaviour seen between these two strains.

The dopamine D3 receptor antagonist PNU-99194A fails to block (+)-7-OH-DPAT substitution for D-amphetamine or cocaine

The present study examined the role of dopamine D3 receptor actions in the stimulus generalization produced by (+)-7-OH-DPAT in rats trained to discriminate either D-amphetamine or cocaine from saline. Twelve male Sprague-Dawley rats were trained to discriminate D-amphetamine (1.0 mg/kg) and 12 rats were trained to discriminate cocaine (5.0 mg/kg) from saline in a two-choice, water-reinforced operant procedure. Stimulus generalization tests were administered with the D3 receptor-preferring agonist, (+)-7-hydroxy-N, N-di-n-propyl-2-aminotetralin ((+)-7-OH-DPAT, 0.01-1.0 mg/kg) as well as the D3-preferring antagonist, 5,6-di-methoxy-2-(dipropylamino)indan-hydrochloride (PNU-99194A, 5-40 mg/kg). PNU-99194A (10-40 mg/kg) was also administered in combination with the training dose of D-amphetamine or cocaine to test for antagonism of each training drug cue. Finally, to assess the role of D3 receptor actions in the stimulus generalization produced by (+)-7-OH-DPAT (0.1 mg/kg), PNU-99194A (10, 20 mg/kg) was tested in combination with this compound in each training group. The results showed complete stimulus generalization with (+)-7-OH-DPAT in rats trained to discriminate D-amphetamine, although only partial stimulus generalization was observed with this compound in rats trained to discriminate cocaine. PNU-99194A produced partial substitution for both training drugs, and failed to block the discriminative stimulus effects of either D-amphetamine or cocaine. Moreover, this compound failed to block the stimulus generalization produced by (+)-7-OH-DPAT in rats trained to discriminate D-amphetamine. These results question the importance of D3 receptor actions in the discriminative stimulus effects of psychostimulants and their similarities to (+)-7-OH-DPAT.

Cocaine sensitization prevents the hypolocomotor effects of high but not low doses of PD 128,907

In this study we examined the effects of the preferential dopamine D3 receptor agonist S(+)-(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]b enzopyrano-[4,3-b]-1,4-oxazin-9-ol (PD 128,907) on locomotion in mice sensitized to cocaine. In mice repeatedly treated with saline, PD 128,907 induced hypoactivity over a wide dose range (0.01-40 mg/kg); however, after repeated treatment with 40 mg/kg cocaine, higher doses of PD 128,907 (2.5-40 mg/kg) no longer induced hypoactivity whereas the effects of lower doses (0.01-0.16 mg/kg) were not altered. Because lower doses of PD 128,907 are thought to induce hypoactivity via activation of dopamine D3 receptors, the present data suggest that, under conditions where cocaine induces marked sensitization to its locomotor effects, the sensitivity of these receptors is not altered. In contrast, because higher doses of PD 128,907 can activate dopamine D2 receptors, it is conceivable that apparent cross-sensitization to its dopamine D2 receptor agonist properties is responsible for the lack of hypolocomotor effects at high doses. Overall, the results indicate that altered dopamine D3 receptor sensitivity does not play an important role in the expression of cocaine-induced sensitization.

Associative and limbic regions of monkey striatum express high levels of dopamine D3 receptors: effects of MPTP and dopamine agonist replacement therapies

The role of the dopamine D3 receptor subtype in the central nervous system is still not well understood. It has a distinct and restricted distribution, mostly associated with limbic territories of the striatum (olfactory tubercle and the shell of nucleus accumbens) in rat brain. Dopaminergic denervation induced by a 6-hydroxydopamine lesion of the nigrostriatal system in rat down-regulates the expression of the D3 receptor. In the present study, we investigated the functional neuroanatomy of the dopamine D3 receptor subtype in the monkey (Macaca fascicularis) basal ganglia. We also studied the effect of administration of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and chronic D1-like (SKF 82958) or D2-like (cabergoline) agonist treatments on dopamine D3 receptor levels using receptor autoradiography. Our results clearly show that the distribution of D3 receptors in the monkey is more closely related to associative and limbic components of the striatum (caudate-putamen), as compared with its sensorimotor counterpart. Hence, D3 receptors may be more specifically involved in cognitive and motivational aspects of striatal functions, which are elaborated in prefrontal, temporal, parietal, cingulate and limbic cortices. Moreover, MPTP administration significantly decreased levels of D3 receptors and this effect was reversed or compensated by a chronic treatment with a D1-like, but not a D2-like, receptor agonist. The D3 receptor may represent an important target for adjunct or direct therapy designed to improve cognitive deficits observed in patients with Parkinson's disease, schizophrenia and other illnesses with frontal lobe cognitive disturbances.

The antisense strategy applied to the study of dopamine D3 receptor functions in rat forebrain

1. The authors have investigated the effects of a dopamine D3 receptor antisense oligodeoxynucleotide (ODN), on neuropeptides (neurotensin and dynorphin) and transcription factor (c-fos) mRNA levels in rat forebrain. 2. Intracerebroventricular injections of ODNs were made into the lateral ventricle (5 and 10 micrograms/h, for 5 days). Effect of antisense administration on dopamine D2 and D3 receptor binding were measured by means of receptor autoradiography. Neuropeptides and c-fos mRNA levels were evaluated by in situ hybridization using specific complementary RNA probes. 3. Dopamine D3 receptor densities were dose-dependently reduced in the shell of nucleus accumbens of rats that received the D3 antisense ODN. Sense and missense controls remained without effect. No significant effect was observed on D2 receptor binding in any of the ODN groups studied, as measured with [3H]raclopride binding. Concomitant reductions of dynorphin and neurotensin mRNA levels were observed in the shell of nucleus accumbens after D3 antisense ODN administration. Interestingly, the D3 antisense administration also reduced c-fos mRNA levels in the cingulate cortex of these animals. 4. The results show that D3 receptors may tonically regulate basal transcription factor, as well as neuropeptides, gene expression in the rat forebrain. These results clearly demonstrate that an antisense strategy could be useful to identify molecular targets under control of specific dopamine receptor subtypes.

Discriminative stimulus properties of the putative dopamine D3 receptor agonist, (+)-PD 128907: role of presynaptic dopamine D2 autoreceptors

The putative D3 receptor agonist, (+)-PD 128907, is widely used to study the functional relevance of D3 receptors in vivo. Given that non-selective D2/3/4 receptor agonists serve as effective discriminative stimuli in rats we have trained animals to discriminate (+)-PD 128907 (30 microg kg(-1), s.c.) from saline and examined the pharmacological specificity of the response. Consistent with a D3 receptor mediated response, the non-selective D2/3 receptor agonist apomorphine and the D3 preferring agonists 7-OH-DPAT and (-) quinpirole generalised to the cue whilst the D2/3 receptor antagonists haloperidol, raclopride, spiperone and (+)-butaclamol antagonised drug lever responding. In contrast, the D1 selective agonist (+/-)-SKF 81297 and D1/5 selective antagonist, R-(+)-SCH 23390 had no effect. Results also suggest that presynaptic dopamine receptors are involved. Thus the dopamine depleting agent alpha-methyl-p-tyrosine potentiated the effects of a submaximal dose of (+)-PD 128907 whereas amphetamine failed to generalise per se and blocked (+)-PD 128907 lever selection. However, studies using subtype selective antagonists argue against a role for the D3 receptor. Thus the 10-fold selective D2 receptor antagonist L-741,626 blocked the (+)-PD 128907 discriminative stimulus whereas L-745,829 and GR 103,691, antagonists > 40 and > 100-fold selective for D3 receptors, failed to modify the response. These results suggest that presynaptic D2 receptors mediate the discriminative stimulus properties of (+)-PD 128907 and highlight the lack of selectivity of (+)-PD 128907 for D3 receptors in vivo.

Effects of the substituted (S)-3-phenylpiperidine (-)-OSU6162 on PET measurements in subhuman primates: evidence for tone-dependent normalization of striatal dopaminergic activity

(-)-OSU6162 is a substituted (S)-3-phenylpiperidine derivative which exhibits some affinity to the dopamine D2 receptor family. In vivo, the compound displays a unique normalizing profile on psychomotor activity by an intriguing mixture of stimulatory and inhibitory properties. In the present investigation, some of the effects of (-)-OSU6162 on central dopaminergic function were studied by positron emission tomography (PET) and L-[11C]DOPA in anaesthetized female rhesus monkeys. (-)-OSU6162 displayed a dopaminergic tone-dependent effect with a reduction in the striatal L-[11C]DOPA influx rate in monkeys with high baseline values and an increased striatal L-[11C]DOPA influx rate in animals with low baseline values. Infusion of (-)-OSU6162 for a whole day resulted in a stable effect with no evidence of tolerance. (-)-OSU6162 also stabilized dopaminergic function by attenuating the upregulation of the striatal L-[11C]DOPA influx rate which has previously been shown to occur following 6R-BH4 or 6R-BH4 + L-tyrosine infusions. This "Protean" effect of (-)-OSU6162 on the striatal dopaminergic function corresponds to previous behavioral observations in intact animals and demonstrates a true functional correlation to the measures obtained with L-[11C]DOPA and PET. The normalizing and stabilizing profile of (-)-OSU6162 should be of value in treating a variety of disorders where an underlying dysregulation or disruption of dopaminergic function can be assumed.

Anxiolytic effects of dopamine receptor ligands: I. Involvement of dopamine autoreceptors

The anxiolytic-like properties of dopamine agonists and antagonists with different receptor profiles were investigated in the ultrasonic vocalization test in rats after subcutaneous administration. Only dopamine D2 receptor agonists inhibited ultrasonic vocalization with the following ED50 values: apomorphine (0.07 mg/kg), quinelorane (0.01 mg/kg), quinpirole (0.04 mg/kg), pramipexole (0.09 mg/kg), roxindole (0.04 mg/kg), talipexole (0.04 mg/kg), (+/-)-7-OH-DPAT (0.05 mg/kg), (+/-)-PPHT (0.03 mg/kg), (-)-TNPA (0.06 mg/kg), PD128907 (0.13 mg/kg). The D2 antagonists haloperidol, mazapertine, raclopride, remoxipride, L745870, U99194A, U101958 and S(-)-DS121, the partial agonists PD143188 and preclamol, the selective D1 agonist R(+)-SKF38393 and the D1 antagonist SCH23390, and the uptake inhibitors GBR12909, GBR12935 and indatraline lacked significant inhibitory effects on ultrasonic vocalization. Because at least some of the D2 receptor agonists investigated have selectivity for dopamine autoreceptors, it is speculated that the dopamine autoreceptor may be a target for the development of new antianxiety drugs.

Behavioral and neurochemical effects of the preferential dopamine D3 receptor agonist cis-8-OH-PBZI

In the present study we investigated the in vivo pharmacological profile of the benz[e]indole cis-8-hydroxy-3-(n-propyl)],2,3a,4,5,9b-hexahydro-1H-benz[e]indole (cis-8-OH-PBZI), which has been described as a preferential dopamine D3 receptor agonist in vitro. The compound inhibited spontaneous locomotor activity in mice, an effect which was antagonized by the dopamine D3 receptor antagonist 5,6-dimethoxy-2-(di-u-propylamino) indan (U99194A). Moreover, cis-8-OH-PBZI inhibited conditioned avoidance responding in rats, a preclinical test indicative of antipsychotic efficacy, at doses which did not induce catalepsy. Doses of cis-8-OH-PBZI (6 and 12 mg/kg) that inhibited spontaneous locomotor activity in rats did not affect interstitial levels of dopamine and dihydroxyphenylacetic acid (DOPAC) in the nucleus accumbens or dorsolateral striatum. In contrast to the effect of the dopamine receptor agonist (+/-)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphythalene (7-OH-DPAT), cis-8-OH-PBZI did not induce locomotor activity in reserpinized mice. In conclusion, cis-8-OH-PBZI exhibits a pharmacological profile that suggests it has antipsychotic activity but lacks the motoric side effects often associated with antipsychotic medication. The data suggest a mechanism requiring the activation of postsynaptic dopamine D3 receptors and support the hypothesis that these receptors mediate inhibitory behavioral effects.

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.

D3 dopamine receptor-deficient mouse: evidence for reduced anxiety

Mice without functional D3 dopamine receptors were examined in two animal models for anxiety: the open-field test and the elevated plus-maze test. In the open field, D3 receptor-deficient mice (D3-/-) entered the center significantly more often than normal (D3+/+) littermates, suggesting an anxiolytic-like effect of the D3 receptor mutation. Consistent with this finding, D3-/- mice entered open arms of the plus maze significantly more often and longer than D3+/+ littermates, but did not differ in closed-arm entries, an index of general activity. Heterozygous (D3 +/-) animals showed intermediate behavioral changes. We interpret these results as indicative of reduced anxiety in mice without D3 receptors. Our findings thus suggest that D3 dopamine receptors are involved in the regulation of anxiety.

Effects of the putative dopamine D3 receptor antagonist PNU 99194A on motor behavior and emotional reactivity in C57BL/6J mice

Due to the regional expression of D3 dopamine receptors in limbic areas of the brain, there has been considerable interest in the potential role of this receptor subtype in mediating emotional behavior. Previous studies in habituated rats have shown that the putative dopamine D3 receptor antagonist 5,6-dimethoxy-2-(di-n-propylamino)indan (PNU 99194A) increased locomotor behavior. The present study examined the effects PNU 99194A on motor and emotional behaviors in C57BL/6J mice. Motor behavior was assessed in both habituated and nonhabituated mice. Emotional behavior was assessed using the elevated plus-maze and a social context involving an isolated C57BL/6J mouse and a nonaggressive conspecific. In mice habituated to the activity chamber prior to drug administration, PNU 99194A increased locomotion and rearing at lower doses (5, 10 mg/kg) whereas higher doses (20, 30 mg/kg) reduced these behaviors early in the test session. Thigmotaxis was increased independently of the effects on motor behavior. In mice exposed to the activity chamber for the first time, PNU 99194A produced a weak motor activation at lower doses and an initial decrease in motor behavior at higher doses that was followed by an increase in locomotion later in the test session. PNU 99194A had no systematic effects on activity in the elevated plus-maze, but dose-dependently increased flight reactivity in the social reactivity paradigm. These and previous findings raise questions about the role of dopamine D3 receptors in mediating motor behavior and emotional reactivity as well as the pharmacology of this putative dopamine D3 receptor antagonist.

Selective effects on prefrontal cortex serotonin by dopamine D3 receptor agonism: interaction with low-dose haloperidol

1. Negative symptoms of schizophrenia are characterized by amotivation, anhedonia and anergia. These aspects of the symptom profile can be modeled by D3 agonism in animal behavioral models. 2. Serotonergic systems have been implicated in pathophysiologic substrates for this disorder; most notably, in deficit state schizophrenia, as newer 'atypical' neuroleptics which are especially efficacious for treating this syndrome antagonize central 5-HT2 receptors. 3. FC regions may also be important in chronic negative symptoms, as hypofrontality has been associated with these schizophrenic features. 4. The author examined effects of a behaviorally-active dose of the D3 agonist, 7OH, on 5-HT metabolism in FC, and the ability of a low-dose neuroleptic treatment to antagonize this biochemical effect. 5. Acute administration of 7OH induced a selective decrease of 5-HT turnover in the FC without affecting metabolism of this transmitter in more subcortical DA regions. 6. Hal, which has previously been demonstrated to antagonize electrophysiologic, biochemical and behavioral effects of 7OH, was without effect on agonist-induced decreases in 5-HT turnover. 7. The biochemical association between D3 agonism and reductions of FC 5-HT may be significant for pathophysiologic mechanisms of negative symptoms, and antagonism of this effect may differ for neuroleptics with varying efficacy in alleviating these symptoms.

Dopamine D3 receptor mutant mice exhibit increased behavioral sensitivity to concurrent stimulation of D1 and D2 receptors

The dopamine D3 receptor is expressed primarily in regions of the brain that are thought to influence motivation and motor functions. To specify in vivo D3 receptor function, we generated mutant mice lacking this receptor. Our analysis indicates that in a novel environment, D3 mutant mice are transiently more active than wild-type mice, an effect not associated with anxiety state. Moreover, D3 mutant mice exhibit enhanced behavioral sensitivity to combined injections of D1 and D2 class receptor agonists, cocaine and amphetamine. However, the combined electrophysiological effects of the same D1 and D2 agonists on single neurons within the nucleus accumbens were not altered by the D3 receptor mutation. We conclude that one function of the D3 receptor is to modulate behaviors by inhibiting the cooperative effects of postsynaptic D1 and other D2 class receptors at systems level.

Synthesis of [11C-methyl]-(-)-OSU6162, its regional brain distribution and some pharmacological effects of (-)-OSU6162 on the dopaminergic system studied in the rhesus monkey by positron emission tomography

The labelling of the presynaptic dopamine receptor antagonist (-)-OSU6162, ((S)-(-)-3-(3-(methylsulfonyl)phenyl)-1-propylpiperidine) was performed by an alkylation with [11C]methyl iodide of the thio anion (-)-OSU1281, followed by a selective oxidation to the corresponding methyl sulfone, [11C-methyl]-(-)-OSU6162. The total radiochemical yield calculated from the produced [11C]carbon dioxide to final product was about 25% and the time of synthesis was in the range of 40 min from end of bombardment. The synthesis of the precursor, (-)-OSU1281, was performed from (-)-3PPP in a three-step synthesis. The regional brain distribution of (-)-OSU6162 radiolabelled with 11C was studied in rhesus monkeys by means of positron emission tomography, PET. [11C-Methyl]-(-)-OSU6162 was rapidly and uniformly distributed to gray matters of the brain, and no decrease of radioactivity uptake in the brain was seen after pretreatment with 1 to 3 mg/kg/h of (-)-OSU6162. The effect of doses of 1 to 3 mg/kg/h of (-)-OSU6162 on the dopamine binding was studied by PET using [11C-methyl]raclopride. Radioactivity in the striatum was significantly and dose-dependently decreased by (-)-OSU6162 (r = 0.88), supporting competition with dopamine for selective binding to dopamine receptors.

Excitation of type II anterior caudate neurons by stimulation of dopamine D3 receptors

Previous studies have demonstrated that both direct- and indirect-acting dopamine (DA) receptor agonists excite type II neurons in the anterior caudate (CN) by stimulation of DA receptors belonging to the D2 receptor subfamily (D2, D3, D4 receptor subtypes). In the present study, pramipexole, a D3-preferring DA agonist effective in treating Parkinson's disease, excited type II anterior CN neurons. As with other direct-acting agonists, excitation of the CN neurons occurred only at doses above those that silenced DA neurons in the substantia nigra pars compacta (SNPC). Although more potent than pramipexole in inhibiting SNPC cells, PNU-91356A, a D2-preferring agonist, did not excite type II CN cells. The D3-preferring antagonist (+)-AJ76 was weaker than haloperidol, a D2-preferring antagonist, in reversing the effects of amphetamine on firing rates in dopaminergic neurons in both the SNPC and the CN. However, in relationship to its potency in the SNPC, (+)-AJ76 was more potent than haloperidol in the CN. PNU-101387, a selective D4 antagonist, did not alter amphetamine-induced stimulation of type II CN neurons. We conclude that DA agonists may excite type II anterior CN neurons via D3 receptor activation. The stimulation of these neurons may contribute to the anti-parkinsonian effects of pramipexole.

Perinatal asphyxia induces long-term changes in dopamine D1, D2, and D3 receptor binding in the rat brain

We have investigated the long-term effects of 15-16 min or 19-20 min of perinatal asphyxia on D1, D2, and D3 receptors (analyzed by quantitative autoradiography) in the mesotelencephalic dopamine systems of the 4-week-old rat. Perinatal asphyxia reduced D1 antagonist binding ([3H]SCH 23390 in the presence of ketanserine) in the accumbens nucleus, the olfactory tubercle, and the substantia nigra and increased D1 agonist binding ([3H]dopamine in the presence of spiperone) in the accumbens nucleus and the olfactory tubercle. No changes in D2 antagonist binding ([123]iodosulpride) were found, whereas D2 agonist binding ([3H]N-propylnorapomorphine, [3H]NPA) was reduced in the posterior part of the caudate-putamen, and following 19-20 min of asphyxia it was also reduced in the accumbens nucleus. D3 agonist binding (R/S-(+/-)-2-(N,N-di[2,3(n)-3H] propylamino)-7-hydroxy-1,2,3,4-tetrahydronaphthalene, [3H]7-OH-DPAT) was increased in the anterior part of the caudate-putamen following 15-16 min but not 19-20 min of asphyxia. The results indicate that perinatal asphyxia reduced the number of D1 receptors and increased D1 agonist affinity in the accumbens nucleus and the olfactory tubercle and reduced the number of D1 receptors in the substantia nigra. The number of D2 receptors was unchanged by asphyxia, whereas the D2 agonist affinity was reduced in the caudate-putamen and in the accumbens nucleus. D3 agonist binding was increased in the caudate-putamen selectively following 15-16 min of asphyxia. In conclusion, asphyxia during birth induces long-term changes in the binding characteristics of dopamine receptors in the mesotelencephalic dopamine systems, which may contribute to previously reported behavioral changes.

Evidence to suggest that agonist modulation of hyperlocomotion is via post-synaptic dopamine D2 or D3 receptors

It has been suggested that a sub-population of dopamine D3 receptors is located pre-synaptically and these serve as autoreceptors in dopamine projection areas such as the nucleus accumbens/ventral striatum. To study further the physiological role and synaptic location of the dopamine D3 receptor, we have investigated the in vivo effect of the D3/D2 receptor agonist quinelorane on amphetamine-induced hyperactivity and extracellular dopamine release from the nucleus accumbens of the conscious rat. Amphetamine increased dopamine release to 202 +/- 34% of pre-injection control values, but quinelorane at 2.5 micrograms/kg, a dose which effectively blocked amphetamine-induced hyperlocomotion, had no significant effect on amphetamine-induced dopamine release. These data suggest that hyperlocomotion is mediated via post-synaptic rather than pre-synaptic dopamine receptors. Since quinelorane has significant affinity for the dopamine D3 receptor, these effects may be via post-synaptic D3 receptors; however, D2 receptor effects cannot be disregarded. In summary, these data indicate that the quinelorane effect on amphetamine-stimulated hyperlocomotion is not mediated via D3 or D2 autoreceptors, but rather a population of receptors located post-synaptically, which appear to mediate the inhibition of rat locomotor activity.

Characterization of the effect of dopamine D3 receptor stimulation on locomotion and striatal dopamine levels

By examining the effect of dopamine (DA) D3 receptor stimulation on locomotor activity and extracellular levels of DA in striatum we show that inhibition of locomotor activity induced by DA D3 receptor-selective agonists is mediated by two interacting mechanisms: (1) directly via the stimulation of DA D3 receptors that inhibit locomotor activity, and (2) indirectly via a decrease in extracellular levels of DA. Thus, the moderately DA D3 receptor-selective agonist R-(+)-7-OH- DPAT (R-(+)-7-hydroxy-2-(N,N-di-n-propylamino)tetralin) decreased locomotor activity after administration of 10 nmol/kg and extracellular DA levels in accumbens and striatum after administration of 30 nmol/kg. A decrease in locomotor activity that coincided with a decrease in extracellular DA levels in striatum was observed after administration of 100 nmol/kg of the DA D3 receptor-selective agonist PD128907 ((+)-trans-3,4,4a,10b-tetrahydro-4-propyl-2H,5H-[1]benzopyrano[4,3 b]-1,4-oxasin-9-ol. In combination with the partial, DA D3 receptor-selective agonist PD151328 (2-[4[3-(4-phenyl)-1- piperazinyl)propoxy]phenyl]-benzamidazole), a reversal of the attenuating effect of PD128907 on locomotor activity was observed, without an effect on extracellular levels of DA. In combination with a low--10 nmol/kg--dose of haloperidol, a reversal of the inhibitory effect of PD128907 on locomotor activity was observed that coincided with an increase in extracellular levels of DA. In the presence of 0.5 mg/kg amphetamine, PD128907 decreased amphetamine-induced locomotor activity. This effect could be reversed by PD151328.

Effect of chronic antipsychotic drug treatment on preprosomatostatin and preprotachykinin A mRNA levels in the medial prefrontal cortex, the nucleus accumbens and the caudate putamen of the rat

In situ hybridization histochemistry was used to study the expression of preprosomatostatin (PPSOM) and preprotachykinin A (PPT-A) mRNA in the medial prefrontal cortex (mPFC), the nucleus accumbens (NAC) and the caudate putamen (CP) of the rat after chronic (21 days) treatment with the classical antipsychotic drug haloperidol (1 mg/kg i.p.), the atypical antipsychotic drugs clozapine (15 mg/kg i.p.) and amperozide (5 mg/kg i.p.), and the selective dopamine (DA)-D2/D3 receptor antagonist raclopride (2 mg/kg i.p.). Whereas amperozide markedly elevated the numerical density of PPSOM mRNA expressing neurons in the mPFC (52%), the other drugs did not significantly affect PPSOM mRNA levels in any of the brain regions studied. Amperozide also altered PPT-A mRNA expression in the mPFC, i.e. a decrease (22%) was found. Of the other drugs tested only haloperidol significantly decreased PPT-A mRNA levels in the NAC shell (14%), in the dorso-lateral CP (19%) and in the medial CP (15%). In view of the differences between amperozide and the other drugs studied, as regards both pre-clinical and clinical characteristics, we suggest that the specific effects of amperozide on PPSOM and PPT-A mRNA in the mPFC may be related to its 5-HT releasing action in the frontal cortex, an effect possibly caused by its alpha2-adrenoceptor blocking activity. This effect, in turn, may be related to an antidepressant-like action that this compound exhibits in animal studies. The decrease in PPT-A mRNA levels seen after the haloperidol treatment is probably due to its potent DA-D2 receptor antagonism and may be related to side-effects, rather than therapeutic effects of this drug.

DAD1- and DAD2-like agonist effects on motor activity of C57 mice: differences compared to rats

Studies on rats indicate that DAD1- and DAD2-like agonists produce a biphasic action on motor activity, with low doses reducing activity below control levels, and higher doses initially reducing, then elevating, activity for a prolonged period. Although some of the reported effects of DAD1- and DAD2-like receptor agonists on motor activity of mice are consistent with their effects on rats, the possibility of species differences is also apparent. In the current study the effects of DAD1- and DAD2-like agonists on motor activity of C57BL/6 (C57) mice were determined to establish species consistencies and differences with respect to their effects on rats. The partial DAD1-like agonist SKF 38393 reduced the activity of C57 mice at low doses and elevated activity above control levels at higher doses, if the mice were thoroughly habituated to the test chamber. The full DAD1 agonist SKF 82958 also increased the activity of C57 mice, and along with the SKF 38393 results indicates a response to DAD1 receptor stimulation similar to that reported for rats. In contrast to the species similarity in response to DAD1 stimulation, the DAD2-like agonist quinpirole produced only a dose-responsive monotonic reduction in the activity of C57 mice, whether the animals were nonhabituated or well-habituated to the testing environment, male or female, young or mid-aged, injected intraperitoneally (i.p.) or subcutaneously (s.c.), and with either low or high doses. This apparent species difference in response to quinpirole might reflect distinguishable functional properties of the DA subreceptor systems.

Effects of the dopamine D3 receptor agonist, R(+)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin, on memory processes in mice

The putative dopamine D3 receptor agonist, R(+)-7-hydroxy-N,N-di-n-propyl-2-aminotetralin (R(+)-7-OH-DPAT) (0.1-100 microg/kg, s.c.), administered before training, immediately after training, and before retention significantly shortened step-down latency of passive avoidance learning, indicating the amnesic effects of R(+)-7-OH-DPAT. Neither the dopamine D1 receptor antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzaz epine maleate R(+)-SCH23390) (2.5 and 5 microg/kg, i.p.), nor the dopamine D2 receptor antagonist, S(-)-sulpiride (10 and 30 mg/kg, i.p.), markedly influenced the R(+)-7-OH-DPAT (10 and 100 microg/kg, s.c.)-induced amnesia. In addition, only a 1000 microg/kg dose of R(+)-7-OH-DPAT decreased locomotor activity; 1 and 100 microg/kg doses of the drug were ineffective. These results suggest that the amnesic effects of the dopamine D3 receptor agonist, R(+)-7-OH-DPAT, are not mediated via dopamine D1 or D2 receptors in the brain.

The behavioural effects of pramipexole, a novel dopamine receptor agonist

Pramipexole (SND 919; 2-amino-4,5,6,7-tetrahydro-6-propyl-amino-benzthiazole-dihydroc hlo ride) is a novel dopamine D2 family receptor agonist with a predominant action on D2 autoreceptors and with some D3 vs. D2 receptor preference. The central behavioural effects of pramipexole given subcutaneously to rats (male Wistar) and mice (Albino Swiss) are presented in this paper. Used in low doses (0.001-0.1 mg/kg), pramipexole induced locomotor hypoactivity which was antagonized by a low dose of spiperone; at higher doses (0.3, 1 mg/kg) it evoked hyperactivity which was inhibited by haloperidol, sulpiride and clozapine, but not by SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3- benzazepine hydrochloride). Pramipexole (0.1-1.0 mg/kg) antagonized the akinesia induced by combined pretreatment with reserpine (5 mg/kg) and alpha-methyl-p-tyrosine (250 mg/kg). Pramipexole (0.1-1 mg/kg) potentiated the hyperkinetic effect of L-DOPA (L-3,4-dihydroxyphenylalanine) (50 and 200 mg/kg, together with benserazide, 50 mg/kg) in naive and monoamine-depleted (reserpine + alpha-methyl-p-tyrosine) rats. The higher doses of pramipexole (1 and 3 mg/kg) evoked stereotypy which was antagonized by pretreatment with sulpiride or clozapine. The catalepsy induced by haloperidol, spiperone or fluphenazine was antagonized by pramipexole (1-3 mg/kg). Pramipexole (1 mg/kg) induced hypothermia in mice, which was antagonized by sulpiride. The obtained results indicate that pramipexole: (i) at low doses stimulates the dopamine D2 presynaptic autoreceptors; (ii) at higher doses stimulates dopamine D2 postsynaptic receptors. An effect on the dopamine D3 receptor cannot be excluded. At low doses pramipexole may have antipsychotic activity, and at higher ones antiparkinsonian activity.

Induction of dopamine D3 receptor expression as a mechanism of behavioral sensitization to levodopa

In rats with unilateral lesions of the nigrostriatal dopamine pathway with 6-hydroxydopamine, the motor stimulating effects of levodopa, an indirect dopamine receptor agonist, evidenced by contraversive rotations, become enhanced upon repeated intermittent administration. However, the mechanisms of this behavioral sensitization are essentially unknown. We show that development of sensitization is accompanied by a progressive appearance of D3 receptor mRNA and binding sites, visualized by in situ hybridization and 7-[3H] hydroxy-N,N-di-n-propyl-2-aminotetralin autoradiography, respectively, occurring in the denervated caudate putamen, a brain area from which this receptor subtype is normally absent. Development and decay of these two processes occur with closely parallel time courses, whereas there were no marked changes in D1 or D2 receptor mRNAs. D3 receptor induction by levodopa is mediated by repeated D1 receptor stimulation, since it is prevented by the antagonist SCH 33390 and mimicked by the agonist SKF 38393, but not by two D2 receptor agonists. The enhanced behavioral response to levodopa is mediated by the newly synthesized D3 receptor, since it is antagonized by nafadotride, a preferential D3 receptor antagonist, in low dosage, which has no such effect before D3 receptor induction. D3 receptor induction and behavioral sensitization are also accompanied by a sustained enhancement of prodynorphin mRNA level and a progressively decreasing expression of the preprotachykinin gene. We propose that imbalance between dynorphin and substance P release from the same striatonigral motor efferent pathway, related to D3 receptor induction, is responsible for behavioral sensitization.

Locomotor inhibition, yawning and vacuous chewing induced by a novel dopamine D2 post-synaptic receptor agonist

The N-n-propyl analog of dihydrexidine ((+/-)-trans-10, 11-dihydroxy-5,6,6a,7,8,12b-hexahydrobenzo[a]phenanthridine) is a dopamine receptor agonist with high affinity for dopamine D2 and D3 receptors (K0.5 = 26 and 5 nM, respectively). Members of the hexahydrobenzo[a]phenanthridine structural class are atypical because they display high intrinsic activity at post-synaptic dopamine D2 receptors, but low intrinsic activity at dopamine D2 autoreceptors. The present study examined the effects of (+/-)-N-n-propyl-dihydrexidine on unconditioned behaviors in rats. The most striking results observed were large, dose-dependent decreases in locomotor activity (e.g., locomotor inhibition), and increases in vacuous chewing; yawning was also increased at the highest dose of (+/-)-N-n-propyl-dihydrexidine. The locomotor inhibition and yawning induced by (+/-)-N-n-propyl-dihydrexidine were blocked by pre-treatment with (-)-remoxipride (S(-)-3-bromo-N-((1-ethyl-2-pyrrolidinyl)-methyl)-2, 6-dimethoxybenzamide), a dopamine D2 receptor antagonist, but not by the dopamine D1 receptor antagonist (+)-SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1 H-3-benzazepine). Vacuous chewing was decreased by both (-)-remoxipride and (+)-SCH23390. These data support the hypothesis that a subpopulation of post-synaptic dopamine D2 receptors has a critical role in decreases in locomotor activity and induction of vacuous chewing and yawning.

The dopamine D3 receptor antagonist, (+)-S 14297, blocks the cataleptic properties of haloperidol in rats

In contrast to haloperidol, the selective dopamine D3 receptor antagonist, (+)-S 11566 [(+/-)-[7-(N,N-dipropylamino)-5,6,7,8-tetrahydro- naphtho(2,3b)dihydro,2,3-furanel]] and its active isomer, (+)-S 14297, induced neither catalepsy nor reduced conditioned avoidance responses in rats. (+)-S 11566 and (+)-S 14297 did, however, dose-dependently abolish the cataleptic actions of haloperidol. This action was expressed stereospecifically inasmuch as (-)-S 17777, the inactive distomer of (+)-S 14297, was ineffective. Further, the influence of haloperidol upon conditioned avoidance responses was not affected by (+)-S 14297. These data suggest that blockade of dopamine D3 receptors may inhibit the extrapyramidal but not-as based on the conditioned avoidance response paradigm-antipsychotic actions of neuroleptics.

Mechanism of action of clozapine-induced modification of motor behavior in an animal model of the "super-off" phenomenon

We tested the effects of clozapine, and "atypical" neuroleptic with high affinity for the D4 (dopaminergic), and the 5-HT1c and 5-HT2 (serotonergic) receptor subtypes on locomotor activity in an animal model of Parkinson's disease showing a bimodal response curve to increasing doses of a D2 agonist. Sulpiride (D2 antagonist) and ritanserin (5-HT1c and 5-HT2 antagonist) were used for comparison. The D1 agonist SKF 38393 at a dose of 8 mg/kg significantly reversed the akinesia induced by chronic reserpine treatment (1 mg/kg for 5 days) and alpha-methyl-p-tyrosine pretreatment (300 mg/kg). In this model, the addition of a low dose of a D2 agonist, LY 171555 (quinpirole, 1 microgram/kg), inhibited the effects of SKF 38393, whereas the same drug at higher doses (5-50 microgram/kg) restored and potentiated the stimulatory response to D1 stimulation. Clozapine inhibited the inhibitory phase and potentiated the stimulatory phase of the curve. Sulpiride inhibited both phases of the dose-response curve (inhibitory/stimulatory), whereas ritanserin had no effect. We believe these results may reflect a disinhibition phenomenon possible mediated by the blockade by clozapine of a subpopulation of inhibitory, dopamine (DA) receptors belonging to the D2 "family."

Dopamine D2 receptors play a role in the (-)-apomorphine-like discriminative stimulus effects of (+)-PD 128907

The discriminative stimulus effects of the dopamine D3 receptor-preferring agonist, S(+)-(4aR,10bR)-3,4,4a,10b-tetrahydro-4-propyl -2H, 5H-[1]benzopyrano-[4,3-b]-1,4-oxazin-9-ol ((+)-PD 128907), were examined in rats trained to discriminate (-)-apomorphine (0.16 m/kg) from saline in a two-lever, fixed ratio 10 drug-discrimination paradigm. Both (-)-apomorphine and (+)-PD 128907 produced dose-related (-)-apomorphine-lever selection, with full substitution observed at 0.16 mg/kg, i.p. (ca. 0.5 mumol/kg). Drug-appropriate responding produced by either (-)-apomorphine or (+)-PD 128907 was antagonized by the putative dopamine D3 receptor antagonists, (1S,2R)-cis-5-methoxy-1-methyl-2-(n-propylamino)tetralin) ((+)-AJ76) and cis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino)tetralin ((+)-UH 232), as well as by the dopamine D2 receptor antagonist haloperidol. Because haloperidol was approximately 30-150-times more potent than (+)-AJ76 or (+)-UH-232 in blocking the effects of either receptor agonist, the results indicate that dopamine D2 receptors play a role in the discriminative stimulus effects of (+)-PD 128907.

Interactions between cocaine and (-)-DS 121: studies with 2-deoxyglucose autoradiography and microdialysis in the rat brain

(-)-DS 121 [S-(-)-3-(3-cyanophenyl)-N-n-propyl piperidine], a dopamine autoreceptor preferring antagonist, has been shown to stimulate locomotor activity and induce conditioned place preference. However, the drug fails to facilitate intracranial self-stimulation or substitute for cocaine in cueing experiments, and it blocks cocaine self-administration. In the present study using 2-deoxyglucose autoradiography, (-)-DS 121 (at 50 but not 15 mg/kg i.p.) significantly and selectively increased local cerebral glucose utilization in the olfactory cortex, medial and lateral septum, hippocampal areas, substantia nigra pars reticulata, caudate, and mammillary body. Local cerebral glucose utilization was depressed in caudal areas of the cortex. Interestingly, however, both doses of (-)-DS 121 blocked the increases in local cerebral glucose utilization produced by 5 mg/kg i.v. cocaine. The present study also evaluated the effects of (-)-DS 121 of extracellular striatal dopamine levels using microdialysis in freely moving rats. By itself, 15 mg/kg of (-)-DS 121 increased extracellular striatal dopamine levels to approximately 300% of controls. Cocaine (5 mg/kg i.v.) produced a 370% increase in striatal dopamine levels. When rats were pretreated with (-)-DS 121, a subsequent dose of cocaine augmented the increase in extracellular striatal dopamine to 870% of controls. The results support the contention that (-)-DS 121 possesses weak cocaine-mimetic effects and that its antagonism of cocaine's subjective effects are due to interactions with dopamine at postsynaptic sites. It is hypothesized that, like other preferential autoreceptor antagonists, (-)-DS 121 may be useful as a pharmacotherapy in drug addiction.