Dopamine D3 receptor mRNA and binding sites in human brain

Human D3 dopamine receptor in the medulloblastoma TE671 cell line: cross-talk between D1 and D3 receptors

In search of a cell line in which the D3 dopamine receptor is expressed endogenously, we found that the neuron-derived human medulloblastoma cell line TE671 expresses the human D3 (hD3) and D1 (hD1) receptor, but neither the D2 or D4 receptors. Exposure of TE671 cells to the D3 agonist 7-OH-DPAT (DPAT), or to the D1 agonist SKF-38393 (SKF) increased the expression of hD3 or hD1 mRNA, respectively. Moreover, whereas DPAT had no effect on hD1 mRNA levels, stimulating the cells with SKF caused an increase in both hD1 and hD3 transcript levels. These results suggest (i) that following ligand stimulation, hD3 and hD1 receptors are upregulated to enhance their own receptor expression, and (ii) that upregulation of hD1 receptor transcripts leads to a stimulation of the hD3 dopamine receptor transcripts.

NGB 2904 and NGB 2849: two highly selective dopamine D3 receptor antagonists

N-(4-[4-¿2, 3-dichlorophenyl¿-1-piperazinyl]butyl)-3-fluorenylcarboxamide and N-(4-[4-¿2, 3-dichlorophenyl¿-1-piperazinyl]butyl)-2-biphenylenylcarboxamide were prepared in several steps from 2,3-dichloroaniline. These compounds were identified as highly selective dopamine D3 receptor antagonists.

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.

Identification of the dopamine D3 receptor in oligodendrocyte precursors: potential role in regulating differentiation and myelin formation

Expression of the dopamine D3 receptor (D3r) was found in primary mixed glial cultures from newborn brain and in the corpus callosum in vivo during the peak of myelination. Expression of the D3r mRNA, but not D2r mRNA, was detected as early as 5 d in vitro (DIV) by RT-PCR. Immunoblot studies revealed D3r protein was also expressed in the cultures. Double immunofluorescence analysis for the D3r and for surface markers of specific stages of oligodendrocyte development indicated that D3r expression occurred in precursors and in immature oligodendrocytes but not in mature oligodendrocytes (i.e. , A2B5(+) 007(-) 01(-) and A2B5(+) 007(+) 01(-) cells but not A2B5(-) 007(+) 01(+) cells). Confocal microscopic analysis indicated that D3r was associated with cell bodies and cell membranes but not with the processes emanating from cell somas. Immunohistochemistry of brain sections revealed the presence of D3r in some oligodendrocytes located mainly within the genu and radiato of the corpus callosum during the active period of myelination. Treatment of cultures with 20 microM quinpirole led to decreased numbers of O1(+) oligodendrocytes possessing myelin-like membranes as well as an increase in the number of precursors in 14 DIV cultures. This effect was prevented by the dopamine antagonist haloperidol. These results show that the D3r expression is not restricted to neurons but it is also expressed in differentiating oligodendrocytes before terminal maturation. It also suggests that dopamine or some other D3r ligand may play a role in oligodendrocyte differentiation and/or the formation of myelin by mature oligodendrocytes.

Mapping of dopamine D3 receptor binding site by pharmacological characterization of mutants expressed in CHO cells with the Semliki Forest virus system

Nine mutants and the wild-type human dopamine D3 receptor were expressed at high levels in BHK and CHO cells using the Semliki Forest virus system and were analysed for receptor binding with several structurally different dopamine D3 ligands. The mutation His349Leu showed a significant decrease in pKi values for raclopride, dopamine and GR218231, but an increase in affinity for GR99841. Thr369Val had an increase in pKi for both GR99841 and 7-OH-DPAT. The receptor modelling based on sequence alignment with bacteriorhodopsin indicated that Thr369 and His349 are located on the inside of the ligand binding pocket and the effect of the mutagenesis was therefore expected. The change in binding affinity for Thr369Val could be due to the location in the transmembrane domain VII close to the aspartate residue in domain III, the postulated counter ion for dopamine.

D3 dopamine receptor mRNA is widely expressed in the human brain

Considerable attention has been given to the association of the D3 dopamine receptor subtype and limbic function based on the abundant localization of D3 receptor sites and mRNA expression in the islands of Calleja and nucleus accumbens in experimental animals. Though most human anatomical studies have focused on the role of D3 receptors in limited brain structures, detailed information about the overall anatomical organization of the D3 receptor in the human brain is still, however, not available. In the current study, we examined the anatomical distribution of D3 receptor mRNA expression at different levels of the human brain in whole hemisphere horizontal cryosections using in situ hybridization. This approach made it possible to establish for the first time the wide and heterogenous expression of the D3 receptor gene throughout the human brain. As expected, the most abundant D3 mRNA expression levels were found in the islands of Calleja and discrete cell cluster populations within the ventral striatum/nucleus accumbens region. High levels were also evident within the dentate gyrus and striate cortex. Low to moderate D3 mRNA expression levels were apparent in most brain areas including all other cortical regions (highest in the anterior cingulate/subcallosal gyrus), caudate nucleus, putamen, anterior and medial thalamic nucleus, mammillary body, amygdala, hippocampal CA region, lateral geniculate body, substantia nigra pars compacta, locus coeruleus, and raphe nuclei. While the current anatomical map of D3 receptor mRNA expression in the human brain does confirm previous reports that D3 receptors may play important roles in limbic-related functions such as emotion and cognition, the findings also suggest other non-limbic functions for D3 mRNA-expressing cell populations such as processing of motor and sensory information.

Evidence for enhancement of gap junctional coupling between rat island of Calleja granule cells in vitro by the activation of dopamine D3 receptors

1. Using patch-clamp techniques, we have studied actions of dopamine and related compounds on granule neurones within the islands of Calleja in vitro, in slices if approximately 200 microns thickness or as groups of varying cell number following enzymic digestion. 2. Prior to agonist application, island of Calleja granule cells displayed spontaneous stepwise shifts in whole-cell conductance ranging from 104 to 632 pS. The reversal potentials of these conductance changes ranged widely and matched the distribution of the cells' membrane potentials. Reversal potentials and membrane potentials shifted equally when cells were uniformly depolarized in 24 mM external K+. 3. Bath-applied dopamine elicited, after a delay of 4-9 min, an exaggerated form of the spontaneous behaviour that frequently gave way to a sudden large (up to thirtyfold) conductance change. At concentrations of 100-300 nM, a range of agonists with increasing affinity for the D3 receptor (apomorphine, quinpirole, 7-OH DPAT and PD 128907) triggered the response. The actions were neither mimicked by SKF-38393 nor antagonized by SCH-23390 (a selective D1 agonist and antagonist, respectively). Haloperidol reversibly blocked responses elicited by the D3/D2 agonist quinpirole. The action of effective agonists was maintained when transmitter release was abolished. Given the reported lack of D2 receptors in the islands of Calleja, these findings indicate a direct action of dopamine at the D3 receptor. 4. The dopaminergic effects were not affected by Gd3+ or substantial replacement of external Na+ with TEA, Tris or choline, eliminating stretch-activated channels but suggesting that if transmembrane channels were to be involved in this dopaminergic action they posseses a non-selective permeability to large cations. The reported presence of gap junctions in the islands of Calleja offers the explanation that these effects derive from enhanced activity of such channels or their hemi-constituents. 5. In testing the possible involvement of gap junctional coupling the following experimental observations were made: (i) alkalinization of slices mimicked the effect of D3 agonists; (ii) in cell groups, recording from pairs provided evidence of intercellular coupling, and mechanical separation of recorded neurones from neighbouring cells during the agonist-evoked response caused shutdown of the additional conductance; (iii) when applied to slices, the gap junctional blocker, 18 alpha-glycyrrhetinic acid, whilst not preventing the full-blown dopamine response, significantly reduced both the variance of recorded granule cell input conductance and the cells' apparent capacitance. 6. Taken together the results indicate a D3 action in granule cells, which is best explained by a dopaminergic promotion of intercellular coupling. The physiological relevance of such a mechanism is discussed.

Layer V neurons bear the majority of mRNAs encoding the five distinct dopamine receptor subtypes in the primate prefrontal cortex

In situ hybridization histochemistry was used to determine the laminar distribution of D1, D2, D3, D4, and D5 dopamine receptor mRNAs in the primate prefrontal cortex and to compare striatal and cortical levels of these messages within the same tissue sections. All five subtypes of dopamine receptor mRNA are present in both the monkey striatum and the cerebral cortex but in different proportions within each structure. Thus, levels of D1 and D2 mRNAs are noticeably stronger in the striatum than in the cortex, whereas D4 and D5 expression is clearly higher in the cortex. The D3 transcripts appear nearly equivalent in the striatum and the cortex. A major finding is that, within the prefrontal cortex, mRNAs encoding all dopamine receptor subtypes are expressed most strongly in layer V. This laminar pattern of mRNA distribution does not hold in all cortical regions. The relatively high levels of mRNAs encoding known dopamine receptor subtypes in the primate cerebral cortex, including the D4 receptor, underscore the importance of this structure as a target for therapeutic actions of antipsychotic drugs. Further, their prominence in layer V of the prefrontal cortex, which contains the corticostriatal and corticotectal projection neurons, provides a neural basis for dopaminergic regulation of the descending control systems.

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.

Antipsychotic regulation of dopamine D1, D2 and D3 receptor mRNA

A range of antipsychotic drugs, both "typical" and "atypical", was administered to rats over a time course and at several different dosages. The mRNA levels of dopamine D1, D2 and D3 receptor were measured in either whole brain or dissected brain regions. D3 receptor mRNA was up-regulated in whole brain by clozapine (10 and 30 but not 3 mg/kg/day), sulpiride (50 and 100 but not 20 mg/kg/day). haloperidol (3 but not 1 or 0.3 mg/kg/day), flupenthixol (3 but not 1 or 0.3 mg/kg/day), pimozide (4.5 but not 1.5 or 0.5 mg/kg/day) and loxapine (1.2 and 4 mg/kg/day but not 0.4 mg/kg/day). Sulpiride (100 mg/kg/day), clozapine (30 mg/kg/ day) and haloperidol (3 mg/kg/day) all up-regulated the D3 receptor mRNA in nucleus accumbens and olfactory tubercles but not striatum. D1 and D2 receptor mRNA was up-regulated in whole brain by haloperidol and loxapine only, and in the case of haloperidol this was localized to striatum and prefrontal cortex. Haloperidol, clozapine and sulpiride all down-regulated D1 mRNA in hippocampus and additionally haloperidol and sulpiride down-regulated it in the cerebellum. This work shows that all the drugs tested up-regulated D3 receptor, but effects on D1 and D2 receptors were less general.

Dopamine receptor gene expression in an animal model of 'behavioral dependence' on ethanol

The steady-state levels of messenger RNA (mRNA) of five cloned dopamine (D) receptors were measured in five brain regions in rats in a recently developed animal model of 'behavioral dependence' on ethanol. One group of rats was given the choice between ethanol and water over a 9-month period and developed 'behavioral dependence' on ethanol (group a). This group was compared with a group given the choice between ethanol and water for only 2 months (not yet behaviorally dependent, group b), a group forced to consume ethanol as sole fluid over a 9-month period (not behaviorally dependent, group c) and ethanol-naive control rats. All groups were sacrificed 1 month after ethanol withdrawal. The concentrations of mRNA of D3-receptors in the limbic forebrain (which included the nucleus accumbens) were significantly lowered in groups a and b, but unchanged in group c. D3 mRNA levels were reduced in the hippocampus of group b and unchanged in the cortex, amygdala and striatum. No significant changes in the mRNA concentrations of D1-, D2-, D4- or D5-receptors were seen in the five brain regions in any group. In conclusion, chronic consumption of ethanol under the 'free-choice condition', which may best induce the drug-rewarding effect, leads to specific changes in the D3-receptor gene expression which were not seen after forced ethanol administration. Changes in D3 mRNA levels were, however, not a specific correlate of 'behavioral dependence', as they were also detected in rats not yet 'behaviorally dependent' (group b).

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.

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.

Up-regulation of D3 dopamine receptor mRNA in the nucleus accumbens of human cocaine fatalities

The effects of chronic cocaine use on D3 receptor mRNA expression in the human nucleus accumbens was assessed by reverse transcription-polymerase chain reaction. D3 receptor/cyclophilin mRNA ratios in the nucleus accumbens were increased 6-fold in cocaine overdose victims as compared to age-matched and drug-free control subjects. This finding demonstrates that chronic cocaine exposure leads to adaptive increases in the expression of D3 receptor mRNA in a critical reward center in brain.

Limbic circuits and monoamine receptors: dissecting the effects of antipsychotics from disease processes

There is considerable evidence for the involvement of brain dopaminergic and serotonergic systems in schizophrenia pathology. However, post-mortem studies have been limited by difficulties in separating the effects of chronic exposure to antipsychotics from that of the disease process. Our recent studies directly explored this by comparing groups that were free from antipsychotic treatment for up to a year prior to death and that were maintained on antipsychotics. We have used this approach to identify that there are prominent effects of both disease and of antipsychotic treatment. There appears to be a high association for schizophrenics between elevations of D3 receptors in target regions of the mesolimbic dopamine (DA) system and elevated numbers of 5-HT(1A) receptors in prefrontal cortex (PFc). Antipsychotic treatment was correlated with a reduction of D3 receptors in the ventral striatum and its output structures. It also led to a reduction in the number of 5-HT2 receptors in some regions of the PFc without modifying the concentration of 5-HT(1A) receptors. The limbic loop interconnecting the PFc and ventral striatum may be the site of antipsychotic regulation of certain symptoms in schizophrenia, particularly anhedonia and depression. The positive symptoms of schizophrenia are more likely to be associated with disturbances in the temporal lobe. However, dopaminergic systems in the temporal lobe have historically been thought to be underdeveloped compared to that in the basal ganglia and unlikely to be the target of antipsychotics. Our studies of the expression of the DA D2 receptor in the temporal lobe has shown a complex organization in the perirhinal and temporal cortices that is disrupted in schizophrenia. The disturbances, which might be of neurodevelopmental origin and are unrelated to antipsychotic treatment, include altered laminar distribution of the D2 receptor and modified modular organization of D2 receptors in the superior temporal gyrus. We hypothesize that modified expression of D2 receptors in these regions play a key role in the genesis of hallucinations. Treatment with antipsychotics leading to D2 receptor blockade in temporal cortex may reduce the presence of positive symptoms.

Adaptive increase in D3 dopamine receptors in the brain reward circuits of human cocaine fatalities

The mesolimbic dopaminergic system plays a primary role in mediating the euphoric and rewarding effects of most abused drugs. Chronic cocaine use is associated with an increase in dopamine neurotransmission resulting from the blockade of dopamine uptake and is mediated by the activation of dopamine receptors. Recent studies have suggested that the D3 receptor subtype plays a pivotal role in the reinforcing effects of cocaine. The D3 receptor-preferring agonist 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) is a reinforcer in rhesus monkeys trained to self-administer cocaine, but not in cocainenaive monkeys. In vitro autoradiographic localization of [3H]-(+)-7-OH-DPAT binding in the human brain demonstrated that D3 receptors were prevalent and highly localized over the ventromedial sectors of the striatum. Pharmacological characterization of [3H]-(+)-7-OH-DPAT binding to the human nucleus accumbens demonstrated a rank order of potency similar to that observed for binding to the cloned D3 receptor expressed in transfected cell lines. Region-of-interest analysis of [3H]-(+)-7-OH-DPAT binding to the D3 receptor demonstrated a one- to threefold elevation in the number of binding sites over particular sectors of the striatum and substantia nigra in cocaine overdose victims as compared with age-matched and drug-free control subjects. The elevated number of [3H]-(+)-7-OH-DPAT binding sites demonstrates that adaptive changes in the D3 receptor in the reward circuitry of the brain are associated with chronic cocaine abuse. These results suggest that the D3 receptor may be a useful target for drug development of anticocaine medications.

Endogenous dopamine limits the binding of antipsychotic drugs to D3 receptors in the rat brain: a quantitative autoradiographic study

[3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin was used as a radioligand for the autoradiographic measurements of dopamine D3 receptors in rat and human brain. Preincubation of the brain sections was necessary to obtain binding of the radioligand in the islands of Calleja and in the nucleus accumbens, but not in cerebellar lobules 9/10 of the rat. D3 receptors were also totally occluded in unwashed sections of the human striatum. The radioligand binding to D3 receptors was maximal after preincubating the sections for at least 10 min. Pretreatment of the animals with reserpine or tetrabenazine, which results in a severe depletion of endogeneous monoamines, strongly reduces the occlusion of D3 receptors in unwashed brain sections. The occlusion of dopamine D3 receptors in brain sections suggests that the in vivo access to D3 receptors may be locally inhibited by endogenous dopamine. The in vitro binding affinities of 12 antipsychotic drugs for D2 and D3 receptors were evaluated in competition binding experiments, using both rat and cloned human receptors. Most of the compounds showed only a slightly lower affinity for D3 than for D2 receptors in vitro. Affinities of the antipsychotic drugs for cloned human D21 and D3 receptors were very close to their affinities for the rat receptors. In vivo occupancy of these receptors in the rat brain was measured ex vivo by quantitative autoradiography, 2 hours after subcutaneous drug administration. For most compounds, occupancy of D3 receptors, as compared to D2 receptor occupancy, was lower than expected from the corresponding in vivo affinity ratios. For the new antipsychotic risperidone, in vivo occupancy of D3 receptors was measured both in the islands of Calleja and in the cerebellar lobules 9/10. This compound was three times less potent for the occupancy of D3 receptors in the islands of Calleja than in the cerebellum, an area lacking endogenous dopamine (ED50 = 28 and 10 mg kg-1, respectively). Based on the observations in the rat brain, it may reasonably be supposed that therapeutic dosages of antipsychotic drugs will induce in patients only a minor occupancy of D3 receptors in brain areas containing high dopamine concentrations. The role of dopamine D3 receptors as a target of antipsychotic drugs may therefore be less important than previously thought.

Cholinergic innervation in the human striatum: a three-compartment model

The mammalian striatum is divided into compartments that are anatomically and neurochemically distinct. The dorsal striatum has been described as containing two compartments, striosomes and matrix, while the ventral striatum is thought to have a more complex, multi-compartmental organization. In this study, we sought to characterize the compartmentalization of the dorsal and ventral portions of the human striatum using choline acetyltransferase as a marker. Image analysis was used to assess relative densities of immunostaining, and three distinct, choline acetyltransferase-immunostained compartments were demonstrated: intensely immunostained, moderately immunostained and weakly immunostained areas. The dorsomedial portion of the striatum was made up of moderately immunostained regions embedded within a densely immunostained background, thus manifesting the characteristic striosome/ matrix organization of the dorsal striatum. However, the ventral and lateral two-thirds of the striatum were made up of a mixture of densely immunostained, moderately immunostained and weakly immunostained areas, with the moderately immunostained region forming the bulk of the background tissue, and smaller, densely immunostained and weakly immunostained regions embedded within it. These compartments were compared to regions defined by distinct levels of acetylcholinesterase immunostaining in adjacent sections; the staining patterns produced by the two cholinergic markers were found to be identical except in some portions of the nucleus accumbens, where acetylcholinesterase immunostaining was found to be more intense than choline acetyltransferase immunostaining. The immunoreactive somata were mapped within sections stained for choline acetyltransferase taken from different rostrocaudal levels of the striatum, and the distributions and densities of immunoreactive somata within these three cholinergic compartments were determined. In general, the densities of cholinergic somata roughly correlated with immunostaining intensity of regions, e.g. the most intensely immunostained compartment also had the highest densities of cholinergic somata. However, in the rostroventral striatum, the densities of cholinergic somata in the weakly immunostained compartment roughly equalled the densities of cholinergic somata in the moderately immunostained compartment, suggesting that local axonal arborizations of cholinergic cells may differ in density or orientation between the two compartments, or, alternatively, that some of the cholinergic cells in the weakly immunostained compartment may project outside of the striatum. The large proportion of striatum displaying ventral striatal characteristics (a complex, multi-compart-mental organization) in humans relative to that observed in other mammals suggests that the role of the ventral striatum may be expanded and more highly differentiated in the human brain.

D3 receptor expression within the basal ganglia is not affected by Parkinson's disease

Quantitative receptor autoradiography and in situ hybridization were used to investigate the expression of dopamine D3 receptors in sections of human brain containing limbic (nucleus accumbens) and striatal (caudate nucleus, putamen) regions. High levels of dopamine D3 receptor mRNA and specific [3H](+/-)7-hydroxy-N,N-di-N-propyl-2-aminotetralin ([3H]7-OH-DPAT) binding sites were detected in the nucleus accumbens with lower levels in the caudate nucleus and putamen. No difference in D3 receptor expression was observed between normal and parkinsonian brain. These results indicate that D3 receptor expression is not altered in Parkinson's disease. In addition, they suggest that dopamine release in striatal and limbic areas is not necessary for maintenance of D3 receptor expression.

Scopolamine augments c-fos and zip/268 messenger RNA expression induced by the full D(1) dopamine receptor agonist SKF-82958 in the intact rat striatum

It is generally accepted that the widely used, partial dopamine D(1) receptor agonist, SKF-38393, does not induce immediate early gene expression in striatal projection neurons unless D(1) receptors are sensitized and uncoupled from D(2) receptors by 6-hydroxydopamine lesions or reserpine treatment. In contrast, this study demonstrates, using quantitative in situ hybridization, that the full D(1) receptor agonist, SKF-82958, induced robust expression of c-fos and zif/268 messenger RNAs in the intact rat striatum, especially in the entire shell and medial and ventral core areas of the nucleus accumbens and olfactory tubercle, and in the cerebral cortex, 45 min after one injection. The induction of the striatal immediate early genes is characterized by (i) induction in only medium-sized spiny neurons, (ii) dose-dependent induction, which correlates well with dose-dependent increases in motor activity, and (iii) blockade by the D(1) receptor antagonist, SCH-23390. The muscarinic cholinergic receptor antagonist, scopolamine, which itself did not alter striatal gene expression, profoundly augmented the behaviors and expression of the two immediate early genes in the ventral and dorsal striatum induced by 0.1, 0.5 and 2.0 mg/kg SKF-82958. However, scopolamine attenuated basal, and SKF-82958-stimulated, expression of c-fos and zif/268 messenger RNAs in the cortex. Scopolamine also enabled SKF-38393 to induce locomotor stimulation and c-fos and zif/268 messenger RNA expression in the normosensitive striatum of the rat when SKF-38393 alone caused no such changes. These data demonstrate an ability of SKF-82958 to induce immediate early gene messenger RNA expression in normosensitive dorsal and ventral striatum. Furthermore, intrinsic muscarinic receptor-mediated cholinergic transmission in the striatum may provide an activity-dependent inhibitory control on striatal D(1) receptor stimulation.

Burst stimulation of the medial forebrain bundle selectively increase Fos-like immunoreactivity in the limbic forebrain of the rat

The present study was designed to evaluate the postsynaptic functional consequences of different presynaptic activity patterns in midbrain dopamine systems using electrical stimulation of the rat medial forebrain bundle and subsequent determination of c-fos expression, used as a marker for neuronal activation, in dopamine target areas, by means of Fos immunohistochemistry. Nerve terminal dopamine release evoked by electrical stimulation of the medial forebrain bundle was monitored in the same animals using in vivo voltammetry. A 5 Hz stimulation consisting of 60 trains of five pulses and lasting 1 min was applied to the medial forebrain bundle. This stimulation was repeated 15 times every 3 min. Its pattern was defined by the interpulse interval which was either 70 ms or 200 ms for burst or regularly spaced stimulation, respectively. Our results show that burst stimulation of the medial forebrain bundle, which increase release of dopamine in target areas, increases the basal Fos-like immunoreactivity in the stimulated hemisphere, while regular stimulation does not affect expression of this protein. Moreover, the increase in Fos-like immunoreactivity induced by burst stimulation is restricted to limbic related structures, i.e. nucleus accumbens shell and intermediate aspect of the lateral septum, and the major island of Calleja, but is not observed in motor related structures (nucleus accumbens core and striatum). Pretreatment with the D1 dopamine receptor antagonist, SCH23390 (0.1 mg/kg, i.p.), blocked the increase in Fos-like immunoreactivity induced by burst stimulation of the medial forebrain bundle, suggesting a role for these receptors in the observed effects. Pretreatment with the 5-hydroxytryptamine2A/2C receptor antagonist ritanserin (0.4 mg/kg, i.p.) did not affect the increase in Fos-like immunoreactivity induced by burst stimulation in the nucleus accumbens shell or in the lateral septum, although it blocked the stimulated enhancement of Fos-like immunoreactivity in the major island of Calleja. The present data indicate that, rather than the absolute mean discharge rate of midbrain dopamine neurons, the temporal organization of the action potentials they generate conveys information to their target areas.

The locomotor effects of a putative dopamine D3 receptor agonist in developing rats

Dopamine receptors have been categorized into subfamilies D1 and D2, each with separate roles in dopamine-mediated behaviors. Of the D2 subfamily, the dopamine D3 receptor has been cloned, but the behavioral effects of selectively stimulating the D3 receptor are largely unknown. The purpose of this study was to quantify the locomotor responses of developing rats to the putative dopamine D3 receptor agonist, 7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT). One of three doses of 7-OH-DPAT (0.01, 0.10, 1.00 mg/kg) or saline was injected subcutaneously into rats at the age of 10,20,30, or 60 days. Five minutes after the injection, rats were placed in automated activity monitors which recorded locomotor behavior at 5 min intervals for 2 h. The high dose of 7-OH-DPAT increased locomotor activity in rats of all ages. The medium and low doses increased activity in 10- and 20-day-old rats but not in 30- or 60-day-old rats. The level of drug-induced activation peaked at 20 days of age. In 30- and 60-day-old rats, but not 10- and 20-day-old rats, a period of locomotor suppression preceded the activation in response to the high dose of 7-OH-DPAT. In rats aged 20 days and older, the middle and low doses decreased locomotion early in the test session, but activation did not ensue. This dose-response pattern across ontogeny closely resembles that induced by quinpirole, an agonist at the dopamine D2 receptor subfamily.

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

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

A targeted mutation of the D3 dopamine receptor gene is associated with hyperactivity in mice

While most effects of dopamine in the brain are mediated by the D1 and D2 receptor subtypes, other members of this G protein-coupled receptor family have potentially important functions. D3 receptors belong to the D2-like subclass of dopamine receptors, activation of which inhibits adenylyl cyclase. Using targeted mutagenesis in mouse embryonic stem cells, we have generated mice lacking functional D3 receptors. A premature chain-termination mutation was introduced in the D3 receptor gene after residue Arg-148 in the second intracellular loop of the predicted protein sequence. Binding of the dopamine antagonist [125I]iodosulpride to D3 receptors was absent in mice homozygous for the mutation and greatly reduced in heterozygous mice. Behavioral analysis of mutant mice showed that this mutation is associated with hyperactivity in an exploratory test. Homozygous mice lacking D3 receptors display increased locomotor activity and rearing behavior. Mice heterozygous for the D3 receptor mutation show similar, albeit less pronounced, behavioral alterations. Our findings indicate that D3 receptors play an inhibitory role in the control of certain behaviors.

Dopamine D3 receptors in the basal ganglia of the common marmoset and following MPTP and L-DOPA treatment

The distribution of the dopamine D3 receptor was studied by receptor autoradiography using [3H]7-OH-DPAT in striatal and extrastriatal brain regions of the common marmoset (Callithrix jacchus). Saturation studies demonstrated that [3H]7-OH-DPAT bound with similar affinity to different regions of marmoset brain. In normal marmosets, specific [3H]7-OH-DPAT binding was found in both striatal and extrastriatal regions. Very high levels of specific [3H]7-OH-DPAT binding were detected in the islands of Calleja and nucleus accumbens but in addition high levels of binding were detected in rostral caudate nucleus and putamen. In common marmosets treated with the selective nigral neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), the levels of specific [3H]7-OH-DPAT binding in striatal and extrastriatal regions were not different to those in normal animals. Chronic treatment of MPTP-treated marmosets with L-DOPA/ carbidopa did not alter the levels of specific [3H]7-OH-DPAT binding in any brain region. These results demonstrate that in common marmosets D3 receptors are located in both striatal and limbic regions. The receptor density is not altered by dopaminergic denervation or by chronic L-DOPA administration. The D3 receptor may, therefore, be important in both the therapeutic and adverse effects of drugs used to treat Parkinson's disease.

Dopamine agonists used in the treatment of Parkinson's disease and their selectivity for the D1, D2, and D3 dopamine receptors in human striatum

1. It has been suggested that an ideal antiparkinsonian treatment requires stimulation of both D1 and D2 dopamine receptors. Bromocriptine and lisuride are regarded as pure D2 receptor agonists, whereas pergolide and apomorphine are thought to stimulate both D1 and D2 receptors. 2. The aim of this study was to compare the affinities of bromocriptine, lisuride, pergolide, and apomorphine for the D1, D2, and D3 receptors in postmortem human striatum. The dissociation constants (Ki values) of the dopamine agonists were determined from competition binding experiments with selective radioligands. 3. The Ki values of the orally administered agonists--bromocriptine, pergolide, and lisuride--for the D2 receptors were proportional to their optimal doses against parkinsonism. Ki(D1)/Ki(D2) ratios were 23 for lisuride, 67 for pergolide, 60 for bromocriptine, and 2.6 for apomorphine. Ki(D3)/Ki(D2) ratios were 0.4 for lisuride, 1 for pergolide, 5.4 for bromocriptine, and 21 for apomorphine. 4. The present results support the hypothesis that the antiparkinsonian effect of dopamine agonists is mediated primarily by D2 receptors. Apomorphine is a mixed D1/D2 agonist, but pergolide has no more D1 agonist properties than bromocriptine and lisuride. The role of the D3 receptors is unknown, but their activation might either be associated with the generation of psychiatric side-effects or dyskinesias, or alternatively add to antiparkinsonian activity.

7-OH-DPAT injected into the accumbens reduces locomotion and sucrose ingestion: D3 autoreceptor-mediated effects?

7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) injected bilaterally in the nucleus accumbens (NAC) resulted in profound, noncatatonic, dose-dependent (0.3-3 mg total dose) hypolocomotion but without inducing yawning. It also decreased intake of a highly preferred 3% sucrose solution (1 microgram total dose). Systemic injection of 7-OH-DPAT (0.1-3.0 mg/kg, i.p.) similarly induced hypolocomotion while failing to induce yawning. In none of these studies did rats show any signs of hyperlocomotion or any stereotyped responses normally associated with D2 or mixed D1/D2 receptor stimulation. These data suggest that hypolocomotion elicited by 7-OH-DPAT in the NAC may be mediated at the D3 receptor as distinct from the D2 dopamine receptor. We discuss the possibility that the behavioural effects we observed are mediated at D3 autoreceptors.

Acute administration of typical and atypical antipsychotic drugs induces distinctive patterns of Fos expression in the rat forebrain

Fos expression in the rat brain was investigated by immunohistochemistry after i.p. administration of single doses of a wide range of typical neuroleptic antipsychotic drugs (including the potent dopamine D2 antagonist haloperidol and the mixed monoamine antagonist chlorpromazine) and atypical antipsychotic drugs (including the weak dopamine D2 antagonists clozapine and thioridazine, the relatively pure D2 antagonist raclopride and the mixed D2 and serotonin S2 antagonist risperidone). For comparison to the effects of the antipsychotic drugs and also because the unique clinical therapeutic effects of clozapine have been attributed to S2 blockade, the S2 antagonist ritanserin was also studied. The single shared effect of all antipsychotic drugs tested was the induction of significantly increased Fos immunoreactivity in the nucleus accumbens (NAc). Fos-positive neurons in the NAc were mostly localized in patches throughout its rostrocaudal extent. Haloperidol, chlorpromazine, raclopride and risperidone all significantly increased Fos expression in the medial and lateral striatum. Fos-positive neurons in the striatum were distributed more lateral than medial and declined from rostral to caudal levels. Haloperidol, thioridazine and risperidone also markedly increased Fos expression in the lateral septum. Distinguishing it from the other neuroleptics, clozapine did not increase Fos expression in the lateral striatum, but induced a significant increase in Fos expression in the prefrontal cortex. Ritanserin did not induce Fos expression in any brain region examined, suggesting that S2 antagonism is not responsible for the effects of antipsychotic drugs observed here. Our results suggest that there are distinctive patterns of Fos expression in the forebrain induced by typical and atypical antipsychotic drugs. Notably, Fos expression in the NAc, as a shared property of all the antipsychotic drugs, may be related to the actions mediating the therapeutic effects of these drugs in the treatment of psychotic disorders. The density of Fos-positive neurons stimulated by antipsychotic drugs in the striatum appeared to be correlated with the relative severity of extrapyramidal side-effects produced by these drugs and may be related to the mechanisms mediating these effects.

Novel dopamine receptors half a decade later

Only half a decade ago, the effects of dopamine were all attributed to activation of two receptor subtypes, the D1 and D2, with opposing effects on adenylate cyclase, and for which apparently selective ligands were available. From the end of 1988, however, the application of homology cloning techniques starting from sequences of the seven transmembrane domain catecholamine receptors, particularly that of the D2 receptor, led to the identification of 'novel', previously uncharacterized dopamine receptors. In this article, Pierre Sokoloff and Jean-Charles Schwartz discuss the functional significance of such diversity, as well as the new therapeutic perspectives it offers.

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

The present experiments were designed to test further the idea that 7-OH-DPAT (7-hydroxy-N,N-di-n-propyl-2-aminotetralin), a putative dopamine (DA) D3 agonist, has effects at DA autoreceptors to reduce intracranial DA levels and to reduce behaviours that are DA-dependent. Rats were trained to respond on a self-stimulation protocol for electrical stimulation to the ventral tegmental area (VTA). Each press of a lever delivered a 0.5 s train of square wave, 1.5 ms duration, 100 Hz, 90-120 mA stimulation. Systemic administration of 7-OH-DPAT at 0.01-0.3 mg/kg i.p., quickly dose-dependently reduced responding. Electrical stimulation using similar parameters to those that supported self-stimulation were then applied to the VTA of anaesthetized rats. Fast cyclic voltammetry (FCV) revealed that this stimulation released DA in the nucleus accumbens (NAC). 7-OH-DPAT i.p. (0.1-3.0 mg/kg) quickly and potently reduced the size of the DA-generated voltammetric signal. This effect of 0.3 mg/kg 7-OH-DPAT was not blocked by sulpiride (60 mg/kg, i.p.) a D2-specific antagonist that may preferentially block D2 autoreceptors. These data are discussed with reference to the possibility that 7-OH-DPAT reduces the release of dopamine in the NAC, at D3, but not at D2, autoreceptors and that this in turn may reduce the rewarding effect of VTA stimulation.

The putative dopamine D3 receptor agonist 7-OH-DPAT: lack of mesolimbic selectivity

7-Hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT), an agonist with relative selectivity for the dopamine D3 receptor, was examined in several electrophysiological assays to determine whether it exhibits preferential effects in the mesolimbic versus nigrostriatal dopamine systems. Extracellular single unit activities of substantia nigra pars compacta (A9) and ventral tegmental area (A10) dopamine neurons, and caudate-putamen and nucleus accumbens neurons, were recorded in male rats anesthetized with chloral hydrate. Intravenous (+/-)-7-OH-DPAT potently and completely inhibited the firing of both A9 and A10 dopamine neurons (ED50's: 3.5 +/- 0.7 micrograms/kg and 3.9 +/- 0.9 micrograms/kg, respectively). The active enantiomer, (+)-7-OH-DPAT, was 2 to 3 times more potent than the racemic drug (ED50's: 1.2 +/- 0.3 micrograms/kg and 1.7 +/- 0.4 micrograms/kg for A9 and A10 cells, respectively). There were no significant differences in potency for either form in inhibiting A9 and A10 dopamine neurons. In other studies, iontophoretically applied (+)-7-OH-DPAT caused current-dependent inhibitions of spontaneously active or glutamate-driven caudate-putamen and nucleus accumbens neurons (I50 values, 6.5 and 7.9 nA, respectively). Again, no difference in potency between cell populations was noted. Finally, in cell-attached patch-clamp recordings from freshly dissociated rat caudate-putamen neurons, an 85 pS K+ channel known to be activated by dopamine and the "D2-like" agonist quinpirole was also observed with (+/-)-7-OH-DPAT (0.2-1 microM) applied in the patch pipette.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute antagonism of dopamine D2-like receptors by amisulpride: effects on hormone secretion in healthy volunteers

Amisulpride is a selective D2-like dopamine receptor antagonist with a high affinity for the cloned D2 and D3 receptors. At low doses it may improve depressive and negative schizophrenic symptoms whereas antipsychotic effects on positive schizophrenic symptomatology require higher dosages. Acute endocrine effects were studied for two doses of amisulpride with regard to the daytime secretion of prolactin, thyroidea stimulating hormone (TSH), growth hormone (GH), luteinizing hormone (LH) and cortisol. Amisulpride was administered i.v. to eight healthy male volunteers in a single-blind trial under a randomized cross-over, placebo-controlled design using doses of 20 mg or 100 mg, or saline. The drug was injected at 09:00 h, and plasma samples were withdrawn from 08:30 h to 16:00 h at intervals of 15 and 30 min, respectively. At both dosages, prolactin was significantly elevated to the eight- to ten-fold of baseline levels. Likewise, a significant 50% elevation of TSH concentrations with a trend to a greater increase under the 100 mg dose was observed. Plasma levels of LH and cortisol were not significantly affected by amisulpride. With regard to GH secretion, there was a trend to a decrease only with the 20 mg dose. These results indicate that the neuroendocrinological side-effect profile of acute amisulpride administration may be similar to conventional neuroleptics, and that there are only minor dose-dependent differential effects on hormone secretion in the dose range investigated.

Autoradiographic distribution of D3-type dopamine receptors in human brain using [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin

The regional distribution of D3 dopamine receptors was studied in human brain by quantitative autoradiography with [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT). The highest densities were found in the ventral striatum/nucleus accumbens, followed by the remainder of the neostriatum, cerebral cortex, and cerebellar cortex. Moderate amounts were found in the substantia nigra. Low densities of D3 dopamine receptors were visualized in pituitary gland (posterior lobe > anterior lobe), amygdala, and hippocampus. The globus pallidus and thalamus contained lower densities. The distribution pattern is much more widespread as detected by in situ hybridisation histochemistry for D3 mRNA in human and in rat brain. Our data confirm a predilection of D3 receptors in brain areas involved in cognitive and emotional functions. The presence of D3 receptors in non-limbic parts of the neostriatum and substantia nigra suggests that they also play a role in the dopaminergic control of motor processes. Its precise function in cerebellum and pituitary gland is at present uncertain.

D1 dopamine receptor immunoreactivity in human and monkey cerebral cortex: predominant and extrasynaptic localization in dendritic spines

Antibodies to the D1 dopamine receptor were used to localize this protein in several areas of human and monkey cerebral cortex with light and electron microscopy. In addition to cell body labeling in monkeys, all areas of humans and monkeys had a neuropil label with a laminar distribution predicted by previous D1 receptor autoradiography studies. Using electron microscopy, this neuropil label was seen in numerous dendritic spines, in dendritic shafts, and in occasional axon terminals. While labeled spines were common, they represented only a subset of all cortical spines. Serial sectioning through labeled spines showed that the diaminobenzidine reaction product was usually not at postsynaptic densities but instead was displaced to the side of the large asymmetric (presumed glutamatergic) synapse. Furthermore, most labeled spines did not receive synapses with dopaminergic features, suggesting that many D1 receptors are at extrasynaptic sites, possibly receiving dopamine via diffusion in the neuropil. Similarly, double labeling failed to reveal D1 labeling at synapses of tyrosine hydroxylase immunoreactive axons. Localization to numerous dendritic spines suggests that a primary role of D1 receptors is modulation of glutamatergic input to cortical pyramidal cells.

Clozapine and haloperidol produce a differential pattern of immediate early gene expression in rat caudate-putamen, nucleus accumbens, lateral septum and islands of Calleja

Acute administration of the typical neuroleptic haloperidol (HAL, 2 mg/kg) induced the immediate-early gene proteins (IEGPs) c-Fos, Fos-related antigens (FRAs), FosB, JunB, JunD and Krox24 in the striatum and nucleus accumbens of the rat brain. In contrast, acute administration of the atypical antipsychotic drug clozapine (CLOZ, 30 mg/kg) induced only FRAs, JunB and Krox24 IEGPs in the striatum, and c-Fos, FRAs, and Krox24 IEGPs in the nucleus accumbens. c-Jun was not induced by acute administration of HAL or CLOZ in the rat brain. Differential induction of IEGs by HAL and CLOZ was also observed in the lateral septal nucleus and the islands of Calleja complex of the rat brain. These differences in IEG induction by HAL and CLOZ may be related to the different clinical profiles of the two drugs. Specifically, CLOZ induces FRAs in the islands of Calleja and lateral septum and this action may be involved in its therapeutic effects on the negative symptoms of schizophrenia, whereas HAL produces a coordinate induction of Fos and JunB in striatal neurons and this dimer combination may be involved in producing the extrapyramidal side-effects of typical neuroleptics.

Differential effects of dopamine D2 and D3 receptor antagonists in regard to dopamine release, in vivo receptor displacement and behaviour

To establish possible functional differences between the dopamine D2 and D3 receptor we investigated the relation between the ability, for a set of nine mixed dopamine D2 and D3 receptor antagonists, to displace N, N-dipropyl-2-amino-5,6-dihydroxy tetralin (DP-5,6-ADTN) from striatal binding sites and the subsequent behavioural consequences in vivo. Dopamine D2 receptor preferring antagonists are powerful displacers of DP-5,6-ADTN from the striatum. Maximal displacement is followed by strong hypomotility. Displacement of the agonist by the D3 preferring antagonist U99194A is only partial and results in synergistic increases in locomotor activity. Superimposing haloperidol upon GBR12909 leads to a synergistic increase in striatal dialysate dopamine concentrations. This effect is absent when combining GBR12909 with the putative D3 antagonist U99194A. These data give support for the hypothesis that the dopamine D3 receptor is functionally relevant at the postsynaptic level. Here, in contrast to the D2 receptor, it is proposed to exert an inhibitory influence on psychomotor functions.

Etiopathogenesis and treatment of psychosis

Psychotic illnesses (schizophrenia and schizoaffective and affective psychosis) have a lifetime prevalence of 2-3% and probably occur at a similar rate in all human societies. No etiologically significant environmental precipitants have been identified, and this suggests that these diseases are primarily genetic. Brain studies reveal that in schizophrenic patients, development of cerebral asymmetry is arrested, which may be associated with a small reduction in cortical mass. Episodes of illness can be ameliorated by dopamine (in particular D2) antagonists, drugs that are antipsychotic rather than merely antischizophrenic. The discovery of at least five dopamine receptor subtypes and their genes paves the way for new approaches to treatment. However, whether psychotic patients undergo a primary disturbance of dopaminergic transmission remains unclear.

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.

[3H]7-OH-DPAT labels both dopamine D3 receptors and sigma sites in the bovine caudate nucleus

The binding of [3H]7-hydroxy-N,N-di-n-propyl-2-aminotetralin (7-OH-DPAT) to membranes of the bovine caudate nucleus was characterized and shown to be heterogenous. While [3H]7-OH-DPAT labels the dopamine D3 receptor in this tissue (Ki = 4.8 nM), as shown by its sensitivity to dopamine and (-)-quinpirole, it also binds with high affinity (Ki = 48 nM) to sigma recognition sites. The present results demonstrate that [3H]7-OH-DPAT is a useful radioligand to label native D3 receptors in the bovine caudate, although care must be taken to assure its selectivity.

The dopamine D3 receptor and autoreceptor preferring antagonists (+)-AJ76 and (+)-UH232; a microdialysis study

The in vivo neurochemical profiles of haloperidol, raclopride and the dopamine D3 and autoreceptor preferring dopamine receptor antagonists (+)-UH232 and (+)-AJ76 on dopamine release and metabolism in the dorsal striatum and in the nucleus accumbens are described. It is shown that both (+)-UH232 and especially (+)-AJ76 have different effects on brain dialysate dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) as compared to haloperidol or raclopride. It is suggested that the relative increase in dialysate dopamine over the relative increase in DOPAC is a neurochemical fingerprint, unique for different dopamine receptor antagonists. As a consequence the increased release and metabolism of dopamine after systemic administration of dopamine receptor antagonists may be controlled by different receptors and different dopamine antagonists can partly distinguish between these receptors. This may be due to their different interactions with different dopamine D2 type receptors. It is finally concluded that (+)-UH232 and especially (+)-AJ76 seem to prefer release regulating autoreceptors at the level of the axon terminals.

Differential visualization of dopamine D2 and D3 receptor sites in rat brain. A comparative study using in situ hybridization histochemistry and ligand binding autoradiography

At least five members of the dopamine receptor family have been characterized at the gene level. D2, D3 and D4 dopamine receptors are related pharmacologically. In order to visualize the differential expression of D1, D2 and D3 receptors in rat brain we have combined in situ hybridization histochemistry with receptor autoradiography. Regions enriched with D3 messenger RNA (mRNA) included the islands of Calleja (ioC) and nucleus accumbens. Very low or undetectable levels were present in the caudate-putamen. In contrast, no D2 transcripts were observed in the islands of Calleja, but there were high levels in the nucleus accumbens, caudate-putamen (CP) and pyramidal layer of the olfactory tubercle. A comparison of the binding pattern of six dopamine receptor radioligands hitherto regarded as D2 receptor-selective showed that the islands of Calleja were intensely labelled by [125I]iodosulpride, [3H]CV 205 502 and [3H]SDZ 205 501, while the binding of [3H]spiperone, [3H]raclopride and [3H]YM 09151-2 was much lower or undetectable. Pharmacological analysis of the binding of D2/D3 ligands to the islands of Calleja and caudate-putamen suggests that binding sites in these two regions are of different pharmacology, consistent with the presence of D3 sites in the islands of Calleja and the predominance of D2 sites in the caudate. These results demonstrate the expression of D3 binding sites in the rat brain and provide a procedure to differentiate D2 and D3 receptor populations in binding studies.

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

We report on the pharmacological effects of the 20 fold D3 vs. D2 dopamine receptor preferring compound U99194A. It is shown that U99194A increases rat locomotor activity at doses that do not increase release or utilisation of dopamine in the striatum or the nucleus accumbens significantly. The data do not support any direct agonist action of U99194A at dopamine receptors. It is suggested that U99194A can antagonise a population of postsynaptic dopamine receptors involved in the suppression of some aspects of psychomotor activity. These postsynaptic receptors presumably belong to the D3 receptor subtype.