Localization of dopamine D3 receptors to mesolimbic and D2 receptors to mesostriatal regions of human forebrain.

We characterized the binding of [125I]epidepride to dopamine D2-like and D3-like receptors in tissue sections of human striatum. The competition for binding of [125I]epidepride by domperidone, quinpirole, and 7-hydroxy-N,N-di(1-propyl)-2-aminotetralin (7-OH-DPAT) was best fit by assuming one site in the caudate but two sites in nucleus accumbens. Guanosine 5'-[beta, gamma-imido]triphosphate showed a large modulatory influence in agonist inhibition of [125I]epidepride binding in caudate but not in nucleus accumbens. The binding of [125I]epidepride in the presence of 7-OH-DPAT (1000-fold selective for D3-like versus D2-like sites) and domperidone (20-fold selective for D2-like versus D3-like sites) was used to quantify the numbers of D2-like and D3-like receptors in areas of human brain. The distribution of D2-like and D3-like receptors was largely nonoverlapping. Binding of [125I]epidepride to D3-like receptors was negligible in the dorsal striatum but was concentrated in islands of dense binding in the nucleus accumbens and ventral putamen that aligned with acetylcholinesterase-poor striosomes. Binding to D3-like receptors was also enriched in the internal globus pallidus, ventral pallidum, septum, islands of Calleja, nucleus basalis, amygdalostriatal transition nucleus of the amygdala, central nucleus of the amygdala, and ventral tegmental area. Binding of [125I]epidepride to D2 but not D3 receptors was detected in cortex and hippocampus.     

Substituted benzamides as ligands for visualization of dopamine receptor binding in the human brain by positron emission tomography.

Two substituted benzamides, FLB 524 and raclopride, were labeled with 11C and examined for their possible use as ligands for positron emission tomography (PET)-scan studies on dopamine-2 (D-2) receptors in the brains of monkeys and healthy human subjects. Both ligands allowed the in vivo visualization of D-2 receptor binding in the corpus striatum caudate nucleus/putamen complex in PET-scan images. [11C]Raclopride showed a high ratio of specific striatal to nonspecific cerebellar binding, and the kinetics of binding of this ligand made it optimal for PET studies. The in vivo binding of [11C]raclopride in the striatum of cynomolgus monkeys was markedly reduced by displacement with haloperidol. This and previous in vitro data indicate that [11C]raclopride binds selectively to striatal D-2 dopamine receptors. In healthy human subjects, [11C]raclopride binding in the caudate nucleus/putamen was 4- to 5-fold greater than nonspecific binding in the cerebellum. In comparison with previously available ligands for PET-scan studies on central dopamine receptors in man, [11C]raclopride appears to be advantageous with regard to (i) specificity of binding to D-2 receptors, (ii) the high ratio between binding in dopamine-rich (caudate, putamen) and dopamine-poor (cerebellum) human brain regions, and (iii) rapid association and reversibility of specific binding. [11C]Raclopride should be a valuable tool for characterizing D-2 receptors in the brains of patients with neuropsychiatric disorders.     

Immunohistochemical localization of the D1 dopamine receptor in rat brain reveals its axonal transport, pre- and postsynaptic localization, and prevalence in the basal ganglia, limbic system, and thalamic reticular nucleus.

D1 dopamine receptor localization was examined by immunohistochemistry using a polyclonal anti-peptide antibody which (i) immunoprecipitated a protein fragment encoded by a D1 receptor cDNA and (ii) on Western blots of solubilized striatal and hippocampal membranes recognized two proteins of approximately 50 kDa and 75 kDa, corresponding to reported sizes of D1 receptor proteins. Immunoreactivity overlapped with dopamine-containing pathways, patterns of D1 receptor binding, and mRNA expression. Staining was concentrated in prefrontal, cingulate, parietal, piriform, entorhinal, and hippocampal cortical areas and subcortically in the basal ganglia, amygdala, septal area, substantia inominata, thalamus, hypothalamus, and neurohypophysis. Prominent labeling was seen in the thalamic reticular nucleus, a region known to integrate ascending basal forebrain inputs with thalamocortical and corticothalamic pathways and in fiber bundles interconnecting limbic areas. In striatal neuropil, staining appeared in spines (heads and necks), at postsynaptic sites in dendrites, and in axon terminals; in the pars reticulata of the substantia nigra, labeling was prevalent in myelinated and unmyelinated axons and dendrites. These data provide direct evidence for the regional and subcellular distribution of D1 receptor protein in the brain and for its pre- and postsynaptic localization in the basal ganglia. The prominent immunoreactivity seen in the limbic system and thalamic reticular nucleus supports an important role for this receptor subtype in mediating integrative processes involved with learning, memory, and cognition.     

Stimulation of high-affinity adenosine A2 receptors decreases the affinity of dopamine D2 receptors in rat striatal membranes.

Since high-affinity adenosine A2 receptors (A2a) are localized exclusively in dopamine-rich regions in the central nervous system and mediate inhibition of locomotor activity, we have examined the effect of A2a receptor activation on D1 and D2 receptor binding in membrane preparations of the rat striatum. The A2a agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'- N-ethylcarboxamidoadenosine (CGS 21680) increased the Kd of the dopamine D2 agonist L-(-)-N-[3H]propylnorapomorphine without affecting the Bmax. The increase in Kd was maximal (40%) at 30 nM CGS 21680. CGS 21680 (30 nM) decreased the dopamine-induced inhibition of [3H]raclopride (a D2 antagonist) binding due to an increase (about 3-fold) in KH and KL, the dissociation constants of high- and low-affinity binding sites. The effects of CGS 21680 were antagonized by the adenosine antagonist 8-phenyltheophylline (10 microM). (-)-N6-(2-Phenylisopropyl)adenosine produced an effect similar to that of CGS 21680, provided the concentration used was high enough to stimulate A2a receptors (300 nM). GTP (50 microM) also decreased the dopamine-induced inhibition of [3H]raclopride binding but, in contrast to CGS 21680, GTP decreased the proportion of D2 receptors in the high-affinity state. CGS 21680 (30 nM) did not affect the Kd or Bmax of [3H]raclopride and failed to affect ligand binding to D1 receptors. Thus, stimulation of A2a receptors potently reduces the affinity of D2 agonist binding sites within the plasma membrane of striatal neurons. This A2a-D2 interaction may underlie the neuroleptic-like actions of adenosine agonists and the enhancing effects of adenosine antagonists, such as caffeine, on locomotor activity.     

Dopamine receptors in human and calf brains, using [3H]apomorphine and an antipsychotic drug.

In order to develop a better dopamine receptor radioligand, [3H[apomorphine was prepared and tested for dopamine-like binding properties in both calf and human brain tissues. Specific binding of [3H]apomorphine was defined as that binding which occurred in the presence of 1 muM (-)-butaclamol (an inactive neuroleptic) minus that occurring in the presence of 1 muM (+)-butaclamol (active neuroleptic). The specific binding was saturable, the number of sites being double that of specific [3H]dopamine binding, and occurred primarily in dopamine-rich regions of postmortem human brains. The binding had a dissociation constant of 0.9 nM for human caudate (2 nM for calf caudate) and was blocked by dopamine and norepinephrine, but not by isoproterenol or (-)-propranolol, distinguishing it from a beta-adrenergic receptor. Since there was little desorption of [3H]apomorphine, the ligand permits extensive washing during routine assays for dopamine receptors, and facilitates biochemical purification of the receptor.     

Cloning, molecular characterization, and chromosomal assignment of a gene encoding a second D1 dopamine receptor subtype: differential expression pattern in rat brain compared with the D1A receptor.

Multiple D1 dopaminergic receptor subtypes have been postulated on the basis of pharmacological, biochemical, and genetic studies. We describe the isolation and characterization of a rat gene encoding a dopamine receptor that is structurally and functionally similar to the D1 dopamine receptor. The coding region, which is intronless, encodes a protein of 475 amino acids (Mr 52,834) with structural features that are consistent with receptors coupled to guanine nucleotide-binding regulatory proteins. The expressed protein binds dopaminergic ligands and mediates stimulation of adenylyl cyclase with pharmacological properties similar to those of the D1 dopamine receptor. The gene encoding the human homologue of this receptor subtype is located to the short arm of chromosome 4 (4p16.3), the same region as the Huntington disease gene. In striking contrast to the previously cloned D1 receptor, little or no mRNA for the receptor described here was observed in striatum, nucleus accumbens, olfactory tubercle, and frontal cortex. High levels of mRNA for this receptor were found in distinct layers of the hippocampus, the mammillary nuclei, and the anterior pretectal nuclei, brain regions that have been shown to exhibit little or no D1 dopamine receptor binding. On the basis of its properties we propose that this dopamine receptor subtype be called D1B.     

Localization of D1 dopamine receptor mRNA in brain supports a role in cognitive, affective, and neuroendocrine aspects of dopaminergic neurotransmission.

Expression of a D1 dopamine receptor was examined in the rat brain by using a combination of in situ hybridization and in vitro receptor autoradiography. Cells expressing D1 receptor mRNA were localized to many, but not all, brain regions receiving dopaminergic innervation. The highest levels of hybridization were detected in the caudate-putamen, nucleus accumbens, and olfactory tubercle. Cells expressing D1 receptor mRNA were also detected throughout the cerebral cortex, limbic system, hypothalamus, and thalamus. D1 receptor mRNA was differentially expressed in distinct regions of the hippocampal formation. Dentate granule cells were labeled in dorsal but not ventral regions, whereas the subicular complex was prominently labeled in ventral but not dorsal regions. Intermediate to high levels of D1 binding sites, but no hybridizing D1 receptor mRNA, were detected in the substantia nigra pars reticulata, globus pallidus, entopeduncular nucleus, and subthalamic nucleus. In these brain regions, which are involved in the efferent flow of information from the basal ganglia, D1 receptors may be localized on afferent nerve terminals originating in other brain regions. These results indicate that in addition to a role in control of motor function, the D1 receptor may also participate in the cognitive, affective, and neuroendocrine effects of dopaminergic neurotransmission.     

Characterization of subtype-specific antibodies to the human D5 dopamine receptor: studies in primate brain and transfected mammalian cells.

To achieve a better understanding of how D5 dopamine receptors mediate the actions of dopamine in brain, we have developed antibodies specific for the D5 receptor. D5 antibodies reacted with recombinant baculovirus-infected Sf9 cells expressing the D5 receptor but not with the D1 receptor or a variety of other catecholaminergic and muscarinic receptors. Epitope-tagged D5 receptors expressed in mammalian cells were reactive with both D5 antibodies and an epitope-specific probe. A mixture of N-linked glycosylated polypeptides and higher molecular-mass species was detected on immunoblots of membrane fractions of D5-transfected cells and also of primate brain. D5 receptor antibodies intensely labeled pyramidal neurons in the prefrontal cortex, whereas spiny medium-sized neurons and aspiny large interneurons of the caudate nucleus were relatively lightly labeled. Antibodies to the D5 dopamine receptor should prove important in experimentally determining specific roles for the D5 and D1 receptors in cortical processes and diseases.     

Human striatal dopamine receptors are organized in compartments.

Dopamine (D2) receptors visualized in postmortem human striatum by quantitative autoradiography of [3H]spiroperidol binding are organized into circumscribed zones of low receptor density separated from other such zones by regions of higher D2 density. The D2-rich zones of the caudate nucleus and putamen contain twice the binding of D2-poor zones. The Hill coefficient, obtained from saturation analysis of [3H]spiroperidol binding to thin sections of human striatum, gave a value near unity, indicating the binding was occurring to a single type of site. The patchiness of [3H]spiroperidol binding was unaltered by postincubation removal of lipid from the tissue sections, indicating that a differential absorption of tritium in white and grey matter does not account for the heterogeneous distribution. The D2-rich and D2-poor regions appear to form labyrinths oriented in the anterior-posterior axis and are typically aligned with, respectively, acetylcholinesterase-rich and -poor compartments as visualized on stained adjacent sections. Thus, the distribution of dopamine D2 receptors conforms to the "striosomal" organization of the human caudate-putamen, a finding that suggests that this receptor subtype may mediate the influence of dopamine on distinct neurochemical compartments within the structure.     

Estrogenic properties of raloxifene,but not tamoxifen,on D2 and D3 dopamine receptors in the rat forebrain

The present study investigated the estrogenic specificity of the modulation of dopamine D(2) and D(3) receptors by comparing the effects of estradiol with tamoxifen or raloxifene. These compounds have estrogenic and/or antiestrogenic activity depending on the target tissue. Two weeks after ovariectomy of female rats, we observed a 60% decrease in the uterine weight, which was prevented by a replacement therapy of 2 weeks with 17beta-estradiol. A tamoxifen or raloxifene treatment of 2 weeks increased uterine weights by 35 and 15%, respectively, but significantly less than estradiol treatment. Ovariectomy decreased dopamine D(2) receptor specific binding (20%) in the dorsolateral part of the anterior striatum and these receptors were left unchanged in the other parts of the striatum as well as in the olfactory tubercle and the nucleus accumbens. 17beta-Estradiol and raloxifene, but not tamoxifen treatment prevented this decrease. Ovariectomy left dopamine D(3) receptor specific binding unchanged. However, estradiol and raloxifene treatment decreased dopamine D(3) receptor binding in the islands of Calleja, the core and shell of the nucleus accumbens and the dorsal part of the anterior striatum, compared with ovariectomized rats. Our results show that raloxifene, but not tamoxifen, has an agonist estrogenic activity on dopamine receptors. Furthermore, estradiol and raloxifene have opposite effects on specific binding to dopamine D(2) and D(3) receptors.     

Effects of chronic alcohol and repeated deprivations on dopamine D1 and D2 receptor levels in the extended amygdala of inbred alcohol-preferring rats

BACKGROUND: Dopaminergic (DA) activity in the extended amygdala (EA) has been known to play a pivotal role in mediating drug and alcohol addiction. Alterations of DA activity within the EA after chronic exposure to alcohol or substances of abuse are considered a major mechanism for the development of alcoholism and addiction. To date, it is not clear how different patterns of chronic alcohol drinking affect DA receptor levels. Therefore, the current studies investigated the effects of chronic ethanol consumption, with or without deprivations, on D1 and D2 receptor densities within the EA. METHODS: Inbred alcohol-preferring (iP) rats were divided into 3 groups with the following treatments: (1) water for 14 weeks; (2) continuous alcohol (C-Alc) for 14 weeks [24-hour concurrent access to 15 and 30% (v/v) ethanol]; or (3) repeatedly deprived of alcohol (RD-Alc) (24-hour concurrent access to 15 and 30% ethanol for 6 weeks, followed by 2 cycles of 2 weeks of deprivation of and 2 weeks of reexposure to ethanol access). At the end of 14 weeks, the rats were killed for autoradiographic labeling of D1 and D2 receptors. RESULTS: Compared with the water control group, both the C-Alc and the RD-Alc groups displayed increases in D1 receptor binding density in the anterior region of the Acb core, whereas the RD-Alc group displayed additional increases in D1 receptor binding density in anterior regions of the lateral and intercalated nuclei of the amygdala. Additionally, both C-Alc and RD-Alc rats displayed increases in D2 receptor binding density in anterior regions of the Acb shell and core, whereas RD-Alc rats displayed additional increases in D2 receptor binding density in the dorsal striatum. CONCLUSION: The results of this study indicate that 14-week extended alcohol drinking with continuous chronic or repeated deprivations increase binding sites of D1 and D2 receptors in specific regions of the EA with greater sensitivity in the anterior regions. The repeated deprivation has greater effect on altering D1 and D2 receptor binding sites in the Acb, dorsal striatum, and subamygdala regions. The current result indicates that the two drinking paradigms may have common as well as differential mechanisms on alteration of dopamine receptor-binding sites in specific regions of the EA.     

Occurrence of dopaminergic (D(2)) receptors within the rabbit pulmonary circulation

The pharmacological characteristics and the microanatomical localization of dopamine D(2)-like receptors, or more correctly spiroperidol binding sites, in the rabbit pulmonary circulation were studied using combined marker binding and light microscopy autoradiography with [((3))H]-spiroperidol (spiperone) as marker. The marker was bound to the samples of the pulmonary artery in a manner consistent with the labelling of dopamine D(2)-like receptors with an equilibrium dissociation constant (K(d)) of about 2.4+/-0.07 nmol/l and a maximum density of binding sites of 65+/-4.5 fmol/mg tissue. Samples of bronchial artery show the same results as those of the pulmonary artery. In contrast, binding experiments made with samples of rabbit lung (capillary of the microcirculation), of pulmonary veins and/or of bronchial veins did not allow the evaluation of specific binding.Autoradiography, observed with light microscopy, showed the development of specific silver grains within the whole wall of extraparenchymal branches of the pulmonary artery and/or of the bronchial artery. Development of silver grains was inhibited by compounds active on the dopamine receptors. The greater sensitivity to displacement by domperidone, haloperidol, and bromocriptine than to displacement by N-propyl-nor-apomorphine, quinpirole and clozapine suggests that the binding sites observed in extraparenchymal, large and medium-sized branches of the rabbit pulmonary and bronchial arteries belong, likely, to the dopamine D(2) receptor subtype. Quantitative analysis of images let us count the amount of these receptors in many samples of the pulmonary and/or bronchial arteries.     

Morphine induces c-fos and junB in striatum and nucleus accumbens via D1 and N-methyl-D-aspartate receptors.

Morphine induced the c-fos and junB immediate early genes in neurons of the medial and ventral striatum and nucleus accumbens. Induction of c-fos and junB mRNA and Fos protein was blocked by naloxone, the D1 dopamine (DA) receptor antagonists SCH23390 and SCH39166, and the N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK801. SCH23390 attenuated morphine induction of AP-1 binding in striatum, suggesting that c-fos and junB contribute to AP-1 binding. SCH23390 and MK801 did not block morphine induction of c-fos and junB in septum. Since the morphine induction of c-fos and junB in striatum and nucleus accumbens (NA) was similar to that observed with cocaine and amphetamine, these data support current concepts that limbic striatum and NA are among the brain regions that mediate drug abuse. Furthermore, since DA and NMDA receptors may mediate opiate reward and opiate induction of c-fos and junB, the DA/NMDA regulation of c-fos and junB and their target genes may produce long-term changes in the striatal and NA circuits that contribute to opiate drug abuse.     

An astroglia-linked dopamine D2-receptor action in prefrontal cortex

Typical neuroleptic drugs elicit their antipsychotic effects mainly by acting as antagonists at dopamine D2 receptors. Much of this activity is thought to occur in the cerebral cortex, where D2 receptors are found largely in inhibitory GABAergic neurons. Here we confirm this localization at the electron microscopic level, but additionally show that a subset of cortical interneurons with low or undetectable expression of D2 receptor isoforms are surrounded by astrocytic processes that strongly express D2 receptors. Ligand binding of isolated astrocyte preparations indicate that cortical astroglia account for approximately one-third of the total D2 receptor binding sites in the cortex, a proportion that we found conserved among rodent, monkey, and human tissues. Further, we show that the D2 receptor-specific agonist, quinpirole, can induce Ca(2+) elevation in isolated cortical astrocytes in a pharmacologically reversible manner, thus implicating this receptor in the signaling mechanisms by which astrocytes communicate with each other as well as with neurons. The discovery of D2 receptors in astrocytes with a selective anatomical relationship to interneurons represents a neuron/glia substrate for cortical dopamine action in the adult cerebral cortex and a previously unrecognized site of action for antipsychotic drugs with affinities at the D2 receptor.     

Localization of D1 and D2 dopamine receptors in brain with subtype-specific antibodies.

Five or more dopamine receptor genes are expressed in brain. However, the pharmacological similarities of the encoded D1-D5 receptors have hindered studies of the localization and functions of the subtypes. To better understand the roles of the individual receptors, antibodies were raised against recombinant D1 and D2 proteins and were shown to bind to the receptor subtypes specifically in Western blot and immunoprecipitation studies. Each antibody reacted selectively with the respective receptor protein expressed both in cells transfected with the cDNAs and in brain. By immunocytochemistry, D1 and D2 had similar regional distributions in rat, monkey, and human brain, with the most intense staining in striatum, olfactory bulb, and substantia nigra. Within each region, however, the precise distributions of each subtype were distinct and often complementary. D1 and D2 were differentially enriched in striatal patch and matrix compartments, in selective layers of the olfactory bulb, and in either substantia nigra pars compacta or reticulata. Electron microscopy demonstrated that D1 and D2 also had highly selective subcellular distributions. In the rat neostriatum, the majority of D1 and D2 immunoreactivity was localized in postsynaptic sites in subsets of spiny dendrites and spine heads in rat neostriatum. Presynaptic D1 and D2 receptors were also observed, indicating both subtypes may regulate neurotransmitter release. D1 was also present in axon terminals in the substantia nigra. These results provide a morphological substrate for understanding the pre- and postsynaptic functions of the genetically defined D1 and D2 receptors in discrete neuronal circuits in mammalian brain.     

Identification, characterization, and localization of the dopamine D3 receptor in rat brain using 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin.

We have identified 7-[3H]hydroxy-N,N-di-n-propyl-2-aminotetralin ([3H]7-OH-DPAT) as a selective probe for the recently cloned dopamine D3 receptor and used it to assess the presence of this receptor and establish its distribution and properties in brain. In transfected Chinese hamster ovary (CHO) cells, it binds to D3 receptors with subnanomolar affinity, whereas its affinity is approximately 100-, 1000-, and 10,000-fold lower at D2, D4, and D1 receptors, respectively. Specific [3H]7-OH-DPAT binding sites, with a Kd of 0.8 nM and a pharmacology similar to those at reference D3 receptors of CHO cells, were identified in rat brain. D3 receptors differ from D2 receptors in brain by their lower abundance (2 orders of magnitude) and distribution, restricted to a few mainly phylogenetically ancient areas--e.g., paleostriatum and archicerebellum--as evidenced by membrane binding are autoradiography studies. Native D3 receptors in brain are characterized by an unusually high nanomolar affinity for dopamine and a low modulatory influence of guanyl nucleotides on agonist binding. These various features suggest that D3 receptors are involved in a peculiar mode of neurotransmission in a restricted subpopulation of dopamine neurons.     

Content of Dynorphins and k-Opioid Receptors in Distinct Brain Regions of C57BL/6 and DBA/2 Mice

Differences in the activity of various components of the endogenous opioid system under basal conditions and after ethanol exposure have been reported between strains and lines of animals showing either high or low ethanol consumption. The objective of the present studies was to investigate the presence of differences in (a) the density of k-opioid binding sites, (b) the content of prodynorphin mRNA, and (c) the content of dynorphin peptides in distinct brain regions between the C57BL/6 (ethanol-preferring) and the DBA/2 (ethanol-avoiding) mice. Results indicated that the C57BL/6 mice have a higher content of k-opioid binding sites and dynorphin A1-13 in the amygdala, and dynorphin A 1–8 in the ventral tegmental area, whereas the DBA/2 mice presented a significantly higher content of k-opioid binding sites, prodynorphin mRNA, as well as dynorphin A 1–13 and dynorphin A 1–8 peptides in the nucleus accumbens and septum. In addition, the DBA/2 mice presented a higher content of k-opioid receptors in the periaqueductal gray and dynorphin A1-13 and dynorphin A 1–8 in the caudate putamen. Because enhanced stimulation of the k-opioid receptors at the level of the nucleus accumbens has been associated with decreased dopamine release and aversive states, the higher content of k-opioid receptors, prodynorphin mRNA, and dynorphin peptides (the endogenous ligand of k-binding sites) in regions of the limbic system of the DBA/2 mice may play an important role in determining their low alcohol consumption.     

D1 and D2 dopamine receptor mRNA in rat brain.

Physiological and pharmacological criteria have divided dopamine receptors into D1 and D2 subtypes, and genes encoding these subtypes have recently been cloned. Based on the sequences of the cloned receptors, we prepared oligodeoxynucleotide probes to map the cellular expression of the corresponding mRNAs in rat brain by in situ hybridization histochemistry. These mRNAs showed largely overlapping yet distinct patterns of expression. The highest levels of expression for both mRNAs were observed in the caudate-putamen, nucleus accumbens, and olfactory tubercle. Within the caudate-putamen, 47 +/- 6% and 46 +/- 5% of the medium-sized neurons (10-15 microns) expressed the D1 and D2 mRNAs, respectively, and only the D2 mRNA was observed in the larger neurons (greater than 20 microns). The D1 and D2 mRNAs were expressed in most cortical regions, with the highest levels in the prefrontal and entorhinal cortices. Within neocortex, D1 mRNA was observed primarily in layer 6 and D2 mRNA in layers 4-5. Within the amygdala, D1 mRNA was observed in the intercalated nuclei, and D2 mRNA in the central nucleus. Within the hypothalamus, D1 mRNA was observed in the suprachiasmatic nucleus and D2 mRNA in many of the dopaminergic cell groups. Within the septum, globus pallidus, superior and inferior colliculi, mammillary bodies, and substantia nigra only D2 mRNA was detected. These data provide insight into the neuroanatomical basis of the differential effects of drugs that act on D1 or D2 receptors.