Computer-assisted autoradiographic localization of subtypes of serotonin1 receptors in rat brain

In vitro light microscopic receptor autoradiography was carried out to localize two subtypes of 5-HT1 receptors in certain regions of the rat central nervous system (CNS). The data were analyzed by computerized digital image processing to reveal subfields of high receptor densities within the septal area, the prefrontal and frontal cortex, and the hippocampus. The septum and hippocampus showed interesting localization of serotonin binding which can be subtyped into 5-HT1-A and 5-HT1-B sites. The differential localization of these sites may be helpful in shedding some light as to the functional significance of multiple serotonin receptors.     

D-1 dopamine receptors in the rat brain: a quantitative autoradiographic analysis

The distribution of dopamine D-1 receptors has been determined in the rat brain by a quantitative in vitro light-microscopic autoradiographic method. The binding of [N-methyl-3H]-SCH 23390 to slide-mounted tissue sections takes place with characteristics expected of a substance that recognizes D-1 receptors. The binding is saturable, has high affinity, and exhibits an appropriate pharmacology and stereospecificity in several discrete microscopic brain regions as determined by quantitative autoradiography. The highest density of D-1 receptors occurs in the caudate-putamen, accumbens nucleus, olfactory tubercle, and the substantia nigra pars reticulata. High concentrations of D-1 receptors were associated with the intercalated and medial nuclei of the amygdala, entopeduncular nucleus, and major island of Calleja. Furthermore, moderate to low concentrations were observed in several other structures, such as the frontal cortex, subthalamic nucleus, and several thalamic, hypothalamic, and hippocampal areas. The distribution of D-1 receptors correlates very well with projection areas of dopaminergic pathways. This technique furnishes a powerful assay for the accumulation of detailed pharmacologic and anatomical data about D-1 receptors, and the results suggest possible CNS sites of action of D-1 dopamine receptor selective compounds.     

Autoradiographic localization of dopamine D1 and D2 receptors in the brain of several mammalian species

Summary Dopamine D 1 and D 2 receptor distributions were studied in the brain of the mouse, rat, guinea pig, cat and monkey by means of in vitro quantitative autoradiography using [³H]SCH 23390 and [³H]CV 205–502 to label D 1 and D 2 subtypes respectively.The distribution of both subtypes of receptors was similar within the basal ganglia of all species investigated. The highest densities for both subtypes were found in the nucleus caudatus, putamen, nucleus accumbens, olfactory tubercle and substantia nigra.Outside of the basal ganglia, differences in the distribution of both receptors were found among the species examined in regions such as cerebellum, cortex, hippocampus, superior colliculus and olfactory bulb.In all species D 1 receptor densities were higher than those of D 2. The absolute amount of both subtypes, however, varied among species.These results indicate that dopamine receptor distribution is well preserved in the basal ganglia during evolution, although differences among species exist in their distribution outside the basal ganglia and their absolute amount.Abbreviations used in figures A corpus amygdalae - Acc nucleus accumbens - CA 1 CA 1 subfield of the hippocampus - CA 2 CA 2 subfield of the hippocampus - CgC cingular cortex - Cls claustrum - Cd nucleus caudatus - CPu caudate-putamen - DA dopamine - DG dentate gyrus of the hippocampus - EP nucleus entopeduncularis - GP globus pallidus - GPl globus pallidus, pars lateralis - GPm globus pallidus, pars medialis - gr granular layer of the cerebellum - IP nucleus interpeduncularis - LS lateral septum - mol molecular layer of the cerebellum - OT olfactory tubercle - Pk Purkinje cell layer of the cerebellum - Pu putamen - SC superior colliculus - snc substantia nigra, pars compacta - snr substantia nigra, pars reticulata - VTA area ventralis tegmentalis     

Characterization and autoradiographic localization of LHRH receptors in the rat brain

Luteinizing-hormone-releasing hormone (LHRH) is distributed in several extrahypothalamic areas, suggesting that it might act as a neurotransmitter or neuromodulator in the central nervous system. This study was undertaken to characterize and localize LHRH receptors in the rat brain by using slide-mounted frozen sections. The radioligand used was an iodinated stable LHRH agonist, [D-Ser(TBU)6, des-Gly-NH2(10)]LHRH ethylamide. It was clearly established that LHRH receptors with characteristics similar to those of pituitary LHRH receptors were present in the brain. They were found in high concentrations in the dorsal hippocampus, amygdala, septum, and subiculum and in very low amounts in the hypothalamus. Generally, a good correlation could be observed between receptor distribution and immunohistologically localized LHRH. These results strongly suggest that LHRH has multiple sites of action in the rat brain and reinforce the hypothesis that the peptide could act as a neurotransmitter/neuromodulator in the central nervous system.     

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. I. Serotonin-1 receptors

The distribution of serotonin-1 (5-HT1) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with [3H]serotonin (5-[3H]HT), 8-hydroxy-2-[N-dipropylamino-3H]tetralin (8-OH- [3H]DPAT), [3H]LSD and [3H]mesulergine, and the densities quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Competition experiments for 5-[3H]HT binding by several serotonin-1 agonists led to the identification of brain areas enriched in each one of the three subtypes of 5-HT1 recognition sites already described (5-HT1A, 5-HT1B, 5-HT1C). The existence of these 'selective' areas allowed a detailed pharmacological characterization of these sites to be made in a more precise manner than has been attained in membrane-binding studies. While 5-[3H]HT labeled with nanomolar affinity all the 5-HT1 subtypes, the other 3H-labeled ligands labeled selectively 5-HT1A (8-OH-[3H]DPAT), 5-HT1C ([3H]mesulergine) and both of them ([3H]LSD). Very high concentrations of 5-HT1 receptors were localized in the choroid plexus, lateroseptal nucleus, globus pallidus and ventral pallidum, dentate gyrus, dorsal subiculum, olivary pretectal nucleus, substantia nigra, reticular and external layer of the entorhinal cortex. The different fields of the hippocampus (CA1-CA4), some nuclei of the amygdaloid complex, the hypothalamic nuclei and the dorsal raphé, among others, also presented high concentrations of sites. Areas containing intermediate densities of 5-HT1 receptors included the claustrum, olfactory tubercle, accumbens, central grey and lateral cerebellar nucleus. The nucleus caudate-putamen and the cortex, at the different levels studied, presented receptor densities ranging from intermediate to low. Finally, in other brain areas--pons, medulla, spinal cord--only low or very low concentrations of 5-HT1 receptors were found. From the areas strongly enriched in 5-HT1 sites, dentate gyrus and septal nucleus contained 5-HT1A sites, while globus pallidus, dorsal subiculum, substantia nigra and olivary pretectal nucleus were enriched in 5-HT1B. The sites in the choroid plexus, which presented the highest density of receptors in the rat brain, were of the 5-HT1C subtype. The distribution of 5-HT1 receptors reported here is discussed in correlation with the distribution of serotoninergic neurons and fibers, the related anatomical pathways and the effects which appear to be mediated by these sites.     

Neuroleptic and dopamine receptors: autoradiographic localization of [3H]spiperone in rat brain.

Rats were administered [3H]spiperone (SP: spiroperidol) by tail vein injection and 2 h later the brain was processed for light microscopic autoradiography. High densities of autoradiographic grains were found in all areas known to have a dopaminergic innervation, including the olfactory tubercles, nucleus accumbens, nucleus caudate-putamen, lateral septum, zona incerta, nucleus subthalamicus, arcuate nucleus, nucleus of the central amygdala, areas in the ventral tegmentum and the claustrum. There were also increased autoradiographic grain densities in other areas such as the midbrain and the frontal cortex indicating that binding occurred to other neurotransmitter receptors besides dopamine receptors. These studies delineate with a high resolution at an anatomical level the major binding sites for neuroleptic drugs in the forebrain. They suggest which areas of the brain are the most involved in neuroleptic drug action and they add further evidence that important regions are those receiving a dense dopaminergic innervation.     

Quantitative autoradiographic localization of neuropeptide Y receptors in the rat lower brainstem

Using quantitative autoradiography, we have characterized the binding of ¹²⁵I-Bolton-Hunter coupled neuropeptide Y ([¹²⁵I]NPY) and observed the localization of ¹²⁵I-NPY receptors in the rat lower brainstem. [¹²⁵I]NPY bound to the receptors in a specific, saturable and reversible manner with high affinity. The binding was blocked only by unlabeled NPY but not by NPY-related peptides i.e. peptide YY, pancreatic polypeptide (avian and human), nor by neurotensin. [¹²⁵I]NPY receptors were revealed to be coupled to the guanosine triphosphate (GTP)-binding protein. Regional distribution study showed that [¹²⁵I]NPY has a distinctive pattern of distribution in the rat lower brainstem, being particularly concentrated in the area postrema and the medial subnucleus of the nucleus tractus solitarii. These results suggest that such NPY receptors have an important role in cardiovascular regulation.     

Postnatal development of dopamine D1-like receptors in rat cortical and striatolimbic brain regions: An autoradiographic study

Postnatal development of dopamine D1-like (D1/D5) receptors in rat caudate-putamen (CPu), nucleus accumbens (NAc), hippocampus, frontal and entorhinal cerebral cortex was assessed between postnatal days (PD) 7-60 by in vitro receptor autoradiography. Density of [3H]SCH-23390 binding to D1-like receptors increased from PD-7 to a peak at PD-28 in CPu (11-fold) and NAc (23-fold), then declined by 20-40% in both regions over PD-35-60, to adult levels. In hippocampus, frontal and entorhinal cortex, D1-like receptors increased by lesser amounts (3- to 4-fold) from PD-7 to stable, maximal adult levels at PD-60. Evidently, excess D1-like receptors were eliminated during maturation of CPu and NAc, but not in the other forebrain regions. Postnatal D1-like receptor development in rat forebrain paralleled that of D2- and D4-like receptors in the same regions.     

Quantitative autoradiographic mapping of serotonin receptors in the rat brain. II. Serotonin-2 receptors

The distribution of serotonin-2 (5-HT2) receptors in the rat brain was studied by light microscopic quantitative autoradiography. Receptors were labeled with four ligands: [3H]ketanserin, [3H]mesulergine, [3H]LSD and [3H]spiperone, which are reported to show high affinity for 5-HT2 receptors. Co-incubation with increasing concentrations of several well-known 5-HT2-selective drugs, such as pirenperone, cinanserin and ketanserin, resulted in an inhibition of the binding of the four 3H-labeled ligands to the same areas. However, all of them recognized, in addition to 5-HT2 sites, other populations of binding sites. Receptor densities were quantified by microdensitometry with the aid of a computer-assisted image-analysis system. Our results reveal a heterogeneous distribution of 5-HT2 receptor densities in the rat brain. Very high concentrations were localized in the claustrum, olfactory tubercle and layer IV of the neocortex. The anterior olfactory nucleus, piriform cortex and layer I of neocortex were also rich in 5-HT2 receptors. Intermediate concentrations of receptors were found in caudate putamen, nucleus accumbens, layer V of neocortex, ventral dentate gyrus and mammillary bodies. Areas containing only low concentrations of receptors included the thalamus, hippocampus, brainstem, medulla, cerebellum and spinal cord. The specificity of the different ligands used is discussed in terms of the other populations of sites recognized by them. The distribution of 5-HT2 receptors here reported is discussed in correlation with (a) the known distribution of serotoninergic terminals, (b) the specific anatomical systems and (c) the central effects reported to be mediated by 5-HT2-selective drugs.     

Ontogeny of dopamine D1 receptors in rat forebrain: a quantitative autoradiographic study

The development of dopamine D1 receptors during the early postnatal period is examined in rat forebrain, using quantitative autoradiography and [3H]SCH 23390 as ligand. Dopamine D1 receptors are present in many regions at birth. In general, regions with the highest densities of D1 receptors in adults have the highest densities of receptors in neonates. For most regions in the forebrain there is a steady increase in the density of D1 receptors, as measured in fmol/mg tissue, from day 1 to day 28 of age. This is most obvious in the regions with the greatest number of receptors, such as the caudate-putamen, the nucleus accumbens, and the olfactory tubercles. The more caudal regions examined in this study had a relatively greater portion of their receptors present at day 1 compared to day 28 than more rostral regions. In general they had about 50% of their receptors present at birth, whereas most regions studied had receptor levels at day 1 about 20% of those found at day 28. In the most anterior regions, the development of receptors was somewhat slower. Receptor number in the frontal cortex region did not begin to increase until about 10 days of age. The present studies indicate that dopamine D1 receptors develop in the forebrain of the rat in a steady pattern. There are no dramatic increases or decreases in receptor number throughout the postnatal period.     

Characterization and autoradiographic localization of TRH receptors in sections of rat brain

Receptors for thyrotropin-releasing hormone (TRH) in rat brain have been localized autoradiographically by exposing tritium-sensitive film to sections labeled with [3H]3-Me-His2-TRH. Greatest grain density was found over certain nuclei of the amygdala, with considerable density over several other forebrain areas. Properties of TRH receptor binding in frozen sections closely resembled those of receptors in fresh membrane fragments.     

Effects of repeated injections of cocaine on D1 and D2 dopamine receptors in rat brain

In order to determine if chronic administration of cocaine produced long-lasting alterations in dopamine receptor binding, rats were treated with single daily injections of cocaine (0, 10, or 20 mg/kg) for 15 consecutive days and killed either 20 min or 2 weeks after the last injection. The density of D1 binding sites in frontal cortex was either unchanged (10 mg/kg) or slightly increased (20 mg/kg) 20 min after the last daily injection, but was decreased 2 weeks later. D1 sites in striatum were decreased both immediately and 2 weeks after the injection regimen. Decreases in D1 binding site density in nucleus accumbens were observed only immediately after the last injection. In contrast to these effects on D1 binding sites, D2 binding sites were decreased in striatum and frontal cortex and increased in the nucleus accumbens 20 min after repeated cocaine, but were unaffected 2 weeks after repeated cocaine. Computer-assisted analysis of the saturation isotherms revealed that chronic administration of cocaine did not affect the affinity (Kd) of the radioligands used to label D1 or D2 sites. These findings suggest that repeated administration of cocaine results in long-term decreases in D1 binding sites in striatum and frontal cortex and transient decreases in D2 binding sites. Furthermore, cocaine caused opposite, transient effects on D1 and D2 site density in nucleus accumbens.     

Autoradiographic localization of D1 dopamine receptors in the rat brain with [3H]SCH 23390

The regional distribution of D1 dopamine (DA) receptors in the rat brain has been studied by quantitative autoradiography using the specific D1 antagonist [3H]SCH 23390 as a ligand. The binding of [3H]SCH 23390 to striatal sections was saturable, stereospecific, reversible and of high affinity (Kd = 2.05 nM); it occurred at a single population of sites and possessed the pharmacological features of the D1 DA receptor. The highest densities of [3H]SCH 23390 binding sites were found in the caudate-putamen, olfactory tubercle, nucleus accumbens and substantia nigra (especially in the pars compacta). High densities were also observed in the nucleus interstitialis striae terminalis, the anterior olfactory nucleus, the entopeduncular nucleus, the subthalamic nucleus, the claustrum and the amygdalohippocampal area. An intermediate labelling was found in the anteromedial and suprarhinal DA terminal fields of the cerebral cortex, the basolateral, medial and lateral amygdaloid nuclei, the endopiriform nucleus, the primary olfactory cortex, the globus pallidus, the superior colliculus (especially the superficial layer), the nucleus amygdaloideus corticalis and the dorsal hippocampus (molecular layer of the CA1 and dentate gyrus). In the anteromedial and suprarhinal cortices, [3H]SCH 23390 binding was more concentrated in layers V and VI. Moderate levels of [3H]SCH 23390 were found in the thalamus, hypothalamus, the habenula, the ventral tegmental area, the posterior cingulate and entorhinal cortices, the supragenual dopamine terminal system and the cerebellum (molecular layer). This regional distribution of [3H]SCH 23390 closely correlated (except for the cerebellum) with the reported distribution of dopaminergic terminals. The topographical distribution of [3H]SCH 23390 has also been studied in detail in striatal subregions. The density of D1 receptors was much greater in the ventrolateral sector and medial margin of the striatum than in the ventromedial and dorsolateral sectors. A rostrocaudal decrease in the densities of D1 sites was also found along the rostrocaudal axis of the caudate-putamen. These lateral to medial and anteroposterior gradients overlapped with the density of the dopaminergic afferents.     

Loss of D1 and D2 dopamine receptors and muscarinic cholinergic receptors in rat brain following in vitro polytron homogenization

The receptor specific binding of [3H]SCH23390 and [3H]spiperone to rat striatal membranes and [3H]QNB to rat cortical membranes was significantly reduced if the membrane suspensions were subjected to polytron homogenization. The degree of loss of the binding sites was a function of the polytron speed (setting) and the length of time of homogenization. Although the loss of binding sites could be reduced by maintaining a cold temperature of the tissue suspension during polytron homogenization, significant loss of binding sites could not be prevented. As many as 75% of the binding sites could be lost following polytron homogenization under the conditions employed in the present experiments. However, no significant change in the affinity of the remaining binding sites was observed. The loss of binding sites is not likely due to small tissue fragments passing through the filters. Care should be taken during tissue homogenization using a polytron or similar equipment to ensure that loss of receptor binding sites is minimized or remains constant between individual samples.     

Muscarinic cholinergic receptor subtypes in the rat brain. I. Quantitative autoradiographic studies

The binding characteristics of N[³H]methylscopolamine (³H]NMS) to slide-mounted tissue sections were studies using quantitative autoradiography. Binding of [³H]NMS was saturable, reversible of high affinity (K_d = 0.26nM). The inhibition of [³H]NMS binding produced by several muscarinic agonists and antagonists was analyzed in 29 discrete brain regions by constructing complete displacement curves. Comparison of IC₅₀ values obtained both biochemically and by autoradiography demonstrated to a very close agreement, supporting the validity of the autoradiographic approach. The competition curves for the agonists carbachol, oxotremorine and 2-ethyl-8-methyl-2,8-diazaspiro-[4,5]-decan-1,3-dion-hydrobromide (RS 86) fitted to a two-site model, with comparable affinity values from region to regions, although different proportions of high- and low-affinity sites were seen in the different areas studied. The distribution of high- and low-affinity sites was similar for the three agonists. Atropine showed monophasic curves presenting similar affinities in all regions studied. In contrast, pirenzepine differentiated between high- and low-affinity sites which showed a distribution opposite to that observed for the agonists. Gallamine, a ligand for a putative regulatory site in the muscarinic receptor, inhibited [³H]NMS binding in a biphasic manner. The calculated IC₅₀ values for the gallamine high- and low-affinity sites did not vary from region to region and the distribution of these sites correlated well with that observed for the agonists. High-affinity pirenzepine sites (also called M₁ sites) were localized mainly in forebrain areas, such as striatum, hippocampus and cortex, and their regional distribution correlated with that of the low-affinity sites for the agonists and gallamine. On the other hand, low-affinity sites for pirenzepine (named M₂ sites) were mainly found in the brainstem and parts of the thalamus. A good correlation was found between pirenzepine low-affinity sites and agonist and gallamine high-affinity sites. The significance of these findings is discussed in relation to the known and possible effects of selective M₁ and M₂ centrally acting agents.     

Autoradiographic localization of dopamine D1 and D2 receptors in rat cerebral cortex following unilateral neurotoxic lesions.

Relative to dopaminergic innervation of cortex, dopamine D1 and D2 receptors may be located on presynaptic terminals and/or postsynaptically on cortical neurons. To assess the relative distribution of these sites, quantitative in vitro receptor autoradiography was performed following injection of 6-hydroxydopamine (6-OHDA) into the median forebrain bundle (MFB; which lesions presynaptic DA terminals) and ibotenic acid into the prefrontal and anterior cingulate cortices (which lesions neurons whose cell bodies are intrinsic to cortex). Receptor autoradiography was performed ten days after injection of neurotoxins with [3H]SCH 23390 (a D1 probe) and [125I]epidepride (a D2 probe). Both DA receptor subtypes were found in all layers of anterior cingulate and prefrontal cortices but were concentrated in deeper layers V and VI. Ibotenic acid lesion of cortex reduced D1 and D2 receptors by 55-80%, although the concentrations of DA and its major metabolite dihydroxyphenylacetic acid (DOPAC) were unchanged. Lesion of MFB produced no significant change in D1 and D2 receptors, but was associated with a 49-52% decrease in DA and DOPAC levels relative to the contralateral side. These results suggest that the majority of D1 and D2 receptors in prefrontal and anterior cingulate cortices are located postsynaptically on neurons intrinsic to the cortex.     

β-Adrenoceptor subtypes in the human brain: autoradiographic localization

The distribution and characteristics of beta-adrenoceptors in postmortem human brain was studied using quantitative autoradiographic techniques. 125I-Cyanopindolol was used as a ligand. High densities of beta-adrenoceptors were found in the caudate, putamen, different cortical areas and layers and the hippocampal formation. Low densities were present in other areas such as the thalamus, hypothalamus, midbrain and cerebellar cortex. Specific beta 1 and beta 2 antagonists were used to visualize and quantify separately the two subtypes of beta-adrenoceptors. Computer analysis of the competition curves obtained revealed that the putamen was enriched in beta 1 sites while the cerebellum contained predominantly beta 2 adrenoceptors. The regional distribution of beta-adrenoceptor subtypes was found to be similar to that seen in the rat brain.     

Localization of dopamine D2 receptor mRNA and D1 and D2 receptor binding in the rat brain and pituitary: an in situ hybridization-receptor autoradiographic analysis

Several lines of evidence suggest the existence of multiple dopamine receptor subtypes, referred to as D1 and D2. The present study examines the distribution of these dopamine binding sites in the rat brain and pituitary in relation to the distribution of D2 receptor mRNA using a combination of in vitro receptor autoradiographic and in situ hybridization techniques. 3H-Raclopride and 3H-SCH23390 (in the presence of 1 microM ketanserin) were used to label D2 and D1 receptor binding sites, respectively, while a 495 bp cRNA probe synthesized from the Sac I-Bgl II fragment of a rat D2 receptor cDNA was used to visualize the D2 receptor mRNA. Analysis of adjacent tissue sections in which receptor autoradiography and in situ hybridization had been performed revealed several brain regions where the D2 binding site and corresponding mRNA appear to be similarly distributed, including the caudate-putamen, nucleus accumbens, olfactory tubercle, globus pallidus, substantia nigra, and ventral tegmental area. In the pituitary gland, D2 binding sites and mRNA appear to be codistributed with very dense levels in the intermediate lobe and individually labeled cells in the anterior lobe. Brain regions demonstrating a lack of correspondence between the distribution of the D2 binding site and D2 receptor mRNA include the olfactory bulb, neocortex, paleocortex, hippocampus, and zona incerta. Several hypotheses are discussed to explain the lack of correspondence in certain brain regions; these include the localization of receptor binding sites on both fibers and cell bodies and receptor transport. These studies provide a better understanding of the anatomical distribution of the D2 receptor and serve as a framework for future regulatory and anatomical mapping studies. By focusing on specific brain regions, such as the nigrostriatal system, hippocampus, and olfactory bulb, they provide insights into D2 receptor synthesis, transport, and insertion into cell membranes.     

Topographic nonoverlapping distribution of D1 and D2 dopamine receptors in the amygdaloid nuclear complex of the rat brain.

The distribution of D1 and D2 dopamine (DA) receptors in the nuclei and subnuclear zones of the rat amygdaloid complex was mapped using quantitative light microscopic autoradiography. [125I]iodosulpiride and [125I]SCH 23982 (in the presence of 50 nM ketanserin) were used to label D2 and D1 DA receptors, respectively. The DA receptor subtypes exhibited a topographic, nonoverlapping distribution which generally conformed to the cytoarchitectonic boundaries of the component nuclei and subnuclear zones of the amygdaloid complex. The highest density of [125I]iodosulpiride binding sites was observed in the main intercalated cell group and the central amygdaloid nucleus where a medial to lateral gradient of binding sites was localized to its subnuclear zones. [125I]SCH 23982 binding sites were localized in the main intercalated cell group and the basolateral amygdaloid nucleus with a uniform low density in the central nucleus. The functional topography of mesoamygdaloid DA neurons may therefore be mediated, in part, at the level of DA receptor subtypes. The pattern of distribution of [125I]iodosulpiride binding sites in subdivisions of the central amygdaloid nucleus and bed nucleus of the stria terminalis suggests that the functions of the "extended amygdala," a major system of the functional organization of the basal forebrain, may be regulated by DA afferents at multiple key sites of D2 receptor action.     

Pharmacological characterization and autoradiographic localization of dopamine D1-like receptors in the thymus

The present study was designed to identify the pharmacological profile and the anatomical localisation of dopamine D₁-like receptor sites in the rat thymus using [³H]SCH 23390 as a ligand. [³H]SCH 23390 was specifically bound to sections of the thymus. Binding was time, temperature and concentration-dependent belonging in the range of concentrations of radioligand used to a single class of high affinity sites. The dissociation constant was 1.6 nM and the maximal density of binding sites averaged to 170 fmol/mg tissue. The pharmacological profile of [³H]SCH 23390 binding to sections of the rat thymus is consistent with the labelling of dopamine D₁-like sites. Dopamine was able to compete with [³H]SCH 23390 binding to sections of rat thymus in the range of nanomolar concentrations. This suggests the labelling of dopamine D₅ receptor sites. Light microscope autoradiography revealed the localisation of [³H]SCH 23390 binding sites primarity in the cortex of the thymus and in lesser amounts at the level of thymic corpuscles. The possible functional significance of dopamine D₁-like receptors in the rat thymus is discussed.