Neurotransmitter receptors: neuroanatomical localization through autoradiography

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.

Autoradiographic analysis of GABA-benzodiazepine receptors in an animal model of acute hepatic encephalopathy

To complement analogous studies using conventional ligand-membrane binding assays, the densities of gamma-aminobutyric acid and benzodiazepine receptors in the brain have been assessed using an autoradiographic technique in an animal model of hepatic encephalopathy. Hepatic encephalopathy due to fulminant hepatic failure was induced in rabbits by the intravenous injection of galactosamine. The specific binding of three radiolabeled ligands was assessed densitometrically in several microregions of cerebral cortex, hippocampus and cerebellum. [3H]Muscimol was used to assess gamma-aminobutyric acid receptor density and [3H]flunitrazepam or [3H]Ro 15-1788 was used to assess benzodiazepine receptor density. No significant differences were observed between the magnitude of binding of the three ligands to each of the microregions of brain from control rabbits and rabbits in Stage III or IV hepatic encephalopathy. These findings suggest that the behavioral expression of hepatic encephalopathy in the model studied is not dependent upon an increase in the number of gamma-aminobutyric acid or benzodiazepine receptors, but do not conflict with the hypothesis that gamma-aminobutyric acid-ergic tone is increased in hepatic encephalopathy.

Tracing of dopamine receptors in hemiparkinsonism with positron emission tomography (PET)

In 13 hemiparkinson patients possible changes in dopamine receptors were investigated in vivo with PET, using [11C]methylspiperone as the receptor labeling ligand. Though in individual patients the specific binding of the tracer differed between both striata, no consistent difference was found. However, the specific binding in the striatum of patients with longterm dopaminergic medication was significantly higher than in non-medicated patients, and close to normal. No correlation with the predominantly unilateral clinical pathology was found. Since the postsynaptic receptor changes in hemiparkinson patients appear not to be uniform, these results suggest that the clinical asymmetry is not only caused by a dysfunction of nigral dopaminergic cells, but by pathology in other cells and probably in other neurotransmitter systems as well. The observed bilateral increase in receptor binding during dopaminergic therapy most likely results from a symmetrical change in local pharmacokinetics or from alteration of the receptor binding. Further studies with PET are needed to determine the exact nature of this change.

Autoradiographic localization and quantification of dopamine D2 receptors in normal human brain with [3H]N-n-propylnorapomorphine

The precise distribution of dopamine receptors has been studied autoradiographically in the normal human brain using [3H]N-n-propylnorapomorphine ([3H]NPA) as a ligand. Preliminary experiments aimed at optimizing the binding assay conditions revealed that preincubation washing of caudate nucleus sections was a prerequisite to obtain a good ratio of specific to non-specific binding. The binding of [3H]NPA to caudate-putamen sections was saturable, stereospecific, reversible, of high affinity (Kd = 0.27-0.35 nM) and occurred at a single population of sites. Competition experiments with various drugs indicated that in the caudate-putamen the specific [3H]NPA binding sites possess the pharmacological features of the dopamine D2 receptor. The highest levels of [3H]NPA binding sites were found in the caudate nucleus, putamen, globus pallidus and nucleus accumbens. There were also intermediate to low concentrations of the 3H-ligand in the hippocampus, the insular and cingular cortices and in the occipito-temporal gyrus, while almost undetectable levels of binding were found in the anterior frontal cortex. Thorough examination of the subregional distribution of [3H]NPA binding sites in the caudate-putamen-pallidum complex revealed heterogeneous patterns of radioactivity. In these brain regions, the distribution of autoradiographic grains was punctate and islands of high and low densities were observed. Moreover, in the caudate nucleus, there was a subtle high lateral to low medial gradient in the topography of the [3H]NPA binding sites and a more pronounced gradient along the rostrocaudal axis; the highest levels of binding being located at the midbody of the nucleus. No gradients of [3H]NPA binding were observed in the putamen. The present data indicate that [3H]NPA is a suitable ligand for accurate autoradiographic labeling of dopamine D2 receptors in human postmortem brain tissue and that dopamine receptors are heterogeneously distributed and topographically organized in patches and gradients in the basal ganglia regions.

Ontogeny of dopamine, serotonin and spirodecanone receptors in rat forebrain--an autoradiographic study

Sections from freshly frozen neonatal rat brain, ages 0-21 days, were incubated with [3H]spiperone (SP). Initial studies characterized the binding sites for SP in terms of association and dissociation rates, saturability and pharmacology. The binding sites were found to be predominantly dopamine D2 receptors in sections centered in the striatum and these receptors were similar to receptors in adult brain. Autoradiographic studies using in vitro techniques examined the anatomic distribution of [3H]SP binding sites. Using domperidone, ketanserin and (+)butaclamol it was possible to differentiate dopamine D2, serotonin S2 and spirodecanone receptors. Dopamine receptors were found at birth in striatum and nucleus accumbens and increased in density with age. In the first two weeks postnatally there was an apparent dorsolateral to ventromedial gradient in the striatum. Serotonin S2 receptors were found in the cortex, lateral olfactory tubercles and nucleus accumbens and claustrum. These receptors increased in density with age but to a much lesser extent than dopamine receptors. Spirodecanone receptors were first apparent in the piriform cortex by day 5. They became very dense with age in particular regions, i.e., layer II of cortex, piriform cortex, medial olfactory tubercles, lateral septum and in patches in the nucleus accumbens. These studies delineate with a high resolution at an anatomical level the major receptor sites for neuroleptic drugs in the developing rat forebrain. They describe the developmental pattern of these receptors and provide a basis for further studies on their control and function during development.

Heterogeneous distribution of dopamine D2 receptors within the rat striatum as revealed by autoradiography of [3H]N-n-propylnorapomorphine binding sites

The precise topographical distribution of dopamine D2 receptors in the rat striatum has been studied autoradiographically through the use of [3H]N-n-propylnorapomorphine (NPA) as a ligand. The highest densities of [3H]NPA binding sites were found in the ventrolateral aspect and medial margin of the striatum, while sparse grain densities were present in the medial and latero-dorsal striatum. Moreover, a clear gradient of [3H]NPA binding was observed along the rostro-caudal axis of the striatum. These heterogeneities in the distribution of dopamine receptors provide further evidence for a functional compartmentalization of the striatum.

Quantitative autoradiography of 3H-nomifensine binding sites in rat brain

The distribution of 3H-nomifensine binding sites in the rat brain has been studied by quantitative autoradiography. The binding of 3H-nomifensine to caudate putamen sections was saturable, specific, of a high affinity (Kd = 56 nM) and sodium-dependent. The dopamine uptake inhibitors benztropine, nomifensine, cocaine, bupropion and amfonelic acid were the most potent competitors of 3H-nomifensine binding to striatal sections. The highest levels of (benztropine-displaceable) 3H-nomifensine binding sites were found in the caudate-putamen, the olfactory tubercle and the nucleus accumbens. 6-Hydroxydopamine-induced lesion of the ascending dopaminergic bundle resulted in a marked decrease in the 3H-ligand binding in these areas. Moderately high concentrations of the 3H-ligand were observed in the bed nucleus of the stria terminalis, the anteroventral thalamic nucleus, the cingulate cortex, the lateral septum, the hippocampus, the amygdala, the zona incerta and some hypothalamic nuclei. There were low levels of the binding sites in the habenula, the dorsolateral geniculate body, the substantia nigra, the ventral tegmental area and the periaqueductal gray matter. These autoradiographic data are consistent with the hypothesis that 3H-nomifensine binds primarily to the presynaptic uptake site for dopamine but also labels the norepinephrine uptake site.

Comparison of three 18F-labeled butyrophenone neuroleptic drugs in the baboon using positron emission tomography

The butyrophenone neuroleptics spiroperidol, benperidol, and haloperidol were radiolabeled with fluorine-18 and studied in baboon brain using positron emission transaxial tomography (PETT). Pretreatment of the baboon with a high pharmacological dose of (+)-butaclamol reduced the specifically bound component of radioactivity distribution in the striatum to approximately the radioactivity distribution found in the cerebellum. Comparative studies of brain distribution kinetics over a 4-h period indicated that either [18F]spiroperidol or [18F]benperidol may be suitable for specific labeling of neuroleptic receptors. In an 8-h study with [18F]spiroperidol, striatal radioactivity did not decline, suggesting that spiroperidol either has a very slow dissociation rate or that it binds irreversibly to these receptors in vivo. [18F]Haloperidol may not be suitable for in vivo PETT studies, because of a relatively high component of nonspecific distribution and a faster dissociation from the receptor. Analysis of 18F in plasma after injection of [18F]spiroperidol indicated rapid metabolism to polar and acidic metabolites, with only 40% of the total radioactivity being present as unchanged drug after 30 min. Analysis of the metabolic stability of the radioactively labeled compound in rat striatum indicated that greater than 95% of [18F]spiroperidol remains unchanged after 4 h.

Autoradiographic localization of dopamine receptors in the spinal cord of the rat using [3H]-N-propylnorapomorphine

Dopamine receptors were localized autoradiographically in the rat spinal cord after in vitro labelling using 3H-N-propylnorapomorphine (NPA). The highest densities of 3H-NPA binding sites were found in the substantia gelatinosa and in a zone of the grey matter immediately ventral to the dorsal corticospinal tract. Other areas of the grey matter presented only moderate or low receptor densities while no 3H-NPA binding sites could be found in the white matter. The localization of 3H-NPA binding sites is compatible with a role for spinal cord dopamine in the processing of sensory information.