C-TERMINAL TAIL OF BETA-ADRENERGIC RECEPTORS - IMMUNOCYTOCHEMICAL LOCALIZATION WITHIN ASTROCYTES AND THEIR RELATION TO CATECHOLAMINERGIC NEURONS IN N-TRACTUS-SOLITARII AND AREA POSTREMA

Beta-adrenergic receptors (beta-AR) in the medial nuclei of tractus solitarii (m-NTS) and area postrema (AP) may bind to catecholamines released from neurons, whereas only the AP has fenestrated capillaries allowing access to circulating catecholamines. Since varied autonomic responses are seen following beta-AR activation of the dorsal vagal complex, including the m-NTS and AP, we hypothesized that there might be a cellular basis for varied responses to beta-AR stimulation that depends on the differential access to circulating catecholamines. Therefore, we comparatively examined the ultrastructural localization of the beta-AR in relation to catecholaminergic neurons in these regions. An antibody directed against the C-terminal tail (amino acids 404-418) of hamster beta-adrenergic receptor (beta-AR404) was used in this study. The localization of beta-AR404 was achieved by the avidin-biotin peroxidase complex (ABC) technique in combination with a pre-embed immunogold labeling method to localize tyrosine hydroxylase (TH), the catecholamine-synthesizing enzyme. Within m-NTS and at subpostremal border, labeling for beta-AR404 was evident along the intracellular surface of pl asma membranes of small, apparently distal, astrocytic processes, Astrocytic processes with beta-AR404-immunoreactivity formed multiple, thin lamellae around TH-labeled and non-TH neuronal cell bodies and dendrites. Beta-AR404-immunoreactive astrocytes also extended end-feet around blood vessels and surrounded groups of axon terminals that were directly juxtaposed to each other. Some, but not all, of these axons demonstrated TH-immunoreactivity. Fewer beta-AR404-immunoreactive astrocytes were detected in AP, regardless of their proximity to catecholaminergic processes or blood vessels. The present astrocytic localization of beta-AR404, together with the earlier, neuronal localization of beta-AR's third intracellular loop, suggest that the beta-AR may be substantially different between neurons and astrocytes. The regional difference in the prevalence of beta-AR404-immunoreactive astrocytes suggests that these receptive sites may either: (i) be preferentially activated by catecholamines released from terminals rather than circulating catecholamines; or (ii) be down-regulated in AP due to blood-born substances, such as catecholamines. The extensive localization of beta-AR in the border between m-NTS and AP also suggests that catecholaminergic activation of these astrocytes may dictate the degree of diffusion of catecholamines which are of neuronal or vascular origin. The specific localization of beta-AR404-immunoreactivity to the more distal portions of astrocytes suggests the possibility that astrocytes have restrictive distributions of beta-AR and that the beta-adrenergic activation lead to morphological or chemical changes that are also localized to the distal portions of astrocytes. Additionally, the detection of beta-AR404 in astrocytes contacting non-TH-immunoreactive neurons suggests the possibility for catecholaminergic modulation of non-catecholaminergic neurons via the activation of astrocytes.