Neuronal and glial localization of alpha(2A)-adrenoceptors in the adult zebrafish (Danio rerio) brain.

The alpha(2A)-adrenoceptor (AR) subtype, a G protein-coupled receptor located both pre- and postsynaptically, mediates adrenaline/noradrenaline functions. The present study aimed to determine the alpha(2A)-AR distribution in the adult zebrafish (Danio rerio) brain by means of immunocytochemistry. Detailed mapping showed labeling of alpha(2A)-ARs, in neuropil, neuronal somata and fibers, glial processes, and blood vessels. A high density of alpha(2A)-AR immunoreactivity was found in the ventral telencephalic area, preoptic, pretectal, hypothalamic areas, torus semicircularis, oculomotor nucleus (NIII), locus coreruleus (LC), medial raphe, medial octavolateralis nucleus (MON), magnocellular octaval nucleus (MaON), reticular formation (SRF, IMRF, IRF), rhombencephalic nerves and roots (DV, V, VII, VIII, X), and cerebellar Purkinje cell layer. Moderate levels of alpha(2A)-ARs were observed in the medial and central zone nuclei of dorsal telencephalic area, in the periventricular gray zone of optic tectum, in the dorsomedial part of optic tectum layers, and in the molecular and granular layers of all cerebellum subdivisions. Glial processes were found to express alpha(2A)-ARs in rhombencephalon, intermingled with neuronal fibers. Medium-sized neurons were labeled in telencephalic, diencephalic, and mesencephlic areas, whereas densely labeled large neurons were found in rhombencephalon, locus coeruleus, reticular formation, oculomotor area, medial octavolateralis and magnocellular octaval nuclei, and Purkinje cell somata. The functional role of alpha(2A)-ARs on neurons and glial processes is not known at present; however, their strong relation to the ventricular system, somatosensory nuclei, and nerves supports a possible regulatory role of alpha(2A)-ARs in autonomic functions, nerve output, and sensory integration in adult zebrafish brain.