CELLULAR AND SUBCELLULAR-DISTRIBUTION OF ALPHA(2A)-ADRENERGIC RECEPTORS IN THE VISUAL-CORTEX OF NEONATAL AND ADULT-RATS

Activation of alpha(2)-adrenergic receptors ((alpha(2)AR) in the cereb ral cortex has been shown to modulate visually guided delayed response tasks as well as anxiety and depression. We used an antiserum directe d specifically against the A subtype of alpha(2)AR (alpha(2A)AR) to de termine the cell types and subcellular sites for noradrenergic recepti on mediated by this receptor in the adult and the developing rat visua l cortices. Light microscopic examination of adult tissue revealed num erous labeled perikarya in layers II-VI, many of which appeared distin ctly pyramidal. A few perikarya in layer I also were immunoreactive. I n all layers, alpha(2A)AR immunoreactivity (alpha(2A)AR-ir) was presen t within proximal dendrites and fine processes. In neonatal tissue, th ere was an intense, distinct band of immunoreactivity spanning the lay er composed of tightly packed immature cell bodies, i.e., the cortical plate. The band dissipated as this tier differentiated postnatally in to the supragranular layers. Electron microscopy showed that the supra granular layers, which contain the highest density of noradrenergic fi bers, also contain the highest areal density of labeled postsynaptic j unctions beyond 2 weeks of age. Throughout the ages, the majority of i mmunoreactivity occurred at sites which, in single ultrathin sections, appeared to be nonjunctional sites of axons, dendrites, and in glial processes. Our observations indicate that (1) both pyramidal and nonpy ramidal neurons are receptive to norepinephrine via alpha(2A)AR, (2) a lpha(2A)AR synthesis is robust prior to synaptogenesis, and (3) alpha( 2A)AR operates both pre-and postsynaptically. (C) 1996 Wiley-Liss, Inc .