Cellular and subcellular distribution of α2A‐adrenergic receptors in the visual cortex of neonatal and adult rats

Abstract

Activation of α2-adrenergic receptors (α2AR) in the cerebral cortex has been shown to modulate visually guided delayed response tasks as well as anxiety and depression. We used an antiserum directed specifically against the A subtype of α2AR (α2AAR) to determine the cell types and subcellular sites for noradrenergic reception mediated by this receptor in the adult and the developing rat visual cortices. Light microscopic examination of adult tissue revealed numerous labeled perikarya in layers II–VI, many of which appeared distinctly pyramidal. A few perikarya in layer I also were immunoreactive. In all layers, α2AAR immunoreactivity (α2AAR-ir) was present within proximal dendrites and fine processes. In neonatal tissue, there was an intense, distinct band of immunoreactivity spanning the layer composed of tightly packed immature cell bodies, i.e., the cortical plate. The band dissipated as this tier differentiated postnatally into the supragranular layers. Electron microscopy showed that the supragranular layers, which contain the highest density of noradrenergic fibers, also contain the highest areal density of labeled postsynaptic junctions beyond 2 weeks of age. Throughout the ages, the majority of immunoreactivity 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 nonpyramidal neurons are receptive to norepinephrine via α2AAR, (2) α2AAR synthesis is robust prior to synaptogenesis, and (3) α2AAR operates both pre-and postsynaptically. © 1996 Wiley-Liss, Inc.