Electron microscopic observation of pituitary adenylate cyclase-activatingpolypeptide (PACAP)-containing neurons in the rat retina

The distribution and localization of pituitary adenylate cyclase-activating polypeptide (PACAP) in the rat retina were studied by immunocytochemistry with both light and electron microscopy. PACAP-like immunoreactivity (PACAP -LI) was detected in the amacrine and horizontal cells as well as in the in ner plexiform layer, the ganglion cell layer and the nerve fiber layer. PAC AP-LI seemed to be concentrated predominantly in the neuronal perikarya and their processes, but not in other cells in the retina. At the ultrastructu ral level, PACAP-LI was visible in the plasma membranes, rough endoplasmic reticulum, and cytoplasmic matrix in the PACAP-positive neurons in the inne r nuclear layer. In the inner plexiform layer, PACAP-positive amacrine cell processes made synaptic contact with immunonegative amacrine cell processe s, bipolar cell processes, and ganglion cell terminals. These findings sugg est that PACAP may function as a neurotransmitter and/or neuromodulator. (C ) 2000 Elsevier Science Inc. All rights reserved.