Synaptic and neurochemical features of calcitonin gene-related peptide containing neurons in the rat accessory optic nuclei

Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), inclu ding the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neu rons are involved in visual circuitry, electron microscopic preparations we re analyzed from normal rats and rats with optic nerve transections. A co-l ocalization analysis was also made because CGRP-labeled neurons had feature s of GABAergic neurons. Thus, sections were prepared for light microscopy t o determine whether CGRP-containing neurons also had glutamate decarboxylas e (GAD) and other markers for GABAergic neurons, such as calcium binding pr oteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron micr oscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that wer e postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating th at retinal ganglion cells form synapses with CGRP-labeled neurons in the AO N. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric sy napses with unlabeled somata as well as dendritic shafts and spines. Consis tent with this type of synapse being GABAergic were the co-localization dat a showing that about 90% of the CGRP-labeled neurons co-localized GAD in th e AON. Many CGRP-labeled neurons showed immunostaining for CR (40%) whereas only a few had labeling for CB (5%). No CGRP-labeled neurons had PV. These data show that CGRP-containing neurons receive direct retinal input and re present a subpopulation of GABAergic neurons which differentially co-expres s calcium-binding proteins. (C) 1999 Elsevier Science B.V. All rights reser ved.