Within the rodent visual system, calcitonin gene-related peptide (CGRP) is selectively expressed in neurons in the accessory optic nuclei (AON), including the dorsal terminal nucleus (DTN), lateral terminal nucleus (LTN) and medial terminal nucleus (MTN). To determine whether CGRP-immunoreactive neurons are involved in visual circuitry, electron microscopic preparations were analyzed from normal rats and rats with optic nerve transections. A co-localization analysis was also made because CGRP-labeled neurons had features of GABAergic neurons. Thus, sections were prepared for light microscopy to determine whether CGRP-containing neurons also had glutamate decarboxylase (GAD) and other markers for GABAergic neurons, such as calcium binding proteins: calbindin (CB), calretinin (CR) and parvalbumin (PV). Electron microscopy of the DTN and LTN showed CGRP-labeled somata and dendrites that were postsynaptic to axon terminals forming asymmetric synapses. Many of these axon terminals degenerated following optic nerve transection indicating that retinal ganglion cells form synapses with CGRP-labeled neurons in the AON. In the DTN, LTN and MTN, CGRP-labeled axon terminals formed symmetric synapses with unlabeled somata as well as dendritic shafts and spines. Consistent with this type of synapse being GABAergic were the co-localization data showing that about 90% of the CGRP-labeled neurons co-localized GAD in the 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 represent a subpopulation of GABAergic neurons which differentially co-express calcium-binding proteins.

The enzyme glutamic acid decarboxylase (GAD) has been localized in sections of rodent brains (gerbil, rat) using conventional immunocytochemical techniques. Our findings demonstrate that large numbers of GAD-positive neurons and axon terminals (puncta) are present in the visual relay nuclei of the pretectum and the accessory optic system. The areas of highest density of these neurons are in the nucleus of the optic tract (NOT) of the pretectum, the dorsal and lateral terminal accessory optic nuclei (DTN, LTN), the ventral and dorsal subdivisions of the medial terminal accessory optic nucleus (MTNv, MTNd), and the interstitial nucleus of the posterior fibers of the superior fasciculus (inSFp). The findings indicate that 27% of the NOT neurons are GAD-positive and that these neurons are distributed over all of the NOT except the most superficial portion of the NOT caudally. The GAD-positive neurons of the NOT are statistically smaller (65.9 microns2) than the total population of neurons of the NOT (84.3 microns2) but are otherwise indistinguishable in shape from the total neuron population. The other visual relay nuclei that have been analyzed (DTN, LTN, MTNv, MTNd, inSFp) are similar in that from 21% to 31% of their neurons are GAD-positive; these neurons are smaller in diameter and are more spherical than the total populations of neurons. The data further show that a large proportion of the neurons in these visual relay nuclei are contacted by GAD-positive axon terminals. It is estimated that approximately one-half of the neurons of the NOT and the terminal accessory optic nuclei receive a strong GABAergic input and have been called "GAD-recipient neurons". Further, the morphology of the GAD-positive neurons combined with their similar distribution to the GAD-recipient neurons suggest that many of these neurons are acting as GABAergic, local circuit neurons. On the other hand, the large number of GAD-positive neurons in the NOT and MTN (20-30%) in relation to estimates of projection neurons (75%) presents the possibility that some may in fact be projection neurons. The overall findings provide morphological evidence which supports the general conclusion that GABAergic neurons play a significant role in modulating the output of the visually related NOT and terminal accessory optic nuclei.