GABAergic neurons comprise a major cell type in rodent visual relay nuclei: an immunocytochemical study of pretectal and accessory optic nuclei.

R.A Giolli,R.H.I Blanks,C.E Ribak,J.H Fallon,G.M Peterson,H.M Mcdonald

doi:10.1007/bf00235635

Abstract

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.

Highlights

Previous studies have established the presence of GABAergic, local circuit neurons Within the visual thalamic nuclei and the visual cortex of mammals (Ribak 1978; Sterling and Davis 1980; McDonald et al 1981; Hendrickson et al 1983; Ohara et al 1983; Somogyi et al 1983; Fitzpatrick et al 1984; Montero and Singer 1984; Penny et al 1984)
The morphology of the glutamic acid decarboxylase (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
The large number of GAD-positive neurons in the nucleus of the optic tract (NOT) and Medial terminal accessory optic nucleus (MTN) (20-30%) in relation to estimates of projection neurons (75%) presents the possibility that some may be projection neurons

Summary

Introduction

Previous studies have established the presence of GABAergic, local circuit neurons Within the visual thalamic nuclei and the visual cortex of mammals (Ribak 1978; Sterling and Davis 1980; McDonald et al 1981; Hendrickson et al 1983; Ohara et al 1983; Somogyi et al 1983; Fitzpatrick et al 1984; Montero and Singer 1984; Penny et al 1984). The present study directs attention to another portion of the visual system which is located along the mesodiencephalic junction and includes the nucleus of the optic tract (NOT) (Scalia 1972), the dorsal, lateral and medial terminal accessory optic nuclei (DTN, LTN, MTN) (Hayhow et al 1960), and the associated interstitial nucleus of the posterior bundle of the superior fasciculus (inSFp) (Giolli et al 1984, 1985). In rodents, these nuclei are abundantly supplied with retinal afferents through which they receive information related to the speed and direction of visual surround movement (see Simpson et al 1979). It is likely that these GABAergic neurons (local circuit and/or projection neurons) play a major role in modulating the output of this interrelated group of visual relay nuclei

Results

Discussion

Conclusion