GABAergic neurons and circuits in the pretectal nuclei and the accessory optic system of mammals

Two classes of GABAergic cell bodies have been described. They probably can be divided into GABAergic local interneurons and GABAergic projection neurons. GABAergic cell bodies receive few terminals which is in contrast to non-GABAergic somata, which receive many synaptic contacts. GABAergic dendrites that originate from GABAergic cell bodies, however, receive numerous terminals, both GABAergic and nonGABAergic. It can therefore be concluded that somatic inhibition is not present on GABAergic neurons, but does occur on nonGABAergic neurons. Furthermore, dendrites traverse large parts of the NOT/DTN forming a complex network that enables sampling and integration from a wide area. The projection to the IO is not GABAergic itself, but cells projecting to the IO receive a substantial GABAergic input, that probably originates in part from the MTN. Further investigation on the distribution of this input over a completely identified neuron would provide the quantitative data that are required to verify the above mentioned hypothesis. A GABAergic projection that originates in the pretectal nuclei is directed towards the superficial layers of the SC in the cat (Appell and Behan, 1990) and rat (Van der Want et al., 1991). A second GABAergic projection derives from the pretectum and reaches the LGN (Cucchiaro et al., 1991). Whether this projection originates from the same GABAergic cell bodies that project to the SC and the LGN or is derived from different populations remains to be determined. The ultrastructural studies of the NOT/DTN complex have shown that GABAergic terminals with different morphological characteristics are present and that the GABA positive F and P terminals are widely distributed over somata and the adjacent neuropil. The P terminals probably originate from dendrites of GABAergic interneurons while the F types originate from GABAergic projection and interneurons (Van der Want and Nunes Cardozo, 1988). One of these sources is located in the MTN differ from the intrinsic GABAergic terminals with respect to their relation to R terminals. GABAergic MTN terminals were never observed to receive R terminal input. This is in contrast with other GABAergic terminals which frequently do receive direct contact from R terminals. Within glomeruli triadic arrangements, formed by a single retinal terminal, a dendritic profile and second axonal profile dendritic profile and second axonal profile synapsing with the dendrite, were frequently encountered in the OPN (Campbell and Lieberman, 1985), but only occasionally in the NOT/DTN (Nunes Cardozo and Van der Want, 1987).(ABSTRACT TRUNCATED AT 400 WORDS)