CHARACTERIZATION OF A DIRECTIONAL SELECTIVE INHIBITORY INPUT FROM THEMEDIAL TERMINAL NUCLEUS TO THE PRETECTAL NUCLEAR-COMPLEX IN THE RAT

The receptive field properties of neurons in the medial terminal nucle us of the accessory optic system (MTN) that project to the ipsilateral nucleus of the optic tract (NOT) and dorsal terminal nucleus (DTN), a s identified by antidromic electrical activation, were analysed in the anaesthetized rat. The great majority (88%) of MTN neurons that were antidromically activated from NOT and DTN preferred downward directed movement of large visual stimuli while the remaining cells preferred u pward directed stimulus movement. Distinct retrograde tracer injection s into the NOT/DTN and the ipsilateral inferior olive (IO) revealed th at no MTN neurons project to both targets. MTN neurons projecting to t he ipsilateral NOT/DTN were predominantly found in the ventral part of the MTN, whereas those projecting to the IO were found in the dorsal part of the MTN. In situ hybridization for glutamic acid decarboxylase (GAD) mRNA was used as a marker for GABAergic neurons. Up to 98% of M TN neurons retrogradely labelled from the ipsilateral NOT/DTN also exp ressed GAD mRNA. Earlier studies have shown that MTN neurons that pref er upward directed stimulus movements are segregated from MTN neurons that prefer downward directed stimulus movements. It also has been dem onstrated that directionally selective neurons in the NOT/DTN prefer h orizontal stimulus movements and receive an inhibitory input from ipsi lateral MTN. Our results indicate that this input is mediated by GABAe rgic cells in the ventral part of MTN, which to a large extent prefer downward directed stimulus movements, and that the great majority of M TN neurons that prefer upward directed stimulus movements project to o ther targets one of which possibly is the IO.