SEGREGATION OF DIRECTION-SELECTIVE NEURONS AND SYNAPTIC ORGANIZATION OF INHIBITORY INTRANUCLEAR CONNECTIONS IN THE MEDIAL TERMINAL NUCLEUS OF THE RAT - AN ELECTROPHYSIOLOGICAL AND IMMUNOELECTRON MICROSCOPIC STUDY

A combined electrophysiological and morphological investigation of the medial terminal nucleus (MTN) in the rat was undertaken, aimed at a b etter understanding of the relationship between structure and function in this nucleus. The locations of upward and downward direction selec tive units in the MTN were documented with extracellular electrophysio logical recording. By means of tracer experiments, with Phaseolus vulg aris-leucoagglutinin, biocytin, and cholera toxin subunit B-horseradis h peroxidase, the internal connections of the MTN, its retinal afferen ts, and the projection neurons to the inferior olive were visualized. Terminals originating from the retina and from internal connections we re characterized at the ultrastructural level. Their termination patte rn on cells in the MTN, including identified inferior olive projection neurons, were determined. Additionally, postembedding GABA immunocyto chemistry was performed to identify GABAergic elements. From reconstru ctions of the positions of electrophysiologically recorded units in th e MTN, a local segregation between upward and downward direction selec tive units was revealed. Upward direction selective units were found i n the dorsal part and ventromedially, whereas downward direction selec tive units were found ventral and laterally in the MTN. The MTN receiv es optic fibers via two separate routes which, based on their trajecto ry, presumably terminate in different parts of the MTN: the inferior f ascicle of the accessory optic tract in the dorsal part, and the poste rior fiber bundle of the superior fascicle in the ventral part of the MTN. A correspondence has been found between the segregation of direct ion selective units and the areas in the MTN where the retinal fibers from the two pathways distribute. It is, therefore, proposed that the inferior fasciculus conveys upward direction selectivity and the poste rior fiber bundle downward direction selectivity, and that the two fib er bundles terminate segregated in the MTN. After anterograde tracing from the eye, retinal terminals were found evenly distributed througho ut the MTN. They are characterized as GABA negative R-type terminals. After retrograde tracing from the inferior olive, identified MTN-infer ior olive projection neurons were found in the dorsal MTN and medially in the ventral MTN. Their location in the MTN suggests that MTN-infer ior olive projection neurons are upward direction selective. MTN-infer ior olive projection neurons are large non-GABAergic cells, with a var iable form. A majority of both F- and R-type terminals were found to m ake synaptic contacts on the dendrites of MTN cells. MTN-inferior oliv e projection neurons did not differ from other neurons in this respect .