THE ROSTRAL DORSAL CAP AND VENTROLATERAL OUTGROWTH OF THE RABBIT INFERIOR OLIVE RECEIVE A GABAERGIC INPUT FROM DORSAL GROUP-Y AND THE VENTRAL DENTATE NUCLEUS

The dorsal cap and ventrolateral outgrowth of the inferior olive are i nvolved in the control of eye movements. The caudal dorsal cap is pred ominantly involved in the horizontal optokinetic reflex; it receives m ost of its GABAergic input from the nucleus prepositus hypoglossi. In the present study, we determined the source of a major inhibitory inpu t to the rostral dorsal cap and the ventrolateral outgrowth, which are the olivary subnuclei mainly involved in the ''vertical'' optokinetic reflexes. We studied these subnuclei in the rabbit with the use of re trograde tracing of horseradish peroxidase and anterograde tracing of wheat germ agglutinin-coupled horseradish peroxidase combined with pos tembedding immunocytochemistry. The ventral dentate nucleus of the cer ebellum and dorsal group y project contralaterally to the rostral dors al cap and ventrolateral outgrowth; this projection is entirely GABAer gic. The terminals of this input form predominantly symmetric synapses with extraglomerular and intraglomerular dendrites; the remaining ter minals are axosomatic. In addition, the dorsal cap and ventrolateral o utgrowth contain significantly more crest synapses than any other oliv ary subnucleus. The terminals that form these crest synapses are deriv ed from dorsal group y and/or the ventral dentate nucleus. None of the terminals in the dorsal cap or ventrolateral outgrowth was glycinergi c. (C) 1991 Wiley-Liss, Inc.