CEREBELLAR INFLUENCES ON ACCESSORY OCULOMOTOR NUCLEI OF THE RAT - A NEUROANATOMICAL, IMMUNOHISTOCHEMICAL, AND ELECTROPHYSIOLOGICAL STUDY

With the aim to evaluate a possible neocerebellar control on eye movem ents, the projections from the cerebellar lateral nucleus (LN) to the accessory oculomotor nuclei (i.e., the nucleus of posterior commissure , the nucleus of Darkschewitsch, and the interstitial nucleus of Cajal ), the putative neurotransmitters subserving this pathway, and the nat ure of the synaptic influences exerted by these projections were studi ed in adult rats. We used the orthograde transport of horseradish pero xidase conjugated with wheat germ agglutinin (WGA-HRP) to identify the mesencephalic areas where cerebellofugal fibers terminate, and retrog rade labeling with the fluorescent dye fluoro-gold to estimate the inc idence of cerebellar neurons projecting to the accessory oculomotor nu clei. Orthograde labeling showed that only a small contingent of cereb ellofugal fibers reaches the contralateral accessory oculomotor nuclei . The retrogradely labeled cells were located primarily in the small-c elled part of LN. By immunohistochemistry, we observed that all the ce lls retrogradely labeled from the accessory oculomotor area were also stained by using glutamate or aspartate antisera, but none of them wer e double-stained with a GABA antiserum. Electrical stimulation of the contralateral LN elicited changes in firing rate of a significant frac tion of cells belonging to the accessory oculomotor nuclei (36.4% in t he nucleus of posterior commissure, 47.1 % in the nucleus of Darkschew itsch, and 44.6% in the interstitial nucleus of Cajal). In 57.8% of th e cases, the responses were excitations, most of which had latencies a nd response characteristics compatible with a monosynaptic linkage. Th e remaining 42.2% of the cases were inhibitions with latencies ranging between 5 and 22 ms. Extracellular field potential recordings within the contralateral accessory oculomotor nuclei were interpreted as aris ing from impulses propagating along excitatory axons projecting in a b undle from the cerebellum. Stimulation of LN area in rats following in tranuclear injection of kainic acid was not capable of evoking short l atency excitations, so these responses can be considered to depend on the activation of LN efferents. The LN projection on accessory oculomo tor nuclei could be part of the final precise control exerted by the n eocerebellum on those brain structures concerned with movements of the eyes. (C) 1993 Wiley-Liss, Inc.