NEUROTRANSMITTER PROFILE OF SACCADIC OMNIPAUSE NEURONS IN NUCLEUS RAPHE INTERPOSITUS

Saccadic omnipause neurons (OPNs) are essential for the generation of saccadic eye movements. In primates OPNs are located near the midline within the nucleus raphe interpositus (rip). In the present study we u sed several different neuroanatomical methods to investigate the trans mitters associated with OPNs in the monkey. Immunolabeling for the cal cium-binding protein parvalbumin was employed to mark OPNs in the monk ey and define the homologous cell group in cat and human. The use of a ntibodies against GABA, glycine (GLY), glutamate (GLU), serotonin (5-H T), and tyrosine hydroxylase revealed that the somata of OPNs are GLY immunoreactive, but they are devoid of GABA and 5-HT immunostaining. I n situ hybridization with the GAD(67) mRNA probe confirmed the negativ e GABA immunostaining of OPNs. H-3-GLY was injected into a projection field of OPNs, the rostral interstitial nucleus of the medial longitud inal fascicle (riMLF)-the vertical saccadic burst neuron area. This re sulted in selective retrograde labeling of the OPNs in rip, while no l abeling was found in the superior colliculus, which sends an excitator y projection to the riMLF. The somata and dendrites of putative burst neurons in the riMLF were contacted by numerous GLY-immunoreactive ter minals. The quantitative analysis of immunoreactive terminal-like stru ctures contacting OPNs revealed a strong input from GLY- and GABA-posi tive terminals on somata and dendrites, whereas GLU-positive puncta we re mainly confined to the dendrites. Very few 5-HT and catecholaminerg ic terminals contacted OPN somata. Our findings suggest that OPNs use GLY as a neurotransmitter, and they receive numerous contacts from GAB Aergic, glycinergic, and glutaminergic afferents, and significantly fe wer from monoaminergic inputs.