EXPERIMENTAL NEUROLOGY 86,93- 104 ( 1984) Projections to Eye Movement-Related Pause Neuron Region in Cat Using HRP JUICHI ITO, CHARLES H. MARKHAM, ANDIAN S. CURTHOYS**' UCLA School of Medicine. Department of Neurology, Reed Neurological Research Center, Los Angeles California 90024, and *University of Sydney, New South Wales, Australia Received February 18. 1984; revision received May 25, 1984 The origins of the afferent connections to the eye movement-related pause neurons in cats were examined using retrograde transport of horseradish peroxidase with wheat germ agglutinin. The major input sources to the pause neuron region were nucleus prepositus hypoglossi, medial vestibular nucleus, gigantocellular reticular nucleus, parvocellular reticular nucleus, nucleus reticularis tegmentis pontis, nucleus reticularis pontis caudalis, nucleus reticularis pontis oralis, and superior colliculus. Less heavily labeled sources included the lateral and descending vestibular nuclei, the locus ceruleus, and the dorsal raphe. INTRODUCTION Many investigators have shown that certain neurons in the medial pontine reticular nuclei play important roles in the regulation of rapid eye movements (9-l 1, 13-16, 19, 2 1, 22, 26, 28, 32, 33, 41, 44, 45, 49). Among them, pause neurons (PNs), which are clustered in the cat in a small region near the midline and adjacent to the rostra1 pole of the abducens nucleus (11, 14, 45), exhibit a tonic discharge during the slow phase of vestibular nystagmus or when eyes are not moving, and cease firing prior to and during the quick phase in both directions (9, 13-l 5, 32, 4 1). On stimulation of the PN region, saccadic eye movements and the quick phase of nystagmus Abbreviations: PN-pause neuron; IBN-inhibitory burst neuron; WGA-HRP-wheat germ agglutinin-horseradish peroxidase; PH-prepositus hypoglossi; MVN, LVN, DVN-medial, lateral, descending vestibular nuclei; NRTP, NRPC, NRPO-nucleus reticularis tegmenti pontis, pontus caudalis, pontus oralis; SC-superior colliculis. ’ We express appreciation to Mrs. S. Kashitani for secretarial help and to Mr. Dan Stoller for laboratory assistance. Send reprint requests to Dr. C. H. Markham. Dept. of Neurology, UCLA School of Medicine, Los Angeles, CA 90024. 93 0014-4886/84 $3.00 Copyright 0 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.