GABA-immunoreactive internuclear neurons in the ocular motor system of lampreys

The presence of internuclear neurons in the abducens and oculomotor nuclei of lampreys [Gonzalez, M.J., Pombal, M.A., Rodicio, M.C. and Anadon, R., In ternuclear neurons of the ocular motor system of the larval sea lamprey, J. Comp. Neurol. 401 (1998) 1-15] indicates that coordination of eye movement s by internuclear neurons appeared early during the evolution of vertebrate s. In order to investigate the possible involvement of inhibitory neurotran smitters in internuclear circuits, the distribution of gamma-aminobutyric a cid (GABA) in the extraocular motor nuclei of the lamprey was studied using immunocytochemical techniques. Small GABA-immunoreactive (GABAir) neurons were observed in the three ocular motor nuclei. Numerous GABAir neurons wer e observed in the group of internuclear neurons of the dorsal rectus oculom otor subnucleus. A second group of GABAir neurons was observed among and be low the trochlear motoneurons. Two further groups of GABAir interneurons, p eriventricular and lateral, were located in the abducens nucleus among the cells of the caudal rectus and the ventral rectus motor subnuclei, respecti vely. In addition to the presence of GABAir neurons, in all the ocular moto r nuclei the motoneurons were contacted by numerous GABAir boutons. Taken t ogether, these results suggest that CABA is involved as a neurotransmitter in internuclear pathways of the ocular motor system of lampreys. (C) 2000 E lsevier Science B.V. All rights reserved.