INTERNUCLEAR NEURONS OF THE OCULAR MOTOR SYSTEM OF THE LARVAL SEA LAMPREY

The internuclear neurons of the ocular motor system of lampreys are ch aracterized here for the first time. Horseradish peroxidase (HRP), flu orescein-, or Texas red- (TRDA) coupled dextran-amine applied into the oculomotor nucleus of larval lamprey (Petromyzon marinus) retrogradel y labeled two populations of contralateral abducens interneurons, one lateral and the other periventricular. Tracer application to the abduc ens nucleus anterogradely labeled thick contralateral fibers that spec ifically contact the medial rectus motor subnucleus by means of large boutons. Local application of TRDA to this subnucleus allowed identifi cation of the lateral abducens interneurons as the origin of this proj ection. Electron microscopy of the medial rectus motor subnucleus show ed large boutons bearing round synaptic vesicles that contact on the p erikarya, as well as small boutons with pleomorphic vesicles. This lat eral rectus (abducens)- medial rectus (oculomotor) internuclear projec tion of lampreys appears to be similar to those involved in the coordi nation of horizontal eye movements in mammals. The periventricular abd ucens interneurons projected bilaterally to other oculomotor subnuclei . Tracer application to the abducens nucleus labeled a group of small interneurons in the ipsilateral dorsal rectus motor subnucleus. Antero grade labeling indicates that oculomotor interneurons project ipsilate rally to the ventral rectus abducens subnucleus, thus, corresponding t o oculomotor interneurons found in mammals and frogs. The interneurons of the dorsal rectus and ventral rectus motor subnuclei are probably involved in the control of conjugate vertical eye movements. The prese nt results strongly suggest that the internuclear coordination of conj ugate eye movements appeared in the earliest vertebrates. The homologi es of extraocular muscles of lampreys and gnathostomes were reexamined . (C) 1998 Wiley-Liss, Inc.