NEURONS OF THE POSTERIOR SUBDIVISION OF THE NUCLEUS PREOPTICUS PERIVENTRICULARIS PROJECT TO THE PREGLOMERULAR NUCLEUS IN THE WEAKLY ELECTRIC FISH, APTERONOTUS-LEPTORHYNCHUS

By using an in vitro tract-tracing technique, the neural connections b etween two diencephalic cell groups, the posterior subdivision of the nucleus preopticus periventricularis (PPp) and the preglomerular nucle us (PG), was examined in the weakly electric gymnotiform fish Apterono tus leptorhynchus, Neurons of the PPp project to one area within PG, t he ventromedial cell group of the medial subdivision of the preglomeru lar nucleus (PGm-vmc). Axons of these cells reach the ipsilateral PGm- vmc via the basic hypothalamic tract, while collaterals decussate via the postoptic commissure to innervate the contralateral PGm-vmc. We hy pothesize that those neurons within PPp that project to the PGm-vmc ar e homologous to neurons of the medial preoptic area of mammals. As par t of an elaborate circuit, PPp and PG may participate, as in mammals, in the control of complex social behavior patterns. (C) 1997 Elsevier Science B.V.