HYPOPHYSEAL AND EXTRAHYPOPHYSIAL PROJECTIONS OF THE NEUROSECRETORY-SYSTEM OF CARTILAGINOUS FISHES - AN IMMUNOCYTOCHEMICAL STUDY USING A POLYCLONAL ANTIBODY AGAINST DOGFISH NEUROPHYSIN

Immunocytochemistry using antibodies against the neurohypophysial nona peptides has given equivocal results regarding relevant aspects of the classical neurosecretory system of elasmobranchs. The lack of antibod ies reacting with the elasmobranch neurophysins (Nps) has prevented th e study of this neurosecretory system by Nps immunocytochemistry. This led us to purify Nps from Scyliorhinus canicula, and to use them to r aise a polyclonal antibody. This antibody reacted strongly with the el asmobranch neurophysin neurons, revealing their most delicate and dist ant hypophysial and extrahypophysial projections. A detailed mapping o f the neurosecretory system of five elasmobranch species (Etmopterus s pinax, Squalus acanthias, Scyliorhinus canicula, Galeus melanostomus, Raja radiata) and one holocephalian species (Hydrolagus colliei) was p erformed. In elasmobranchs, the magnocellular neurophysin cells formed a distinct preoptic nucleus, whereas in Hydrolagus the immunoreactive cells were scattered. Distinct parvicellular neurophysin cells were p resent in the preoptic nucleus. In Raja the nucleus ''O'' contained pa rvicellular Nps-immunoreactive neurons. The findings at the pituitary level point to the possibility that neurophysin neurons, in addition t o releasing nonapeptides into the systemic capillaries of the neural l obe, also participate in the regulation of the function of the rostral , medial and intermediate lobes of the adenohypophysis by a dual mecha nism, i.e., a neurovascular pathway and a direct neural input. The ext rahypophysial projections of the neurophysin neurons were highly devel oped to a degree not comparable to any other vertebrate group. The tar gets of these projections were located in the telencephalon, diencepha lon and hindbrain. The evolutionary and functional implications of thi s phenomenon are discussed. (C) 1996 Wiley-Liss, Inc.