Hypophysial 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 nonapeptides has given equivocal results regarding relevant aspects of the classical neurosecretory system of elasmobranchs. The lack of antibodies reacting with the elasmobranch neurophysins (Nps) has prevented the study of this neurosecretory system by Nps immunocytochemistry. This led us to purify Nps from Scyliorhinus canicula, and to use them to raise a polyclonal antibody. This antibody reacted strongly with the elasmobranch neurophysin neurons, revealing their most delicate and distant hypophysial and extrahypophysial projections. A detailed mapping of the neurosecretory system of five elasmobranch species (Etmopterus spinax, Squalus acanthias, Scyliorhinus canicula, Galeus melanostomus, Raja radiata) and one holocephalian species (Hydrolagus colliei) was performed. In elasmobranchs, the magnocellular neurophysin cells formed a distinct preoptic nucleus, whereas in Hydrolagus the immunoreactive cells were scattered. Distinct parvicellular neurophysin cells were present in the preoptic nucleus. In Raja the nucleus "O" contained parvicellular Nps-immunoreactive neurons. The findings at the pituitary level point to the possibility that neurophysin neurons, in addition to releasing nonapeptides into the systemic capillaries of the neural lobe, also participate in the regulation of the function of the rostral, medial and intermediate lobes of the adenohypophysis by a dual mechanism, i.e., a neurovascular pathway and a direct neural input. The extrahypophysial projections of the neurophysin neurons were highly developed to a degree not comparable to any other vertebrate group. The targets of these projections were located in the telencephalon, diencephalon and hindbrain. The evolutionary and functional implications of this phenomenon are discussed.