Secretory glycoproteins of the subcommissural organ of the dogfish (Scyliorhinus canicula): evidence for the existence of precursor and processed forms

The probable presence of oxytocin in the hypothalamo-hypophysial system of two reptilian species, the snake Natrix maura and the turtle Mauremys caspica, was re-investigated. A high-pressure liquid chromatographic analysis of the turtle neural lobe revealed the existence of vasotocin, mesotocin, and a third compound co-eluting with oxytocin. Brains from both species were fixed by vascular perfusion with Bouin's fluid. Adjacent paraffin sections were immunostained using antisera against the following substances: (1) bovine oxytocin-neurophysin; (2) a mixture of bovine oxytocin-neurophysin and vasopressin-neurophysin; (3) dogfish neurophysins; (4) oxytocin; (5) arginine-vasotocin; (6) mesotocin; (7) somatostatin. Immunoreactivity against oxytocin was found in parvocellular neurons of the snake suprachiasmatic nucleus and cerebrospinal-fluid contacting neurons of the medial nucleus of the infundibular recess of both species, the latter immunoreactivity being much more conspicuous in the turtle. Numerous fibers containing immunoreactive oxytocin extended between the medial nucleus of the infundibular recess, and the internal region of the medium eminence and the neural lobe. The oxytocin-immunoreactivity in all locations was completely abolished by preabsorption of the anti-oxytocin serum with three different oxytocin preparations. None of the neurons of the suprachiasmatic and medial nucleus of the infundibular recess, including the oxytocin-immunoreactive elements, reacted with either the antineurophysin sera used, or the anti-vasotocin or anti-mesotocin antibodies. The possible existence of a reptilian oxytocin-neurophysin is discussed. The alternative that, in the reptilian hypothalamus, neurons synthesize a compound closely related to, but different from oxytocin is also considered.