THE NITRIC-OXIDE SYNTHASE (NOS)-LIKE IMMUNOREACTIVE EXTRAHYPOPHYSIAL PROJECTIONS OF THE NEUROSECRETORY PREOPTIC NUCLEUS OF THE ELECTRIC RAY(ELASMOBRANCHS) SUGGEST A NEUROREGULATORY ROLE FOR THIS NUCLEUS

The extrahypophysial projections of the neurosecretory preoptic nucleu s (PON) of the electric ray were studied with the aid of an antibody a gainst nitric oxide synthase (NOS). PON neurons were the only NOS-like -immunoreactive (NOS-ir) cells in the brain. These neurons gave rise t o both hypophysial and extrahypophysial NOS-ir projections. Some fibre s coursed from the PON to the neurointermediate lobe in the preoptic-h ypophysial tract. Other NOS-ir fibres coursed either rostrally or caud ally forming terminal fields in the telencephalon (subpallial region), diencephalon (preoptic nucleus, ventrolateral thalamus and posterior recess nucleus), tuberal region (area tegmentalis ventralis and substa ntia nigra), mesencephalon (lateral tegmentum), rhombencephalon (isthm al nucleus, vagal viscerosensory column and ventrolateral reticular ar ea) and the spinal cord (intermediate horn). The possible involvement of the extrahypophysial PON projections in neuroregulation of visceral centres is discussed.