Choline acetyltransferase immunoreactivity in the hypothalamoneurohypophysial system of the lamprey

The cholinergic innervation of the neurohypophysis of the lampreys Petromyz on marinus and Lampetra fluviatilis was studied by means of immunocytochemi cal techniques with antibodies directed against the enzyme choline acetyltr ansferase (ChAT). The results obtained in both species were basically simil ar. A rich innervation by ChAT immunoreactive fibres was found throughout t he neurohypophysis. These fibres originate from cholinergic neurons located in the preoptic region and the paraventricular nucleus. Some of these chol inergic neurons are in contact with the cerebrospinal fluid. Numerous axona l swellings were evident in the tuberal region of the sea lamprey, but not in the river lamprey. The possible pathways of cholinergic release in the l amprey hypophysis are discussed.