Choline acetyltransferase immunoreactivity in the hypothalamoneurohypophysial system of the lamprey

The cholinergic innervation of the neurohypophysis of the lampreys Petromyzon marinus and Lampetra fluviatilis was studied by means of immunocytochemical techniques with antibodies directed against the enzyme choline acetyltransferase (ChAT). The results obtained in both species were basically similar. A rich innervation by ChAT-immunoreactive fibres was found throughout the neurohypophysis. These fibres originate from cholinergic neurons located in the preoptic region and the paraventricular nucleus. Some of these cholinergic neurons are in contact with the cerebrospinal fluid. Numerous axonal 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 lamprey hypophysis are discussed.