Abstract
Evidently opiates inhibit the release of oxytocin and vasopressin by acting on nerve terminals in the neurohypophysis. Extracts of neurohypophysis contain substantial amounts of Met- and Leu-enkephalin, and Leu-enkephalin-immunoreactive (IR) nerve fibers originating in the magnocellular hypothalamic nuclei were described in the neural lobe, where the 2 hormones are secreted. The distributions of oxytocin, vasopressin and enkephalin immunoreactivity (IR) in the neurohypophysis of the rat were compared; Met-enkephalin-IR is invariably associated with nerve terminals that contain oxytocin-IR whereas the terminals that contain vasopressin-IR often, but not invariably, are Leu-enkephalin immunoreactive.