Immunocytochemical evidence for vasopressin receptors.

An electron microscopic study was made of mouse pituitaries immunocytochemically stained with anti-lysine vasopressin (LVP) as the primary antiserum in the unlabeled antibody peroxidase-anti-peroxidase procedure. Vasopressin (VP) was identified in the neurosecretory granules of the neural lobe which stained with peroxidase anti-peroxidase molecules. Electron density was induced in secretory granules of the pars intermedia (PI), both in the melanocyte stimulated hormone and ACTH cell types, probably indicating VP molecules attached to binding (receptor) sites. Omission of anti-LVP abolished staining both in the neural lobe and the PL Anti-LVP absorbed with antigen, by admixing with LVP, abolished staining in the neural lobe but not in the PI; according to optical density measurements the PI showed a +/- 22% staining increase over controls. Staining intensity in the PI probably reflects occupancy of binding (receptor) sites for VP. Exposure of PI granules to LVP before the usual staining sequence resulted in +/- 48% increased staining. In water-deprived mice with high endogenous VP titers, staining was +/- 33% and +/- 40% more intense than in normal mice. Solid phase absorbed and eluted antibodies to LVP provided additional proof that staining in both neural lobe and PI could be attributed to anti-LVP. Results indicate that binding or receptor sites for VP are located on secretory granules in the PL Possible physiological significance is discussed.