Central vasopressin system mediation of acute pressor effect of gamma-MSH.

Intravenous (iv) administration of gamma-melanocyte-stimulating hormone (gamma-MSH) produces central sympathetically mediated pressor and cardioaccelerator effects and increases the activity of hypothalamic vasopressinergic neurons. The autonomic actions are similar to infusion of vasopressin (Vp) into the hindbrain of 4th ventricle (Ven). To ascertain whether activation of the central Vp system is the proximate cause of the pressor effects of gamma-MSH, we investigated the effects of gamma-MSH in rats pre- and postblockade of central nervous system Vp receptors and in rats with a hereditary lack of vasopressin (Brattleboro strain). Central Vp receptor blockade significantly reduced (80%) the pressor effects of iv gamma-MSH. As a control, iv administration of the antagonist, while effective in blocking the pressor effect of iv Vp, had no effect on the gamma-MSH pressor response. When compared with their genetic controls (Long-Evans strain), Brattleboro rats also had greater than 80% reduction in their pressor response to iv gamma-MSH. The results indicate that circulating gamma-MSH activates the central Vp system to produce its sympathoexcitatory pressor effects.