Bimodal effects of noxious stimuli on vasopressin secretion in rats

Effects of electric foot shocks (FSs) on plasma immunoreactive vasopressin (ir-VP) were studied in relation to FS frequency in conscious and anesthetized rats under a hypertonic condition. The plasma ir-VP level was significantly lower after low-frequency FSs (0.05-0.2 Hz) but significantly higher after high-frequency FSs (5 Hz) in conscious rats. Plasma ir-VP was significantly higher in anesthetized rats after FSs at frequencies of either 0.2 or 50 Hz compared to unshocked control rats. These data support the hypothesis that FSs activate two distinct neural mechanisms: one potentiates and the other suppresses VP secretion.