Central effects of dopamine on vasopressin release in the normally hydrated and water-loaded rat.

The effect of intracerebroventricular (I.C.V.) micro-injections of dopamine on vasopressin (AVP) release was investigated in normally hydrated and hydrated rats anaesthetized with urethane, hormone concentrations being determined by radioimmunoassay. Dopamine given in doses of less than 25 micrograms had little effect on AVP concentrations already elevated as a result of anaesthesia and surgery. Doses of over 25 micrograms produced a transient increase in AVP concentrations followed by a fall. Both the increase and the fall were statistically significant. Pimozide (400 micrograms/kg) blocked the fall in AVP concentrations following dopamine. A fall was still seen after the administration of haloperidol (400 micrograms/kg) but it was only significant 20 min after the injection of dopamine. The changes in AVP concentration after the administration of naloxone (400 micrograms/kg) were not statistically significant. In water-loaded rats I.C.V. micro-injections of dopamine produced a dose-dependent antidiuresis over the range 1-25 micrograms. An injection of 25 micrograms dopamine in these animals produced an increase in AVP concentrations to 1.8 +/- 0.51 microunits/ml and a fall in urine flow which could be approximately matched by an infusion of vasopressin of 15 microunits/min. The antidiuresis in response to dopamine could be blocked by haloperidol. The response to dopamine in the anaesthetized animals depends on a number of factors including the initial activity of the neurohypophysial system.