Tolerance to Acute Ethanol Inhibition of Peptide Hormone Release in the Isolated Neurohypophysis

Abstract

Acute ethanol (EtOH) exposure reduces the evoked release of vasopressin (AVP) and oxytocin (OT) from excised neurohypophyses and from dissociated neurohypophysial terminals of the rat. Rats placed on a diet that maintained blood levels of 30 mM EtOH for 20 to 40 days developed tolerance to acute EtOH inhibition of release. In the presence of 10 mM EtOH, high (50 mM) K+-induced release of AVP from isolated neurohypophysial terminals of EtOH-naive rats was reduced by 77.7+/-1.4%, whereas in the chronic EtOH group, release was reduced by only 9.4+/-8.7%. Similar tolerance was evident during acute challenge with 75 mM EtOH, as well as for release of OT from isolated terminals. Animals treated with an intraperitoneal injection of EtOH and sacrificed 90 min postinjection did not exhibit the reduced EtOH inhibition of release from dissociated terminals during a 75 mM EtOH acute challenge. The altered component responsible for the tolerance to inhibition of release resides in the isolated terminal, because tolerance measured in vitro from intact neurohypophyses was similar to that seen in isolated terminals. The failure of EtOH-injected animals to exhibit reduced inhibition of release in response to an acute EtOH challenge indicates that short-term elevated blood alcohol level does not induce this tolerance. The finding of tolerance to EtOH-induced inhibition of release from the intact neurohypophysis and isolated terminals provides a physiological preparation in which to examine the molecular targets of acute drug action modified after chronic exposure to the drug.