GABA-A AGONIST MUSCIMOL INHIBITS STIMULATED VASOPRESSIN RELEASE IN THE POSTERIOR PITUITARY OF SPRAGUE-DAWLEY, WISTAR, WISTAR-KYOTO AND SPONTANEOUSLY HYPERTENSIVE RATS

Vasopressin-containing neurosecretory cells are partly regulated by GA BAergic neurons present both at the hypothalamic and the pituitary lev el. In the present work, we compared GABA effects on vasopressin relea se from posterior pituitaries of Sprague-Dawley (SPD), Wistar (W), Wis tar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Release of vasopressin was studied in vitro by placing neurointermediate lobes in perfusion chambers. It was stimulated twice by 50 mM KCl (S1 and S2) and the ratio between the first and the second stimulation was calcula ted (S2/S1). The basal and stimulated release of vasopressin was enhan ced in the SHR. There was no difference in vasopressin content in the pituitary between the WKY and the SHR but the levels were lower compar ed to the SPD rat. Muscimol, a GABA-A receptor agonist, was added duri ng S2. Muscimol inhibited in a dose-dependent manner the stimulated re lease of vasopressin, with an ED50 about 3 muM. The effect of muscimol was not statistically different between the strains expressed in rati os. The actual inhibition was apparently greater in the SHR, as both b asal and stimulated vasopressin release was larger.