THE INHIBITORY EFFECT OF PICROTOXIN ON BASAL AND COLD-INDUCED THYROTROPIN SECRETION INVOLVES SOMATOSTATIN MEDIATION

This work was undertaken to investigate whether the inhibitory tone ex erted by GABA on somatostatin (SRIH) release operates in the control o f thyrotropin (TSH) secretion in both basal and cold-stimulated condit ions. In a first group of animals (G1) undergoing both carotid and thi rd ventricle push-pull cannulation, i.vt. injection of picrotoxin (10( -5) M) induces a significant decrease in plasma TSH level under basal conditions (0.09 +/- 0.02 versus 0.27 +/- 0.4 ng/100 mu 1; P < 0.03, n = 5). In a median eminence OME) push-pull cannulated group of rats (G 2), picrotoxin, peripherally administered, blocks cold-induced inhibit ion of SRIH release (35.0 +/- 1.8 versus 7.4 +/- 3.3 pg/15 min; P < 0. 005; n = 5). In a third group of intact rats (G3), peripheral administ ration of picrotoxin (2 mg/kg i.p.) blunts the cold-induced TSH releas e (0.17 +/- 0.03 versus 0.46 +/- 0.04 ng/100 mu 1; P < 0.001; n = 5). Our results strongly suggest that a decrease in SRIH release is involv ed in the GABAergic control of basal and cold-induced TSH secretion.