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 exerted by GABA on somatostatin (SRIH) release operates in the control of thyrotropin (TSH) secretion in both basal and cold-stimulated conditions. In a first group of animals (G1) undergoing both carotid and third 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 microliters; P < 0.03, n = 5). In a median eminence (ME) push-pull cannulated group of rats (G2), picrotoxin, peripherally administered, blocks cold-induced inhibition 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 administration of picrotoxin (2 mg/kg i.p.) blunts the cold-induced TSH release (0.17 +/- 0.03 versus 0.46 +/- 0.04 ng/100 microliters; P < 0.001; n = 5). Our results strongly suggest that a decrease in SRIH release is involved in the GABAergic control of basal and cold-induced TSH secretion.