Modification of GABAergic Activity and Thyrotropin Secretion in Male Rats

The effect of GABA on basal and stimulated TSH secretion was studied in male rats. The effects of drugs on basal TSH levels were not consistent. Muscimol(0.5 mg/kg subcutaneously, but not 2 mg/kg) increased whereas baclofen (10 mg/kg intraperitoneally), amino-oxyacetic acid (AOAA, 20 mg/kg intraperitoneally) and bicuculline (2 mg/kg intraperitoneally, but not 1 or 4 mg/kg) decreased basal TSH concentrations. Muscimol, AOAA and baclofen dose-dependently reversed the TSH cold-response, as did a large dose of di-n-propylacetate (DPA, 400 mg/kg intraperitoneally) and 500 mg/kg (but not 50, 100 or 1500 mg/kg intraperitoneally) of GABA itself. Bicuculline was not effective alone. Neither did it modify the effects of muscimol, AOAA and GABA on the cold-stimulated TSH response. None of the drugs studied (AOAA, GABA, bicuculline) modified TRH-induced (100 ng intraperitoneally) TSH-response. GABA injected into the third ventricle (5-50 microgram/rat) or into the medial basal hypothalamus (MBH, 5 microgram/rat) had no effect on the basal TSH levels. However, the TSH cold-response was inhibited when GABA (5 microgram/rat) was infused into the MBH but not when it was infused into the third ventricle (5-50 microgram/rat). The results suggest that GABAergic pathways may have an inhibitory effect on the stimulated TSH secretion in male rats. The locus of this inhibition is not situated in the anterior pituitary, but possibly in the MBH.