Inhibition of the TSH secretion by histamine in male rats

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Abstract
The effect of histamine (HA) on the cold-stimulated and the TRH-induced TSH secretion was studied in male rats utilizing HA and specific HA antagonists and histaminergic drugs as tools. L-histidine (1000 and 1500 mg/kg i.p.) inhibited the cold-stimulated but not the TRH-induced TSH secretion. Mepyramine (5-50 mg/kg i.p.) and cimetidine (50-250 mg/kg i.p.) had no effect on the TSH-cold-response, but the TRH-induced TSH secretion was enhanced by them. HA (1 mg/kg s.c.), 2-pyridylethylamine (2PEA; 10 mg/kg i.p.) and impromidine (1 mg/kg s.c.) decreased the TRH-induced TSH secretion but did not affect the TSH cold-response. When infused into the 3rd ventricle HA (1-50 .mu.g/rat), impromidine (0.1 and 1.0 .mu.g/rat) and 2PEA (50 .mu.g/rat) decreased the cold-stimulated TSH-secretion. The locus of the inhibitory action of HA seems to be both at the anterior pituitary, where it is mediated through H1- and H2-receptors and in the hypothalamus, where the effect appears to be mediated through H2-receptors. At least a part of this particular action might be explained by other mechanisms since neither H1- nor H2-receptor antagonists were able to counteract the effect of HA.