Thyrotropin releasing hormone (TRH): its changes in discrete hypothalamic areas after treatment with triiodothyronine, thyroidectomy and acute cold exposure

In order to elucidate the specific thyrotropic area in the hypothalamus, thyrotropin releasing hormone (TRH) content and concentration were measured in discrete hypothalamic nuclei and areas after triiodothyronine (T3) administration (T3 10 micrograms/rat/day for 6 days), thyroidectomy (TX) and acute cold exposure in male rats. In th TX and T3 groups, serum TSH levels were significantly increased in TX group and markedly decreased in T3 and TX with T3 groups as compared to the sham operated control group (Sham). TX produced a slight but nonsignificant decrease in TRH content in most of the hypothalamic nuclei examined as compared with the Sham group. However, a significant increase in TRH contents was seen in the anterior hypothalamic nucleus (AHN), median eminence (ME) and posterior pituitary (PP) in TX with T3 group as compared to the rats with only TX. In the acute cold stress experiments, serum TSH levels were elevated from 15 to 30 min of 4 degrees C exposure. Together with these peripheral changes, TRH content and concentration in the suprachiasmatic nucleus (SC) were increased significantly at 15 min and had returned to the normal level by 30 min after 4 degrees C cold exposure. However, in the paraventricular nucleus (PV) and dorsal premammillary nucleus (PMD), marked decrease in TRH concentrations were observed with this stress. Therefore, 1) decreased TSH release in TX rats treated with T3 was induced by the block of TRH release from the AHN and ME as compared with the TX group, and 2) elevated serum TSH levels in 4 degrees C cold stress might be induced by the release of TRH from the PMD and PV. These experiments demonstrate that the specific hypothalamic area for TSH release was located in some of the anterior and posterior hypothalamic nuclei and in the ME.