THYROTROPIN-RELEASING-HORMONE - INHIBITORY FUNCTION ON GROWTH-HORMONETHROUGH BOTH SOMATOSTATIN AND GROWTH HORMONE-RELEASING FACTOR NEURONS

Double labelling immunohistochemistry using antibodies to thyrotropin releasing hormone (TRH) and somatostatin (SS) was undertaken in the an terior hypothalamus in 6 rats. Light microscopic quantitation revealed that 94.5% of SS immunopositive perikarya in the preoptic anterior hy pothalamic area (PO/AHA) and 97.5% in the paraventricular nucleus appe ared to be contacted by one or more TRH immunopositive terminals. In t he chronically cannulated unanaesthetised male rat, unilateral microin jections of a range of doses of TRH were made in the PO/AHA, where SS neurons are located, or in the medial basal hypothalamus, where growth hormone (GH)-releasing factor (GRF) neurons are located. Transient re ductions in GH plasma levels occurred only after injections of the hig hest (10 nmol) dose of TRH in both sites. The function of TRH inputs t o both somatostatin and GRF neurons appears to be inhibitory for GH. T he physiological conditions in which these inputs function remain to b e defined.