Neuroanatomic localization of the inhibitory effect of TRH on growth hormone secretion.

The inhibitory effect of centrally administered thyrotropin-releasing hormone (TRH) on the plasma growth hormone (GH) response to GH-releasing hormone (GHRH) in the rat was studied in relation to the anatomic loci involved. Experiments were performed in animals with bilateral electrolytic lesions in the medial preoptic (MPO) area or with anterolateral hypothalamic deafferentation and in sham-operated controls. Blood samples were obtained every 10 to 20 min from and drugs were injected into freely moving animals with indwelling cannulas in the right atrium and lateral cerebral ventricle. In control animals, the plasma GH response to GHRH, 1 microgram iv, was almost completely inhibited by TRH, 1 microgram icv, injected 5 min previously. In animals with either MPO lesions or anterolateral hypothalamic deafferentation in which median eminence somatostatin immunochemical staining was almost completely eliminated, the GH response to GHRH was enhanced and TRH did not exhibit any inhibitory effect. These results, together with the previous observation that the inhibitory effect of TRH is blocked by prior treatment with anti-somatostatin serum, suggest that the effect of TRH is mediated by stimulation of somatostatin-containing neurons in the periventricular nucleus of the MPO area.