Evidence for the involvement of non-androgenic testicular factors in the regulation of hypothalamic somatostatin and GHRH mRNA levels.
The growth hormone (GH) secretory pattern is dependent on sex and developmental stage. It is generally accepted that in the male rat this pattern is markedly influenced by androgens secreted by the Leydig cells. Recent findings, however, point to the existence of other non-androgenic testicular factors produced by the Sertoli cells and which regulate in vivo the GH responses to growth hormone releasing hormone (GHRH). The aim of this work was to investigate the role played by non-androgenic testicular factors on hypothalamic somatostatin (SST) and GHRH mRNA levels. Seventy-day-old male Sprague-Dawley rats were used throughout the work. They were divided into five groups: (1) control rats; (2) gonadectomized rats; (3) gonadectomized rats supplemented with exogenous administration of dihydrotestosterone (DHT); (4) ethylene dimethane sulphonate (EDS)-treated rats; (5) EDS-treated rats supplemented with exogenous administration of DHT. EDS is a cytotoxic agent that specifically destroys the Leydig cells. The rats were killed after 15 days of treatment. Hypothalamic SST mRNA levels were determined by Northern blot and by in situ hybridization, and GHRH mRNA levels assessed by Northern blot. We found that selective removal of Leydig cells with EDS greatly reduced the SST mRNA content in the periventricular nucleus of the hypothalamus. These levels were significantly lower than those found in gonadectomized rats. Furthermore, replacement treatment with dihydrotesterone (DHT) did not completely restore SST mRNA levels in EDS-treated rats, contrasting with the complete recovery of SST mRNA levels in gonadectomized rats. On the other hand, gonadectomy and EDS treatment produced a significant reduction in GHRH mRNA levels. DHT administration reversed the action of gonadectomy, but did not restore GHRH mRNA content in EDS-treated rats. These data suggest that, in addition to testosterone, as yet unidentified non-androgenic testicular factors can significantly influence SST and GHRH mRNA levels. This may indicate that non-androgenic testicular factors acting at hypothalamic level may be important in the neuroregulation of GH secretion and in the maintenance of sexual dimorphism in GH secretory pattern.