Involvement of hypothalamic somatostatin in the suppression of growth hormone secretion by central corticotropin-releasing factor in conscious male rats.

The role of central corticotropin-releasing factor (CRF) in the regulation growth hormone (GH) secretion was studied in freely moving conscious male rats with indwelling intra-atrial and intracerebroventricular (i.c.v.) cannulae. GH measurements in blood samples obtained every 20 min from 10.00 to 14.00 h in control animals injected with saline either intravenously (i.v.) or into the lateral cerebral ventricle revealed that spontaneous GH secretion was pulsatile, and occurred regularly at around 12.00 h. When ovine CRF (10 micrograms) was injected i.c.v., spontaneous GH secretion was inhibited (mean plasma GH [11.20-13.00 h]: 20 +/- 7 ng/ml vs. control: 126 +/- 22 ng/ml, p less than 0.01). In contrast, the intravenous injection of CRF (10 micrograms) did not affect spontaneous GH secretion (mean plasma GH [11.20-13.00 h]: 162 +/- 25 ng/ml vs. control: 193 +/- 31 ng/ml). This GH suppressive action of central CRF was blocked by the i.v. injection (0.5 ml) of antisomatostatin serum (AS), but not of normal sheep serum (NS), (mean plasma GH [11.20-13.00 h]: NS + CRF: 15 +/- 2 ng/ml vs. AS + CRF: 202 +/- 30 ng/ml, p less than 0.01). The mean plasma GH value [11.20-13.00 h] in animals receiving AS and CRF was not significantly different from those in animals receiving saline (i.v.) together with AS. These results suggest a potential inhibitory role of central CRF in the regulation of spontaneous GH secretion in the rat which is mediated by the stimulation of hypothalamic somatostatin.