Growth hormone-releasing factor increases somatostatin release and mRNA levels in the rat periventricular nucleus via nitric oxide by activation of guanylate cyclase.

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Abstract
Previous work has shown that growth hormone-releasing factor (GRF) stimulates cGMP production and somatostatin [somatotropin (growth hormone)-release-inhibiting factor, SRIF] release without altering cAMP accumulation by fragments of median eminence incubated in vitro. Therefore, this study was undertaken to evaluate the effect of GRF and cGMP on SRIF mRNA and SRIF release in the periventricular nuclei of male rats in vitro. SRIF mRNA levels were determined in explants of periventricular nuclei incubated for 6 hr in Waymouth's medium in the presence of various substances. Steady-state levels of SRIF mRNA were measured by an S1 nuclease protection assay using a 32P-labeled rat SRIF RNA probe. SRIF release and cGMP formation were measured at 30 min and 6 hr by RIA. SRIF mRNA levels and SRIF release were significantly (P < 0.025) increased (approximately 2-fold) by 1 microM dibutyryl cGMP, whereas sodium butyrate had no effect. This augmentation was not influenced by cycloheximide, an inhibitor of protein synthesis. Sodium nitroprusside (10 microM), an activator of the guanylate cyclase pathway via its release of nitric oxide, augmented (P < 0.001) SRIF mRNA levels and significantly increased (P < 0.05) SRIF release. GRF (1 nM) increased SRIF mRNA (P < 0.001) and stimulated the release of SRIF at 30 min (P < 0.05) and 6 hr (P < 0.01). This stimulation was abolished by 10 microM NG-monomethyl-L-arginine (L-NMMA), a specific inhibitor of nitric oxide synthase, but not by NG-monomethyl-D-arginine (D-NMMA, the inactive isomer). GRF also increased cGMP formation. This effect was completely blocked by incubation with L-NMMA but not D-NMMA. These results indicate that GRF releases nitric oxide. The nitric oxide diffuses to the adjacent SRIF neurons, where it activates guanylate cyclase, leading to increased formation of cGMP. This cGMP increases SRIF mRNA and SRIF release in the periventricular nuclei of male rats.