GROWTH-HORMONE RELEASE INDUCED BY GROWTH HORMONE-RELEASING HEXAPEPTIDE IS NOT MEDIATED BY THYROTROPIN-RELEASING-HORMONE IN NEONATAL RATS

GH-releasing hexapeptide (GHRP-6) and nursing stimulate GH secretion i n rat pups via GH-releasing factors (GRFs) distinct from GH-releasing hormone (GHRH). It was determined whether GH secretion induced by GHRP -6 or nursing was mediated by TSH-releasing hormone (TRH) in 2-d-old r ats. In vitro, GHRP-6 and TRH stimulated GH secretion of neonatal pitu itary glands. At their maximally effective doses, GHRP-6 and TRH evoke d approximately equal GH responses. Treatment with a combination of th e maximally effective doses of GHRP-6 and TRH resulted in a GH respons e comparable to that evoked by either treatment alone. GHRP-6 in vivo induced a greater GH response than did TRH. Treatment in vivo with a c ombination of the maximally effective doses of GHRP-6 and TRH synergis tically increased serum GH levels. Unlike GHRP-6, TRH was an effective stimulus of prolactin secretion either in vitro or in vivo. Nursing w as an effective stimulus for GH secretion, but only marginally increas ed serum prolactin levels. The effects of either of the peptides and n ursing on GH secretion were additive. These results suggest that GHRP- 6 stimulates GH secretion both by acting directly on the pituitary gla nd and indirectly via a hypothalamic GRF. The indirect effect appears to be greater. The alternative GRFs released by GHRP-6 or nursing are distinct from each other and from TRH. These findings suggest that alt ernative GRFs play a significant role in the regulation of GH secretio n in neonatal rats.