HYPOTHALMIC PITUITARY-AXIS OF THE SPONTANEOUS DWARF RAT - AUTOFEEDBACK REGULATION OF GROWTH-HORMONE (GH) INCLUDES SUPPRESSION OF GH RELEASING-HORMONE RECEPTOR MESSENGER-RIBONUCLEIC-ACID/

In this study, the spontaneous dwarf rat (SDR) has been used to examin e GHRH production and action in the selective absence of endogenous GH . This dwarf model is unique in that GH is not produced because of a p oint mutation in the GH gene. However, other pituitary hormones are no t obviously compromised. Examination of the hypothalamic pituitary-axi s of SDRs revealed that GHRH messenger RNA (mRNA) levels were increase d, whereas somatostatin (SS) and neuropeptide Y (NPY) mRNA levels were decreased, compared with age- and sex matched normal controls, as det ermined by Northern blot analysis (n = 5 animals/group; P < 0.05). The elevated levels of GHRH mRNA in the SDR hypothalamus were accompanied by a 56% increase in pituitary GHRH receptor (GHRH-R) mRNA, as determ ined by RT-PCR (P < 0.05). To investigate whether the upregulation of GHRH-R mRNA resulted in an increase in GHRH-R function, SDR and contro l pituitary cell cultures were challenged with GHRH (0.001-10 nM; 15 m in), and intracellular cAMP concentrations were measured by RLA. Inter estingly, SDR pituitary cells were hyperresponsive to 1 and 10 nM GHRH , which induced a rise in intracellular cAMP concentrations 50% greate r than that observed in control cultures (n = 3 separate experiments; P < 0.05 and P < 0.01, respectively). Replacement of GH, by osmotic mi nipump (10 mu g/h for 72 h), resulted in the suppression of GHRH mRNA levels (P < 0.01), whereas SS and NPY mRNA levels were increased (P < 0.05), com pared with vehicle-treated controls (n = 5 animals/treatmen t group). Consonant with the fall in hypothalamic GHRH mRNA was a decr ease in pituitary GHRH-R mRNA levels. Although replacement of insulin- like growth factor-I (IGF-I), by osmotic pump (5 mu g/h for 72 h), res ulted in a rise in circulating IGF-I concentrations comparable with th at observed after GH replacement, IGF-I treatment was ineffective in m odulating GHRH, SS, or NPY mRNA levels. However, IGF-I treatment did r educe pituitary GHRH-R mRNA levels, compared with vehicle-treated cont rols (P < 0.05). These results further validate the role of GH as a ne gative regulator of hypothalamic GHRH expression, and they suggest tha t SS and NPY act as intermediaries in GH-induced suppression of hypoth alamic GHRH synthesis. These data also demonstrate that increases in c irculating IGF-I are not responsible for changes in hypothalamic funct ion observed after GH treatment. Finally, this report establishes modu lation of GHRH-R synthesis as a component of GH autofeedback regulatio n.