Hypothalamic growth hormone‐releasing factor (GRF) in higher mammals, including human GRF, is a 44 amino acid residue peptide and is highly homologous in structure. By contrast, mouse GRF (mGRF) recently deduced by cDNA cloning consists of only 42 residues and shows relatively low homology to the GRFs of higher mammals and the same rodent species, rat. To characterize and localize the predicted mature mGRF peptide in the hypothalamus, we have generated its antiserum and developed a homologous radioimmunoassay. Immunoreactive mGRF in the acid hypothalamic extract was eluted as a single peak at a position identical to that of synthetic peptide on both gel filtration chromatography and reverse‐phase high‐performance liquid chromatography (HPLC). Secretion of immunoreactive mGRF from incubated hypothalami increased several fold in response to 50 mM K⁺, and this rise was abolished in the absence of medium Ca²⁺. Only a single peak of immunoreactive mGRF that coeluted with synthetic replicate was observed after the K⁺ ‐stimulated medium was extracted on Bond Elut C18 cartridges and applied on reverse‐phase HPLC. Immunohistochemistry identified many mGRF‐positive cell bodies in the arcuate nucleus and dense bundles of immunoreactive fibers in the median eminence. Treatment of mice with gold thioglucose (GTG), a chemical agent known to cause hypothalamic lesions, markedly depleted both content and in vitro secretion of immunoreactive mGRF. The decline in mGRF secretion was greater in GTG obese than in nonobese mice, whereas somatostatin secretion was not affected by GTG treatment. As compared to the previous findings obtained on rat GRF, mouse had approximately the same content of hypothalamic GRF, but retained only one third the capacity to release GRF in response to the same depolarizing stimulus.

These findings demonstrate that mouse hypothalamus contains and secretes a mGRF‐like molecule which has immunological and chromatographic characteristics identical to that predicted by molecular cloning. The depolarization‐evoked, Ca²⁺‐dependent release of immunoreactive mGRF and its neuronal localization in the arcuate nucleus‐median eminence region indicate that it functions as a hypophysiotropic hormone of physiological importance. GTG produces hypothalamic GRF deficiency, probably sparing hypophysiotropic somatostatin neurons, and could be used to generate a mouse model for human GRF‐deficient dwarfism.