Regional distribution of rat growth hormone releasing factor-like immunoreactivity in rat hypothalamus.

A heterologous RIA for rat GH-releasing factor (rGRF) was established using synthetic rGRF-(1-43)OH for both standard reference and radioiodination with the antiserum produced against human GRF-(1-44) NH2. The regional distribution of rGRF-like immunoreactivity (LI) in rat hypothalamus was examined according to the Palkovits microdissection method and compared with that of somatostatin (SRIF)-LI. Both rGRF- and SRIF-LI contents (mean +/- SE; nanograms per mg protein) were highest in the median eminence (rGRF, 32.28 +/- 11.42; SRIF, 109.9 +/- 19.2) and next most abundant in the arcuate nucleus (rGRF, 3.50 +/- 0.47; SRIF, 12.88 +/- 0.60). Only a small amount of rGRF-LI was found in the ventromedial (1.41 +/- 0.51) and dorsomedial (1.16 +/- 0.15) nuclei and the anterior hypothalamic area (1.29 +/- 0.42), whereas rGRF-LI was not detected in the other nuclei of the hypothalamus. A considerable amount of SRIF-LI was contained in the periventricular, ventromedial, and paraventricular nuclei and the anterior hypothalamic area, in accordance with other reports. Gel filtration chromatography revealed that hypothalamic extracts contained a major peak of rGRF-LI corresponding to rGRF-(1-43)OH and two peaks of SRIF-LI equivalent to SRIF-(1-28) and SRIF-(1-14), respectively. These findings indicate that rGRF-LI is localized in the median eminence and arcuate nucleus in the rat and that rGRF-, SRIF-(1-28)-, and SRIF-(1-14)-LI are present in a 1:2.10:6.29 ratio on a molar basis.