REGIONAL DIFFERENCES IN THE DISTRIBUTION OF GONADOTROPIN-RELEASING-HORMONE CELLS BETWEEN RAPIDLY GROWING AND GROWTH-RESTRICTED PREPUBERTAL FEMALE SHEEP

Growth retardation induced by dietary restriction in the lamb results in a low GnRH pulse frequency, and thus, puberty is delayed. in our ex perimental model, in which ovariectomized lambs are maintained at wean ing weight (similar to 20 kg BW), hypothalamic GnRH is present and rel easable, suggesting that central mechanisms limit the release of GnRH during chronic growth restriction. Our study compared the number and d istribution of GnRH-containing neurons in growth-restricted (n = 5) an d rapidly growing (n = 5) ovariectomized prepubertal female lambs at 2 4 weeks of age normal age of puberty is about 30 weeks). Immunoreactiv e cells were labeled using LR-1 antiserum (R. Benoit) and an avidin-bi otin-immunoperoxidase procedure. GnRH neurons were localized in 60-mu m coronal sections from the level of the diagonal band of Broca to the mammillary bodies. The estimated total number of GnRH neurons in the growth-restricted and rapidly growing lambs was similar (3364.8 +/- 51 3.8 vs. 3151.2 +/- 279.8, respectively). In addition, tile percent dis tributions of GnRH neurons in the diagonal band of Broca, the anterior hypothalamus, the lateral hypothalamus, and the posterior hypothalamu s were not different. A trend (P = 0.07) toward a smaller percent dist ribution in the preoptic area was noted in growth-restricted lambs (30 .6 +/- 3.6) compared to rapidly growing lambs (44.0 +/- 5.2). By contr ast, the percent distribution of GnRH neurons in the medial basal hypo thalamus was significantly greater in the growth-restricted lambs comp ared with the rapidly growing lambs (17.7 +/- 2.2 vs. 6.7 +/- 1.4, res pectively; P < 0.005). It is of interest that the percent distribution of GnRH-containing neurons in the medial basal hypothalamus of the hy pogonadotropic growth-restricted lamb is similar to that observed in t he fetal lamb, whereas the eugonadotropic rapidly growing lamb is more similar to the adult female. In this context, decreased GnRH accretio n and delayed puberty during diet-induced growth restriction may arise from alterations in the GnRH neurosecretory system.