Increased galanin and neuropeptide-Y immunoreactivity within the hypothalamus of ovariectomised ewes following a prolonged period of reduced body weight is associated with changes in plasma growth hormone but not gonadotropin levels.

Abstract

Manipulation of diet is known to affect the secretion of the gonadotropins and growth hormone (GH). The former are under the direct regulation of hypothalamic gonadotropin-releasing hormone (GnRH) and the latter is under the dual control of GH-releasing hormone (GHRH) and somatostatin (SRIH). At the level of the hypothalamus, both galanin (GAL) and neuropeptide Y (NPY) are thought to regulate the secretion of the above releasing and inhibiting factors. Both peptides are also potent orexigenic agents. We have studied ovariectomised ewes that were either well-fed (HIGHs) or underfed (LOWs) and used immunocytochemistry and image analysis to measure the levels of GAL and NPY in hypothalamic nuclei in which GnRH, GHRH and SRIH are found and which are also involved in the regulation of appetite and feeding. The sheep were given a normal diet or a restricted diet for 15 months. Four pairs of ewes were then blood-sampled to measure GH, luteinising hormone (LH), and follicle-stimulating hormone (FSH) and then killed for recovery of the brains. After perfusion, cryostat sections were cut through the entire hypothalamus, mounted, and stained fro NPY or GAL. All treatments and analyses were performed in pairs. The number of immunoreactive cells, density of terminals and total immunoreactivity (IR) were quantified by image analysis by sampling 6-16 subareas (depending on region) on sections through the pre-optic area (POA), paraventricular nucleus (PVN), arcuate nucleus (ARC) and median eminence (ME). Mean (+/- SEM) live weight of the LOWs was significantly (p < 0.0001) lower than that of the HIGHs (37.6 +/- 0.6 kg vs. 60.6 +/- 0.5 kg). There was no difference in the plasma levels of LH and FSH but the area under the GH curve (ng/ml/h) was significantly (p < 0.0001) greater in the LOWs (320 +/- 40.9 vs. 67.3 +/- 16.1). There was an increased number of cells staining for NPY but not GAL in the ARC/ME of the LOWs. Nevertheless, the oveall level of immunostaining for both peptides was increased in the LOWs. GAL IR was restricted to the mediobasal hypothalamus. In the LOWs, the density of NPY terminal fields in each area of the ARC was significantly (p < 0.05) increased. Food restriction also increased the density of NPY terminals in the POA and PVN (p < 0.025) but not in the ME. These data indicate that a dietary manipulation which affects GH secretion but not the gonadotropins may be mediated by NPY and GAL neuronal systems in specific brain regions within the hypothalamus.