Concentration and distribution of neuropeptide Y, galanin, beta-endorphin,vasoactive intestinal peptide and gonadotrophin-releasing hormone in the hypothalamus of gilts during oestrogen-induced surge secretion of luteinizing hormone

The relationship of neuropeptide Y (NPY), galanin (GAL), beta-endorphin (be ta-END) and vasoactive intestinal peptide (VIP) to GnRH neurons were determ ined during the estradiol-induced LH surge. In experiment 1, 16 ovariectomi zed (OVX) gilts received 15 mu g estradiol benzoate (EB)/kg BW at 0800 h an d were slaughtered at either 24 h (n = 5), 48 h (n = 6) or 72 h (n = 5) lat er and five were injected with corn oil vehicle (0 h controls). Concentrati ons of neuropeptides were determined in tissue extracts by RIA. In experime nt 2, nine OVX gilts were injected with EB as in experiment I and killed at either 24, 48 or 72 h (n = 3) later and three were not injected with EB (0 h controls). Frozen sections were processed to localize neuropeptides. In experiment 1, all measured neuropeptides were highest in pituitary stalk me dian eminence (SME). The GnRH concentration was not different at any time p oint in medial basal hypothalamus (MBH), preoptic area (POA) or SME. The NP Y content in MBH was lower at 24, 48 and 72 h after EB than at 0 h (p < 0.0 01), and lower in SME at 48 and 72 h than at 24 h (p < 0.05) and 0 h (p < 0 .01), respectively. Concentration of GAL in SME was four times higher at 72 h than at 0, 24 or 48 h (p < 0.001). The VIP concentration increased in PO A (p < 0.05) and MBH (p < 0.001) at 24 h and 72 h (p < 0.05). Concentration of VIP in SME was lower at 24 and 48 h than at 0 h (p < 0.05) and increase d to more than twice (P < 0.05) by 72 h. Concentrations of beta-END were no t different at any time point in POA and MBH but the highest content of bet a-END in SME occurred at 24 h (p < 0.001). In experiment 2 a moderate numbe r of GnRH-immunoreactive (IR) fibres were found in the periventricular area of the POA and in organum vasculosum of the laminae terminalis (OVLT). The GnRH-IR fibres formed networks in the external and internal layer of the m edian eminence (ME). At 24 h, GnRH-IR neurons and fibres in the POA and ME were more numerous and noticeable differences were found in the arcuate nuc leus (ARC) and ventromedial nucleus (NVM). At 48 and 72 h. numbers of IR ne urons and fibres were higher in the ARC and NVM, but no changes occurred in the POA and ME. The ARC contained a moderate number of NPY-IR fibres, but less numerous small cell bodies. Only a few NPY-IR perikarya and fibres wer e in the NVM and fibre density was similar at all times after EB injection. VIP-IR fibres were scarcely distributed mostly in the posterior POA and th e internal layer of ME. The number of VIP-IR fibres was similar at all time points and regions. A moderate number of varicose beta-END fibres supplied the POA, and they were especially dense near the OVLT, but the cell bodies were moderate in number and did not show strong immunoreactivity. In ME, A RC and NVM, the number of beta-END immunoreactive structures was greater at 24 and 48 h than at 0 h. The number of beta-END-IR nerve fibres in POA was higher at 72 h than at 0 h. Levels of all neuropeptides studied were simil ar in the POA and MBH and content of NPY, GAL and beta-END was very high in the SME of the pig forebrain. The dynamic changes of NPY, GAL, VIP and bet a-END content in pig hypothalamus during the oestrogen-induced negative and positive feedback phases of LH secretion indicate their potential role in modulating GnRH release from the median eminence.