Hypothalamic arcuate neuropeptide Y-neurons decrease periventricular somatostatin-neuronal activity before puberty in the female lamb: morphological arguments.

摘要

It is assumed that hypothalamic somatostatin plays a dominant role in the regulation of growth of developing lambs. On the other side, neuropeptide Y (NPY) neurons of the arcuate (ARC) nucleus are potentially involved in the control of gonadotrophins in prepubertal lambs and also of growth hormone (GH) secretion in adults. This study therefore investigated whether the transition from the prepubertal to the peripubertal period is accompanied by changes in NPY-ir and NPY mRNA content in neurons of the ARC nucleus and their putative projections to somatostatin neurons in both the ARC and periventricular (PEV) nuclei. The hypothalami of prepubertal (17-week-old) and peripubertal (32-week-old) female lambs were compared using single and double-labelling immunohistochemistry, and hybridisation in situ for NPY. Single-labelling for NPY mRNA and NPY-ir was quantified by image analysis using a light microscope and expressed as the percent area stained and/or the integral density of the reaction. Double-labelling for NPY-somatostatin relationships was analysed by confocal microscopy. Our data suggest that there are no detectable changes in NPY-ir in the PEV nucleus in the period leading up to puberty, whereas both the distributional area and intensity of NPY-labelling in the ARC are significantly higher in peripubertal compared to prepubertal sheep. In contrast, NPY mRNA levels are higher in prepubertal than in peripubertal ewes in the ARC nucleus. Confocal microscopy suggests the existence of NPY-somatostatin axo-somatic contacts in both PEV and ARC nuclei. In the PEV nucleus, the number of close appositions between NPY-ir fibres and somatostatin-ir perikarya is higher in prepubertal than in peripubertal ewes, but in the ARC no such difference was observed. In conclusion, our observations suggest that there is decreased activity of the NPY neurons of the ARC nucleus closely related to somatostatin neurons in the PEV nucleus at the onset of puberty. The withdrawal of this NPY effect may allow a higher release of somatostatin, which consequently inhibits GH secretion and stops growth. Both peptides are involved in the transmission of signals leading to stop growth at puberty.