Many peptides that are present in the external zone of the median eminence are not secreted into the hypophysial portal blood of sheep.

Abstract

Apart from the well recognized factors that are produced by the hypothalamus and secreted into hypophysial portal blood to regulate pituitary function, there is a range of neuropeptides that are present in the median eminence and could be secreted to serve a modulatory function. In this study we have collected hypophysial portal blood and jugular venous blood from sheep in an attempt to identify which of these putative modulatory peptides might be secreted from the median eminence. We have measured neuropeptide Y (NPY), substance P (SP), galanin (GAL), neurokinin A (NKA), peptide histidine isoleucine (PHI), vasoactive intestinal peptide (VIP), neurotensin (NT) and cholecystokinin (CCK). We also examined the sheep median eminence using immunohistochemistry for NPY, SP and GAL and determined degradation profiles of NPY, SP, GAL and NKA in portal and jugular plasma. In no instance did we find that levels of the above peptides were consistently higher in portal blood than in peripheral blood. In some cases levels of peptide were lower in portal plasma e.g. for NPY (6/10 sheep). In one experimental series SP levels in portal plasma were significantly (p < 0.05) lower than levels in jugular plasma but this was not found in another experimental series. Galanin levels were significantly (p < 0.01) lower in portal plasma compared to levels in jugular plasma. We conducted in vitro studies to determine whether or not the above peptides are selectively degraded in portal blood but were unable to show any differences between the rates of degradation in portal and jugular plasma. Immunohistochemistry revealed projections into the external zone of the median eminence for NPY, GAL and SP. This study shows that none of the above peptides are secreted into the hypophysial portal blood of sheep. For some peptides e.g. GAL, enzymes from the endothelial cells of the portal vessels may enhance degradation. Projections into the external zone of the median eminence of neuronal systems containing these peptides may serve to modulate the secretion of the well recognized release and inhibiting factors by acting on the neurosecretory terminals.