Abstract
Intravenous injection of synthetic substance P caused a significant increase in plasma prolactin (PRL) and growth hormone (GH) in urethane-anesthetized male rats. Simultaneous administration of either ι-dopa or nicotine significantly suppressed plasma GH increase induced by substance P, whereas plasma PRL responses to substance P were blunted by ι-dopa but not by nicotine. Plasma PRL and GH were also elevated by intravenous injection of neurotensin and xenopsin. Both β-endorphin and α-endorphin injected into the lateral ventricle significantly elevated plasma PRL and GH. β-endorphin is more potent than α-endorphin. Plasma PRL and GH responses to these opioid peptides were significantly blunted by naloxone, an opiate antagonist. Intraventricular injection of Vasoactive Intestinal Polypeptide (VIP) caused a significant and dose-related increase in plasma PRL, whereas plasma GH were not affected at the dose examined. Increases in plasma PRL induced by VIP were significantly blunted not only by ι-dopa but also by naloxone injected intravenously. These results suggest that substance P, neurotensin and endorphins stimulate the secretion of both PRL and GH, whereas VIP may stimulate PPL but not GH secretion in the rat.
