Effects of tachykinins on luteinizing hormone release in female rats: potent inhibitory action of neuropeptide K.

Tachykinins, a family of biologically active related peptides, are found in variable amounts in the rat hypothalamus. We assessed the effects of five tachykinins, substance P (SP), neurokinin A (NKA), neuropeptide K (NPK), neuropeptide gamma (NP gamma), and neurokinin B (NKB), on LH release in different experimental model systems in ovariectomized rats. In the first series of experiments rats were ovariectomized and implanted with permanent cannulae in the third cerebroventricle of the rat brain. Two weeks later, the effects of intracerebroventricular injection of 0.5 or 1.25 nm various tachykinins on LH release were studied. The results showed that whereas SP, NKA, and NKB were ineffective, and NP gamma was marginally effective, NPK produced a long-lasting suppression of LH release. NPK decreased LH release in a dose- and time-related fashion. Similarly, in the second series of experiments, whereas SP and NKA were inactive, NPK completely suppressed the LH surge induced by progesterone in estrogen-primed ovariectomized rats. In the third series of experiments we observed that NK-2 receptor agonist [Nle10]NKA4-10, and not NK-1 receptor agonist [Sar9,Met(O2)11]SP, suppressed both the release of LH in vivo and basal and KCl-induced hypothalamic LHRH release in vitro. These results show that NPK is the most effective tachykinin in suppressing LH release, and the inhibitory response is mediated by hypothalamic NK-2 receptors. These findings are in accord with the hypothesis that NPK may serve as a hypothalamic inhibitory neurotransmitter/neuromodulator of LHRH secretion.