In awake male rats, tiny IV doses of the peptide kisspeptin-10 quickly trigger strong bursts of the hormone LH, which controls reproductive function. The effect works even when the rats are fasting, and giving the peptide repeatedly keeps the LH spikes steady. A longer version of the peptide (kisspeptin-52) does the same thing but a bit stronger.

The ability of kisspeptins, ligands of the G protein-coupled receptor 54, to potently elicit LH secretion is now undisputed. Yet, most of the pharmacological characterization of their gonadotropin-releasing effects has been conducted after intracerebral administration. In contrast, the effects of peripheral injection of kisspeptin remains less well defined. In this study, dynamic LH secretory responses to iv administration of kisspeptin-10 in different experimental settings are presented, and compared with those evoked by kisspeptin-52, using a protocol of serial blood sampling in conscious, freely moving male rats. LH responsiveness to peripheral administration of kisspeptin appeared extremely sensitive, as doses as low as 0.3 nmol/kg (0.1 microg/rat) evoked robust LH bursts, the magnitude of which was dose-dependent and apparently maximal in response to 3.0 and 30 nmol/kg kisspeptin-10. The ability of kisspeptin-10 to stimulate LH release was fully preserved, and even doubled in terms of relative increases, after short-term fasting despite suppression of prevailing LH levels. Repeated injections of kisspeptin-10 (four boluses, at 75-min intervals) evoked associated LH secretory pulses, the magnitude of which remained constant along the study period. Moreover, in this setting, in vivo LH responses to a terminal injection of GnRH were preserved, whereas basal and depolarization-induced GnRH release ex vivo was significantly enhanced. Finally, iv administration of kisspeptin-52 elicited dynamic LH responses analogous to that of kisspeptin-10; yet, their net magnitude and duration was slightly greater. In summary, we present in this study a series of experiments on the effects of systemic (iv) injection of single or repeated doses of kisspeptin upon dynamic LH secretion in conscious male rats. Aside from potential physiologic relevance, our present data might contribute to setting the basis for the rational therapeutic use of kisspeptin analogs in the pharmacological manipulation of the gonadotropic axis.