Ultrastructural evidence of kisspeptin-gonadotrophin-releasing hormone (GnRH) interaction in the median eminence of female rats: implication of axo-axonal regulation of GnRH release.

The present study was conducted to determine the morphological and functional interaction between kisspeptin and gonadotrophin-releasing hormone (GnRH) neuronal elements at the median eminence in female rats to clarify a possibility that kisspeptin directly stimulates GnRH release at the nerve end. A dual immunoelectron microscopic study of kisspeptin and GnRH showed that the kisspeptin-immunoreactive nerve element directly abutted the GnRH-immunoreactive nerve element, although no obvious synaptic structure was found between kisspeptin and GnRH neurones in the median eminence. The current retrograde tracing study with FluoroGold (FG) indicates that kisspeptin neurones are not in contact with fenestrated capillaries because no FG signal was found in kisspeptin neurones when the FG was injected peripherally. This peripheral FG injection revealed the neuroendocrine neurones projecting to the median eminence because FG-positive GnRH neuronal cell bodies were found in the preoptic area. Synthetic rat kisspeptin (1-52)-amide stimulated GnRH release from the median eminence tissues in a dose-dependent manner. Thus, the present results suggest that kisspeptin at least partly exerts stimulatory effects on GnRH release from the neuronal terminals of GnRH neurones by axo-axonal nonsynaptic interaction in the median eminence.

Oestrogen-dependent stimulation of luteinising hormone release by galanin-like peptide in female rats.

Galanin-like peptide (GALP), a ligand for three types of galanin receptor, is reported to have a role in regulating luteinising hormone (LH) release in male rodents and primates, but its role in LH release in female rodents remains controversial. The present study was conducted to test whether GALP has a stimulatory role in regulating LH secretion in female rats. The effect of i.c.v. infusion of GALP (5 nmol) on pulsatile LH release was investigated in Wistar-Imamichi strain female rats, or lean and obese Zucker rats. In oestradiol-17beta (oestradiol)-primed ovariectomised (OVX) Wistar-Imamichi female rats, i.c.v. infusion of GALP caused a gradual increase in LH release for the first 1.5 h after the infusion followed by an increased LH pulse frequency during the next 1.5 h, resulting in a significant increase in the mean LH concentrations and baseline levels of LH pulses throughout the sampling period and in the frequency of LH pulses at the last half of the period compared to vehicle-treated controls. The stimulatory effect of GALP was oestrogen-dependent because the same GALP treatment did not affect LH release in OVX rats in the absence of oestradiol. In lean Zucker rats, LH pulses were found in oestradiol-primed OVX individuals and central GALP infusion increased mean LH concentrations in the last half of the period. By contrast, few LH pulses were found in oestradiol-primed OVX obese Zucker rats reportedly with lower hypothalamic GALP expression. Central GALP infusion caused an apparent but transient increase in LH release, resulting in the significant increase in all pulse parameters of LH pulses compared to vehicle-treated controls in the first half of the sampling period. These results suggest that hypothalamic GALP is likely involved in stimulating GnRH/LH release, and that the stimulatory effect of GALP on LH release is oestrogen-dependent in female rats.