Neuroanatomical distribution and properties of Kiss1 neurones in male rats

Abstract

Kisspeptin is synthesised in the brain in known regions such as the arcuate nucleus (ARC) and anteroventral periventricular nucleus (AVPV). Kisspeptin has been classified as a key player in puberty and acts centrally in the upstream regulation of GnRH neurone secretion via Kiss1 receptors (Kiss1r) to elicit release of GnRH peptide for further stimulation of LH secretion from the pituitary and consequently of the gonads to trigger spermatogenesis, follicle development, and sex steroid synthesis. The KNDy (kisspeptin/neurokinin B/dynorphin) compendium of neurones in the ARC hints at the possible site of the GnRH pulse generator. Moreover, Kiss1 cells in different rodent brain areas respond to the absence of circulating steroids in castrate models. Projections of Kiss1 neurones are found throughout the brain and other kisspeptinergic populations exist in important areas that regulate metabolism, the circadian rhythm, and stress. Taken together, these observations suggest that the scope of kisspeptin actions is not only limited to the modulation of reproductive function and different Kiss1 cell populations in several brain regions implies association with major neuropeptides/neurotransmitters and involvement in other physiological processes. Firstly, the immunocytochemical protocol and antiserum were optimized and validated to assess true kisspeptin immunoreactive detection of cells and fibres. Kisspeptin cells were found in specific areas and fibres were located virtually everywhere throughout the rat forebrain. Kisspeptin neurones were predominantly based in the ARC, AVPV, medial amygdala, bed nucleus of the stria terminalis and other hypothalamic regions. Sex steroid regulation of Kiss1 neurones was then investigated by examining the effects of a chemical castration on those with and without steroid rescue. GnRH antagonist s.c. injection down-regulated the number Kiss1 neurones in various brain regions while it enhanced the number of cells in the ARC Kiss1 population. Testosterone replacement reversed the antagonist effects on Kiss1 neurones. LH plasma concentration decreased in rats injected with the GnRH antagonist with or without testosterone administration. These studies therefore confirm importance of sex steroids on the different kisspeptin populations. Then, the effects of fasting and re-feeding on Kiss1 neurones in the ARC, rostral part of the third ventricle (RP3V), and dorsomedial nucleus (DMN) were investigated. Re-feeding after a 24 hour fast induced no Fos expression in RP3V Kiss1 neurones and very mild Fos activation in ARC and DMN neurones. Thus, Kiss1 cells are found in various brain regions whereby distinct physiological actions occur and each individual population responds differently to the effects of steroid and metabolic regulation.