Intrinsic Neuronal Plasticity of Kiss1 Neurons in Mice around Puberty
Puberty is a critical stage in mammalian development, characterized by rapid somatic growth, sexual maturation, and the final maturation of the central nervous system. The timing of puberty is crucial, as initiating it either too early or too late can lead to adverse outcomes. Precocious puberty can result in attenuated growth, while delayed puberty can lead to mental health problems. Therefore, understanding puberty is essential for gaining insights into fertility, post-pubertal health, and species continuity. Puberty is initiated by the hypothalamic-pituitary-gonadal (HPG) axis, a key hormonal regulatory system in mammals. Gonadotropin-releasing hormone (GnRH) is released from the hypothalamus by GnRH neurons, stimulating the anterior pituitary to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones act on the gonads to promote gametogenesis and the production of sex steroids, which provide negative feedback to the hypothalamus and anterior pituitary to maintain appropriate hormone levels. Kiss1 neurons govern the pubertal process and are critical for the transition from an infertile to a fertile state. Mature Kiss1 neurons in the arcuate nucleus (ARC) and anteroventral periventricular nucleus (AVPV) project to GnRH neurons to induce GnRH pulses and surges, respectively. This thesis aimed to test the hypothesis that Kiss1 neurons in the hypothalamus exhibit neuronal plasticity during puberty in mice. Kiss-CRE-tdTomato female mice begin puberty at around 4.5-5 weeks of age. Mice were divided into three age groups: 3-week pre-pubertal, 4-week pre-pubertal, and post-pubertal (6-8 weeks). To test their intrinsic plasticity, whole-cell current-clamp recordings on Kiss1 neurons (labelled with tdTomato in brain slices) was used. Kiss1ARC neurons in 3-week pre-pubertal mice showed a significantly lower number of action potentials, and a higher firing frequency compared to Kiss1ARC neurons in 4-week and post-pubertal mice. In contrast, in male mice, Kiss1ARC neurons between 3w and 6-8w did not show many significant differences, suggesting that the plasticity of electrophysiological properties of Kiss1ARC neurons is sexually dimorphic. In female mice, the medium after- hyperpolarization (mAHP) following a train of spikes was significantly larger in post- pubertal Kiss1ARC neurons. For single action potentials, the threshold, the trough, and the time from peak to trough were significantly lower in post-pubertal mice, indicating differences in the expression of ion channels between 3w and 6-8w Kiss1ARC neurons. Quantitative polymerase chain reaction (qPCR) was used to investigate the expression levels of specific ion channel genes in Kiss1ARC neurons of female mice. Scn2a (the gene for NaV1.2), Kcnq2 (the gene for KV7.2) and Lrrc55 gene (the gene for BKγ3 subunits) were higher in 3-week pre-pubertal mice than in post-pubertal mice, while Hcn1 expression was lower in 3-week pre-pubertal mice. The implications of these changes for neuronal excitability are discussed. In the AVPV, there were very few neurons labelled by tdTomato in 3-week female mice. In contrast to Kiss1ARC neurons, pre-pubertal 4-week Kiss1AVPV neurons had regular action potentials with more spikes while post-pubertal Kiss1AVPV neurons tended to have bursts of action potentials with fewer spikes. In conclusion, this thesis provides evidence of neuronal plasticity in Kiss1 neurons during puberty. In the ARC, female Kiss1 neurons show an increase in the number of action potentials, with a sustained but lower frequency of firing, at puberty. This plasticity may enable post-pubertal Kiss1ARC neurons to fire action potentials in a continuous and regular pattern, enabling modulation of firing and thus facilitating the generation of GnRH pulses.
electrophysiology, Kisspeptin, neuronal plasticity, patch clamp, Puberty
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