Abstract
Preterm birth, a consequence of early activation of the uterus, has a negative impact on the health of the offspring and the mother. The stimulus for uterine contractions is through the action of the posterior pituitary hormone, oxytocin, also responsible for the milk let-down during suckling. Little is known about how oxytocin neurons are regulated by the brain during pregnancy to prepare for birth and lactation. Evidence points the role of kisspeptin (peptides that is a ligand of the G-protein coupled receptor 54 (GPR54) essential for the onset of puberty and fertility) in affecting the rate of firing of action potentials by the oxytocin neurons. This thesis aimed to use mouse as the animal model to test the hypothesis that the increased response of oxytocin neurons to kisspeptin is a critical element of the physiological processes that underpin the increased activity of oxytocin neurons required for normal birth and lactation. Prolactin is another pregnancy hormone that prepares women for birth and milk production, but how does prolactin influence the stimulation of oxytocin cells was of interest. Levels of prolactin (or the closely related placental lactogen) are elevated during pregnancy and lactation, and both oxytocin neurons and kisspeptin neurons express prolactin receptors. Thus, to test the role of prolactin in increasing kisspeptin fiber expression around the oxytocin neurons in late pregnancy, virgin mice were chronically administered with high doses of prolactin to emulate the hyperprolactinemia seen in pregnancy and lactation. However, no conclusive results could be drawn from this study. Kisspeptin fibers were observed to closely appose onto the oxytocin neurons in both the paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the mouse hypothalamus. Through immunohistochemical labeling for neurokinin B (NKB, a marker for kisspeptin neurons originating from the arcuate nucleus), the origin of the kisspeptin fiber innervation of oxytocin neurons was determined to be from the anteroventral periventricular nucleus/periventricular nucleus (AVPV/PeN). The AVPV/PeN kisspeptin neurons innervating the oxytocin neurons in the PVN and SON of the mouse hypothalamus were hypothesized to be gamma-aminobutyric acid (GABA) in nature in late pregnancy. To test this hypothesis, the vesicular GABA transporter (VGAT) Cre-mice (non-pregnant and pregnant) with a tdTomato reporter gene (VGAT is used as a marker for GABA cells) were employed. A remarkable finding from this chapter was the reduction in the number of AVPV/PeN kisspeptin neurons in late-pregnant mice compared to non-pregnant mice, and 80% of the kisspeptin neurons in late-pregnant mice were GABAergic, as evidenced by the co-localization of VGAT-positive cells on kisspeptin-positive neurons. Thus, the AVPV/PeN kisspeptin neurons that projects and closely appose onto the oxytocin neurons in the PVN and SON, that are GABAergic in nature in late pregnancy was hypothesized to be essential in regulating the onset and timing of parturition and maternal adaptations in mice. By employing viral-based cell ablation technique, it was demonstrated that the AVPV/PeN kisspeptin neurons are not necessary to determine the onset or timing of birth of pups, pup survival, or maternal behaviors in dams during pregnancy and lactation. Nevertheless, this study was crucial in confirming the necessity of the AVPV/PeN kisspeptin neurons in regulating fertility, as evidenced by the lack of the ability of female mice to get pregnant (> 60% failed to get pregnant) despite the successful event of copulation. Hence, while the initial data from this thesis do not show any conclusive results, there is now evidence for a significant decline in the AVPV/PeN kisspeptin neuronal number in late pregnant mice, and these kisspeptin neurons may not be after all, necessary for regulating birth or lactation in mice, as hypothesized.