Chapter 5: Neuroanatomy and neuroendocrinology of the GnRH system of Cape mole-rats and the Natal mole-rats

This study mapped the distribution and morphology of the GnRH neuronal systems of Cape and Natal mole-rats and the GnRH-immunoreactivity of the median eminence was quantified. A comparison was made between the winter and summer seasons in both species and also between dominant and subordinate animals in the Natal mole-rat species. Although the Cape mole-rat is larger than the Natal mole-rat, it has a smaller number of GnRH neurons. No differences were found in the number or size of these neurons across season in either species or reproductive status in the Natal mole-rat. In both species, the size of GnRH-ir neurons was similar in the different seasons and no difference was detected according to reproductive status. The GnRH neurons and fibres are loosely distributed along the septo-preoptico-infundibular pathway in both species. Dense clusters of fibres are visible in the area of the organum vasculosum of the lamina terminalis and the median eminence. The species differed with regard to the incidence of GnRH-ir neurons along the rostral-caudal axis of the brain. In the Cape mole-rat, almost 90% of the GnRH-ir perikarya are present in the medial septum or preoptic area, and only 10% in the mediobasal hypothalamus. In the Natal mole-rat, a much larger proportion (40%) of the total GnRH-ir neurons is located in the mediobasal hypothalamus with the remainder in the medial septum or preoptic area. In the Cape mole-rat females there is no difference in GnRH-immunoreactivity in the median eminence in and out of the breeding season. In the Natal mole-rat, dominant animals of both sexes had significantly less GnRH-immunoreactivity in the median eminence than the Chapter 5 – Neuroanatomy and neuroendocrinology 80 subordinate animals. This suggests that GnRH is retained in the median eminence in these subordinate, behaviourally suppressed animals. Behaviourally, both of these species display regulated reproduction, breeding in the Cape mole-rat is restricted to a specific part of the year, while subordinate individuals of the Natal mole-rats are reproductively suppressed by dominant animals. However, neuroendocrinologically, this is only reflected in subordinate animals of the Natal mole-rat.