Abstract:
The Damaraland mole-rat is a subterranean mammal exhibiting extreme reproductive skew
with a single reproductive female in each colony responsible for procreation. Non-reproductive
female colony members are physiologically suppressed while in the colony exhibiting reduced
concentrations of plasma luteinizing hormone (LH) and a decreased response of the pituitary,
as measured by the release of bioactive LH, to an exogenous dose of gonadotrophin releasing
hormone (GnRH). Removal of the reproductive female from the colony results in an elevation
of LH and an enhanced response of the pituitary to a GnRH challenge in non-reproductive
females comparable to reproductive females, implying control of reproduction in these
individuals by the reproductive female. The Damaraland mole-rat is an ideal model for
investigating the physiological and behavioural mechanisms that regulate the hypothalamopituitary-
gonadal axis. In contrast, we know less about the control of reproduction at the level
of the hypothalamus. The immunohistochemistry of the GnRH system of both reproductive
and non-reproductive female Damaraland mole-rats has revealed no significant differences with
respect to morphology, distribution or numbers of immunoreactive GnRH perikarya. We
examined whether the endogenous opioid peptide beta-endorphin was responsible for the
inhibition of the release of the GnRH from the neurons indirectly by measuring LH
concentrations in these non-reproductive females following single, hourly and eight hourly
injections of the opioid antagonist naloxone. The results imply that the endogenous opioid
peptide, beta-endorphin, is not responsible for the inhibition of GnRH release from the
perikarya in non-reproductive females. Preliminary data examining the circulating levels of
cortisol also do not support a role for circulating glucocorticoids. The possible role of
kisspeptin is discussed.
