LH RESPONSES TO SINGLE DOSES OF EXOGENOUS GNRH BY FRESHLY CAPTURED DAMARALAND MOLE-RATS, CRYPTOMYS-DAMARENSIS

Pituitary function in reproductive and nonreproductive colony members of Damaraland mole-rats, Cryptomys damarensis, was investigated by mea suring the LH responses to single doses of 2 mug exogenous GnRH and ph ysiological saline in 29 females and 37 males (31 of these animals fro m two entire colonies). In females, basal LH concentrations were signi ficantly greater in reproductive (n = 9) than in nonreproductive anima ls (n = 11): 7.6 +/- 1.0 versus 4.3 +/- 0.6 miu ml-1, respectively (P < 0.001). Reproductive females had a significantly greater LH response to 2.0 mug GnRH (7.6 +/- 1.0 to 37.7 +/- 6.2 miu ml-1; n = 9) than di d nonreproductive females (4.3 +/- 0.6 to 11.8 +/- 1.0 miu ml-1; n = 1 1, P < 0.001). In contrast, there was no significant difference in bas al LH concentrations between reproductive (n = 8) and nonreproductive males (n = 20): 5.3 +/- 4.3 versus 3.2 +/- 1.2 miu ml-1, respectively. There was also no difference in LH response to the administration of 2.0 mug GnRH between reproductive and nonreproductive males (5.3 +/- 4 .3 to 21.8 +/- 8.6 miu ml-1; n = 8; versus 3.2 +/- 1.2 to 21.1 +/- 8.5 miu ml-1; n = 21; P = 0.5). When the results from the two entire colo nies were analysed separately, LH responses to GnRH in the 11 nonrepro ductive females were less than in the two reproductive females. In con trast, the response of two reproductive males in the colonies did not differ from that of 16 nonreproductive males, although these latter co mparisons could not be validated statistically. The following conclusi ons were made. First, the anterior pituitary of nonreproductive female mole-rats is less sensitive to GnRH than is that of reproductive fema les, possibly resulting from a lack of priming by endogenous GnRH. Sec ond, the hypothalamic-pituitary axis is more active in reproductive fe males, giving rise to higher concentrations of LH in plasma. Thus supp ression in nonreproductive females is due to inhibition of GnRH render ing them temporarily infertile. Third, the lack of differentiation in response to GnRH in males suggests that nonreproductive males are beha viourally inhibited from breeding.