LH responses to single doses of exogenous GnRH by freshly captured Damaraland mole-rats, Cryptomys damarensis

Abstract

Pituitary function in reproductive and nonreproductive colony members of Damaraland mole-rats, Cryptomys damarensis, was investigated by measuring the LH responses to single doses of 2 μg exogenous GnRH and physiological saline in 29 females and 37 males (31 of these animals from two entire colonies). In females, basal LH concentrations were significantly greater in reproductive (n = 9) than in nonreproductive animals (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 μg GnRH (7.6 ± 1.0 to 37.7 ± 6.2 miu ml-1; n = 9) than did nonreproductive females (4.3 ± 0.6 to 11.8 ± 1.0 miu ml -1; n = 11, P < 0.001). In contrast, there was no significant difference in basal 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 μg 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 ± 8.5miu ml-1; n = 21; P = 0.5). When the results from the two entire colonies were analysed separately, LH responses to GnRH in the 11 nonreproductive females were less than in the two reproductive females. In contrast, the response of two reproductive males in the colonies did not differ from that of 16 nonreproductive males, although these latter comparisons could not be validated statistically. The following conclusions were made. First, the anterior pituitary of nonreproductive female mole-rats is less sensitive to GnRH than is that of reproductive females, possibly resulting from a lack of priming by endogenous GnRH. Second, the hypothalamic-pituitary axis is more active in reproductive females, giving rise to higher concentrations of LH in plasma. Thus suppression in nonreproductive females is due to inhibition of GnRH rendering them temporarily infertile. Third, the lack of differentiation in response to GnRH in males suggests that nonreproductive males are behaviourally inhibited from breeding.