Regulation of LH in the female common brushtail possum (Trichosurus vulpecula)

Abstract

The seasonal control of reproduction, specifically the hypothalamic-pituitary-ovarian axis, has not been fully investigated in the common brushtail possum (Trichosurus vulpecula). This research aimed to characterise the control of pulsatile LH release and the seasonal change of LH in response to oestradiol in the possum through the development of two main hypotheses. The first hypothesis proposed that LH is secreted in pulses from the anterior pituitary gland in response to discrete pulses of GnRH released from the hypothalamus, both of which are modulated by the negative feedback effect of oestradiol. The second hypothesis proposed that the annual rhythm of reproduction in the possum is controlled via the hypothalamic-pituitary axis by an interaction between photoperiod and oestradiol.

To achieve the study aims, preliminary work aimed to identify an anaesthetic that would allow blood collection from possums without compromising the hormones being measured. Ovariectomised female possums were anaesthetised with halothane, a ketamine xylazine combination or left conscious, and were challenged with GnRH. Ketamine-xylazine was found to be the most appropriate anaesthetic as it had little effect on serum LH concentrations before and during the GnRH challenge.

The first hypothesis addressed the regulation of pulsatile LH in the female possum. To test whether the regulation of LH release occurs at the level of the hypothalamus or the pituitary gland, ovariectomised possums were anaesthetised with halothane or left untreated. Blood samples drawn at 10 minute intervals for 4 h showed that LH was released in distinct pulses (ranging between 0.75/h and 0.92/h). Halothane suppressed LH pulse frequency and amplitude. It is suggested that LH secretion is controlled at the level of the brain, as the pituitary gland was still responsive to exogenous GnRH during anaesthesia. To establish the site(s) of action of oestradiol within the hypothalamic-pituitary axis, ovariectomised possums were treated with an oestradiol-17β implant (0.001 mg released over 90 days) or a sham procedure. Blood samples drawn at 10 minute intervals for 4 h, showed that LH pulsing was either eliminated or reduced by exogenous oestradiol, suggesting that this steroid acted at the level of the brain. In addition, the response of LH to exogenous GnRH was suppressed in the presence of oestradiol, indicating an action at the level of the pituitary gland.

The second hypothesis investigated if oestradiol could exert an inhibitory effect on the pituitary gland when administered at pharmacological concentrations. Ovariectomised possums were injected with three doses of oestradiol benzoate and a placebo over four intervals during both the breeding and non-breeding season. During the breeding season, plasma LH concentration was significantly reduced following an oestradiol challenge. This reduction occurred across all doses, except the placebo. During the non-breeding season there was no discernable reduction in plasma LH concentration in response to oestradiol. However, failure to observe a decline in LH can be attributed to the already suppressed basal serum LH concentrations during the non-breeding season in the ovariectomised possum.

To determine if oestradiol was required to elicit a seasonal change in tonic LH secretion, ovariectomised and ovariectomised oestradiol-treated possums were monitored for 14 months in outdoor enclosures. The breeding season was characterised by minimal suppression of LH and the non-breeding season by maximal suppression of LH. These changes occurred in the absence of ovarian steroids, indicating the possible presence of an inhibitory neuroendocrine system that suppress GnRH and hence LH in the possum. The mechanism of seasonal reproductive control in the possum as revealed by this study differs from the steroid-dependent mechanism reported in sheep.

Finally, was LH secretion controlled by a circannual rhythm entrained by photoperiod in the possum? Ovariectomised oestradiol-treated animals were monitored for 19 months under artificial and short day phofoperiod regimes. As this study started before the previous experiment, the consequences of oestradiol replacement in the possum were not known. LH levels were suppressed for the duration of the study with no detectable difference between photoperiod treatments. However, this experiment did reveal that a low does of oestradiol (0.001 mg released over 90 days), caused a significant increase in the size and weight of the urogenital tract.

In conclusion, the regulation of LH secretion in the possum may occur at the level of the hypothalamus. LH is secreted in pulses from the pituitary gland. Oestradiol exerts a negative feedback effect at the level of the brain, and to a lesser extent on the pituitary gland during the breeding season. Seasonal changes in LH secretion in the female possum appear to be regulated by a steroid independent mechanism within the brain.