Peripheral androgen levels in the male brush-tail possum (Trichosurus vulpecula)

Radioimmunoassays were established for the measurement of total androgens and the specific measurement of testosterone and 5 alpha-dihydrotestosterone in peripheral plasma of the brush-tail possum. Androgen concentrations were measured in blood collected from indwelling jugular cannulae to determine whether the normal pattern of androgen secretion in this species was episodic and to attempt to relate total androgen and the pattern of testosterone secretion to the changes previously reported in prostatic, but not epididymal, weight in the breeding season. Blood was collected from restrained animals at varying time-intervals during daylight hours and darkness. Despite an apparent good adaptation to the sampling procedure there was generally a progressive decline in plasma androgen level during the collection period. This was true for animals bled during or out of the breeding season. There was no significant seasonal effect on the androgen concentration in the initial blood sample. When less restraint was used, two of three animals showed fluctuations in androgen levels over the 7-h sampling period. Testosterone levels in blood obtained by cardiac puncture were four- to nine-fold higher than those of 5 alpha-dihydrotestosterone but levels of these androgens in samples obtained during the breeding season were not significantly different from those obtained out of season. The results do not argue for a pulsatile release of testosterone in the possum but do demonstrate a marked capacity for changes in the peripheral androgen concentration. There was a poor correlation between testosterone and 5 alpha-dihydrotestostosterone levels and prostatic weight.