Summary. Male voles were reared from birth to the age of 56 days in photoperiods of 16L:8D or 6L:18D. In 16L:8D testes increased 10-fold in size between the age of 14 and 56 days, and there were concomitant increases in diameter of seminiferous tubules and seminal vesicle weight. Spermatozoa were present in tubules by 35 days. In 6L:18D no significant changes with age occurred in testis size, diameter of seminiferous tubules or seminal vesicle weight. LH secretion increased with age in 16L:8D, but not in 6L:18D, and pituitary and plasma levels were higher in the former than in the latter by 35 days. Pituitary levels of FSH were high up to 21 days in 16L:8D, and up to 35 days in 6L:18D. At 35 days the mean pituitary level was significantly higher in the 6L:18D than in 16L:8D. Plasma levels of FSH were significantly higher in 16L:8D at 14 and 21 days, and then gradually declined. In 6L:18D plasma FSH rose to a maximum at 35 days, but did not reach the levels occurring at an earlier age in voles in 16L:8D. The relationship between pituitary and plasma levels of FSH in the two photoperiods is compatible with the idea of a storage and a readily releasable pool of the hormone within the pituitary gland. In 16L:8D changing levels of LH and FSH seem to be due to the onset of testicular activity with consequent feedback effects of testicular hormones upon the anterior pituitary gland.
R. P. Craven and J. R. Clarke
R. P. Craven and J. R. Clarke
Summary. Male voles were reared from birth to age 28 days in 6L:18D. Pairs of animals showing similar sexual development were assigned at random to 16L:8D or 6L:18D. Treatments continued for a further 56 days. Increase in the activity of the hypothalamo—hypophysial system occurred within 4 days of exposure to 16L:8D, as shown by significant elevation of plasma LH and FSH. Pituitary LH did not increase until Day 7, and pituitary FSH did not increase until Day 21. After exposure to 16L:8D for 4 days, pituitary FSH was lower than in corresponding animals in 6L:18D. These discrepancies between pituitary and plasma values of gonadotrophins indicate that increase in hormone release occurs before synthesis is fully stimulated. Enhanced output of testicular hormones probably began between Day 7 and Day 14, as indicated by an increase in seminal vesicle weight, yet plasma and pituitary concentrations of LH and FSH remained elevated. This suggests that long photoperiods may cause direct stimulation of the hypothalamo—hypophysial system which increasing values of testicular hormones are initially unable to inhibit. The response of this system in voles to an abrupt change from a non-stimulating to a stimulating photoperiod has a time course resembling that for the Soay ram but appreciably slower than for the Japanese quail.
R. F. T. Kinch, R. P. Craven, and B. K. Follett
Summary. Male voles were raised from birth to 100 days of age in photoperiods of 16L:8D or 6L:18D. In the long photoperiod testes increased in size between 15 and 80 days of age, and there was an increase in seminal vesicle weight from 60 days of age. Spermatozoa were present in the testes at 60 days of age. In the short photoperiod testicular growth did not begin until 50 days of age with the seminal vesicles beginning to increase at 80 days of age. Spermatozoa were present in the testes at 100 days of age. Pituitary secretion in vitro of LH and FSH in response to 1 pmol GnRH, as well as hypothalamic GnRH content, rose to peaks at 50 and 80 days of age respectively in animals exposed to long photoperiods. There was no change in pituitary secretion of FSH in response to GnRH stimulation in animals from the short photoperiod. However, pituitary release of LH in response to 1 pmol GnRH rose to a peak at 80 days of age. Hypothalamic GnRH content rose to a peak at 50 days of age and then declined. The relationship between the hypothalamic GnRH and the sensitivity of the pituitary to GnRH stimulation is compatible with the idea that GnRH can mediate its own receptor numbers.