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The response of the pituitary-testicular axis to LH-RH infusion was investigated in rams from birth to sexual maturity. During early postnatal life ram lambs were responsive to LH-RH stimulation and heightened pituitary sensitivity was observed in rams aged 2-3 months. It is suggested that a change in sensitivity of the pituitary-testicular axis occurs at this time and perhaps represents the time of initiation of the pubertal process.

The relationship between the pituitary gland and testis in rams was studied from birth to sexual maturity. The concentrations of LH, FSH and testosterone increased between 5 and 7 weeks of age; the rise was not correlated with any specific cytological change in the testis. An augmented pituitary response to LH-RH was demonstrated as levels of gonadotrophin increased. It is unclear whether this change in sensitivity plays a role in initiation of the pubertal process because Sertoli cell maturation, the earliest detectable change in the seminiferous epithelium, occurs between 17 and 21 weeks of age. Spermatocytes were first seen in biopsies taken at 31-36 weeks and spermatogenesis was established fully by 45 weeks. This second phase of testicular development was characterized by increases in prolactin, testosterone and LH. Leydig cells previously difficult to identify became recognizable at the time of sexual maturation. In newborn rams, castration produced significant increases in LH and FSH levels within 2-3 weeks, but higher basal FSH levels (2- to 3-fold) were observed at 5 than at 3 weeks of age. Cryptorchidism did not elevate LH or FSH significantly during the first year of life. FSH rose after this period to levels 2- to 3-fold higher than in normal rams, while LH and testosterone values remained in the normal range in spite of diminished spermatogenic activity; spermatids were absent and testis size was approximately 60% of that recorded in a normal ram. These studies demonstrate a rise in gonadotrophin and testosterone secretion in rams during the first 5-7 weeks of life, followed by a quiescent period of 8-9 months before a secondary increase occurs coincident with the establishment of sexual maturity.

The levels of plasma LH and FSH were measured in serial blood samples taken at 15-min intervals for 6 h from ewes that had remained fertile after grazing oestrogenic pasture (clover-fertile ewes), from ewes that were permanently affected by clover disease (clover-infertile ewes) and from normal ewes. Two flocks of ewes from different locations were studied. In flock 1, tonic LH secretion (total area under the curve of LH concentration versus time, 1 area unit = 1 ng ml-1 x 1 h) was significantly (P < 0.05) greater in clover-infertile ewes (10.4 area units) during anoestrus than in ewes that had remained fertile after prolonged grazing of oestrogenic clover (5.4 area units). Tonic LH and FSH secretions during the bleeding season and FSH secretion during anoestrus were not significantly different. In flock 2, LH levels during the breeding season were significantly (P < 0.05) elevated in clover-infertile ewes (10.9 area units) compared to normal ewes (5.4 area units) that had never grazed oestrogenic clover. LH secretion in clover-infertile ewes (7.8 area units) was intermediate to that found in infertile and control ewes. Concentrations of FSH, progesterone and ovarian vein oestradiol-17 beta (E2) during the breeding season were similar in the three groups. In another experiment, the positive feedback release of LH following administration of E2 (12.5, 25 or 50 micrograms per ewe) was measured in anoestrous ewes of flock 2. Significantly (P < 0.01) more clover-infertile ewes demonstrated a positive feedback effect than control ewes when given 12.5 micrograms E2 but not when given higher doses. The elevation of LH secretion in permanently affected clover-infertile ewes is inconsistent with the hypothesis that the hypothalamo-pituitary axis of these ewes is less responsive to the negative feedback effect of oestrogen. Furthermore, the patency of the positive feedback loop is consistent with the ability to ovulate.

Neither routine experimental procedures, including vascular cannulation and venipuncture, nor starvation or disturbance caused significant fluctuations in luteinizing hormone (LH) concentrations in ovariectomized ewes. Samples obtained at 5-min intervals revealed episodic LH releases occurring every 10-35 min. Hourly sampling over 3 days revealed large LH variations but no distinct diurnal variation.

Plasma LH, FSH and testosterone concentrations were measured by radioimmunoassays in male crossbred Merino/Corriedale sheep from birth to 45 weeks of age. FSH levels were 11 and 22 ng/ml at birth, increased to peak levels (mean value of 47 ng/ml) at 5 weeks and fluctuated between 25 and 35 ng/ml for the next 40 weeks. Similarly, LH (less than 0-5 ng/ml) and testosterone (less than 38 ng/100 ml) levels were low at birth and were significantly elevated by 5 weeks of age. LH values varied betwen 0-9 and 3-0 ng/ml for the next 30 weeks and then a secondary rise occurred reaching levels of 2-4 ng/ml by the 41st week after birth. Concentrations of LH subsequently fell to levels observed in adult rams. Testosterone levels rose gradually between the 5th and the 25th week, and then increased rapidly to values of 270-517 ng/100 ml by the 41st week after birth, a time coincident with the peak LH levels. Histological examination of testicular biopsies demonstrated that Sertoli cell maturation occurred 17-21 weeks after birth and was followed by activation of spermatogenesis leading to the presence of spermatozoa in the seminiferous epithelium by 39-42 weeks of age.