Seasonal variations in testis size, testosterone and LH basal levels, and pituitary response to luteinizing hormone releasing hormone in the brown hare, Lepus europaeus

In the brown hare, fertile mating takes place from the beginning of December to September. Seasonal variations of testis size, plasma levels of LH and testosterone, and pituitary and testicular responses to a monthly i.v. injection of 5 μg LHRH were studied in adult hares. Testis recrudescence began in October, 2 months before the beginning of the breeding season. The gonads were fully developed between January and July, and regressed thereafter. The increase of testosterone basal levels paralleled testicular growth, despite undetectable levels of LH. These LH levels increased slightly from April to August, when testosterone secretion decreased. Luteinizing hormone releasing hormone (LHRH) injection was always followed by a release of LH; the LH value peaked 15 min after injection and returned to basal concentrations 2 h later. The pituitary response to LHRH was significantly higher in July (76.4 ± 11.2 ng/mL, n = 5) and August (60.1 ± 5.6 ng/mL, n = 5), at the end of the mating season. The LH surge induced by LHRH was always followed by a secretion of testosterone lasting approximately 2 h. This testosterone secretion was maximum in January (88.6 ± 23.9 ng/mL, n = 5) and February (81.3 ± 18.6 ng/mL, n = 5), when the testis is fully developed. This suggests an increase in the pituitary content of LH in July and August, when day length decreases and testosterone negative feedback is less effective. These high LH levels might be responsible for the testis recrudescence 2 months later.