| Abstract: | In the brown hare, fertile mating takes place from the beginning of December to September. Pituitary and ovarian response to a monthly i.v. injection of 5 micrograms LHRH was studied from September 1983 to October 1984 in 2 groups of 6 hares. The basal concentrations of LH remained undetectable until the end of January, rose from 0.23 +/- 0.14 ng/ml from February to a maximum of 1.44 +/- 0.57 ng/ml in July. LHRH injection was always followed by a release of LH. Between September and December, the LH value peaked 15 min after injection and returned to basal concentrations 2 h later. From January, this pattern altered and a second peak of LH appeared 2 h after injection. Peak levels 15 min after LHRH were around 10 ng/ml between September and December, increased from 47.0 +/- 8.0 ng/ml in January to 106 +/- 33 ng/ml in July and decreased in August (69.4 +/- 10.6 ng/ml). The values of the second peak rose from 11.0 +/- 2.2 ng/ml in January to 90.6 +/- 12.4 ng/ml between March and July and decreased in August (24.5 +/- 5.1 ng/ml). The LH surge induced by LHRH was always followed by a transient rise in progesterone. During the breeding season, this progesterone secretion increased considerably. Ovulation was possible between January and August and the number of ovulating females was maximum between March and July. The amount and duration of progesterone secretion during the resulting pseudopregnancies increased during the breeding season. |
