MATURATIONAL CHANGES IN GONADOTROPIN-SECRETION - THE LH RESPONSE TO REALIMENTATION AND A NOCTURNAL INCREMENT IN LH-SECRETION OF FEED-RESTRICTED PREPUBERTAL GILTS

Potential maturational changes in patterns of LH secretion during feed restriction and the LH response to realimentation were examined. Ten sets of four growth-matched littermate gilts were fed ad libitum, to r each either 55, 65, 75 or 85 kg target body weights (one littermate pe r weight). Gilts were then maintenance fed for 8 days (days 1-8), and to appetite on day 9, and kept in a 9 h light (07:001-16:00 h): 15 h d ark photoperiod. Blood samples were obtained every 10 min from 07:00 t o 16:00 h, and from 16:00 to 24:00 h on day 8 to evaluate any nocturna l rise in LH, and from 07:00 to 16:00 h on day 9 to evaluate the LH re sponse to realimentation. Radioimmunoassays for LH, FSH, melatonin and oestradiol were performed on selected plasma samples. In all three sa mpling periods, LH and FSH secretion decreased as gilt weight increase d, in a quadratic manner. During feed restriction, daytime LH, but not FSH, secretion was lower than at nighttime, irrespective of weight. D aytime LH secretion increased in response to realimentation, irrespect ive of weight. Plasma oestradiol concentrations were increased by feed restriction and decreased by realimentation but showed little relatio nship to gilt weight. No consistent relationship was established betwe en plasma melatonin concentrations and ambient illumination. In conclu sion, LH secretion exhibits a diurnal rhythm during feed restriction i n the prepubertal gilt and, in a similar way to FSH, decreases as pube rty is approached. The plane of nutrition influences circulating oestr ogen concentrations. The increase of LH secretion following realimenta tion is not influenced by maturation over the prepubertal period.