Lack of a nocturnal rise in serum concentrations of melatonin as gilts attain puberty

Twenty prepubertal crossbred gilts (Yorkshire x Hampshire x Duroc) weighing 98.1 +/- 4.2 kg at 5 mo of age were placed in an environmentally controlled room having a temperature of 18 degrees C and light:dark cycle of 12 h:12 h. Light intensity measured 700 Ix at eye level to the gilts. Three mature ewes were penned adjacent to the gilts to serve as positive controls for the light-dark cycles. After a 30-d acclimation period, 10 gilts from the pool determined to be prepubertal (serum progesterone < 500 pg/mL) were fitted with surgically implanted jugular catheters. Blood samples were drawn at 1100 (4 h after onset of light), 1130, 1200, 2300 (4 h after onset of darkness), 2330, and 2400 for 4 d. On d 5 of sampling, gilts were transported in an open-bed truck for 15 min, returned to their original environment, and exposed to boars for 20 min. Boar exposure was repeated every day throughout the remainder of the experimental period. Blood samples were drawn from each gilt until 7 d after estrus or for 12 d in those gilts that did not exhibit estrus. Blood samples were drawn by venipuncture from the ewes during the entire experimental period. For each sampling day, within an individual gilt or ewe, means of serum concentrations of melatonin (MEL) for night (scotophase) and day (photophase) samples were calculated. After three replications were conducted, four classes of animals were obtained: ewes (n = 9); nonpubertal gilts (n = 10); and two classes of gilts that ultimately reached puberty (prepubertal [n = 16] and postpubertal [n 16]). Across all gilts, only 65 of 406 bleeding periods (16.0%) had a nocturnal (scotophase) rise in serum MEL. The proportion of gilts expressing a nocturnal rise in serum MEL did not differ as gilts approached puberty (P > .05). Incidence of nocturnal rises of MEL was similar (P > .05) in gilts that attained puberty and gilts that did not attain puberty. Nocturnal rises in MEL were observed in 86.2% of the bleeding periods of ewes housed in the same environment. These data indicate clearly that nocturnal rises in serum MEL are not necessary for a gilt to attain puberty.