Summary. The ability of exogenous melatonin, applied either orally or by implant, to overcome the seasonal inhibition of puberty in domestic gilts was tested in two experiments. In Expt 1, 24 gilts received two melatonin implants at 126 days of age and again at 161 days and 196 days, while 24 gilts acted as controls. All gilts were slaughtered at a mean age of 223 days. Blood samples were collected by venepuncture from eight gilts in each treatment at 126, 144 and 178 days of age and the plasma was assayed for melatonin concentration by direct radioimmunoassay. In Expt 2A, four gilts (125 days of age) were fed either 0, 1, 2 or 4 mg of melatonin at 14:00 h on each of four consecutive days. Blood samples for melatonin assay were collected via indwelling jugular catheters every 30 or 60 min from 12:00 to 22:00 h. In Expt 2B, 27 gilts were fed 1 mg of melatonin at 15:00 h each day from 129 days of age until slaughter at 221 days, while 25 gilts acted as controls. In both experiments, the presence of morphologically normal corpora lutea at slaughter was the criterion for puberty.
In Expt 1, constant-release melatonin implants had no effect on the percentage of gilts which reached puberty. Among the 24 control gilts, two (8·3%) reached puberty compared with one of the 24 (4·2%) gilts with implants. In all the samples from control gilts, and in the samples taken from treated gilts prior to implantation at 126 days of age, mean plasma melatonin concentration was below the sensitivity of the assay (3·6 pg/ml). In the samples taken from treated gilts 18 days after the first implants, mean plasma melatonin concentration had increased to 34·1 pg/ml (P < 0·001). In the samples taken 17 days after the insertion of the second pair of implants, mean plasma melatonin concentration was higher than it had been at 144 days (66·6 vs. 34·1 pg/ml, s.e.m. = 2·03, P < 0·001).
In Expt 2A, mean plasma melatonin concentration in the gilts receiving 0 mg of melatonin was below the sensitivity of the assay throughout the period when the lights were on (12:00–18:00 h). After the lights were turned off, mean plasma melatonin concentration increased (P < 0·05) within 1 h, and was still high at 22:00 h. In gilts fed pellets containing melatonin, mean plasma melatonin concentration was below the sensitivity of the assay before feeding, but it increased (P < 0·001) within 30 min of feeding, and was still high at 22:00 h. There was no significant difference in the mean plasma melatonin concentration in the post-feeding period among the gilts fed 1, 2 or 4 mg of melatonin. In Expt 2B, daily feeding of 1 mg of melatonin increased (P < 0·05) the proportion of gilts which reached puberty. Among the 27 gilts which were fed melatonin, 15 (55·6%) reached puberty compared with six of the 25 (24%) control gilts.
These results show that, in contrast to constant-release melatonin implants, oral melatonin given each afternoon overcomes the seasonal inhibition of attainment of puberty in domestic gilts. These data provide further evidence that photoperiod is the environmental factor mediating seasonality in this species and support the conclusion that the diurnal rhythm of plasma melatonin, rather than the absolute concentration, is the important characteristic of melatonin secretion which pigs use to transduce photoperiodic information.
Keywords: melatonin; puberty; pig; season; gilt