Concentrations of plasma melatonin and luteinizing hormone in domestic gilts reared under artificial long or short days

Plasma melatonin concentrations were measured every 1-2h over 24h and plasma luteinizing hormone (LH) concentrations every 15 min over 12h in domestic gilts reared under artificial light regimens that had previously been used to demonstrate photoperiodic effects on puberty. In Expt 1, the light regimens both commenced at 12h light:12h dark (12L:12D) and either increased (long-day) or decreased (short-day) by 15 min/week until the long-day gilts were receiving 16L:8D and the short-day gilts 8L:16D at sampling. In Expt 2, both light regimens commenced at 12L:12D and either increased (long-day) or decreased (short-day) by 10 or 15 min/week to a maximum of 14.5L:9.5D or a minimum of 9.5L:14.5D before being reversed. Sampling took place when daylength had returned to 14L:10D (long-day) or 10L:14D (short-day). In immature gilts housed at 12L:12D (Expt 1) and in postpubertal (Expt 1) and prepubertal (Expt 2) gilts reared under long-day or short-day light regimens, mean plasma melatonin concentrations were basal (3.6 pg/ml) when the lights were on and increased to peak concentrations >15 pg/ml within 1-2h after dark, before declining gradually to basal concentrations at or near the end of the dark phase. In prepubertal gilts bearing subcutaneous melatonin implants and reared under long-days (Expt 2), mean plasma melatonin concentration in the 6h before dark was 91.9 ± 5.26 pg/ml and 125.0 ± 6.66 pg/ml 1h after dark, but this increase was not statistically significant. In Expt 2, the short-day gilts had fewer LH pulses (2.6 ± 0.25 vs. 4.6 ± 0.24; P < 0.01) in the 12-h sampling period than the long-day gilts, but the amplitude of the pulses (2.28 ± 0.23 vs. 1.26 ± 0.16 ng/ml; P < 0.01) and the area under the LH curve (78.8 ± 5.60 vs. 47.3 ± 6.16: P < 0.01) was greater in the short-day gilts. In the short-day, but not in the long-day gilts, LH pulses were more frequent (2.0 ± 0.0 vs. 0.6 ± 0.25; P < 0.01), but had a smaller area (61.9 ± 7.2 vs. 120.2 ± 23.6; P < 0.05) in the 6h of dark than in the 6h of light, which together made up the 12-h sampling period. These data show that, in pigs, as in other species, the concentration of melatonin in plasma increases in the dark and the duration of the nocturnal increase depends on photoperiod. The implants provided high and variable concentrations of plasma melatonin, above which a nocturnal increase was not observed. The patterns of LH secretion were consistent with the short-day gilts being closer to puberty than the long-day gilts as a consequence of differing rates of sexual maturation due to the light regimens imposed during rearing.