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A. M. Paterson, C. A. Maxwell and A. Foldes

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

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G. P. Pearce, A. M. Paterson and P. E. Hughes

Summary. Prepubertal gilts were fitted with jugular vein and carotid artery catheters at 148 days of age. At 160 days of age the 24 gilts were allocated to treatment in a 2 × 2 factorial design involving intra-carotid infusion of cortisol (10 mg in 40 ml saline) or saline alone with or without i.v. injection of 5 μg synthetic GnRH midway through the 1 h infusion. Plasma cortisol concentrations were elevated in gilts infused with cortisol (P < 0·05). The LH response to exogenous GnRH was reduced by cortisol infusion. Treated gilts released less LH (P < 0·001) and had a lower mean LH peak (P < 0·01) than did control gilts but the timing of the induced LH peak was not affected. In the absence of an exogenous GnRH challenge, cortisol infusion increased the endogenous secretion of LH (P < 0·01). These results suggest that acute elevations in plasma cortisol concentration may be involved in mediating changes in pituitary responsiveness and the secretion of LH in the peripubertal gilt.

Keywords: prepubertal gilts; elevated cortisol; LH secretion

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A. M. Paterson, G. B. Martin, A. Foldes, C. A. Maxwell and G. P. Pearce

Summary. Plasma melatonin concentrations were measured every 1–2 h over 24 h and plasma luteinizing hormone (LH) concentrations every 15 min over 12 h 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 12 h light:12 h 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 1–2 h 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 6 h before dark was 91·9 ± 5·26 pg/ml and 125·0 ± 6·66 pg/ml 1 h 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 6 h of dark than in the 6 h 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.

Keywords: melatonin; LH; gilts; photoperiod; pig