Pituitary responsiveness to diurnal and nocturnal GnRH pulses in melatonin-treated ewe lambs

It has been shown that oral administration of melatonin to Suffolk ewe lambs, from 10 weeks of age onwards, advances the onset of puberty compared with control lambs maintained under the same natural photoperiod. Luteinizing hormone (LH) pulse frequency at 20 and 26 weeks of age was unchanged by melatonin. However, LH pulse amplitudes greater than 1 ng mL(-1) were consistently observed in melatonin-treated lambs, suggesting either a high responsiveness of the pituitary gland to endogenous gonadotrophin-releasing hormone (GnRH) pulses, or a large amount of GnRH released by each pulse. The purpose of the present study was to assess the pituitary responsiveness to six diurnal and six nocturnal exogenous pulses of GnRH (10 ng kg(-1) bodyweight) in melatonin-treated ewe lambs (3 mg melatonin daily at 1600 hours, from 10 weeks of age; n = 5) and control lambs of the same age (n = 5), born in the spring and kept under natural photoperiod. Pulses of GnRH were given intravenously at 60-min intervals by means of an indwelling jugular catheter from 0900 to 1400 hours and from 2100 to 0200 hours to ewe lambs of 20 and 26 weeks of age. Blood samples were collected at 10-min intervals using a contralateral jugular vein catheter from 1 h before and up to 1 h after the last GnRH pulse. The difference (delta) between plasma LH concentrations at 0 min and the greatest concentration of LH after each GnRH pulse was calculated and compared in the same group. The total area under the GnRH response curve (AUC) was also calculated and compared within and between the groups. The AUC of melatonin-treated lambs (66.1 +/- 5.94 and 52.24 +/- 7.42 ng mL(-1)/6 h, diurnal and nocturnal respectively) was greater than that of control lambs (39.42 +/- 4.29 and 32.82 +/- 3.6 ng mL(-1)/6 h diurnal and nocturnal respectively; P<0.05) at 20 weeks of age. At 26 weeks of age, only the diurnal total AUC was greater in melatonin-treated lambs than in control lambs (60.17 +/- 7.98 and 29.8 +/- 5.02 ng mL(-1)/6 h respectively; P<0.05). Delta LH concentrations in response to the first diurnal pulse of GnRH were greater than those in response to the fifth diurnal GnRH pulse (P<0.05) in melatonin-treated lambs of 20 weeks of age. Also, the delta LH concentrations in response to the first three diurnal GnRH pulses were greater than to the last three nocturnal pulses of GnRH (P<0.05). Delta LH concentrations were greater in response to the second diurnal pulse of GnRH than to the last three diurnal GnRH pulses, and greater than the responses to the first and the last four nocturnal GnRH pulses (P<0.05), at 26 weeks of age in melatonin-treated lambs. The response to nocturnal pulses of GnRH was similar. In control lambs, the responses to diurnal and nocturnal GnRH pulses were similar at 20 and 26 weeks of age. These results suggest that melatonin enhances the pituitary responsiveness to GnRH pulses in ewe lambs.