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 photope-riod. 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.
Extra key words: luteinizing hormone, sheep.
Plasma melatonin profiles of Romney Marsh sheep in natural photoperiod and in acutely extended darkness