This study investigated the effect of melatonin treatment of ewe lambs on LH pulsatility
in an attempt to examine the mechanism whereby melatonin advances the onset of puberty.
Six ewe lambs were given intravaginal melatonin implants at 12.8 weeks of age. Another
six lambs received empty implants. All lambs were serially blood sampled every 15
minutes for six hours on several occasions prior to the onset of puberty. One week
after implantation LH pulse frequency and mean LH levels were higher in treated lambs
than the control lambs (pulse frequency 0.13/h vs 0.03/h; mean LH levels 2.0±0.2 ng/ml
vs 1.3±0.1 ng/ml; p < 0.05). Melatonin treatment failed to alter pulse frequency after
the initial increase. Puberty was advanced by 3 weeks in the treated group. In the
second experiment six lambs received melatonin implants at 13 weeks of age and another
six lambs served as control. In this experiment blood samples were taken intensively
during the first few weeks after treatment. Results of this study show that mean plasma
LH levels and LH pulse frequency were again higher during the first week after implantation.
This transient increase in LH release may be part of the mechanism initiating the
eventual advancement of puberty although the significance of this increase is questionable.
In both experiments the LH response to estradiol injection was monitored at various
times after treatment, but no effects of melatonin were found, although the magnitude
of the response increased with age.
Melatonin - LH - Lambs - Puberty
