The effects of a plant compound, 6-MBOA, on reproduction in the wild rabbit Oryctolagus cuniculus (L.)
This study tested the hypothesis that a plant compound, 6-methoxybenzoxazoline (6-MBOA), can affect the annual reproductive cycle of wild rabits by acting as an agonist or antagonist of melatonin. Preliminary investigations into seasonal changes in the reproductive endocrinology of wild-caught adult female rabbits kept in outdoor cages revealed an annual cycle of pituitary (release of LH) and ovarian (release of progesterone) responses to exogenous GnRH. The largest responses were between March and May (during the peak breeding season in the wild), followed by a decrease in July. Pituitary responsiveness returned by late November, whereas ovarian responsiveness did not increase until after the winter solstice. Studies of the responses to GnRH of adult female rabbits kept indoors under regulated photoperiod demonstrated that the decrease in pituitary and ovarian responsiveness in July was due to declining daylength. Seasonal regrowth of the reproductive system at the winter solstice was stimulated by transfer to `long days' (16L:8D). This stimulation by long days was not observed if rabbits were also treated with daily, timed intra-muscular (i.m.) injections of melatonin, but injections of 6-MBOA were ineffective. Daily, timed injections of 6-MBOA did not stimulate the reproductive system of seasonally-anoestrus adult female rabbits in the autumn. Treatment of adult female rabbits with sub-cutaneous (s.c.) implants of 6-MBOA failed to prevent a decrease in pituitary and ovarian responsiveness to GnRH on transfer to `short days' (8L:16D). Sexually mature male rabbits under long days received daily, timed i.m. injections of 6-MBOA or melatonin. Only those treated with melatonin showed testicular regression. A dose-response relationship for 6-MBOA was tested by treating adult male rabbits with 6-MBOA in different size s.c. implants. All sizes tested failed to induce testicular regression. No evidence was found that 6-MBOA can affect the annual reproductive cycle in wild rabbits by acting as an agonist or antagonist of melatonin.