Summary. In Exp. 1, four groups of 8 yearling Soay rams were housed under long days (16L:8D) to induce reproductive quiescence and were treated daily for 12 weeks with: (I) vehicle (2 or 4 ml 50% ethanol/water), (II) ML23 (2 mg), (III) melatonin (2 mg) and (IV) melatonin and ML23 (2 mg of each). All treatments were given orally in the mid-light phase. In the rams receiving melatonin (Group III) there was an earlier increase in the plasma concentrations of FSH and testosterone and regrowth of the testes compared to the controls (time to maximum testicular diameter: 10·0 ± 0·5 and 15·3 ± 1·2 weeks). These differences were reversed after the end of the 12-week treatments when rapid testicular regression occurred in melatonin-treated rams but not in the controls. In the group receiving ML23 and melatonin (Group IV), there was early reactivation and regression of the reproductive axis as in the melatonin group (testis max. 9·9 ± 0·7 and 10·0 ± 0·5 weeks) while in the group receiving ML23 alone (Group II) there was a slower redevelopment and regression as in the controls (testis max. 15·7 ± 1·1 and 15·3 ± 1·2 weeks). The comparison between the 4 groups in the changes in the blood concentrations of prolactin, voluntary food intake and total body weight also indicated that the treatment with ML23 failed to modify the effect of melatonin (combined treatment vs melatonin) or the effect of the long day photoperiod (ML23 vs vehicle).
In Exp. 2, a more soluble derivative of ML23 (called ML23-lysine) was tested in two groups of 4 adult Soay rams which were housed under long days (16L:8D) and treated daily for 12 weeks, as above with: (I) melatonin (2 mg) and (II) melatonin (2 mg) and ML23-lysine (4 mg). In both groups there was a similar reactivation and regression of the testes with no significant differences between the two treatments.
The overall results illustrate that the treatments with melatonin in this protocol were effective at influencing a wide range of reproductive and non-reproductive measures in the ram, but that ML23 was unable to antagonize these biological effects of melatonin.
Keywords: ram; seasonal reproductive cycles; photoperiod; pineal gland; melatonin receptors