EFFECT OF DURATION OF MELATONIN TREATMENT ON THE ONSET AND DURATION OF ESTROUS CYCLICITY IN EWES

Forty-two Scottish Blackface ewes that lambed outdoors in March were removed from their lambs at the end of April and housed under natural daylength at 57-degrees-N, Treatments (n = 7 ewes per treatment) commenced on 1 May and comprised daily oral dosing at 15:00 h with 3 mg melatonin dissolved in water and ethanol (4:1, v/v) for 30, 60, 90, 120 or 150 days. Control ewes received the vehicle alone. Ovarian activity was assessed by laparoscopy at monthly intervals with an additional interim observation in mid-July. Blood was sampled three times a week by jugular venepuncture and assayed for progesterone, prolactin and follicle-stimulating hormone (FSH). Luteinizing hormone (LH) was determined in blood samples collected at 15 min intervals for 10 h on days 28, 60, 91, 119 and 150.

Thirty days of melatonin treatment delayed (P < 0.01) first ovulation by about 1 month (mean interval +/- SEM from 1 May to progesterone > 1 ng ml-1, 165 +/- 4.5 days versus 132 +/- 9.2 days for controls). None of the ewes that received melatonin for 60 days ovulated before the end of melatonin treatment, but subsequently six of them did; the mean interval from 1 May to increased progesterone concentration was 75 +/- 1.2 days. All ewes receiving melatonin for 90, 120 and 150 days ovulated with corresponding mean intervals of 83 +/- 2.7, 85 +/- 1.3 and 87 +/- 2.2 days, respectively (P < 0.001 compared with controls). Ewes treated for 60, 90 and 120 days returned to anoestrus 4-6 weeks after the end of melatonin treatment; for the 120 day treatment this coincided with the onset of oestrous cyclicity in control ewes.

Melatonin had no effect on the concentrations of FSH. Thirty days of melatonin treatment in May delayed the autumn decline in LH pulse amplitude and increase in pulse frequency, whereas treatment for 120 and 150 days advanced these changes to late July-early August. Prolactin concentrations declined to minimum values after 30 days of melatonin treatment, but began to increase again after about 100 days. After the treatments for 90, 120 and 150 days, prolactin increased to higher concentrations (P < 0.001) than in controls and failed to return to control values before sampling ended in late October, i.e. 95, 65 and 35 days later, respectively.