Control of photoperiodic inhibition of luteinizing hormone secretion by dopaminergic and serotonergic systems in ovariectomized Ile-de-France ewes supplemented with oestradiol.

The role of dopaminergic and serotonergic systems on LH secretion was investigated in Ile-de-France ewes under different artificial inhibitory photoperiodic regimens. All animals were ovariectomized at the end of the breeding season, chronically treated with an oestradiol implant, and subjected to various changes in daylength for 9 months to inhibit or stimulate their LH secretion. Plasma LH concentration was assessed by taking blood samples twice a week throughout the experiment. The effects of acute intravenous injections of the dopaminergic2 receptor antagonist pimozide (0.08 mg kg-1) and the 5-hydroxytryptamine2 (5HT2) receptor antagonist cyproheptadine (3 mg kg-1) on LH pulsatility were assessed during challenges in four different situations: (1) long days (LD); (2) before short-day response (SD); (3) during refractoriness to short days (RSD); and (4) during inhibition by long days (ILD). LH in blood samples collected twice a week remained low during long days (0.59 +/- 0.03; mean +/- SEM), increased 45 +/- 1.5 days after the onset of short days and decreased 132 +/- 4.9 days later when ewes became refractory to short days, whereas ewes subjected to long days after 91 short days stopped their neuroendocrine activity 19 days earlier (113 +/- 4.7) (P < 0.01). In comparison with the pre-injection period, pimozide significantly increased the mean number of pulses in SD and RSD ewes, but not in LD and ILD ewes: SD: 0 versus 0.45 pulses in 4 h (P < 0.02); RSD: 0 versus 0.9 (P = 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)