CONTROL OF PHOTOPERIODIC INHIBITION OF LUTEINIZING-HORMONE SECRETION BY DOPAMINERGIC AND SEROTONERGIC SYSTEMS IN OVARIECTOMIZED ILE-DE-FRANCE EWES SUPPLEMENTED WITH ESTRADIOL

The role of dopaminergic and serotonergic systems on LH secretion was investigated in Ile-de-France ewes under different artificial inhibito ry 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 inhibi t or stimulate their LH secretion. Plasma LH concentration was assesse d 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 day s (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 ons et of short days and decreased 132 +/- 4.9 days later when ewes became refractory to short days, whereas ewes subjected to long days after 9 1 short days stopped their neuroendocrine activity 19 days earlier (11 3 +/- 4.7) (P < 0.01). In comparison with the pre-injection period, pi mozide 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). Cyproheptadine significantly in creased the mean number of pulses in SD and RSD ewes: SD: 0 versus 1 ( P < 0.008); RSD: 0 versus 1.5 (P = 0.03). An effect of cyproheptadine was shown in LD ewes (0 versus 0.5 (P < 0.03)), but it was less marked than in the same ewes under short-day photoperiod (SD ewes; P < 0.05) . In ILD ewes, a small increase was observed (0 versus 0.33 (P = 0.05) ) but it was less than in RSD ewes (P < 0.03). These results support t he hypothesis of an inhibitory role of dopaminergic and serotonergic s ystems on LH pulsatile release and suggest that refractoriness to shor t days is due to activation of these two systems.