Control of photoperiodic inhibition of luteinizing hormone secretion by dopaminergic and serotonergic systems in ovariectomized Ile-de-France ewes supplemented with oestradiol
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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). Cyproheptadine significantly increased 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 the hypothesis of an inhibitory role of dopaminergic and serotonergic systems on LH pulsatile release and suggest that refractoriness to short days is due to activation of these two systems.
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