LH secretion and ovulation following exposure of Arctic charr to different temperature and photoperiod regimes: Responsiveness of females to a gonadotropin-releasing hormone analogue and a dopamine antagonist

Abstract

The timing of ovulation and LH plasma levels were investigated in Arctic charr reared at 5°C and 10°C, exposed to the ambient photoperiod, or short or long-day photoperiod regimes during the prespawning period. The effectiveness of sGnRHa alone, or sGnRHa combined with a dopamine antagonist, in stimulating LH secretion and inducing ovulation was also investigated. With the natural photoperiod, ovulation occurred spontaneously at 5°C, but was inhibited at 10°C. A transition from 10 to 5°C soon resulted in suppression of the inhibition. At 5°C, the effectiveness of sGnRHa was similar to that of sGnRHa combined with pimozide in stimulating LH secretion and inducing ovulation. At 10°C, sGnRHa+pimozide was more effective that sGnRHa alone in stimulating LH secretion and inducing a high rate of ovulation, suggesting that dopamine-induced inhibition of LH secretion could occur naturally in Arctic charr at 10°C. Exposure of Arctic charr to a long day (LD) photoperiod regime in fall and winter did not completely inhibit ovulation, but markedly delayed it and prolonged the ensuing ovulation period. The LD photoperiod also reduced LH plasma levels in females while they were ovulating, but did not modify the responsiveness of the pituitary to GnRHa stimulation compared to a control group exposed to a short-day (SD) photoperiod. There was an interval of several weeks after a transition from LD to SD before LH secretion and ovulation were stimulated.