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Contribution of increasing and decreasing daylength to the photoperiodic control of LH secretion in the Ile-de-France ram

Summary. Sexually mature Ile-de-France rams were exposed to an 8-month light regimen in which the daily light increment and decrement were constant and equal to 7 min/day. Daylength therefore varied from 6 to 20 h. The animals were allotted to two groups of 12 rams each and submitted to the same light regimen but 4 months out of phase. Blood was collected every 40 min for 10 h, on 15 occasions at various intervals during the second light cycle. Plasma LH and on some occasions plasma testosterone concentrations were measured by radioimmunoassay. In both groups the number of LH pulses increased significantly as daylength increased from 11:40 to 20:00 h (P < 0·01) but because their amplitude was low the mean plasma LH increase, although significant, was moderate. As daylength started to decrease (from 20:00 to 18:30 h), the frequency of LH pulses further increased (P < 0·05) and the pulse amplitude doubled so that mean plasma LH values increased abruptly and remained high until the decreasing light photoperiod reached 11:40 h. Thereafter, the frequency and amplitude of LH pulses decreased and the mean plasma LH dropped to one-fourth of its maximal value. Mean plasma testosterone concentrations were low during most of the 8-month cycle but increased steeply when daylength decreased from 11:40 to 6:00 h. These results indicate that increasing daylength provides a slow stimulation of LH release by an increase in the number of LH pulses, while a long decreasing photoperiod (20:00 to 11:40 h) provides a further and strong stimulation of both LH pulse frequency and amplitude in rams submitted to this regimen. The abrupt increase of LH release before and after the shift from increasing to decreasing daylength indicates that this increase is largely dependent on the decreasing daylength per se and not just the result of a photostimulation initiated in increasing daylength. The opposite patterns of plasma LH and testosterone secretions when daylength decreased from 11:40 to 6:00 h also suggest a possible cause-to-effect relationship between the high testosterone and the low plasma LH values recorded at that time.

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