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

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 less than 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)