Photostimulation changes the pattern of luteinizing hormone secretion in turkey hens.

Changes in the secretory pattern of luteinizing hormone (LH) were determined in turkey hens during initiation of reproduction. Photosensitive hens were switched from a short day photoschedule of 6L:18D (light:dark) to 14L:10D and serially bled six times during the next 6 weeks of photostimulation and early after ovulatory cycles had begun. The pattern of LH secretion at 0-1 day of photostimulation was characterized by a low baseline concentration of LH with numerous low frequency, high amplitude pulses. This pattern changed at Days 4-5 and 11-12 of photostimulation (hens photostimulated but not yet laying) to a pattern characterized by a high baseline with few ill defined pulses of very short duration. Once the hens had initiated egg production, ovulatory surges of LH (duration of 358 +/- 104 min) were superimposed on the basic pattern seen during photostimulation. The ascending limb of the ovulatory surge was much shorter in duration than the descending limb. No diurnal variation was detected in baseline concentration before or during photostimulation or after egg production had begun. It was concluded that photosensitive turkey hens secrete LH in low frequency; high amplitude pulses superimposed on a low baseline. During photostimulation, this pattern changes to one characterized by a high baseline with few pulses of low amplitude and short duration. This pattern of secretion is stimulatory to the ovary and continues until egg laying begins when it is modified by ovulatory surges of LH, characterized by short duration ascending limbs and longer duration descending limbs, which are superimposed on the high baseline pattern noted during photostimulation.