Daily fluctuations of hypothalamic LH-RH like activity and pituitary gonadotropic potency were measured from White Leghorn hens at 2 hr. intervals for a single 24 hr. period. Hens were classified into three separate reproductive stages, on the basis of average daily egg production of: (1) above 50%, (2) below 50% and (3) 0%. Pituitary gonadotropic potency remained at a relatively stable level throughout the 24 hr. period for higher producing birds and became progressively more erratic as reproductive stage decreased. Mean pituitary gonadotropin was inversely related to the reproductive stage, with the above 50% stage having the lowest mean 24 hr. level. Hypothalamic LH-RH like activity was highest in the light phase and lowest during the dark phase of the 24 hr. period. The most apparent difference in hypothalamic LH-RH like activity between reproductive stages was the amount of fluctuation between consecutive 2 hr. samples. As reproductive activity decreased the hypothalamic LH-RH activity became more erratic. Data from this experiment demonstrate that hypothalamic LH-RH activity and pituitary gonadotropins are not limiting as reproduction decreases.
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Harrison et al. (1974) studied this question.