Correlative aspects of luteinizing hormone releasing hormone sensitivity and cytoplasmic estrogen receptor concentration in the anterior pituitary and hypothalamus of the cycling rat

Pituitary responsiveness to an intra arterial injection of synthetic LH RH (50 ng) was examined at critical times during the rat 4 day estrous cycle. LH release was maximal and prolonged on proestrus; the response on the other days was of short duration and the order of magnitude was estrus > diestrus 1 > diestrus 2. Sera FSH levels did not rise significantly until 60 min after an inter arterial injection of LH RH. Pituitary and hypothalamic cytoplasmic receptor levels were measured at identical times throughout the estrous cycle using protamine sulfate precipitation and Scatchard plot analysis. Hypothalamic and pituitary estrophile levels showed significant decreases which started on the afternoon of diestrus 2 and continued until mid proestrus. Receptor depletion was followed by a replenishment process which began on late proestrus and continued through mid diestrus 2 at both tissue sites. This pattern correlates well with the changing plasma estradiol level in accordance with the current concept of receptor depletion by nuclear translocation followed by cytoplasmic receptor resynthesis. The authors conclude that the change in pituitary receptor population correlates with the tissue's sensitivity to exogenous LH RH, lending further support to the concept of a direct functional effect of estradiol at the pituitary level.