An investigation into the origin of the pro-oestrus/oestrus change in pituitary responsiveness to LRH of the pseudopregnant rat

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Abstract
: LH-secretory responses, induced by (standard) infusion of LRH in the pro-oestrous rat, are much greater than in the oestrous rat. In the present investigation the factors which accomplish the transition from the pro-oestrus to the oestrus state of responsiveness to LRH of the pituitary were studied in the pro-oestrous pseudo-pregnant (PSP) rat. It was considered that the change in responsiveness might be caused by either the cyclic release of LRH by the hypothalamus (which results in a depletion of pituitary LH content and an altered state to further stimulation), the peri-ovulatory changes in the endocrine milieu, or some combination of these factors. It was observed that LH-secretory responses induced 24 h after ovariectomy (OVX) on pro-oestrus, take an intermediate position between the pro-oestrus and the oestrus responses. However, when treated with oestradiol benzoate (OeB), such animals maintain the pro-oestrus state of responsiveness. Regardless of the presence of OeB or of the ovaries, following the simulation of a 'pre-ovulatory' LH-surge by a 3 h lasting infusion of LRH, the pituitary gland passes into the oestrus state of responsiveness to LRH. When, however, ovulation was induced by hCG, the pituitary passed into the same state of responsiveness as after OVX, i.e. the intermediate position. It is concluded that the transition from the pro-oestrus to the oestrus state of responsiveness of the pituitary results from the exposure of the gland to the pro-oestrous LRH-surge.