Long-lasting desensitizing effect of short-term LRH exposure on the pituitary responsiveness to LRH in the ovariectomized rat

The influence of short-term exposure to LRH on the responsiveness to LRH and on the LH content of the pituitary gland was studied in the 2-weeks ovariectomized rat. Rats were infused with LRH at the rate of either 208 or 1040 ng/h from 12.00 h to 15.00 h on day 0, and injected with 3 μg of oestradiol-benzoate (OeB) or oil at 09.00 h on either day 1 or day 3. LH-responses were induced by a 21-h lasting infusion of LRH at the rate of 104 ng/h, starting at 12.00 h on the day after the OeB/oil treatment. Control rats received only one LRH treatment: they were injected with OeB or oil on day—1 and LH-responses were induced on day 0.Results were evaluated on the basis of two parameters: total amounts of LH released during exposure to LRH (1) and release efficiency, i.e. total amounts of LH released in relation to pituitary LH content (2). The findings obtained indicate that exposure to relatively low LRH concentrations causes a greater loss of release efficiency than exposure to relatively high ones. Loss of release efficiency may even increase long after the blood has been cleared of the releasing hormone. From the present experiments it can thus be inferred that exposure of the pituitary gland to LRH leads to either a marked loss of release efficiency and a relatively small LH depletion (1), or a smaller loss of release efficiency and a relatively large LH depletion (2). The two variables both influence the state of responsiveness of the pituitary gland and the present results indicate that only in situation 2 the LH-responsiveness recovers effectively. However, when after exposure to LRH rats are treated with OeB, the consequences, in terms of total amounts of LH released, of differences in degrees of loss of release efficiency and in depletion of LH are eliminated. It is suggested that during the ovulatory cycle this effect of oestrogen plays an important role in the process of recovery of the pituitary gland from diminished responsiveness to LRH induced by LRH