Ultrastructural changes rapidly induced by somatostatin may inhibit prolactin release in estrogen-primed rat adenohypophysis

Apart from the known hypothalamic controls, which have been well documented, a myriad of compounds both endogenous and exogenous have proved effective in influencing secretion of prolactin (PRL). Recent studies have shown that somatostatin (SRIF), when injected intra-atrially as a bolus, is able to inhibit PRL secretion in vivo. However, the inhibitory effect of SRIF occurs only in adenohypophyses previously primed with estradiol. This research was undertaken to examine the ultrastructural effects of that inhibition using male Sprague-Dawley rats primed for three weeks with subcutaneous implants of estradiol. Within 2 min of injection of SRIF (1 mg/kg), the adenohypophyses were removed and processed for electron microscopy. We found dramatic changes in the estradiol-primed mammotrophs, including 1) an apparent rearrangement of rough endoplasmic reticulum (RER) into concentric cisternae, and 2) the appearance of intracellular bodies closely associated with granules. These changes were not observed in non-estradiol-primed male rats injected with SRIF which is consistent with the fact that in the normal male rats, SRIF failed to inhibit PRL secretion. These findings suggest that SRIF causes reorganization of cellular organelles so that PRL granules are sequestered thereby inhibiting secretion of PRL.