Regional differences in the pituitary distribution of luteinizing hormone in the gonadectomized and proestrous female rat

Previous data have shown that regional differences in the presence of anterior pituitary luteinizing hormone (LH) generally correlate with the comparable disparities in distribution of gonadotropes throughout the gland. In female rats, the differences are apparent over the estrous cycle, but are more prominent during the hours preceding the proestrus surge of LH. The current experiments examined (1) if such regional disparities are present throughout the surge window, (2) if differences are mirrored by release of LH in vitro and (3) if the appearance of regional differences is altered in ovariectomized females. Results showed that a comparative elevation in the rostral portion of the pituitary during the pre-surge period diminishes and finally disappears concurrent with the rise in circulating LH. This increase in rostral LH concentrations is reflected in this region by a comparable effect in vitro on stimulated LH secretion from pituitary fragments, although the effect is somewhat diminished by referencing release against tissue concentrations of LH present in a contralateral rostral fragment. Ovariectomies conducted at 1500 h on proestrus, at a time when a significant regional difference has faded, resulted in a prompt increase in LH across all areas of the pituitary, and the emergence of a marked augmentation in rostral concentrations over the ensuing 72 h. The effect was not seen when ovariectomies were performed on estrus. These data show that, while a regional disparity in anterior pituitary LH is present as circulating concentrations of estradiol rise prior to the LH surge, the removal of this steroid feedback at a time when LH synthesis is normally amplified accentuates the difference between the rostral region and other areas of the pituitary.