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Regional differences in the pituitary distribution of luteinizing hormone in the gonadectomized and proestrous female rat
Goldman, J. M., T. E. Stoker, A. E. Murr, W. K. McElroy, AND R. L. Cooper. Regional differences in the pituitary distribution of luteinizing hormone in the gonadectomized and proestrous female rat. GENERAL AND COMPARATIVE ENDOCRINOLOGY. Academic Press Incorporated, Orlando, FL, 156(3):577-583, (2008).
Perifusion of anterior pituitary fragments has frequently been employed to assess direct effects of various toxicants on the secretion of various pituitary hormones. The presumption has often been that the cells releasing these hormones are evenly distributed throughout the gland, so that random portions of the pituitary could be used to investigate in vitro hormonal release, while others evaluated for tissue concentrations. One focus of this study was to evaluate regional differences in pituitary concentrations of luteinizing hormone in the rat over the day of proestrus (when the ovulatory surge of this hormone occurs) to determine if there could be a dissimilar effect using fragments from different regions of the pituitary to determine potential effects of toxicant exposure on hormonal secretion in vitro. The present manuscript serves to emphasize that a random selection of cut pituitary fragments to assess LH secretion may well contribute to an increase in the variability of results, obscuring a potential effect of toxicant exposure. In addition, the contribution of the sex steroids to such differences was established by surgically removing the ovaries to drastically diminish circulating concentrations of these hormones.
Previous data have shown regional differences in the presence of anterior pituitary luteinizing hormone (LH) that generally correlate with comparable disparities in the distribution of gonadotropes throughout the gland. In female rats, the differences are apparent over the estrous cycle, but are more prominent during the hours prior to the appearance of the proestrous surge of LH. The current experiments examined (1) if such regional disparities are present during the surge window, (2) if differences are mirrored by a release of LH in vitro and (3) if the appearance of regional differences is altered by a depletion in circulating estradiol following ovariectomy. The results showed that a comparative elevation of LH 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 by a comparable effect on stimulated LH secretion in vitro from pituitary fragments in this region, although the effect is somewhat diminished by referencing release against the tissue concentration of LH present in a contralateral rostral fragment. Ovariectomies conducted at 1500h on proestrus, at a time when a statistically significant regional difference had 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 hours. The effect was not seen when ovariectomies were performed on estrus, or in intact females after shorter or extended acyclic periods of persistent estrus. These data show that, while a regional disparity in anterior pituitary LH is present as circulating concentrations of estradiol are rising 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.