Gonadotropin-releasing hormone within the organum vasculosum lamina terminalis in the ovariectomized, estrogen/progesterone-treated rat: a quantitative immunocytochemical study using image analysis.

No specific function has been ascribed to the high gonadotropin-releasing hormone (GnRH) content of the organum vasculosum lamina terminalis (OVLT). The objective of this study was to determine whether levels of GnRH within the OVLT are altered during cyclic gonadotropin secretion. GnRH levels were determined at various times during an estrogen/progesterone (E/P)-induced LH surge. Groups of E/P and sesame oil-treated animals were decapitated at 12.00 h, 14.00 h, 16.00 h, 18.00 h, and 22.00 h following the P or oil treatment. Morphological localization, as well as quantitation of immunoreactive GnRH within discrete regions of the brain was achieved by combining unlabeled antibody immunocytochemistry with computerized image analysis. Analysis of GnRH levels in the OVLT revealed that at any of the 5 times examined, there was: (1) no significant difference among controls, (2) no significant difference among E/P-treated animals, and (3) no significant difference between E/P-treated versus control animals. In contrast, ME GnRH levels in E/P-treated rats showed the expected decrease prior to the onset of the LH surge. These findings suggest that the level of GnRH detected in axon terminals within the OVLT cannot be related directly to the serum LH status of ovariectomized, E/P-treated rats. It is therefore possible that GnRH within the OVLT might function in a neuromodulatory role, rather than as a direct regulator of cyclic gonadotropin secretion.