Gamma amino butyric acid (GABA) is the main inhibitory neurotransmitter controlling LH-releasing hormone (LHRH) secretion in the mammalian hypothalamus. Whether alterations in GABA homeostasis within discrete regions of the neuroendocrine brain known to be targets of GABA action, such as the median eminence, can disrupt the ability of the LHRH releasing system to maintain reproductive cyclicity is not known but amenable to experimental scrutiny. The present experiments were undertaken to examine this issue. Immortalized BAS-8.1 astroglial cells were genetically modified by infection with a regulatable retroviral vector to express the gene encoding the GABA synthesizing enzyme glutamic acid decarboxylase-67 (GAD-67) under the control of a tetracycline (tet) controlled gene expression system. In this system, expression of the gene of interest is repressed by tet and activated in the absence of the antibiotic. BAS-8.1 cells carrying this regulatory cassette, and cultured in the absence of tet ("GAD on"), expressed abundant levels of GAD-67 messenger RNA and GAD enzymatic activity, and released GABA when challenged with glutamate. All of these responses were inhibited within 24 h of exposure to tet ("GAD off"). Grafting "GAD on" cells into the median eminence of late juvenile female rats, near LHRH nerve terminals, did not affect the age at vaginal opening, but greatly disrupted subsequent estrous cyclicity. These animals exhibiting long periods of persistent estrus, interrupted by occasional days in proestrus and diestrus, suggesting the occurrence of irregular ovulatory episodes. Administration of the tetracycline analog doxycycline (DOXY) in the drinking water inhibited GAD-67synthesis and restored estrous cyclicity to a pattern indistinguishable from that of control rats grafted with native BAS-8.1 cells. Animals carrying"GAD on" cells showed a small increase in serum LH and estradiol levels, and a marked elevation in serum androstenedione, all of which were obliterated by turning GAD-67 synthesis off in the grafted cells. Morphometric analysis of the ovaries revealed that both groups grafted with GABA-producing cells had an increased incidence of large antral follicles (>500/μm) compared with animals grafted with native BAS-8.1 cells, but that within this category the incidence of steroidogenically more active follicles (i.e. larger than 600 /μm) was greater in "GAD on" than in "GAD off" rats. These results indicate that a regionally discrete, temporally controlled increase in GABA availability to LHRH nerve terminals in the median eminence of the hypothalamus suffices to disrupt estrous cyclicity in the rat, and raise the possibility that similar local alterations in GABA homeostasis may contribute to the pathology of hypothalamic amenorrhea/oligomenorrhea in humans.
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