Regulation of Glutamic Acid Decarboxylase 65 and 67 Gene Expression by Ovarian Steroids: Identification of Two Functionally Distinct Populations of GABA Neurones in the Preoptic Area

Abstract

GABA neurones in the preoptic area (POA) are critical for oestradiol (E2)-dependent surge release of luteinizing hormone (LH); however, it is not clear which population(s) of POA GABA neurones is involved. The goals of the present studies were: (i) to determine whether E2 regulates GABA neurones similarly in two subdivisions of the POA that play a role in LH surge release, the rostral POA region that contains the organum vasculosum of the lamina terminalis (rPOA/OVLT), and the region containing the anteroventral periventricular nucleus (AVPV) and medial preoptic nucleus (MPN) and (ii) to determine whether GABA neurones in either or both regions exhibit temporal changes consistent with a role in the regulation of LH surge release. To accomplish these goals, we measured glutamic acid decarboxylase (GAD) 65 and 67 mRNA levels at several time points in ovariectomized (OVX), E2-treated OVX rats exhibiting LH surge release, and in E2-treated OVX rats in which LH surge release was blocked by prior administration of progesterone (P4). Our findings demonstrate that, despite their close proximity, GABA neurones in the AVPV/MPN region are regulated differently from those in the rPOA/OVLT. Only neurones in the AVPV/MPN region show temporal changes in GAD 67 mRNA expression that appear to be linked to positive-feedback effects of E2 on luteinizing hormone-releasing hormone (LHRH) and LH release. Our findings also indicate that a morning rise and an afternoon fall in GAD 67 mRNA levels marks two E2-dependent signals required for LHRH and LH surge release. Finally, our results suggest that there are distinct E2-induced signals to the rPOA/OVLT and AVPV/MPN regions and that these signals differentially regulate GAD 65 and 67 gene expression.

Dual-Phenotype GABA/Glutamate Neurons in Adult Preoptic Area: Sexual Dimorphism and Function

“…Importantly, GAD67, but not GAD65, gene expression in these neurons is also upregulated in an activity-dependent manner in the absence of seizures (for review, see Gutierrez, 2003). This finding is similar to our previous observation that changes in GAD67 mRNA levels in AVPV GABA/gluamate neurons also reflect changes in GABA release, whereas levels of GAD65 mRNA do not (Curran-Rauhut and Petersen, 2002). Likewise, changes in levels of VGAT mRNA are linked to GABA release in granule cells (Lamas et al, 2001), and we found that IRVGAT in AVPV projection fields decreased at the time GABA release into the region declines (Jarry et al, 1995).…”

“…This interpretation is supported by recent work demonstrating that a daily rise in cAMP levels occurs in AVPV neurons (Chappell et al, 2000). Importantly, this rise is necessary for the LH surge, and it coincides with a rise in GAD67 gene expression in neurons found specifically in the AVPV (Curran-Rauhut and Petersen, 2002). These changes also coincide with the timing of E 2 -dependent changes in IRVGAT and IRVGLUT2 we observed, as well as in GABA and glutamate release into the POA observed by others using this animal model (Jarry et al, 1995).…”

“…First, glutamatergic (Eyigor et al, 2004) and GABAergic (Flugge et al, 1985) neurons in the AVPV contain ER. Furthermore, GABAergic neurons in the AVPV, but not those surrounding GnRH neurons in the rPOA, exhibit changes in GAD67 gene expression that parallel GABA release on the day of LH surge release (Curran-Rauhut and Petersen, 2002). Consistent with these findings, GABA and glutamate terminals provide most of the synaptic input to GnRH neurons (Herbison, 1998;Kiss et al, 2003;Lin et al, 2003;Petersen et al, 2003;Eyigor et al, 2004;Han et al, 2004), and receptors for GABA and glutamate are among the few types found on GnRH neurons (Petersen et al, 2003).…”

“…Considering that GABA and glutamate provide most of the input to GnRH neurons (Herbison, 2003), it seems likely that they are released from the VGAT-and VGLUT2-containing vesicles we observed in contact with GnRH neurons. Second, the decline in IRVGAT before the onset of LH surge parallels the E 2 -dependent suppression of GABA release into the POA (Jarry et al, 1992(Jarry et al, , 1995, as well as the decline in GAD67 gene expression in AVPV neurons (Curran-Rauhut and Petersen, 2002). Notably, work examining other brain regions shows that VGAT and GAD67 mRNA levels are correlated with GABA release (Litwak et al, 1990;Drengler and Oltmans, 1993;Falkenberg et al, 1997;Lamas et al, 2001;Ramirez and Gutierrez, 2001;Gutierrez, 2002;Kang et al, 2003).…”

Evidence that GABAergic neurons in the preoptic area of the rat brain are targets of 2,3,7,8-tetrachlorodibenzo-p-dioxin during development.

“…We treated dams with 1 µg TCDD/kg body weight on gestational day 15 (GD 15) to replicate conditions previously used to show that TCDD disrupts masculinization and defeminization of sexual behaviors and gonadotropin release patterns (13). We measured GAD 67 mRNA levels as an index of GABAergic activity because in adult rats changes in GAD 67, but not in GAD 65, gene expression are closely linked to changes in GABA release into the POA (44). In the present studies, we used in situ hybridization histochemistry (ISHH) so that we could focus on three subdivisions of the POA that lie in close proximity: a) the rostral POA containing the anteroventral periventricular nucleus (rPOA/AVPV), b) the region containing the rostralmost portion of the medial POA (mPOA), and c) the region containing the caudalmost mPOA.…”