Restoration of the Luteinizing Hormone Surge in Middle-Aged Female Rats by Altering the Balance of GABA and Glutamate Transmission in the Medial Preoptic Area1

Abstract

Hypothalamic glutamate and GABA neurotransmission are involved in the ovarian hormone-induced GnRH-LH surge in rodents. We previously reported that middle-aged rats have significantly less glutamate release in the medial preoptic area than young rats on the day of the LH surge. The present study tested the hypothesis that the delayed and attenuated LH surge in ovariohysterectomized, middle-aged rats primed with ovarian steroids results from reduced hypothalamic glutamate and increased GABAA neurotransmission. Microdialysis results show that middle-aged rats with attenuated LH surges had reduced extracellular glutamate and increased extracellular GABA levels in the medial preoptic area compared to young rats. Blocking GABAA receptors with bicuculline or inhibiting synaptic glutamate reuptake with L-trans-pyrrolidine-2,4-dicarboxylic acid increased extracellular Glu in the medial preoptic area and partially restored LH surge amplitude in middle-aged rats without altering LH surge onset. Complete recovery of LH surge amplitude was observed in middle-aged rats treated with the combination of bicuculline and L-trans-pyrrolidine-2,4-dicarboxylic acid. This treatment also restored the extracellular glutamate:GABA ratio in the medial preoptic area of middle-aged rats to the level of young rats. Immunoblot analysis revealed that estradiol and progesterone treatment reduced SLC32A1(formerly known as vesicular GABA transporter) levels and increased SLC17A6 (formerly known as vesicular glutamate transporter 2) levels in the anterior hypothalamus of ovariohysterectomized young but not middle-aged rats. These data suggest that both reduced availability of glutamate and increased activation of GABAA receptors under estrogen positive feedback conditions contribute to the age-related delay in onset and attenuated amplitude of the LH surge.

Chapter 2 Hypothalamic Neural Systems Controlling the Female Reproductive Life Cycle

“…Moreover, the net effect of decreased GABA turnover might be enhanced synaptic GABA. This possibility is consistent with a recent report by Neal-Perry et al (2008) who showed by microdialysis that GABA release in the POA was increased in middle-aged compared to young ovariectomized, hormone-primed rats.…”

“…These data suggest a change in the synthesis of GABA such that the rhythmicity of its release, important to the timing of the pre-ovulatory surge, is interrupted, thus providing one explanation for the alteration of this mechanism during the aging transition, contributing to an attenuation of the GnRH/LH surge. Consistently, the important study of Neal-Perry et al (2008) confirms that pharmacological blockade of GABA A Rs with bicuculline in middle-aged rats mitigates the attenuation of the steroid-induced LH surge.…”

“…Using ovariectomized young and middle-aged female rats, Neal-Perry et al (2008) showed: 1) extracellular glutamate is decreased, and GABA is increased, in middle-aged rats with attenuated LH surges (measured by microdialysis); 2) pharmacological blockade of the GABA A R enhances both glutamate release and the LH surge; and 3) such treatment in concert with an inhibitor of synaptic glutamate reuptake further enhances glutamate levels and restores the LH surge. These results show a tight interaction of glutamate and GABA, along with GABAergic regulation of glutamate levels, and subsequent effects on the GnRH/LH surge (Figure 2).…”

The Neurobiology of Preovulatory and Estradiol-Induced Gonadotropin-Releasing Hormone Surges

“…Intravenous injection of NMDA increases LH release in intact rhesus macaques (191). Early work measuring glutamate release associated with the LH surge in rats observed no difference in glutamate levels in the preoptic area in OVXϩE animals (192), but subsequent studies have observed increases in glutamate release in the preoptic area during the LH surge in OVXϩE and OVXϩEϩP models (193)(194)(195), and that these surge-associated changes are attenuated in middle-aged, reproductively senescent animals (196,197). Blockade of either NMDA or AMPA glutamate receptors blocks the LH surge in rats (198 -200).…”

“…In intact rats, however, bicuculline was observed to have no effect on the LH surge in one study (220) but advanced the peak of the LH surge by 3 h in subsequent experiments (222). In middleaged OVXϩEϩP rats, which show an attenuated LH surge, extracellular GABA levels in the preoptic area are increased, and bicuculline infusion in the preoptic area increases LH levels (196). Together, these studies suggested that GABA exerts an inhibitory effect on GnRH/LH release, although antagonism of endogenous GABA levels yielded conflicting data.…”