Review articleSexual dimorphism in the GABAergic control of gonadotropin release in intact rats
Section snippets
Sexual dimorphism in gonadotropin release
There is a distinct sex difference in the hypothalamo-pituitary-gonadal (HPG) axis in rats. Female rats have a luteinizing hormone (LH) surge in the afternoon of proestrus (Butcher et al., 1974), while male rats do not show any LH surge (Kalra and Kalra, 1977). The sex difference at the level of hypothalamus stands out in estrogen-primed gonadectomized rats. Estradiol treatment causes a daily surge-like secretion of LH in gonadectomized female rats, while it continuously suppresses serum LH
Neurotransmitters participating in the control of GnRH–LH axis
A number of neuroendocrinological studies have suggested that many neurotransmitters are involved in the control of GnRH neurons. We proposed that there are two subgroups of GnRH neurons in female rats, the GnRH surge generator and GnRH pulse generator, which generate gonadotropin surge and sustain pulsatile secretion of gonadotropin, respectively (Kimura and Funabashi, 1998). The GnRH pulse generator exists in both sexes of rats, but the GnRH surge generator exists only in female rats.
Many
GABA as a potent and direct GnRH-regulating neurotransmitter
Although there is no doubt that multiple neurotransmitter systems participate in the regulation of GnRH neurons, abundant neuroendocrinological evidence suggests a potential role of GABA in regulating GnRH neurons. First, histological studies demonstrated direct GABAergic regulation of the GnRH neurons in rats: GnRH neurons express some specific subunits forming the GABAa receptor (He et al., 2000b, Jung et al., 1998, Petersen et al., 1994, Sim et al., 2000) and receive synaptic inputs from
GABA release in the MPO of cycling female rats
We have reported the 4 day profile of the GABA release in the MPO of cycling female rats (Mitsushima et al., 2002). We found that GABA release in the MPO was closely associated with the serum estradiol concentration: on the day of diestrus 1, the serum estradiol concentration and the GABA release are low (Butcher et al., 1974, Mitsushima et al., 2002). On the day of diestrus 2, the serum estradiol concentration is slightly increased, and the GABA release declines in the afternoon. In the
Circadian signal to regulate the GABA release of female rats
Nevertheless, circulating estrogen is not the sole factor controlling the GABA release, since GABA release significantly decreases in the afternoon in a high estrogen milieu (i.e. diestrus 2 and proestrus). We recently monitored the GABA release in ovariectomized rats 3–5 days after implantation of a capsule containing 17β-estradiol. The serum estradiol concentration was kept at approximately 75 pg/ml, which is similar to the peak concentration in proestrous female rats (Fig. 1). Ovariectomized
GABA release in the MPO of male rats
We have recently reported the 24 h profile of GABA release in intact male rats (Tin-Tin-Win-Shwe et al., 2002). Unexpectedly, the GABA release level in intact male rats was as high as the morning level in proestrous female rats, and an episodic release pattern was observed throughout the day. Two to six episodic GABA releases were found per day and no surge-like secretion of LH was observed. Since serum LH and testosterone concentration changes episodically (Ellis and Desjardins, 1982), the
Peak time and acrophase of the GABA release
Peak time in each rat was defined as the time when the sample showed maximum concentration of GABA or LH as shown in upper panel of Fig. 2. In diestrus 2 and estrous rats, individual peak time was observed at various times of the day. Although diestrus 1 or male rats tended to show peaks in the afternoon, the peak time in diestrus 1 or male rats was not significantly different from that in diestrus 2 or estrus rats. But proestrous female rats consistently showed increase of GABA release in the
Cause of the sexual difference in the GABA release
The question arises as to what causes the sexually dimorphic profile in GABA release. The sexual dimorphism of the GABA release may simply reflect sex difference in the gonad or circulating hormone milieu, or it may be due to the sex difference in hypothalamic neuronal circuitry caused by neonatal androgen exposure (Orikasa et al., 2002, McEwen, 1994). To further determine the cause of the sex difference, we performed in vivo microdialysis studies in gonadectomized male and female rats.
Acknowledgements
This work was supported by a Grant-in-Aid for Encouragement of Young Scientists from the Ministry of Education, Cultute, Sports, Science and Technology of Japan to D. Mitsushima.
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