Watanabe Miho, Sakuma Yasuo, Kato Masakatsu
Department of Developmental Physiology, National Institute for Physiological Sciences, Okazaki, Japan.
Biol Reprod. 2009 Aug;81(2):327-32. doi: 10.1095/biolreprod.108.074583. Epub 2009 Apr 8.
Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. Gamma-amino butyric acid (GABA), the main inhibitory neurotransmitter in the adult brain, has long been implicated in playing key roles in the regulation of GnRH neurons. Two groups reported recently that GABA depolarizes GnRH neurons, although one group reported a hyperpolarizing action of GABA. In this study, we investigated the GABA-induced changes in Ca(2+) of GnRH neurons from GnRH-enhanced green fluorescent protein (GnRH-EGFP) rats both to confirm the depolarizing action of GABA and to further examine the developmental and estrous cycle-dependent modulations of GABA action. GABA increased Ca(2+) in GnRH neurons at all developmental stages of both sexes. GABA also increased Ca(2+) in adult female GnRH neurons prepared in the afternoon at each estrous cycle stage. The percentages of neurons with increased Ca(2+) were 90% in proestrus, 59% in estrus, 84% in diestrus I, and 89% in diestrus II. In GnRH neurons prepared from adult females in the morning, however, the percentage was significantly lower than in those prepared in the afternoon, except in estrus. The percentage was also lower in adult males than in adult females. GABA responses were mimicked by muscimol and blocked by bicuculline. In addition, removal of extracellular Ca(2+) completely suppressed the GABA action, and bumetanide attenuated the response. These results indicate that GABA depolarizes GnRH neurons by activating GABA(A) receptors, thereby activating voltage-gated Ca(2+) channels and facilitating Ca(2+) influx. In addition, the response to GABA is modulated according to the estrous cycle stage, diurnal rhythm, and sex.
促性腺激素释放激素（GnRH）神经元构成了生殖中枢调节的最终共同通路。γ-氨基丁酸（GABA）是成年大脑中的主要抑制性神经递质，长期以来一直被认为在GnRH神经元的调节中起关键作用。最近有两个研究小组报道GABA使GnRH神经元去极化，尽管有一个小组报道了GABA的超极化作用。在本研究中，我们研究了GABA诱导的GnRH增强绿色荧光蛋白（GnRH-EGFP）大鼠GnRH神经元胞内钙离子浓度（Ca(2+)）的变化，以确认GABA的去极化作用，并进一步研究GABA作用的发育和发情周期依赖性调节。GABA在两性的所有发育阶段均增加了GnRH神经元中的Ca(2+)。在每个发情周期阶段的下午制备的成年雌性GnRH神经元中，GABA也增加了Ca(2+)。Ca(2+)增加的神经元百分比在发情前期为90%，发情期为59%，间情期I为84%，间情期II为89%。然而，在早晨从成年雌性制备的GnRH神经元中，该百分比显著低于下午制备的神经元，发情期除外。成年雄性的百分比也低于成年雌性。蝇蕈醇模拟了GABA反应，荷包牡丹碱阻断了该反应。此外，去除细胞外Ca(2+)完全抑制了GABA作用，布美他尼减弱了反应。这些结果表明，GABA通过激活GABA(A)受体使GnRH神经元去极化，从而激活电压门控Ca(2+)通道并促进Ca(2+)内流。此外，对GABA的反应根据发情周期阶段、昼夜节律和性别进行调节。
