Giri M, Kaufman J M
Department of Endocrinology, University Hospital, Ghent, Belgium.
Endocrinology. 1995 Jun;136(6):2404-7. doi: 10.1210/endo.136.6.7750461.
Excitatory amino acids (EAA) have been implicated in the control of LH secretion through facilitation of GnRH release, although disparate findings of an inhibitory effect of EAA on LH secretion in several species after gonadectomy have been reported. Using a static incubation system, we studied the effects of EAA receptor agonists and antagonists on in vitro GnRH release from the isolated medial basal hypothalamus (MBH) of the male guinea pig. In the presence of 0.4 mM glycine, the N-methyl-D-aspartate (NMDA)-specific receptor agonist N-methyl-D,L-aspartic acid (NMA) exerted a dose-dependent inhibition of GnRH output from the MBH of intact guinea pigs, which was significant (P < or = 0.01) at concentrations of 1 and 50 mM. The NMDA-specific receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP-5) significantly stimulated GnRH release at 10(-3) and 1 mM (P < or = 0.001) and was ineffective at 10(-6) mM. NMA (50 mM) and AP-5 (1 mM) produced similar effects when tested at a lower glycine concentration of 10 nM. The presence of the two compounds together resulted in overall unchanged GnRH output. The inhibitory effect of 50 mM NMA was also effectively blocked in the presence of 2 mM Mg2+ at both high and low glycine concentrations. When, for comparative purposes, isolated MBHs of 50-day-old intact male rats were exposed to 50 mM NMA, a response opposite to that seen in the intact guinea pig was observed, with a marked increase in GnRH output (P < or = 0.001). When tested at 10 nM glycine, 50 mM kainic acid, a non-NMDA-specific receptor agonist, on the other hand, had a marked stimulatory effect on GnRH output (P < or = 0.01) from intact guinea pig MBHs, an action that was prevented in the presence of the kainate/quisqualate receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (0.1 mM); 6,7-dinitro-quinoxaline-2,3-dione alone inhibited GnRH release (P < or = 0.01). In a separate series of experiments, the effects of EAA (in the presence of 10 nM glycine) on GnRH release from the MBHs of long term orchidectomized and sham-operated guinea pigs were compared. Orchidectomy led to a dramatic reversal of the NMDA-mediated inhibition of GnRH secretion observed in MBHs of sham-operated animals, with 50 mM NMA producing a marked increase (P < or = 0.0001) and 1 mM AP-5 resulting in a clear inhibition (P < or = 0.0001) of GnRH release. Kainic acid (50 mM), on the other hand, had a similar stimulatory action on GnRH release from the MBHs of both orchidectomized (P < or = 0.0001) and sham-castrated (P < or = 0.001) guinea pigs.(ABSTRACT TRUNCATED AT 250 WORDS)
兴奋性氨基酸（EAA）通过促进促性腺激素释放激素（GnRH）的释放参与促黄体激素（LH）分泌的调控，不过也有报道称在几种动物去势后，EAA对LH分泌具有抑制作用，结果不一。我们采用静态孵育系统，研究了EAA受体激动剂和拮抗剂对雄性豚鼠离体内侧基底部下丘脑（MBH）GnRH体外释放的影响。在存在0.4 mM甘氨酸的情况下，N-甲基-D-天冬氨酸（NMDA）特异性受体激动剂N-甲基-D,L-天冬氨酸（NMA）对完整豚鼠MBH的GnRH释放呈剂量依赖性抑制，在1和50 mM浓度时具有显著性（P≤0.01）。NMDA特异性受体拮抗剂D,L-2-氨基-5-磷酸戊酸（AP-5）在10⁻³和1 mM时显著刺激GnRH释放（P≤0.001），在10⁻⁶ mM时无作用。当在较低的10 nM甘氨酸浓度下测试时，50 mM的NMA和1 mM的AP-5产生相似的效果。两种化合物同时存在时，GnRH释放总量不变。在高、低甘氨酸浓度下，2 mM Mg²⁺存在时，50 mM NMA的抑制作用也被有效阻断。为作比较，当50日龄完整雄性大鼠的离体MBH暴露于50 mM NMA时，观察到与完整豚鼠相反的反应，GnRH释放显著增加（P≤0.001）。另一方面，当在10 nM甘氨酸下测试时，50 mM的海人藻酸（一种非NMDA特异性受体激动剂）对完整豚鼠MBH的GnRH释放有显著刺激作用（P≤0.01），在存在海人藻酸/quisqualate受体拮抗剂6,7-二硝基喹喔啉-2,3-二酮（0.1 mM）时该作用被阻断；单独使用6,7-二硝基喹喔啉-2,3-二酮可抑制GnRH释放（P≤0.01）。在另一系列实验中，比较了EAA（在存在10 nM甘氨酸时）对长期去睾和假手术豚鼠MBH中GnRH释放的影响。去睾导致假手术动物MBH中观察到的NMDA介导的GnRH分泌抑制作用发生显著逆转，50 mM NMA使GnRH释放显著增加（P≤0.0001），1 mM AP-5则导致GnRH释放明显抑制（P≤0.0001）。另一方面，50 mM海人藻酸对去睾（P≤0.0001）和假去势（P≤0.001）豚鼠MBH的GnRH释放具有相似的刺激作用。（摘要截短于250字）
