Bernasconi R, Lauber J, Marescaux C, Vergnes M, Martin P, Rubio V, Leonhardt T, Reymann N, Bittiger H
Research and Development Department, Ciba-Geigy, Basel, Switzerland.
J Neural Transm Suppl. 1992;35:155-77. doi: 10.1007/978-3-7091-9206-1_11.
We have investigated whether the pathogenesis of spontaneous generalized non-convulsive seizures in rats with genetic absence epilepsy is due to an increase in the brain levels of gamma-hydroxybutyric acid (GHB) or in the rate of its synthesis. Concentrations of GHB or of its precursor gamma-butyrolactone (GBL) were measured with a new GC/MS technique which allows the simultaneous assessment of GHB and GBL. The rate of GHB synthesis was estimated from the increase in GHB levels after inhibition of its catabolism with valproate. The results of this study do not indicate significant differences in GHB or GBL levels, or in their rates of synthesis in rats showing spike-and-wave discharges (SWD) as compared to rats without SWD. Binding data indicate that GHB, but not GBL, has a selective, although weak affinity for GABAB receptors (IC50 = 150 microM). Similar IC50 values were observed in membranes prepared from rats showing SWD and from control rats. The average GHB brain levels of 2.12 +/- 0.23 nmol/g measured in the cortex and of 4.28 +/- 0.90 nmol/g in the thalamus are much lower than the concentrations necessary to occupy a major part of the GABAB receptors. It is unlikely that local accumulations of GHB reach concentrations 30-70-fold higher than the average brain levels. After injection of 3.5 mmol/kg GBL, a dose sufficient to induce SWD, brain concentrations reach 240 +/- 31 nmol/g (Snead, 1991) and GHB could thus stimulate the GABAB receptor. Like the selective and potent GABAB receptor agonist R(-)-baclofen, GHB causes a dose-related decrease in cerebellar cGMP. This decrease and the increase in SWD caused by R(-)-baclofen were completely blocked by the selective and potent GABAB receptor antagonist CGP 35348, whereas only the increase in the duration of SWD induced by GHB was totally antagonized by CGP 35348. The decrease in cerebellar cGMP levels elicited by GHB was only partially antagonized by CGP 35348. These findings suggest that all effects of R(-)-baclofen are mediated by the GABAB receptor, whereas only the induction of SWD by GHB is dependent on GABAB receptor mediation, the decrease in cGMP being only partially so. Taken together with the observations of Marescaux et al. (1992), these results indicate that GABAB receptors are of primary importance in experimental absence epilepsy and that GABAB receptor antagonists may represent a new class of anti-absence drugs.
我们研究了遗传性失神癫痫大鼠自发性全身性非惊厥性癫痫发作的发病机制是否归因于脑内γ-羟基丁酸（GHB）水平的升高或其合成速率的增加。采用一种新的气相色谱/质谱技术测定GHB或其前体γ-丁内酯（GBL）的浓度，该技术可同时评估GHB和GBL。通过用丙戊酸抑制GHB分解代谢后其水平的升高来估算GHB的合成速率。本研究结果表明，与无棘波-慢波放电（SWD）的大鼠相比，出现SWD的大鼠在GHB或GBL水平及其合成速率方面并无显著差异。结合数据表明，GHB对GABAB受体具有选择性亲和力，尽管较弱，但GBL则不然（IC50 = 150 microM）。在由出现SWD的大鼠和对照大鼠制备的膜中观察到相似的IC50值。在皮质中测得的GHB平均脑水平为2.12±0.23 nmol/g，在丘脑中为4.28±0.90 nmol/g，远低于占据大部分GABAB受体所需的浓度。GHB在局部的蓄积不太可能达到比平均脑水平高30 - 70倍的浓度。注射3.5 mmol/kg GBL（足以诱发SWD的剂量）后，脑内浓度达到240±31 nmol/g（斯奈德，1991年），因此GHB可刺激GABAB受体。与选择性强效GABAB受体激动剂R(-)-巴氯芬一样，GHB可引起小脑环磷酸鸟苷（cGMP）剂量相关的降低。这种降低以及R(-)-巴氯芬引起的SWD增加被选择性强效GABAB受体拮抗剂CGP 35348完全阻断，而只有GHB诱导的SWD持续时间增加被CGP 35348完全拮抗。GHB引起的小脑cGMP水平降低仅被CGP 35348部分拮抗。这些发现表明，R(-)-巴氯芬的所有效应均由GABAB受体介导，而只有GHB诱导SWD依赖于GABAB受体介导，cGMP的降低只是部分依赖。与马雷斯科等人（1992年）的观察结果一起，这些结果表明GABAB受体在实验性失神癫痫中至关重要，并且GABAB受体拮抗剂可能代表一类新型的抗失神药物。
