Department of Comparative Medicine, Stanford University, Stanford, California 94305, USA.
Epilepsia. 2011 Nov;52(11):2057-64. doi: 10.1111/j.1528-1167.2011.03253.x. Epub 2011 Aug 29.
In temporal lobe epilepsy many somatostatin interneurons in the dentate gyrus die. However, some survive and sprout axon collaterals that form new synapses with granule cells. The functional consequences of γ-aminobutyric acid (GABA)ergic synaptic reorganization are unclear. Development of new methods to suppress epilepsy-related interneuron axon sprouting might be useful experimentally.
Status epilepticus was induced by systemic pilocarpine treatment in green fluorescent protein (GFP)-expressing inhibitory nerurons (GIN) mice in which a subset of somatostatin interneurons expresses GFP. Beginning 24 h later, mice were treated with vehicle or rapamycin (3 mg/kg intraperitoneally) every day for 2 months. Stereologic methods were then used to estimate numbers of GFP-positive hilar neurons per dentate gyrus and total length of GFP-positive axon in the molecular layer plus granule cell layer. GFP-positive axon density was calculated. The number of GFP-positive axon crossings of the granule cell layer was measured. Regression analyses were performed to test for correlations between GFP-positive axon length versus number of granule cells and dentate gyrus volume.
After pilocarpine-induced status epilepticus, rapamycin- and vehicle-treated mice had approximately half as many GFP-positive hilar neurons as did control animals. Despite neuron loss, vehicle-treated mice had over twice the GFP-positive axon length per dentate gyrus as controls, consistent with GABAergic axon sprouting. In contrast, total GFP-positive axon length was similar in rapamycin-treated mice and controls. GFP-positive axon length correlated most closely with dentate gyrus volume.
These findings suggest that rapamycin suppressed axon sprouting by surviving somatostatin/GFP-positive interneurons after pilocarpine-induced status epilepticus in GIN mice. It is unclear whether the effect of rapamycin on axon length was on interneurons directly or secondary, for example, by suppressing growth of granule cell dendrites, which are synaptic targets of interneuron axons. The mammalian target of rapamycin (mTOR) signaling pathway might be a useful drug target for influencing GABAergic synaptic reorganization after epileptogenic treatments, but additional side effects of rapamycin treatment must be considered carefully.
在颞叶癫痫中，许多齿状回中的生长抑素中间神经元死亡。然而，一些存活下来并长出轴突侧支，与颗粒细胞形成新的突触。γ-氨基丁酸（GABA）能突触重组的功能后果尚不清楚。开发新的方法来抑制与癫痫相关的中间神经元轴突发芽可能在实验上是有用的。
在表达绿色荧光蛋白（GFP）的抑制性神经元（GIN）小鼠中，通过全身毛果芸香碱处理诱导癫痫持续状态，其中一部分生长抑素中间神经元表达 GFP。从 24 小时后开始，每天用载体或雷帕霉素（3 毫克/千克腹腔内）处理小鼠 2 个月。然后使用立体学方法估计每个齿状回的 GFP 阳性海马神经元数量和分子层加颗粒细胞层中 GFP 阳性轴突的总长度。计算 GFP 阳性轴突密度。测量 GFP 阳性轴突穿过颗粒细胞层的交叉数。进行回归分析以测试 GFP 阳性轴突长度与颗粒细胞数量和齿状回体积之间的相关性。
在毛果芸香碱诱导的癫痫持续状态后，雷帕霉素和载体处理的小鼠的 GFP 阳性海马神经元数量约为对照组的一半。尽管神经元丢失，但与对照组相比，载体处理的小鼠的每个齿状回 GFP 阳性轴突长度增加了两倍以上，这与 GABA 能轴突发芽一致。相比之下，雷帕霉素处理的小鼠和对照组的总 GFP 阳性轴突长度相似。GFP 阳性轴突长度与齿状回体积最密切相关。
这些发现表明，雷帕霉素通过在 GIN 小鼠的毛果芸香碱诱导的癫痫持续状态后抑制存活的生长抑素/GFP 阳性中间神经元的轴突发芽。雷帕霉素对轴突长度的影响是直接作用于中间神经元还是间接作用于中间神经元的轴突，例如通过抑制颗粒细胞树突的生长，而颗粒细胞树突是中间神经元轴突的突触靶标，尚不清楚。雷帕霉素的哺乳动物靶标（mTOR）信号通路可能是影响致痫治疗后 GABA 能突触重组的一个有用的药物靶点，但必须仔细考虑雷帕霉素治疗的其他副作用。
