Demir R, Haberly L B, Jackson M B
Center for Neuroscience, University of Wisconsin Medical School, Madison Wisconsin 53706, USA.
J Neurophysiol. 1998 Nov;80(5):2727-42. doi: 10.1152/jn.1998.80.5.2727.
The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.
梨状皮质是颞叶结构，癫痫易感性极高。为研究癫痫样活动的时空特征，采用电压敏感染料成像电活动的方法对梨状皮质切片进行检测。研究了沿前后轴不同刺激位点和不同切片平面的放电活动。通过使用γ-氨基丁酸-A受体拮抗剂解除抑制或在低氯培养基中诱导短暂的自发爆发来引发癫痫样行为。用荧光染料记录的对照活动具有与先前用微电极报道的对照活动相同的药理学和时间特征。癫痫样放电的同步光学和细胞外微电极记录显示出与先前用微电极描述的相同的持续时间、潜伏期和全或无特性。在所有检测条件下，施加于整个梨状皮质的阈下电刺激诱发了起源于包括内梨状核（先前确定的放电起始位点）的位点的全或无癫痫样放电。在诱导切片而非解除抑制的切片中，起始位点还分别包括来自前梨状皮质和后梨状皮质切片中相邻无颗粒岛叶皮质和梨周皮质的VI层。这些位置以前未被确定为放电起始位点。因此，与内梨状核一样，深层无颗粒岛叶皮质和梨周皮质的癫痫阈值非常低。在诱导性和解除抑制性癫痫发生模型之间还注意到了其他细微差异。确定了起始后放电向外传播至上覆梨状皮质时的速度。还检查了其他方向的传播。在大多数情况下，速度低于动作电位传导速度，这表明反复兴奋和/或电场相互作用在放电传播中起作用。对本研究中确定区域的细胞和组织特性的未来研究应有助于阐明癫痫易感性高的神经生物学基础。
