| Epilepsy is a common chronic neurological disorder and usually lifelong. Recurrent epileptic seizures may lead to accidents and cognitive impairment. Due to the lack of understanding of the complex pathogenesis of epilepsy, clinical treatment of epilepsy is not satisfactory so far. Approximately30%of patients (more in patients with temporal lobe epilepsy, TLE) continue to have seizures and develop intractable epilepsy even a variety of antiepileptic drugs are currently available. Although surgery may be considered as an option in some patients with intractable epilepsy, many are not appropriate candidates for surgery. Therefore, treatment of intractable epilepsy is a major problem for clinical antiepileptic practice.Low-frequency stimulation (LFS,1-3Hz) targeting deep brain region is emerging as a new option for the treatment of intractable epilepsy with the advantages of reversiblity, adjustability and minimally invasive surgery. LFS of epileptic focus and some other brain areas can suppress seizures in patients and experimental animals. However, because of the complex mechanisms underlying LFS treatment for epilepsy, the optimal strategy for LFS remains unclear. Moreover, contradictory reports regarding the effects of LFS on epilepsy limited the clinical application of LFS. To find suitable stimulation targets and modes are important for LFS treatment of epilepsy. The entorhinal cortex (EC) serves as a gateway to the hippocampus and might be a promising target for LFS to treat epilepsy. On the other hand, closed-loop is a novel clinic mode for LFS which means that the stimulation is delivered in response to the electroencephalographic (EEG) activity. Comparing with open-loop mode in which stimulation delivery follows preset programming, closed-loop may minimize the side effects of LFS. However, new technology has brought new problems:the closed-loop mode inevitably has seconds delay for detecting the seizure and giving the stimulation, and such delay may influence the anticonvulsive effects of LFS. Recently, we reported LFS delivered daily immediately after the kindling stimulation inhibits amygdaloid-kindling seizures, while LFS after the cessation of afterdischarge (AD) has no effect or even augments epileptic activities, suggesting it may be reasonable to deliver LFS in the closed-loop mode. Therefore, we first evaluated whether delayed LFS of EC has antiepileptic effect, which is important for the clinical use of LFS in closed-loop mode.Recently, the epileptic networks have been increasingly accepted and aroused wide concern. Epileptic networks include both the seizure focus and the remote brain structures outside of the focus. The effects of LFS on seizure focus (such as kindling focus) and other critical areas of the epileptic networks are poorly known. The dentate gyrus (DG) of hippocampus has been considered as a critical part of epilepsy circuits. Loss of inhibitory interneurons and many granule cell survived with hyper-excitability, abnormal dispersion and mossy fiber sprouting were found in the dentate gyrus both in animal models and patients with TLE. Our unpublished data found a transient increase of glutamine synthetase in the ipsilateral DG during amygdaloid kindling acquisition, and inhibition of GS in the ipsilateral DG to an adequate degree at the appropriate time retards kindling acquisition in the rat. Anatomically, DG granule cells mainly receive direct projections from EC, the perforant pathway (PP), and may filter the inputs from EC. Furthermore, the excitability, structures and regeneration of granule cells can be regulated by the inputs from EC. Thus, LFS EC may interfere with epilepsy by affecting the granule cells of DG. Therefore, we further observed the effect of focal LFS on the EEG activity of the kindling focus and accidently found a polarity-specific effect of LFS; we also investigate the effect of LFS of the EC on neuronal activities and epileptic discharges of DG in maximal dentate activation (MDA) seizure model in rats.1Therapeutic time window of low-frequency stimulation at entorhinal cortex for amygdaloid-kindling seizures in ratsLFS at the EC immediately or4seconds after kindling stimulation exhibited a strong anticonvulsive effect and4seconds-delayed LFS exhibited better effect than immediate LFS on both kindling and kindled seizures. However, LFS delivered after the cessation of AD or10seconds after the kindling stimulation augmented the epileptic activity in kindling progression. In addition, the immediate and4seconds-delayed LFS groups showed less decrease in AD threshold and more increase in the current intensity difference between AD threshold and'generalized seizure threshold. These results provide direct evidence that LFS of the EC can control temporal lobe epilepsy, and confirmed that there is a "time window" in AD duration for LFS therapy, indicating the time delay of closed-loop may be the key factor to LFS clinical treatment and optimization of stimulation timing is crucial.2Polarity-dependent effect of low-frequency stimulation on amygdaloid kindling in ratsBipolar LFS in the same direction of polarity as the kindling stimulation but not in the reverse direction retarded kindling acquisition. And anodal rather than cathodal monopolar LFS inhibited kindling acquisition and kindled seizures. Bipolar LFS showed a stronger anti-epileptic effect than monopolar LFS. Furthermore, anode of LFS (both bipolar and monopolar) deceased, while cathode of LFS increased the power of EEG of the amygdala in normal and kindled rats; the main changes in power of LFP frequency bands were in the delta (0.5-4Hz) band, which was specifically increased during kindling acquisition.Our results provide the first evidence that the effect of LFS at kindling focus is polarity-dependent, which may be due to the different effects of the anode and cathode of LFS on the activity of amygdala, especially on the delta band oscillation. It is likely that the electrode polarity is a key factor affecting the clinical effect of LFS on epilepsy.3Preliminary study on potential circuit mechanism of the antiepileptic effect of low-frequency stimulation at the entorhinal cortex in urethane-anesthetized ratsIn urethane-anesthetized rats, we found that:(1) a single pulse stimulation of the EC and PP pathway induced100-300ms inhibition in neuronal firing in the ipsilateral DG; LFS (15min) of the EC reduced the ipsilateral firing rate of DG neurons with an accumulation effect.(2) pre-treatment with LFS of the ipsilateral EC inhibited MDA induced by kindling stimulation at the contralateral CA3, demonstrated by increased MDA latency, decreased MDA duration and AD duration (ADD); LFS delivered together with kindling stimulation only increased the MDA latency but had no effect on MDA duration and ADD; however, LFS delivered after kindling stimulation prolonged MDA duration and ADD, and even regulated AD frequency. Thus, our results suggest LFS at the EC may control the complex focal TLE, possibly through inhibiting the neuronal activity in the DG, and the timing of LFS onset may be important for using LFS at the EC to treat epilepsy. The DG neurons may be one therapeutic target for clinical treatment of complex focal TLE. We also found the frequency of ADs in the DG, induced by kindling stimulation of the contralateral CA3, can be reglated by LFS of the ipsilateral EC. These results suggest that a seizure may be accomplished by three individual brain regions (the inducing region:the contralateral CA3; the frequency regulating region:the ipsilateral EC; and the discharge region:the ipsilateral DG).In conclusion, we found LFS of the EC and kindling focus inhibit epileptogenesis and epileptic seizures. The antiepileptic effect of LFS of the EC exhibits the "time-window" phenomenon, and inhibition of DG neuronal firing may be involved in the antiepileptic mechanisms. LFS at the kinlding focus shows a polarity dependent property, which may be due to the different effects of the anode and cathode of LFS on the delta band oscillation, suggesting the electrode polarity, especially polarity for anode current, is a key factor affecting the clinical effect of LFS on epilepsy. In addition, we also found kindling stimulation at the contralateral CA3induces ADs at DG, while LFS EC can regulate the AD frequency, suggesting that a seizure may be carried out by three individual brain regions (the inducing region:the contralateral CA3; the frequency regulating region:the ipsilateral EC; and the discharge region:the ipsilateral DG), which provides direct evidence for the network hypothesis of epilepsy.Therefore, the present study reveales some important characteristics and mechanisms for LFS interfering with kindling seizures, and also preliminarily found a focal network characteristic of TLE. These results provide an important experimental basis for clinical application of LFS treatment of epilepsy. |
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