Activation of long-term synaptic plasticity causes suppression of epileptiform activity in rat hippocampal slices

Abstract

Electrical stimulation of cerebral targets for the treatment of epilepsy is an area under active investigation. Recent studies have shown that chronic stimulation of the subthalamic nucleus, fornix, or hippocampus may be effective in attenuating seizure frequency in animal models and in patients with intractable epilepsy. However, many questions exist, such as what are the specific electrical parameters, target sites, and mechanisms, etc., which should be investigated in animal studies before considering the routine use of chronic stimulation in epileptic patients. It is also important to understand what happens to neural activity during repetitive pulse stimulation as well as after stimulation. To this end, we hypothesized: (1) activation of synaptic plasticity suppresses epileptiform activity and (2) low frequency stimulation is an effective stimulation protocol for reducing seizure intensity and frequency. We used rat hippocampal brain slices to study how electrical stimulation affects spontaneous and evoked epileptiform activity. Further, we compared low (1 Hz) versus high (100 Hz) frequency stimulation in the same preparation. We found that orthodromic stimulation of the Schaffer collaterals for 10 min reduces the amplitude of normal responses and diminishes epileptiform activity. The onset of suppression by 1 Hz stimulation was gradual, but persistent, whereas the onset of suppression by 100 Hz was rapid; however, the effects of 100 Hz stimulation were transient. Finally, the NMDA antagonist, AP5 reversed the antiepileptic effects achieved by 1 Hz stimulation. Collectively, these data suggest that using different stimulation parameters prolonged electrical stimulation in the hippocampus may be effective in reducing seizure frequency in patients with epilepsy and that suppression by low frequency stimulation may be mediated by long-term depression (LTD).

Introduction

Currently, little is known about the cellular mechanisms underlying the benefits (or risks) of chronic electrical neurostimulation of brain structures in humans [4], [15], [24]. Further, many questions still need to be addressed, such as what are the optimal stimulation frequencies, electrical parameters, effective target sites, and appropriate age groups before implementing brain stimulation treatment in patients with intractable epilepsy. Moreover, it is essential to understand what happens to neural activity during stimulation versus the time immediately following stimulation, which has important implications for safety and efficacy. For example, so-called seizure activity may be controlled (i.e., a reduction in seizure frequency and severity) as long as electrical stimulation is ongoing, but will there be a “rebound” effect and/or an increased frequency of seizures when stimulation is turned off. In this study we define seizure activity as a disturbance of network function due to a sudden, abnormal, excessive, and disorganized discharge of brain cells, whereas clinically recurrent seizures are usually referred to as “epilepsy” or a “seizure disorder.” Furthermore, we define epileptogenesis as the localization of the seizure onset, and epileptiform activity as electrophysiologic traces that resemble those seen in recordings from patients with epilepsy.

Deep brain stimulation (DBS) in human patients with movement disorders has typically used high frequency stimulation protocols (∼130 Hz) [4]. These protocols have been effective at improving symptoms associated with Parkinsonian tremor, rigor, and bradykinesia. Possible targets for DBS in epilepsy include the thalamus, striatum, hippocampus, and the subthalamic nucleus [4], [21]. Recently, Velasco et al. [28], [29] claimed that subacute hippocampal stimulation in epileptic patients blocks intractable temporal lobe epileptogenesis. Furthermore, Olejniczak [25] found that 30 Hz vagal nerve stimulation in humans reduced interictal epileptiform activity, although this application is in the peripheral nervous system where the mechanism of action is also unknown and may or may not be related to mechanisms associated with DBS.

Animal models have also been used to study the effects of either low or high frequency stimulation on epileptiform activity under a wide variety of conditions in hippocampal brain slices and other targets [3], [6], [11], [12], [13], [14], [20], [33]. Interestingly, experiments have yielded important information about suppressing epileptiform or chaotic activity, but most studies did not compare low vs. high frequency stimulation in the same preparation. Moreover, some prior studies have used direct focal stimulation on brain structures exhibiting epileptiform activity where other investigators have chosen to stimulate seizure-gating networks instead, making the results more challenging to interpret. Additionally, there is some debate whether stimulation of grey matter structures or stimulation of white matter tracks might lead to more effective results. Also, the parameters of stimulation have varied tremendously. For example, Weiss et al. [34] showed that a suppressive or “quenching” effect was achieved in amygdala-kindled animals, but only when certain stimulators that emitted a low level (5–15 μA) direct current were used.

High-frequency stimulation of the subthalamic nucleus or substantia nigra pars reticulata was found to suppress seizure-like activity [30], [32] in vivo. Beurrier et al. [5] has claimed that in rat brain slices that 100–250 Hz produced blockade of subthalamic neurons by direct depression of sodium and calcium currents. However, some reported that high frequency stimulation does not always produce inhibition. In fact, Dostrovsky et al. [10] demonstrated in humans that globus pallidus internal segment (GPi) neurons are inhibited after stimulation, whereas thalamic activity increases after stimulation. In another recent study [31], it was found that low frequency stimulation of the kindling focus delays basolateral amygdala kindling in immature rats. Clearly more studies are needed to expand on these issues.

We chose the hippocampal brain slice model in this study for several reasons. The epileptiform field potential multiple spiking seen in hippocampal slices has been compared to the interictal spike discharge seen in EEG recordings of epilepsy patients [2], [35], [36] and so has been referred to as “interictal-like” spikes. This model also allows us to use direct focal stimulation on selected hippocampal subfields. Finally, this model has been used extensively to study mechanisms of synaptic plasticity at low and/or high frequency stimulation since complex neural network responses have been observed and well documented.

We evaluated synaptic responses during stimulation and also before and after stimulation. Further, we challenged some slices with bicuculline to experimentally induce epileptiform activity and then monitored both evoked and spontaneous responses. Other investigators have previously and successfully used bicuculline to elicit hyperexcitability for modeling seizure-like events.

Here we hypothesized that prolonged stimulation reduces excitability and diminishes epileptiform activity and that suppression by low frequency may be mediated by mechanisms similar to those seen in long-term depression (LTD).