Temporal lobe epilepsy induces intrinsic alterations in Na channel gating in layer II medial entorhinal cortex neurons
Research Highlights
►Layer II medial entorhinal cortex neurons are hyperexcitable in TLE. ►Hyperexcitability is due to network and intrinsic alterations. ►Sodium current inactivation is delayed in layer II mEC neurons in TLE. ►Resurgent and persistent Na current densities are increased in TLE. ►Staining for NaV1.6 and NaV1.2 isoforms is enhanced in TLE.
Introduction
Temporal lobe epilepsy (TLE) is the most common form of adult epilepsy that involves the limbic structures of the temporal lobe including the entorhinal cortex (EC). The EC receives input from the parahippocampus, prefrontal cortex, and frontal cortex (Apergis-Schoute et al., 2006). This activity is then sent to the hippocampus via the perforant path and the temporoammonic path (TAP) (Burwell, 2000). The EC is subdivided into five main cortical layers with layers I-III superficial and layers IV-V deep layers. Layer II consists of non-stellate and stellate neurons and receives excitatory input from the perirhinal cortex, parasubiculum, olfactory structures as well as structures of the EC (Witter et al., 1989) and form the major excitatory input into the dentate gyrus (DG) and CA3 via the perforant path and the TAP.
In TLE, both animal models and patient studies have shown a decreased volume of the EC (Jutila et al., 2001, Bartolomei et al., 2005), corresponding to substantial loss of layer III neurons within the superficial layers of the mEC (Du et al., 1993). Although mEC layer II neurons are spared, they become hyper-excitable, displaying prolonged excitatory synaptic responses to stimulation of the EC deep layers (Bear et al., 1996). This increase in neuronal activity ultimately leads to an excessive excitatory input onto DG neurons of the hippocampus, further exciting the hippocampal–EC circuit (Kobayashi et al., 2003). Potential mechanisms for the hyper-excitability include reduced inhibitory input onto mEC layer II neurons (Kumar and Buckmaster, 2006), hyper-excitability of remaining mEC layer III neurons, providing enhanced synaptic activity via stimulation of the TAP (Ang et al., 2006), and synaptic re-organization within mEC layer II, although the latter has been recently suggested not to exist (Kumar et al., 2007). In addition to altered synaptic networks, changes intrinsic to the neuron, including modulations in ion channel activity, could also be involved.
In hippocampal neurons from animal seizure models, Na channel gating and expression levels are altered in a manner that would favor an increase in neuronal excitability (Ketelaars et al., 2001, Agrawal et al., 2003, Aronica et al., 2001, Whitaker et al., 2001, Vreugdenhil et al., 1998). Na channels are comprised of an α-subunit, and a variable number of auxiliary β-subunits (Catterall, 2000). Neurons are known to express multiple Na channel isoforms (Kress and Mennerick, 2008, Candenas et al., 2006) with the highest expression density along the axonal initial segment (AIS), a specific region near the start of the axon and the site for action potential (AP) initiation. In view of the importance of Na channels in initiating and propagating APs it is not surprising that altered activity and expression of Na channels could be pro-excitatory.
In this study we show that mEC layer II neurons from TLE animals continue to be hyper-excitable when devoid of synaptic input. We hypothesize that alteration's in Na channel activity and expression in TLE neurons account, in part, for this continued hyper-excitability. We propose that the changes in Na channel behavior, together with synaptic network changes, contribute to the hyper-excitability of mEC layer II neurons, altering the threshold for seizure initiation and spread throughout the EC-hippocampal circuit.
Section snippets
Animals
All animal experiments were conducted in accordance with the guidelines established by the National Institutes of Health guide for the Care and Use of Laboratory Animals and were approved by the University of Virginia's Institute of Animal Care and Use Committee. Fifteen adult male Sprague–Dawley rats (250–300 g) received a bipolar twisted pair of stainless steel electrodes to either hemisphere unilaterally in the posterior ventral hippocampus for stimulation and recording (coordinates from
mEC layer II non-stellate and stellate neurons are intrinsically hyper-excitable in TLE
mEC layer II non-stellate and stellate neurons were visually identified using infra red video microscopy and distinguished by their unique AP firing characteristics (Alonso and Klink, 1993, Tahvildari and Alonso, 2005). Although APs in both neuron subtypes had a fast after hyperpolarization (fAHP) followed by a depolarizing after potential (DAP) and a medium after hyperpolarization (mAHP), non-stellate neurons typically had smaller amplitude fAHPs and DAPs than stellate neurons (Fig. 1, Fig. 2
Discussion
Medial entorhinal cortex layer II neurons become hyperexcitable in TLE, leading to potential increased excitatory drive onto the hippocampus (Buckmaster and Dudek, 1997). The mechanisms for this hyper-excitability have focused around alterations in the synaptic network by virtue of the fact that intrinsic changes within the neurons themselves are not thought to occur (Bear et al., 1996, Kobayashi et al., 2003, Kumar et al., 2007). In this study we show that TLE mEC layer II neurons evoke APs at
Conclusion
In animal models of TLE, mEC layer II neurons are spared and become hyper-excitable leading to an increased net excitatory input into the hippocampus via the DG and CA3 (Kumar and Buckmaster, 2006). Here we report an additional intrinsic component for the hyper-excitability of mEC layer II neurons in TLE; namely a change in Na channel activity and expression levels. These changes would be additive to the extensive changes in synaptic activity reported for the mEC in TLE (Kumar et al., 2007,
Acknowledgments
This work was supported by National Institutes of Health-National Institutes of Neurological Disorders and Stroke grants R21NS061069 (MKP & EHB), The Epilepsy Foundation Predoctoral Research Fellowship and 1F31NS064694 NINDS (NJH). We would like to thank Carl Lynch and Suzanne M. Moenter for useful editorial comments and John Williamson, Ravi Katari, and Susanna K. Lynch for expert technical assistance.
References (41)
- et al.
The evolution of a rat model of chronic spontaneous limbic seizures
Brain Res.
(1994) - et al.
Molecular diversity of voltage-gated sodium channel alpha and beta subunit mRNAs in human tissues
Eur. J. Pharmacol.
(2006) From ionic currents to molecular mechanisms: the structure and function of voltage-gated sodium channels
Neuron
(2000)- et al.
Preferential neuronal loss in layer III of the entorhinal cortex in patients with temporal lobe epilepsy
Epilepsy Res.
(1993) - et al.
Molecular and functional changes in voltage-dependent Na(+) channels following pilocarpine-induced status epilepticus in rat dentate granule cells
Neuroscience
(2003) - et al.
Sodium currents in isolated rat CA1 pyramidal and dentate granule neurones in the post-status epilepticus model of epilepsy
Neuroscience
(2001) - et al.
Self-sustaining limbic status epilepticus induced by 'continuous' hippocampal stimulation: electrographic and behavioral characteristics
Epilepsy Res.
(1989) - et al.
Altered subthreshold sodium currents and disrupted firing patterns in Purkinje neurons of Scn8a mutant mice
Neuron
(1997) - et al.
Sodium currents in isolated rat CA1 neurons after kindling epileptogenesis
Neuroscience
(1998) - et al.
Changes in the mRNAs encoding voltage-gated sodium channel types II and III in human epileptic hippocampus
Neuroscience
(2001)
Functional organization of the extrinsic and intrinsic circuitry of the parahippocampal region
Prog. Neurobiol.
Increased persistent sodium currents in rat entorhinal cortex layer V neurons in a post-status epilepticus model of temporal lobe epilepsy
Epilepsia
Differential electroresponsiveness of stellate and pyramidal-like cells of medial entorhinal cortex layer II
J. Neurophysiol.
Massive and specific dysregulation of direct cortical input to the hippocampus in temporal lobe epilepsy
J. Neurosci.
Ultrastructural organization of medial prefrontal inputs to the rhinal cortices
Eur. J. Neurosci.
Induction of neonatal sodium channel II and III alpha-isoform mRNAs in neurons and microglia after status epilepticus in the rat hippocampus
Eur. J. Neurosci.
Entorhinal cortex involvement in human mesial temporal lobe epilepsy: an electrophysiologic and volumetric study
Epilepsia
Responses of the superficial entorhinal cortex in vitro in slices from naive and chronically epileptic rats
J. Neurophysiol.
Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis
Epilepsia
Network properties of the dentate gyrus in epileptic rats with hilar neuron loss and granule cell axon reorganization
J. Neurophysiol.
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