Dendritic hyperpolarization-activated cation channels in entorhinal cortical neurons and temporal lobe epilepsy

Approximately 50 million people worldwide suffer from epilepsy. Chronic temporal lobe epilepsy (TLE) is one of the most prevalent forms of the syndrome. Currently this disorder is predominantly untreatable with medicines. It often occurs after a traumatic head injury (as, for example, that caused by a car accident) or fits induced by high fever. There is a delay (the so-called ‘latent period’) between the precipitating insult and the occurrence of spontaneous fits (seizures; defined as the onset of chronic TLE). It is during the latent period that changes in the brain leading to spontaneous fits occur. To obtain better treatment for chronic TLE, it is important to understand these changes. Ion channels are specialized proteins present in the membranes of brain nerve cells and are important for determining their function. Using models that replicate many if the features of the the human condition, I have recently shown that during the latent period, a particular ion channel, the h-channel, is persistently reduced in number in the cortex (an area of the brain where seizures are generated). I now wish to investigate more about the role of this channel in seizure generation. I also wish to understand more about how the expression of this channel is regulated in neurons. To do this, I am using a variety of state-of-the art techniques including recording electrical currents produced by activation of ion channels from single, genetically modified cortical nerve cells. The possible ramifications of this research are manifold, including a better understanding the mechanisms underlying TLE and the identification of novel treatment strategies.

Temporal Lobe epilepsy (TLE) is the most common, treatment resistant form of human adult epilepsy. A quiescent period (the ?latent? period) occurs between the precipitating insult (e.g. head injury) and the onset of spontaneous seizures (chronic TLE). Most cellular and network level changes leading to chronic TLE occur during the latent period. However, little is known about these alterations. Understanding these would be beneficial for identifying the underlying mechanisms of this condition.
The entorhinal cortex (EC) is a key area of the brain involved in this disorder. During TLE, the EC layer III neurones, particularly, provides the predominant excitatory drive to the subiculum and hippocampus, the other brain regions involved in seizure generation. I have recently demonstrated that the hyperpolarization-activated cation current (Ih) is persistently reduced in these neurones during the latent period (Shah et al., Neuron, 2004). HCN subunits (molecular correlates of Ih) were also significantly reduced. The decrease in Ih is likely to have contributed to their enhanced excitability during the latent period. Further, Ih downregulation also occurs in human cortical neurons during chronic TLE (Wierschke et al., Proc. Phys. Soc., 2006). It is, however, unknown whether the decline in Ih is a prerequisite for epileptogenesis. The possible molecular mechanisms that underlie HCN downregulation during the latent period are also unknown. The proposed study aims to answer these questions. Specifically, brain slices will be made from transgenic mice either lacking or overexpressing HCN subunits in forebrain neurons only in an inducible manner. Electrophysiological recordings will be made from EC layer III neurones present in these slices to explore the involvement of Ih in determining EC excitability. Using in vivo electroencephalographic recordings, these mice will also be used to investigate how alterations in Ih would affect EC network activity, seizure threshold and the latent period duration. Finally, recent evidence suggests that calcium/calmodulin dependent kinase, CaMKII may regulate HCN expression. As a first step towards understanding the possible molecular mechanisms that may cause HCN downregulation in EC neurones during the latent period, I will investigate whether a decrease in CaMKII expression causes Ih downregulation in these cells using molecular/biochemical methods. Whether there are changes in CAMKII expression or activity during the latent period will also be examined using animal models and biochemical methods. The results will provide important information regarding the significance of Ih plasticity during the latent period and the possible mechanisms that initiate the TLE process.