Abstracts

Rationale: There is strong evidence that cooling can terminate intra- and extracellularly recorded spontaneous epileptiform discharges in slice models of epilepsy. Recently, the presence of transient receptor potential channel V4 (TRPV4) activated at temperatures within the physiological range (>27-34°C) in hippocampal cultures has been reported (J Neurosci. 2007 Feb 14;27(7):1566-75). We have investigated the relationship between TRPV4 channel inactivation and antiepileptic effects of cooling.Methods: Whole-cell patch clamp recordings were carried out from neurons in mouse hippocampal slices and cell cultures perfused with 50 μM 4-aminopyridine or with Mg-free aCSF at an initial temperature of 31-36°C. Cooling was induced at a rate of 0.1–2°C/s by switching to cold aCSF and warming was done via a heater at a slower rate. TRPV4 agonist 4-phorbol-12,13-didecanoate (4α-PDD, Sigma) was dissolved in ethanol and diluted 1:1000 in aCSF.Results: We previously reported that in CA1/CA2 as well as in CA3 HPC neurons, rapid cooling induced changes in membrane current and increased input resistance. In contrast, warming from ambient temperature to 31-36°C increased the whole cell conductance. This effect was also accompanied in many neurons by strong increase in the occurrence of sEPSCs and the amount of background tonic currents, consistent with the opening of cationic channels. In some cells, these effects were mimicked by 4α-PDD (10-20µM) although this effect was greater at warmer temperatures. In some neurons 4α-PDD decreased the effect of cooling in terminating epileptiform discharges. An outward current, reversible upon cooling, was also observed with warming. This was most likely due to alterations in tip or junctional potentials as previously suggested (Payton et al, Science. 1969 Aug 8;165(893):594-7) as it was seen with open tip pipettes.Conclusions: Termination of epileptiform discharges with cooling may be related to reduction of a persistent depolarizing conductance such as that of a TRPV4 channel in some (but not all) neurons of the hippocampal network. The inhibitory effect of a TRPV4 agonist on the effect of cooling most likely reveals a role for transient receptor potential channels in regulating hippocampal network excitability and offers novel potential therapeutic avenues to treat epilepsy.