Abstracts

Rationale: M-channels are voltage-gated K+ channels formed by Kv7 subunits. Loss of function mutations in the Kv7.2/7.3 subunits are associated with the human seizure disorder benign familial neonatal convulsions (BFNC). In contrast, retigabine, an opener of Kv7 channels, is a powerful anticonvulsant. Recently, we demonstrated that I_M is present in hippocampal interneurons, potently controlling the interspike interval (Lawrence et al., 2006). Retigabine hyperpolarizes hippocampal interneurons and shunts action potential (AP) firing. However, it is difficult to assess whether this action on Kv7 channels influences GABAergic transmission because retigabine also acts as a positive modulator of GABA_A receptors. To more directly test how Kv7 channel activation impacts GABAergic transmission, we used the anticonvulsant ICA-027243 (ICA), a selective Kv7 channel opener that lacks effects on GABA_A receptors (Rogawski, 2006; Wickenden et al. 2005 SFN abstract).Methods: Whole-cell recordings were made from acute hippocampal slices of either wild type or transgenic mice expressing GFP in somatostatin-positive neurons (Oliva et al. 2000). Voltage clamp (VC) and current clamp (CC) recordings from CA1 GFP+ stratum oriens (SO) neurons, corresponding to O-LM cells, were performed using a K+-based internal solution (IC) and extracellular solution (EC) containing 25 µM DNQX, 100 µM APV, and 5 µM gabazine. To study rhythmic IPSCs in CA1 pyramidal cells, we used a CsCl-based IC and EC containing DNQX and APV only.Results: In accordance with Lawrence et al. (2006), 1 s long voltage steps from -30 to -50 mV revealed I_M (15.2 ± 0.9 pA) in GFP+ SO cells. ICA (10 μM) enhanced I_M to 30.3 ± 4.4 pA (p=0.04) and increased the holding current at -30 mV (from 113 ± 36 pA to 198 ± 29 pA, p=0.004, n=4). In CC, ICA arrested spontaneous firing of GFP+ SO neurons (9.8 ± 3.5 Hz to 0 ± 0 Hz, p=0.049) and hyperpolarized the cells (from -51.1 ± 2.0 mV to -59.3 ± 1.4 mV; p=0.001; n=5). In addition, ICA shunted AP firing in response to 1 s long current steps from rest (5-25 pA; p<0.05 at each current step, n=5). To determine if ICA reduces interneuron network oscillations, we bath applied 5 µM carbachol, which induced barrages of IPSCs occurring at θ frequency (4-12 Hz) in CA1 pyramidal cells (Reich et al., 2005). Within 4 min application of ICA, carbachol-evoked θ IPSCs were attenuated (mean θ power from 20.2 ± 5.6 pA² to 4.7 ± 2.4 pA²; p=0.006; n=6), which was not different from mean θ power in the absence of 5 µM carbachol (p=0.36; n=6).Conclusions: ICA is a powerful activator of I_M in hippocampal interneurons that can profoundly reduce interneuron network activity, particularly during oscillatory activity. This novel mechanism of ICA could play an important role in reducing pathological synchronization of neuronal networks. By virtue of their actions on both interneurons and principal neurons, Kv7 activators may be particularly effective antiseizure agents. Supported by NICHD Intramural funding to CMcB.