Two pore domain potassium (K2P) channels are responsible for background currents that regulate membrane potential and neuronal excitability. Compounds which alter the activity of these channels are predicted to have therapeutic potential in treating CNS disorders. The TREK family of K2P channels (TREK1, TREK2 and TRAAK) have been shown to play an active role in neuroprotection, schizophrenia, depression and pain, whilst TRESK, with high expression in sensory neurons, has a role in nociception (1). Sipatrigine, a neuroprotective agent and a derivative of the anticonvulsant, lamotrigine, is a known antagonist of TREK channels, whilst lamotrigine is thought to primarily inhibit TRESK channels (2, 3). The aim of this study is to clarify differences in the inhibition of these channels by sipatrigine and lamotrigine and investigate the mechanism of inhibition. Currents through wild-type (WT) and mutated human K2P channels transiently expressed in tsA-201 cells were measured using whole-cell patch-clamp electrophysiology in the presence and absence of sipatrigine (100 µM) and lamotrigine (100 µM). Sipatrigine was a potent inhibitor of TREK1 channels (87 ± 2%, mean ± S.E.M., n=19), whilst lamotrigine inhibited TREK1 channels by 30±6% (n=6). TREK2 channels were similarly inhibited by sipatrigine (73 ± 3%, n=10), whilst lamotrigine had little inhibitory effect (13 ± 3%, n=10). Lamotrigine only inhibited TRESK channels by 34 ± 5% (n=8), a similar inhibition to that seen for TREK1. More surprisingly, sipatrigine was found to potently inhibit TRESK channels by 73 ± 3% (n=17). The recent crystal structure of TREK2 bound to a molecule of the anti-depressant, fluoxetine, has revealed several amino acids important for binding, including leucine (L) at position 320 (4). Mutation of the analogous site on TREK1 (L289A) showed a significantly reduced inhibition of 56 ± 7% (n=9) by sipatrigine, however, inhibition by lamotrigine was unaltered (31 ± 9%, n=6). Homology models for TRESK channels have identified two key cavity-facing residues, F145 and F352 required for the effectiveness of certain channel blocking compounds (5). The double mutation of TRESK (F145A_F352A) substantially reduced inhibition by both sipatrigine (10 ± 2%, n=9) and lamotrigine (5 ± 2%, n=11). Our findings show that lamotrigine does indeed inhibit TRESK channels however the compound also inhibits TREK channels. Furthermore, sipatrigine is a potent inhibitor of both TREK and TRESK channels. Mutations of TREK1 and TRESK have demonstrated sites on these channels important for the inhibitory actions of both sipatrigine and lamotrigine. For TREK1 channels, we hypothesise that L289 is important for sipatrigine binding but not for lamotrigine binding. For TRESK channels, F156 and F364 are important residues for inhibition by both compounds.