M-current (IM) plays a key role in regulating neuronal excitability. Mutations in Kv7/KCNQ subunits, the molecular correlates of M- channels, are associated with a familial human epilepsy syndrome. We recently found evidence for functional M-channels in glutamatergic axons in the hippocampus. However, the molecular identity of the alpha subunits (KCNQ1-5) underlying M-channels in these axons is not known. We studied the subunit composition of axonal M-channels in the hippocampus using transgenic mice that conditionally express dominant-negative KCNQ2 subunits (Peters C. et al. Nat. Neuroscience). In control mice, extracellular recordings of compound action potentials (fibre volley) from Schaffer collateral glutamatergic fibres, in the presence of moderately elevated [K+]o (~10 mM), showed that the specific M-channel opener retigabine increased the amplitude of the presynaptic fibre volley by a factor 1.22 (n=5). Subsequent application of the M-channel blocker XE991 reduced the amplitude of the fibre volley by a factor 0.72 (n=5). However, in KCNQ2 mutant mice the retigabine effect was virtually absent (factor 1.03, n=6, p=0.01) and the XE991 effect significantly reduced (factor 0.85, n=6, p=0.01). Our results indicate KCNQ2 or KCNQ subunits that combine with KCNQ2 to form hereomeric KCNQ- channels, underlie a major component of M- current in hippocampal glutamatergic fibres. [Supported by NFR/SFF/FUGE/STORFORSK]