M-type K(+) currents (I(K(M))) play a key role in regulating neuronal excitability. In sympathetic neurons, M-channels are thought to be composed of a heteromeric assembly of KCNQ2 and KCNQ3 K(+) channel subunits. Here, we have tried to identify the KCNQ subunits that are involved in the generation of I(K(M)) in hippocampal pyramidal neurons cultured from 5- to 7-day-old rats. RT-PCR of either CA1 or CA3 regions revealed the presence of KCNQ2, KCNQ3, KCNQ4 and KCNQ5 subunits. Single-cell PCR of dissociated hippocampal pyramidal neurons gave detectable signals for only KCNQ2, KCNQ3 and KCNQ5; where tested, most also expressed mRNA for the vesicular glutamate transporter VGLUT1. Staining for KCNQ2 and KCNQ5 protein showed punctate fluorescence on both the somata and dendrites of hippocampal neurons. Staining for KCNQ3 was diffusely distributed whereas KCNQ4 was undetectable. In perforated patch recordings, linopirdine, a specific M-channel blocker, fully inhibited I(K(M)) with an IC(50) of 3.6 +/- 1.5 microM. In 70 % of these cells, TEA fully suppressed I(K(M)) with an IC(50) of 0.7 +/- 0.1 mM. In the remaining cells, TEA maximally reduced I(K(M)) by only 59.7 +/- 5.2 % with an IC(50) of 1.4 +/- 0.3 mM; residual I(K(M)) was abolished by linopirdine. Our data suggest that KCNQ2, KCNQ3 and KCNQ5 subunits contribute to I(K(M)) in these neurons and that the variations in TEA sensitivity may reflect differential expression of KCNQ2, KCNQ3 and KCNQ5 subunits.