The KCNQ proteins compose a sub-group of the voltage-activated potassium channel family. The family consists of five members (KCNQ1 to 5--also named Kv7.1 to Kv7.5) encoded by single genes, which all give rise to proteins forming slowly activating potassium-selective ion channels. The physiological importance of the KCNQ channel family is emphasized by the fact that mutations in four of the five genes have been linked to human pathologies (KCNQ1 to 4). Here, we present the cloning and characterization of a novel KCNQ5 ortholog from mouse isolated by homology cloning from total mouse brain RNA (GenBank accession number: AY679158). The predicted protein is 95% identical to human KCNQ5. Upon expression in Xenopus oocytes, these proteins form voltage-dependent slowly activating channels with half-maximal activation at -21 mV. Our functional characterization revealed three novel modes of modulation: pH-dependent potentiation by Zn2+ (EC50 = 21.8 microM at pH 7.4), inhibition by acidification (IC50 = 0.75 microM; pKa = 6.1), and regulation by small changes in cell volume. Furthermore, the channels are activated by the anti-convulsant drug retigabine (EC50 = 2.0 microM) and inhibited by the M-current blockers linopiridine and XE-991. Finally, real-time RT-PCR was used to quantify the expression profile in a wide range of mouse tissues. These experiments revealed a relatively broad expression pattern in the nervous system but also expression in other tissues. Highest overall expression levels were observed in cortex and hippocampus. This study shows that murine KCNQ5 channels, in addition to sharing biophysical and pharmacological characteristics with the human ortholog, are tightly regulated by physiological stimuli such as changes in extracellular Zn2+, pH, and tonicity, thus adding to the complex regulation of these channels.