Indeed, attempts to ameliorate cardiac rhythm disorders have established HCN channels as promising drug targets . However, recent studies have shown that HCN channels are associated not only with cardiac dysfunction, but also with neurological disorders such as neuropathic pain and epilepsy . Thus, pharmacological manipulation of HCN channels represents a great INCB18424 inquirer potential for the development of cures to treat these debilitating diseases. In mammals, HCN channels are encoded by four genes, HCN1�C4, that show distinctive but partially overlapping expression in different tissues and brain regions . In recent years several groups have investigated the phenotypic consequences of knocking out single HCN channel-encoding genes, and genespecific neural defects have been reported . Nevertheless, how the different HCN genes contribute to diverse neurological dysfunctions is still unclear. Moreover, because the Ih blockers developed thus far show no subtype-specificity for the different HCN genes , therapeutic testing of these pharmacological agents will benefit from considering the effects of the total lack of Ih current in different neural outcomes. Taking advantage of the fact that in Drosophila there is only one HCN channel encoding gene, DmIh , we have abolished Ih current by deleting a core region of the channel. This new mutant provides an ideal model to study the possible effects of the lack of the Ih current in the whole organism. In rodents, dopaminergic neurons display characteristic rhythmic spontaneous firing activity, which is dynamically modulated by multiple afferent inputs. Several studies have identified Ih current as an important determinant of this spontaneous firing rate . Moreover, many neurotransmitters target HCN channels to modulate the afferent stimuli-dependent activity of dopaminergic cells . Increasing evidences suggest that HCN channels also play relevant roles in several dopamine-related disorders, such as drug addiction , schizophrenia , or Parkinson disease . In spite of the growing data on Ih modulation of dopaminergic neuronal function , the final consequences of WY 14643 side effects altering this current over the dopamine in vivo output have not been reported. Therefore, we analyzed how impairment of Ih current in DmIh mutant flies may alter dopamine outcome in vivo. DmIh gene is broadly expressed in the Drosophila brain , but precise cell-type localization experiments have not been done. Our results show that indeed Drosophila brain dopaminergic cells express DmIh. Moreover, we provide the first demonstration of significant circadian variation in levels of dopamine in Drosophila head extracts, and show that the daily cycling of dopamine is drastically modified when Ih current is eliminated.