Single-cell mRNA expression of HCN1 correlates with a fast gating phenotype of hyperpolarization-activated cyclic nucleotide-gated ion channels (Ih) in central neurons

Hyperpolarization-activated currents (Ih) are key players in shaping rhythm ic neuronal activity. Although candidate genes for Ih channels have been cl oned (HCN1-HCN4), the subunit composition of different native Ih channels i s unknown. We used a combined patch-clamp and qualitative single-cell rever se transcription multiplex polymerase chain reaction (RT-mPCR) approach to analyse HCN1-4 coexpression profiles in four neuronal populations in mouse CNS. Coexpression of HCN2, HCN3 and HCN4 mRNA was detected in single neuron s of all four neuronal cell types analysed. In contrast, HCN1 mRNA was dete cted in neocortical and hippocampal pyramidal neurons but not in dopaminerg ic midbrain and thalamocortical neurons. HCN1 expression was correlated wit h significantly faster activation kinetics on the level of individual neuro ns. Semiquantitative single-cell RT-mPCR analysis demonstrated that HCN1 mR NA expression is at least eightfold higher in cortical neurons than subcort ical neurons. We show that single neurons possess complex coexpression patt erns of Ih candidate genes. Alternative expression of HCN1 is likely to be an important molecular determinant to generate the different neuronal Ih ch annel species adapted to tune either subcortical or cortical network activi ty.