Functional significance of HCN2/3-mediated I(h) in striatal cells at early developmental stages.

Hyperpolarization-activated cAMP-gated cation currents (I(h)) were recently linked to pre- and postnatal developmental processes in several brain regions, including the ventral telencephalon. To evaluate the role of I(h) in striatal development, we used short-term cultured cells from the lateral ganglionic eminence at embryonic day 14 (E14) and postnatal days 1-3 (P1-3) as well as the embryonic striatal progenitor cell line ST14A. Western blot analysis of the I(h) underlying subunit proteins HCN1-4 revealed strong HCN2 expression in proliferating ST14A cells and weak expression in postmitotic ST14A cells and in cells from the developing brain. We also found HCN3 expression only in ST14A cells at both proliferative and nonproliferative stages but not in short-term cultured striatal cells. In all cases, HCN1 and HCN4 transcripts were below the detection level. Despite the selective protein expression, RT-PCR analysis showed stable expression of HCN2-4 but not HCN1 mRNA in all short-term-cultured striatal cells and in the ST14A cell line. Consistent with the strong protein expression, an I(h) was recorded with features of an HCN2-mediated current in ST14A cells at the proliferative stage and in short-term-cultured E14 cells. Of particular importance is that we detected no currents upon hyperpolarization in the ST14A cells at the nonproliferative stage when only HCN3 protein was present. These results suggest the potential importance of ST14A cells in defining the molecular mechanisms regulating I(h) expression and function.