Protein kinase C bidirectionally modulates Ih and HCN1 surface expression in hippocampal pyramidal neurons

Abstract

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels attenuate excitability in hippocampal pyramidal neurons. Loss of HCN channel-mediated current (Ih), particularly that mediated by the HCN1 isoform, occurs with the development of epilepsy. Previously, we showed that following pilocarpine-induced status epilepticus, there are two independent changes in HCN function in dendrites: decreased Ih amplitude associated with a loss of HCN1 surface expression, and a hyperpolarizing shift in voltage-dependence of activation (“gating”). The hyperpolarizing shift in gating was attributed to decreased phosphorylation due to loss of p38 MAP kinase activity and increased calcineurin activity; however, the mechanisms controlling Ih amplitude and HCN1 surface expression under epileptic or normal physiological conditions are poorly understood. I sought to investigate phosphorylation as a mechanism regulating Ih amplitude and HCN1 surface expression (as it does HCN gating) in hippocampal principal neurons under normal physiological conditions. I discovered that inhibition of either tyrosine phosphatases or the serine/threonine phosphatases PP1 and PP2A decreased Ih at maximal activation in hippocampal CA1 pyramidal dendrites and pyramidal-like principal (PLP) neuron somata from naïve rats. Furthermore, I found that inhibition of PP1/PP2A decreased HCN1 surface expression, while tyrosine phosphatase inhibition did not. Protein kinase C (PKC) activation reduced Ih amplitude and HCN1 surface expression, while PKC inhibition produced the opposite effect. PP1/2A inhibition and PKC activation both increased the serine phosphorylation state of the HCN1 protein. The effect of PKC activation on Ih was irreversible. These results indicate that PKC bidirectionally modulates Ih amplitude and HCN1 surface expression in hippocampal principal neurons.