Role of HCN channels in neuronal hyperexcitability after subarachnoid hemorrhage in rats

Disruption of ionic homeostasis and neuronal hyperexcitability contribute to early brain injury after subarachnoid hemorrhage (SAH). The hyperpolarization-activated/cyclic nucleotide (HCN)-gated channels play critical role in the regulation of neuronal excitability in hippocampus CA1 region and neocortex, in which the abnormal neuronal activities are more readily provoked. This study was to investigate the interactions between HCN channels and hyperneuronal activity after experimental SAH. The present results from wholecell recordings in rat brain slices indicated that (1) perfusion of hemoglobin (Hb)-containing artificial CSF produced neuronal hyperexcitability and inhibited HCN currents in CA1 pyramidal neurons, (2) nitric oxide/Spermine (NO/Sp), a controlled releaser of nitric oxide, attenuated neuronal excitability and enhanced HCN currents in CA1 pyramidal neurons, while L-nitroarginine (L-NNA), an inhibitor of nitric oxide synthase, reduced theHCNcurrents;and(3) the inhibitory action ofHbonHCNcurrentswasreversed by application of NO/Sp, which also reduced neuronal hyperexcitability; conversely, L-NNA enhanced inhibitory action of Hb on HCN currents. Additionally, Hb perfusion scavenged the production of nitricoxide and decreased the expression of HCN1 subunitsin CA1 region. IntheratSAHmodel,theexpressionofHCN1, both at mRNA and protein level, decreased in hippocampus CA1 region at 24 h and more pronounced at 72 h after SAH. These observations demonstrated a reduction of HCN channels expression after SAH and Hb reduced HCN currents in hippocampus CA1 pyramidal neurons. Inhibition of HCN channels by Hb may be a novel pathway for inducing the hyperneuronal excitability after SAH. © 2012 the authors.