Alpha 2 adrenoceptor agonist guanabenz directly inhibits hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels in mesencephalic trigeminal nucleus neurons

Alpha 2 (α2-) adrenoceptor agonists, such as clonidine or dexmedetomidine, have been found to inhibit hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channels, not only by reducing intracellular cyclic AMP levels but also by directly blocking HCN channels. In this study, we examined the inhibitory effect of guanabenz, a centrally acting α2-adrenoceptor agonist with high specificity for α2A-subtype, on HCN channels in mesencephalic trigeminal nucleus (MTN) neurons which robustly express HCN channels and have been suggested to coexpress α2A-adrenoceptors. By performing whole-cell patch-clamp recording on MTN neurons in brainstem slices, hyperpolarization-activated inward current (Ih) was examined during guanabenz treatment. Guanabenz inhibited Ih in a dose-dependent manner, which was likely to be ZD7288-sensitive HCN current as it did not affect barium-sensitive inward rectifying potassium current. Guanabenz not only inhibited Ih but also shifted the voltage-dependent activation curve to hyperpolarizing potentials. Interestingly, Ih inhibition by guanabenz was not reversed by α2-adrenoceptor antagonist atipamezole treatment or by intracellular cyclic AMP perfusion, suggesting that the inhibition may not result from α2A-adrenoceptor signalling pathway but from direct inhibition of HCN channels. Coherent to our electrophysiological results, single-cell RT-PCR revealed that most MTN neurons lack α2A-adrenoceptor mRNA. Our study demonstrates that guanabenz can directly inhibit HCN channels in addition to its primary role of activating α2A-adrenoceptors.