HCN Channel Blockers Act Pre‐ and Post‐Synaptically to Inhibit Synaptic Transmission and Gain in the Rat Superior Cervical Sympathetic Ganglion

Hyperpolarization‐activated cation channels encoded by HCN subunits are permeable to Na + and K + ions and carry inward h‐currents. The h‐current (I h ) was originally discovered in the heart, where it is the principal pacemaker current. Ivabradine, a selective blocker of I h , was approved in the US in 2015 as an adjunct to β‐adrenergic blockade for clinical management of heart failure. Recently we observed that sympathetic neurons in the rat superior cervical ganglion (SCG) express prominent I h responses that can be blocked by ZD7288, another selective antagonist. The goal of the present study was to test the hypothesis that I h modulates synaptic transmission in sympathetic ganglia. We did this using dissociated SCG neurons and using acutely isolated intact ganglia, treated with enzymes to permit whole‐cell recording, together with simultaneous presynaptic stimulation and extracellular recording. When synaptic transmission was elicited by submaximal 1 Hz stimulation in the isolated intact SCG, the extracellular postsynaptic compound action potential was inhibited by about 20% by 10–40 μM Ivabradine and by 10–20 μM ZD7288. In dissociated SCG neurons, we measured I h under voltage clamp, fit the data to a Hodgkin‐Huxley model and then used the model to implement virtual I h under dynamic clamp. When neurons were probed with virtual nicotinic fast EPSPs created with dynamic clamp, blocking I h with 10–15 μM ZD7288 increased the threshold‐synaptic conductance (thresh‐g syn ) from 7.0 ± 0.9 nS to 12.1 ± 2.0 nS (p<0.0001) and decreased synaptic gain from 2.2 to 1.7 in response to noisy patterns of virtual synaptic activity that were designed to mimic physiological activity in vivo. This indicates that the native postsynaptic I h normally enhances synaptic amplification to increase postsynaptic spike output by the SCG. When the native I h was reconstituted with virtual I h , synaptic amplification was restored. Shifting the voltage‐dependence of activation to mimic binding of cyclic AMP to HCN channels further modulated thresh‐g syn and synaptic gain. To assess presynaptic actions of I h , we then measured nicotinic cholinergic synaptic currents evoked by minimal presynaptic stimulation of the intact SCG. Ivabradine and ZD7288 both reduced peak EPSC amplitude and total charge by 40 to 50% while also increasing the failure rate. This indicates that native I h normally enhances the probability of acetylcholine release and the amount of release by preganglionic terminals. Taken together the results suggest that clinical use of Ivabradine may have the heretofore unknown effect of reducing peripheral sympathetic activity through its pre‐ and postsynaptic effects on ganglionic transmission. This may have beneficial therapeutic consequences by countering the sympathetic hyperactivity that can drive human hypertension and that exacerbates the progression of heart failure. Support or Funding Information Supported by the Great Rivers Affiliate of the American Heart Association through Grant‐in‐Aid 13GRNT1685007