HCN pacemaker channels and pain: a drug discovery perspective

This article reviews evidence that hyperpolarization-activated, cation nonselective (HCN) channels, the molecular basis of the Ih current, potentially represent valid targets for novel analgesic agents. Ih is a prominent current in many peripheral sensory nerves, with highest current density typically found in large diameter neurons. Recent data suggest that Ih may represent a valid target for the treatment of spontaneous pain and allodynia associated with nerve injury. The majority of available electrophysiological and molecular evidence suggests that fast activating, weakly cyclic adenosine monophosphate (cAMP) sensitive HCN1-based channels may make a significant contribution to Ih, especially in large diameter, mechanosensitive fibers, where the Ih current appears to support abnormal spontaneous firing after nerve injury. In contrast, HCN4 channels seem to play the predominant role in cardiac pacemaker tissue. These observations raise the possibility that HCN1 selective blockers may inhibit pain associated with nerve injury without dramatic effects on heart rate. Development of novel HCN blocking analgesics presents a number of significant technical challenges. Although a number of HCN blockers are available, such as ZD-7288, ivabradine, and others, these drugs inhibit all HCN isoforms with the same potency. As a result, these compounds have powerful effects on heart rate, severely limiting their utility for non-cardiac indications such as pain. Selectivity challenges, mechanisms of compound interaction with the channel, and assay methods are described in detail.