Abstract

Chronic pain after peripheral nerve injury is associated with afferent hyper-excitability and upregulation of HCN-mediated (hyperpolarization-activated, cyclic nucleotide-regulated) I_H pacemaker currents in sensory neurons. HCN channels thus constitute an attractive target for treating chronic pain. HCN channels are ubiquitously expressed; analgesics targeting HCN1-rich cells in the PNS must spare the cardiac pacemaker current (carried mostly by HCN2 and 4) and the CNS (where all four isoforms are expressed). The alkylphenol general anesthetic propofol (2,6-di-iso-propylphenol) selectively inhibits HCN1 channels versus HCN2-4 and exhibits a modest pharmacokinetic preference for the periphery. Consequently, we hypothesized that propofol, and congeners, should be antihyperalgesic. Alkyl-substituted propofol analogs have different rank-order potencies with respect to HCN1 inhibition, GABA_A receptor (GABA_A-R) potentiation and general anesthesia. Thus, 2,6- and 2,4-di-tert-butylphenol (2,6- and 2,4-DTBP) are more potent HCN1 antagonists than propofol while 2,6- and 2,4-di-sec-butylphenol (2,6- and 2,4-DSBP) are less potent. In contrast, DSBPs, but not DTBPs, enhance GABA_A-R function and are general anesthetics. 2,6-DTBP retained propofol's selectivity for HCN1 over HCN2-4. In a peripheral nerve ligation model of neuropathic pain, 2,6-DTBP and subhypnotic propofol are antihyperalgesic. The findings are consistent with these alkylphenols exerting analgesia via non-GABA_A-R targets and suggest that antagonism of central HCN1 channels may be of limited importance to general anesthesia. Alkylphenols are hydrophobic, and thus potential modifiers of lipid bilayers, but their effects on HCN channels are due to direct drug-channel interactions as they have little bilayer-modifying effect at therapeutic concentrations. The alkylphenol antihyperalgesic target may be HCN1 channels in the damaged PNS.