AI Summary
The key points of the content are: - Prolonged exposure to opioids leads to opioid-induced hyperalgesia (OIH) and opioid-induced tolerance (OIT). - OIH is driven by HCN2 ion channels in peripheral nociceptors, while OIT is driven by a member of the HCN family located in the central nervous system (CNS). - Inhibition of HCN2 channels alleviates OIH, while inhibition of CNS HCN channels alleviates OIT. - OIH is associated with increased neuronal activity in the dorsal spinal cord, which can be prevented by blockage or deletion of HCN2 channels. - Induction of OIH increases cAMP in nociceptive neurons and causes a shift in the activation curve of HCN2, leading to increased firing of nociceptors. - OIH is driven by a constitutively active μ-opioid receptor (MOR) variant that increases cAMP by coupling constitutively to Gs. - HCN2 ion channels are a potential therapeutic target for the treatment of opioid addiction. In summary, the study identifies HCN2 ion channels as drivers of opioid-induced hyperalgesia and tolerance. OIH is driven by H
Prolonged exposure to opioids causes an enhanced sensitivity to painful stimuli (opioid-induced hyperalgesia, OIH) and a need for increased opioid doses to maintain analgesia (opioid-induced tolerance, OIT), but the mechanisms underlying both processes remain obscure. We found that pharmacological block or genetic deletion of HCN2 ion channels in primary nociceptive neurons of male mice completely abolished OIH but had no effect on OIT. Conversely, pharmacological inhibition of central HCN channels alleviated OIT but had no effect on OIH. Expression of C-FOS, a marker of neuronal activity, was increased in second-order neurons of the dorsal spinal cord by induction of OIH, and the increase was prevented by peripheral block or genetic deletion of HCN2, but block of OIT by spinal block of HCN channels had no impact on C-FOS expression in dorsal horn neurons. Collectively, these observations show that OIH is driven by HCN2 ion channels in peripheral nociceptors, while OIT is driven by a member of the HCN family located in the CNS. Induction of OIH increased cAMP in nociceptive neurons, and a consequent shift in the activation curve of HCN2 caused an increase in nociceptor firing. The shift in HCN2 was caused by expression of a constitutively active μ-opioid receptor (MOR) and was reversed by MOR antagonists. We identified the opioid-induced MOR as a six-transmembrane splice variant, and we show that it increases cAMP by coupling constitutively to Gs. HCN2 ion channels therefore drive OIH, and likely OIT, and may be a novel therapeutic target for the treatment of addiction.