HCN-Channel-Dependent Hyperexcitability of the Layer V Pyramidal Neurons in IL-mPFC Contributes to Fentanyl-Induced Hyperalgesia in Male Rats

Opioids are often first-line analgesics in pain therapy. However, prolonged use of opioids causes paradoxical pain, termed opioid-induced hyperalgesia (OIH). The infralimbic medial prefrontal cortex (IL-mPFC) has been suggested to be critical in inflammatory and neuropathic pain processing through its dynamic output from layer V pyramidal neurons. Whether OIH condition induces excitability changes of these output neurons and what mechanisms underlie these changes remains elusive. Here, with combination of patch-clamp recording, immunohistochemistry, as well as optogenetics, we revealed that IL-mPFC layer V pyramidal neurons exhibited hyperexcitability together with higher input resistance. In line with this, optogenetic and chemogenetic activation of these neurons aggravates behavioral hyperalgesia in male OIH rats. Inhibition of these neurons alleviates hyperalgesia in male OIH rats but exerts an opposite effect in male control rats. Electrophysiological analysis of hyperpolarization-activated cation current (Ih) demonstrated that decreased Ih is a prerequisite for the hyperexcitability of IL-mPFC output neurons. This decreased Ih was accompanied by a decrease in HCN1, but not HCN2, immunolabeling, in these neurons. In contrast, the application of HCN channel blocker increased the hyperalgesia threshold of male OIH rats. Consequently, we identified an HCN-channel-dependent hyperexcitability of IL-mPFC output neurons, which governs the development and maintenance of OIH in male rats.

Automated summary

Opioid-induced hyperalgesia (OIH) is a paradoxical pain condition caused by prolonged use of opioids. In this study, we found that layer V pyramidal neurons in the infralimbic medial prefrontal cortex (IL-mPFC) exhibit hyperexcitability in male rats with OIH, leading to aggravated behavioral hyperalgesia. Inhibition of these neurons alleviates OIH pain in males but worsen pain in control rats. We also identified that the reduced hyperpolarization-activated cation current (Ih) and decreased HCN1 expression in IL-mPFC output neurons contribute to the hyperexcitability and maintenance of OIH pain in male rats.