Opioids are often rst-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 in ammatory 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 aggravate 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 identi ed an HCN-channel-dependent hyperexcitability of IL-mPFC output neurons, which governs the development and maintenance of OIH in male rats.

Opioids are the frontline analgesics for managing various types of pain. Paradoxically, repeated use of opioid analgesics may cause an exacerbated pain state known as opioid-induced hyperalgesia (OIH), which hinders effective clinical intervention for sever pain. Although the cellular and molecular mechanisms for OIH have been tested at different level on the pain pathway, little is known about the neural circuits underlying OIH modulation. Previous studies suggest that laterocapsular division of the central nucleus of amygdala (CeLC) is critically involved in the regulation of OIH. The purpose of this study is to clarify the role of the projections from infralimbic medial cortex (IL) to CeLC in OIH modulation. OIH was produced by repeated fentanyl subcutaneous injection in male rats. Immunofluorescence staining revealed that c-Fos-positive neurons were significantly more in the right CeLC in OIH rats than untreated rats. Then, we reported that there were functional projections from glutamate pyramidal neurons in IL to the CeLC and found that IL glutamate release onto CeLC increased after fentanyl administration. However, optogenetic activation of this IL-CeLC circuit prevented OIH by inhibiting the CeLC, while silencing this circuit by chemogenetics exacerbated OIH through activating the CeLC. Combined with the electrophysiology results, the enhanced glutamate release from IL to CeLC was a protective response rather than a reason for OIH generation. We imply that increased glutamate release was a cortical gain of IL to relieve OIH, scaling up IL outputs to CeLC may be an effective neuromodulation strategy to inhibit OIH.

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 aggravate 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.