Injury of peripheral nerves can trigger neuropathic pain, producing allodynia and hyperalgesia via peripheral and central sensitization. Recent studies have focused on the role of the insular cortex (IC) in neuropathic pain. Because the IC is thought to store pain-related memories, translational regulation in this structure may reveal novel targets for controlling chronic pain. Signaling via mammalian target of rapamycin (mTOR), which is known to control mRNA translation and influence synaptic plasticity, has been studied at the spinal level in neuropathic pain, but its role in the IC under these conditions remains elusive. Therefore, this study was conducted to determine the role of mTOR signaling in neuropathic pain and to assess the potential therapeutic effects of rapamycin, an inhibitor of mTORC1, in the IC of rats with neuropathic pain. Mechanical allodynia was assessed in adult male Sprague-Dawley rats after neuropathic surgery and following microinjections of rapamycin into the IC on postoperative days (PODs) 3 and 7. Optical recording was conducted to observe the neural responses of the IC to peripheral stimulation. Rapamycin reduced mechanical allodynia and downregulated the expression of postsynaptic density protein 95 (PSD95), decreased neural excitability in the IC, thereby inhibiting neuropathic pain-induced synaptic plasticity. These findings suggest that mTOR signaling in the IC may be a critical molecular mechanism modulating neuropathic pain.

The anterior cingulate cortex (ACC) is a well-known brain area that is associated with pain perception. Previous studies reported that the ACC has a specific role in the emotional processing of pain. Chronic pain is characterized by long-term potentiation that is induced in pain pathways and contributes to hyperalgesia caused by peripheral nerve injury. The mammalian target of rapamycin (mTOR) signaling, which is involved in synaptic protein synthesis, could be a key factor controlling long-term potentiation in neuropathic pain conditions. Until now, there have been no reports that studied the role of mTOR signaling in the ACC involved in neuropathic pain. Therefore, this study was conducted to determine the relationship of mTOR signaling in the ACC and neuropathic pain. Male Sprague-Dawley rats were subjected to cannula implantation and nerve injury under pentobarbital anesthesia. Microinjection with rapamycin into the ACC was conducted under isoflurane anesthesia on postoperative day (POD) 7. A behavioral test was performed to evaluate mechanical allodynia, and optical imaging was conducted to observe the neuronal responses of the ACC to peripheral stimulation. Inhibition of mTOR by rapamycin reduced mechanical allodynia, down-regulated mTOR signaling in the ACC, and diminished the expressions of synaptic proteins which are involved in excitatory signaling, thereby reducing neuropathic pain-induced synaptic plasticity. These results suggest that inhibiting mTOR activity by rapamycin in the ACC could serve as a new strategy for treating or managing neuropathic pain before it develops into chronic pain.

Recent evidence indicates that motor cortex stimulation (MCS) is a potentially effective treatment for chronic neuropathic pain. However, the neural mechanisms underlying the attenuated hyperalgesia after MCS are not completely understood. In this study, we investigated the neural mechanism of the effects of MCS using an animal model of neuropathic pain. After 10 daily sessions of MCS, repetitive MCS reduced mechanical allodynia and contributed to neuronal changes in the anterior cingulate cortex (ACC). Interestingly, inhibition of protein kinase M zeta (PKMζ), a regulator of synaptic plasticity, in the ACC blocked the effects of repetitive MCS. Histological and molecular studies showed a significantly increased level of glial fibrillary acidic protein (GFAP) expression in the ACC after peripheral neuropathy, and neither MCS treatment nor ZIP administration affected this increase. These results suggest that repetitive MCS can attenuate the mechanical allodynia in neuropathic pain, and that the activation of PKMζ in the ACC may play a role in the modulation of neuropathic pain via MCS.

The insular cortex is an important region of brain involved in the processing of pain and emotion. Recent studies indicate that lesions in the insular cortex induce pain asymbolia and reverse neuropathic pain. Endogenous cannabinoids (endocannabinoids), which have been shown to attenuate pain, are simultaneously degraded by fatty acid amide hydrolase (FAAH) that halts the mechanisms of action. Selective inhibitor URB597 suppresses FAAH activity by conserving endocannabinoids, which reduces pain. The present study examined the analgesic effects of URB597 treatment in the insular cortex of an animal model of neuropathic pain. Under pentobarbital anesthesia, male Sprague–Dawley rats were subjected to nerve injury and cannula implantation. On postoperative day 14, rodents received microinjection of URB597 into the insular cortex. In order to verify the analgesic mechanisms of URB597, cannabinoid 1 receptor (CB1R) antagonist AM251, peroxisome proliferator-activated receptor alpha (PPAR alpha) antagonist GW6471, and transient receptor potential vanilloid 1 (TRPV1) antagonist Iodoresiniferatoxin (I-RTX) were microinjected 15 min prior to URB597 injection. Changes in mechanical allodynia were measured using the von-Frey test. Expressions of CB1R, N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD), and TRPV1 significantly increased in the neuropathic pain group compared to the sham-operated control group. Mechanical threshold and expression of NAPE-PLD significantly increased in groups treated with 2 nM and 4 nM URB597 compared with the vehicle-injected group. Blockages of CB1R and PPAR alpha diminished the analgesic effects of URB597. Inhibition of TRPV1 did not effectively reduce the effects of URB597 but attenuated expression of NAPE-PLD compared with the URB597-injected group. In addition, optical imaging demonstrated that neuronal activity of the insular cortex was reduced following URB597 treatment. Our results suggest that microinjection of FAAH inhibitor into the insular cortex causes analgesic effects by decreasing neural excitability and increasing signals related to the endogenous cannabinoid pathway in the insular cortex.