Animal Studies

Endogenous pain inhibition is less efficient in chronic pain patients. Diffuse noxious inhibitory control (DNIC), a form of endogenous pain inhibition, is compromised in women and older people, making them more vulnerable to chronic pain. However, the underlying mechanisms remain unclear. Here, we used a capsaicin-induced DNIC test and resting state functional MRI to investigate the impact of aging and sex on endogenous pain inhibition and associated brain circuitries in healthy rats. We found that DNIC was less efficient in young females compared to young males. DNIC response was lost in old rats of both sexes, but the brain networks engaged during DNIC differed in a sex-dependent manner. Young males had the most efficient analgesia with the strongest connectivity between anterior cingulate cortex (ACC) and periaqueductal gray (PAG). The reduced efficiency of DNIC in young females appeared to be driven by a widespread brain connectivity. Old males showed increased connectivity between PAG, raphe nuclei, pontine reticular nucleus and hippocampus, which may not be dependent on connections to ACC, while old females showed increased connectivity between ACC, PAG and more limbic regions. These findings suggest that distinct brain circuitries including the limbic system may contribute to higher susceptibility to pain modulatory deficits in the elderly population, and sex may be a risk factor for developing age-related chronic pain.

Infection-induced chronic pain is an under-studied pain condition. One example is apical periodontitis, which evokes considerable mechanical allodynia that persists after treatment in 7-12% of patients. Available analgesics often provide incomplete relief. However, a preclinical model to study pain mechanisms associated with apical periodontitis is not available. Here, we report a mouse model of AP to facilitate studies determining mechanisms mediating persistent infection-induced pain. Mice were anesthetized and the left first molar was exposed to the oral environment for 6 weeks. Bone resorption, as an indicator of apical periodontitis, was quantified using micro-computed tomography. Mechanical allodynia was determined using extraoral von-Frey filaments in both male and female mice. The expression of c-fos in the medullary dorsal horn was assessed using immunohistochemistry. Mice with apical periodontitis developed significant mechanical allodynia by day 7 that was maintained for 42 days. Mechanical thresholds were significantly lower in females compared to males. Administration of ibuprofen, morphine or MK-801 reversed mechanical allodynia. Finally, apical periodontitis triggered an up-regulation of c-fos in the medullary dorsal horn. Collectively, this model simulates signs of clinical pain experienced by patients with apical periodontitis, detects sex differences in allodynia and permits the study of peripheral and central trigeminal pain mechanisms.

Low back pain is a chronic, highly prevalent, and hard-to-treat condition in the elderly. Clinical studies indicate that AXL, which belongs to the tyrosine kinase receptor subfamily, mediates pathological pain. However, it is not clear exactly how AXL regulates pain behaviors. In the present study, we used a model of chronic compression of dorsal root ganglion (CCD)-induced neuropathic pain to recreate clinical intervertebral foramen stenosis and related lumbocrural pain to explore whether AXL in primary sensory neurons contributes to this neuropathic pain in rats. Using double-labeling immunofluorescence, we observed that both phosphorylated AXL (p-AXL) and AXL were localized primarily on isolectin B4 (IB4)-positive and calcitonin gene-related peptide (CGRP)-positive neurons, while AXL was also localized in neurofilament-200 (NF200)-positive neurons. CCD-induced pain was associated with the upregulation of AXL mRNA and protein in injured DRGs. Repeated intrathecal administration of the AXL inhibitor, TP0903, or the AXL small interfering RNA (AXL siRNA), effectively alleviated CCD-induced pain hypersensitivities. Moreover, repeated intrathecal administration of either TP0903, or AXL siRNA, reduced the expression of mTOR in injured DRGs, suggesting that mTOR may mediate AXL's actions. These results indicate that the upregulation of DRG AXL may be part of a peripheral mechanism of neuropathic pain via an intracellular mTOR-signaling pathway. Thus, while AXL inhibitors have so far primarily shown clinical efficacy in tumor treatment, AXL intervention could also serve as a potential target for the treatment of neuropathic pain.

Perioperative sleep disturbance is a risk factor for persistent pain after surgery. Clinical studies have shown that patients with insufficient sleep before and after surgery experience more intense and long-lasting postoperative pain. We hypothesize that sleep deprivation alters L-type calcium channels in the dorsal root ganglia (DRG), thus delaying the recovery from post-surgical pain. To verify this hypothesis, and to identify new predictors and therapeutic targets for persistent postoperative pain, we first established a model of postsurgical pain with perioperative sleep deprivation (SD) by administering hind paw plantar incision to sleep deprivation rats. Then we conducted behavioral tests, including tests with von Frey filaments and a laser heat test, to verify sensory pain, measured the expression of L-type calcium channels using western blotting and immunofluorescence of dorsal root ganglia (an important neural target for peripheral nociception), and examined the activity of L-type calcium channels and neuron excitability using electrophysiological measurements. We validated the findings by performing intraperitoneal injections of calcium channel blockers and microinjections of dorsal root ganglion cells with adeno-associated virus. We found that short-term sleep deprivation before and after surgery increased expression and activity of L-type calcium channels in the lumbar dorsal root ganglia, and delayed recovery from postsurgical pain. Blocking these channels reduced impact of sleep deprivation. We conclude that the increased expression and activity of L-type calcium channels is associated with the sleep deprivation-mediated prolongation of postoperative pain. L-type calcium channels are thus a potential target for management of postoperative pain.