Animal Studies

The proalgesic transient receptor potential ankyrin 1 (TRPA1) channel, expressed by a subpopulation of primary sensory neurons, has been implicated in various pain models in mice. However, evidence in rats indicates that TRPA1 conveys nociceptive signals elicited by channel activators, but not those associated with tissue inflammation or nerve injury. Here, in rats, we explored the TRPA1 role in mechanical allodynia associated with stimulation of peptidergic primary sensory neurons (neurogenic inflammation) and moderate (partial sciatic nerve ligation, pSNL) or severe (chronic constriction injury, CCI) sciatic nerve injury.

The temporomandibular joint osteoarthritis (TMJ-OA) is characterized by progressive cartilage degradation, subchondral bone erosion, and chronic pain, leading to articular damage and chewing dysfunction. Studies have shown that interleukin-1β (IL-1β) plays a critical role in the development of TMJ-OA. Transglutaminase 2 (TG2) has been identified as a marker of chondrocyte hypertrophy and IL-1β was able to increase TG2 expression in chondrocytes. Therefore, the aim of this study was to explore the ability of TG2 inhibitors to suppress TMJ-OA progression.

Chronic pain is a significant health problem associated with disability and reduced quality of life. Current management of chronic pain is inadequate with only modest effects of pharmacological interventions. Thus, there is a need for the generation of analgesics for treating chronic pain. While preclinical and clinical studies demonstrate the analgesic effects of testosterone, clinical use of testosterone is limited by adverse androgenic effects. Selective androgen receptor modulators (SARMs) activate androgen receptors and overcome treatment limitations by minimizing androgenic side effects. Thus, we tested if daily soluble SARMs or a SARM-loaded microparticle formulation alleviated muscle hyperalgesia in a mouse-model of widespread pain (male and female C57BL/6J mice). We tested if the analgesic effects of the SARM-loaded microparticle formulation was mediated through androgen receptors by blocking androgen receptors with flutamide pellets. In vitro and in vivo release kinetics were determined for SARM-loaded microparticles. Safety and toxicity of SARM treatment was determined using serum cardiac and liver toxicity panels, heart histology, and conditioned place preference testing. Subcutaneous daily SARM administration, and 2 injections, I week apart, of SARM-loaded microparticles alleviated muscle hyperalgesia in both sexes and was prevented with flutamide treatment. Sustained release of SARM, from the microparticle formulation, was observed both in vitro and in vivo for 4 weeks. SARM treatment produced no cardiac or liver toxicity and did not produce rewarding behaviors. These studies demonstrate SARM-loaded microparticles alleviate muscle pain, release drug for a sustained period, are safe, and may serve as a potential therapeutic for chronic muscle pain.

Apolipoprotein E (ApoE) is an apolipoprotein involved in lipid metabolism and is primarily responsible for lipid transport and cholesterol homeostasis in the central nervous system (CNS). The aim of this study is to explore the role of ApoE in the pathological development of neuropathic pain. First, we examined the location of ApoE in the dorsal root ganglion (DRG) and spinal cord in male mice using immunohistochemistry, and found that ApoE was predominantly expressed in DRG satellite glial cells (SGCs) and macrophages and spinal cord astrocytes. Using a spinal nerve ligation (SNL)-induced neuropathic pain mouse model, we found that nerve injury caused an increase in ApoE expression in the injured DRGs, but not in the spinal cord after SNL surgery. Furthermore, we observed reduced SNL-induced pain hypersensitivity in ApoE knockout mice compared to wild-type mice. Moreover, an antisense oligonucleotide (ASO) targeting the Apoe gene sequence, which was microinjected into the DRG or administered intrathecally, not only reduced ApoE expression in DRG but also attenuated SNL-induced pain hypersensitivity. Finally, we found that a tyrosine kinase receptor AXL, which was previously demonstrated to contribute to neuropathic pain, may mediate ApoE function under neuropathic pain condition. In conclusion, our data suggest that ApoE in DRG promote pain hypersensitivity via the DRG membrane receptor AXL in neurons under neuropathic pain conditions. This study revealed a novel mechanism between lipid homeostasis and neuropathic pain.

Manipulating the activity of ventral tegmental area dopamine (VTA DA) neurons can drive nocifensive reflexes, and their firing rates are reduced following noxious stimuli. However, the pain-relevant inputs to the VTA remain incompletely understood. In this study, we used male and female mice in combination with identified dopamine and GABA neurons in the VTA that receive excitatory inputs from the periaqueductal grey (PAG), a nexus of ascending pain information. We tested whether PAG-VTA synapses undergo functional plasticity in response to a pain model using optical stimulation in conjunction with slice electrophysiology. We found that acute carrageenan inflammation does not significantly affect the strength of excitatory PAG synapses onto VTA DA neurons. However, at the PAG synapses on VTA GABA neurons, the subunit composition of NMDA receptors is altered; the complement of NR2D subunits at synaptic sites appears to be lost. Thus, our data support a model in which injury initially alters synapses on VTA GABA neurons. Over time, these alterations may increase tonic inhibition of VTA DA neurons leading to their reduced firing.Following a focal injury, the firing rate of dopamine neurons of the ventral tegmental area (VTA) decreases, despite a lack of direct innervation from the periphery. Here we assess the functional changes between a primary node of nociceptive output, the periaqueductal gray (PAG), and the VTA after peripheral inflammation. We find that synaptic strength at PAG-to-VTA dopamine neuron synapses is unaffected following inflammatory injury, but find a change in subunit composition of NMDARs at PAG synapses on the inhibitory neurons of the VTA.

Veratramine may have a potential therapeutic effect for diabetic peripheral neuropathy (DPN).

In the present study the interaction of cannabinoid, PhAR-DBH-Me [(, )-18-((1,4)-5-methyl-2,5-diazabicyclo[2.2.1]heptan-2-yl)-18-oxooctadec-9-en-7-ylphenyl-acetate] and tramadol in two neuropathy models, as well as their possible toxic effects, was analyzed. The anti-allodynic effect of PhAR-DBH-Me, tramadol, or their combination, were evaluated in neuropathic rats. Furthermore, the effective dose 35 (as the 35 % of the anti allodynic effect) was calculated from the maximum effect of each drug. Moreover, the isobolographic analysis was performed to determine the type of interaction between the drugs. A plasma acute toxicity study was carried out to assess the hepatic, renal, and heart functions after an individual or combined administration of the drugs, as well as histology using the hematoxylin-eosin or Masson-trichome method. PhAR-DBH-Me, tramadol, and their combination produced an antiallodynic effect on spinal nerve ligation (SNL) and cisplatin-induced neuropathic pain in rats. Moreover, PhAR-DBH-Me and tramadol combination showed a synergistic interaction in neuropathic pain rats induced by SNL but not for cisplatin-induced neuropathy. On the other hand, changes in renal and hepatic functions were not observed. Likewise, analysis of liver, kidney and heart histology showed no alterations compared with controls. Results show that the combination of PhAR-DBH-Me and tramadol attenuates the allodynia in SNL rats; the acute toxicology analysis suggests that this combination could be considered safe in administered doses.

Chronic pain is undertreated in people with Alzheimer's disease (AD) and better understanding of the underlying mechanisms of chronic pain in this neurodegenerative disease is essential. Neuropathic pain and AD share a significant involvement of the peripheral immune system. Therefore, we examined the development of nerve injury-induced allodynia in TASTPM (APPsweXPS1.M146V) mice and assessed monocytes/macrophages at injury site. TASTPM developed partial allodynia compared to WT at days 7, 14 and 21 days after injury, and showed complete allodynia only after treatment with naloxone methiodide, a peripheralized opioid receptor antagonist. Since macrophages are one of the sources of endogenous opioids in the periphery, we examined macrophage infiltration at injury site and observed that CD206/MHCII cells were more numerous in TASTPM than WT. Accordingly, circulating TASTPM Ly6C (classical) monocytes, which are pro-inflammatory and infiltrate at the site of injury, were less abundant than in WT. In experiments, TASTPM bone marrow-derived macrophages showed efficient phagocytosis of myelin extracts containing amyloid precursor protein, acquired CD206/MHCII phenotype, upregulated mRNA expression of proenkephalin () and accumulated enkephalins in culture media. These data suggest that in TASTPM nerve-injured mice, infiltrating macrophages which derive from circulating monocytes and may contain amyloid fragments, acquire M2-like phenotype after myelin engulfment, and release enkephalins which are likely to inhibit nociceptive neuron activity via activation of opioid receptors.