Animal Studies

Endocannabinoids are showing great promise as effective mediators for controlling joint inflammation and pain. One strategy that could be harnessed to promote endogenous cannabinoid function is to inhibit the enzymatic break down of endocannabinoids locally in the joint. KML29 is an inhibitor of monoacylglycerol lipase (MAGL) activity which has been shown to promote increased 2-arachodonylglycerol (2-AG) levels in the circulation and in peripheral tissues. It is also known that 2-AG can be metabolised via the cyclo-oxygenase-2 (COX-2) pathway leading to the production of pro-inflammatory prostaglandins, which may counteract the effects of 2-AG. Therefore, this study examined the effect of KML29 alone as well as in combination with low-dose celecoxib (CXB) on joint pain and inflammation in the monoiodoacetate (MIA) model of osteoarthritis (OA) pain.

Some individuals recover from the pain of nerve trauma within 12 months or less whereas others experience life-long intractable pain. This transition between reversible pain and the establishment of chronic neuropathic pain is poorly understood. We examined the role of persistent inflammation in the dorsal root ganglia (DRG) in the long-term maintenance of mechanical allodynia; an index of neuropathic pain. Male Sprague-Dawley rats underwent chronic constriction injury (CCI), spared nerve injury (SNI) or sham surgery. Both CCI and SNI animals displayed robust mechanical allodynia in the ipsilateral paw at 7d post-surgery; however, only SNI animals maintained mechanical allodynia at 42d post-surgery. DRGs were extracted at 7d or 42d post-surgery to assess inflammation via rt-qPCR or immunohistochemistry to measure colony stimulating factor 1 (CSF1) expression, satellite glial cell (SGC) activation, presence of Iba1 positive macrophages and interleukin1 β (IL-1β) mRNA levels. Whereas DRGs from SNI animals continued to display inflammatory markers at 42d, those from CCI animals did not. Moreover, the level of allodynia displayed by each individual animal correlated with the extent of DRG inflammation. These data support the hypothesis that the amount of CSF1 immunoreactivity and the persistence of inflammation in ipsilateral DRGs contribute to the difference between transient and persistent mechanical allodynia observed in the CCI and SNI models. We also suggest that feedback loops involving cytokines and neurotransmitters may contribute to increased DRG activity in chronic neuropathic pain. Consequently, targeting persistent CSF1 production and peripheral neuroinflammation may be an effective approach to the management of chronic neuropathic pain.

Previously we reported that a group of inflammatory mediators significantly enhanced resurgent currents in dorsal root ganglion (DRG) neurons. To understand the underlying intracellular signaling mechanism, we investigated the effects of inhibition of extracellular signal-regulated kinases (ERK) and protein kinase C (PKC) on the enhancing effects of inflammatory mediators on resurgent currents in rat DRG neurons. We found that the ERK inhibitor U0126 completely prevented the enhancing effects of the inflammatory mediators on both Tetrodotoxin-sensitive (TTX-S) and Tetrodotoxin-resistant (TTX-R) resurgent currents in both small and medium DRG neurons. U0126 substantially reduced repetitive firing in small DRG neurons exposed to inflammatory mediators, consistent with prevention of resurgent current amplitude increases. The PKC inhibitor Bisindolylmaleimide I also showed attenuating effects on resurgent currents, although to a lesser extent compared to ERK inhibition. These results indicate a critical role of ERK signaling in modulating resurgent currents and membrane excitability in DRG neurons treated with inflammatory mediators. It is also suggested that targeting ERK-resurgent currents might be a useful strategy to reduce inflammatory pain.

Accumulating evidence has demonstrated that the enhanced synaptic plasticity of nociceptive interneurons in the spinal dorsal horn is the basis of central sensitization in neuropathic pain. Our previous results demonstrate that Sirtuin 1 (SIRT1), a nicotinamide adenosine dinucleotide (NAD+)-dependent deacetylase, alleviates neuropathic pain in type 2 diabetes mellitus (T2DM) rats. SIRT1 has also been reported to regulate synaptic plasticity in different brain neurons. However, the role of SIRT1 in synaptic plasticity of spinal dorsal horn neurons remains unknown. In this study, we found that in the spinal dorsal horn of diabetic neuropathic pain (DNP) rats and db/db mice, decreased SIRT1 expression was accompanied by enhanced structural synaptic plasticity. The levels of post-synaptic density protein 95 (PSD-95), growth associated protein 43 (GAP43) and synaptophysin (SYP) increased in the spinal dorsal horn of DNP rats and db/db mice and in high glucose (HG)-cultured primary spinal neurons. Upregulation of spinal SIRT1 by SIRT1 activator SRT1720 relieved pain behavior, inhibited the enhanced structural synaptic plasticity in DNP rats and db/db mice, and decreased the levels of synapse-associated proteins in DNP rats, db/db mice, and HG-cultured spinal neurons. SIRT1 shRNA induced pain behavior, enhanced structural synaptic plasticity in normal rats, and increased synapse-associated proteins levels in normal rats and spinal neurons. Intrathecal AAV-Cre-EGFP into SIRT1 mice also induced pain behavior and enhanced synaptic plasticity of the spinal dorsal horn neurons. These results suggest that SIRT1 plays an important role in the progression of DNP by regulating synaptic plasticity of spinal dorsal horn neurons.

BDNF is a critical contributor to neuronal growth, development, learning, and memory. Although extensively studied in the brain, BDNF is also expressed by primary afferent sensory neurons in the peripheral nervous system. Unfortunately, anatomical and functional studies of primary afferent-derived BDNF have been limited by the availability of appropriate molecular tools. Here, we used targeted, inducible molecular approaches to characterize the expression pattern of primary afferent BDNF and the extent to which it contributes to a variety of pain and itch behaviors. Using a reporter mouse, we found that BDNF is expressed primarily by myelinated primary afferents and has limited overlap with the major peptidergic and non-peptidergic subclasses of nociceptors and pruritoceptors. We also observed extensive neuronal, but not glial, expression in the spinal cord dorsal horn. In addition, because BDNF null mice are not viable and even Cre-mediated deletion of BDNF from sensory neurons could have developmental consequences, here we deleted BDNF selectively from sensory neurons, in the adult, using an advillin-Cre-ER line crossed to floxed BDNF mice. We found that BDNF deletion in the adult altered few itch or acute and chronic pain behaviors, beyond sexually dimorphic phenotypes in the tail immersion, histamine, and formalin tests. Based on the anatomical distribution of sensory neuron-derived BDNF and its limited contribution to pain and itch processing, we suggest that future studies of primary afferent-derived BDNF should examine behaviors evoked by activation of myelinated primary afferents.

Recently, studies have focused on the antihyperalgesic activity of the A3 adenosine receptor (A3AR) in several chronic pain models, but the cellular and molecular basis of this effect is still unknown. Here, we investigated the expression and functional effects of A3AR on the excitability of small- to medium-sized, capsaicin-sensitive, dorsal root ganglion (DRG) neurons isolated from 3- to 4-week-old rats. Real-time quantitative polymerase chain reaction experiments and immunofluorescence analysis revealed A3AR expression in DRG neurons. Patch-clamp experiments demonstrated that 2 distinct A3AR agonists, Cl-IB-MECA and the highly selective MRS5980, inhibited Ca-activated K (KCa) currents evoked by a voltage-ramp protocol. This effect was dependent on a reduction in Ca influx via N-type voltage-dependent Ca channels, as Cl-IB-MECA-induced inhibition was sensitive to the N-type blocker PD173212 but not to the L-type blocker, lacidipine. The endogenous agonist adenosine also reduced N-type Ca currents, and its effect was inhibited by 56% in the presence of A3AR antagonist MRS1523, demonstrating that the majority of adenosine's effect is mediated by this receptor subtype. Current-clamp recordings demonstrated that neuronal firing of rat DRG neurons was also significantly reduced by A3AR activation in a MRS1523-sensitive but PD173212-insensitive manner. Intracellular Ca measurements confirmed the inhibitory role of A3AR on DRG neuronal firing. We conclude that pain-relieving effects observed on A3AR activation could be mediated through N-type Ca channel block and action potential inhibition as independent mechanisms in isolated rat DRG neurons. These findings support A3AR-based therapy as a viable approach to alleviate pain in different pathologies.