Animal Studies

Wnt signaling pathway has been investigated extensively for its diverse metabolic and pain modulating mechanisms and recently its involvement has been postulated in the development of neuropathic pain. However, there are no reports as yet on involvement of Wnt signaling pathway in one of the most debilitating neurovascular complication of diabetes, i.e, diabetic peripheral neuropathy (DPN). Thus, in the present study, involvement of Wnt signaling was investigated in DPN using Wnt signaling inhibitors namely LGK974 (Porcupine inhibitor), NSC668036 (Disheveled inhibitor) and PNU74654 (β-catenin inhibitor). Diabetes was induced by a single intraperitoneal injection of streptozotocin (STZ) (50 mg/kg) to male Sprague Dawley rats. Diabetic rats after six weeks of diabetes induction showed increased expression of Wnt signaling proteins in the spinal cord (L4-L6 lumbar segment), dorsal root ganglions (DRGs) and sciatic nerves. Subsequent increase in inflammation, endoplasmic reticulum (ER) stress and loss of intraepidermal nerve fiber density (IENFD) was also observed, leading to neurobehavioral and nerve functional deficits in diabetic rats. Intrathecal administration of Wnt signaling inhibitors (each at doses of 10 and 30 µM) in diabetic rats showed improvement in pain-associated behaviors (heat, cold & mechanical hyperalgesia) and nerve functions (motor, sensory nerve conduction velocities and nerve blood flow) by decreasing the expression of Wnt pathway proteins, inflammatory marker, matrix metalloproteinase 2 (MMP2), ER stress marker, glucose-regulated protein 78 (GRP78) and improving IENFD. All these results signify the neuroprotective potential of Wnt signaling inhibitors in DPN. Perspective: This study emphasizes the involvement of Wnt signaling pathway in diabetic peripheral neuropathy (DPN). Blockade of this pathway using Wnt inhibitors provided neuroprotection in experimental DPN in rats. This study may provide a basis for exploring the therapeutic potential of Wnt inhibitors in DPN patients.

Neuropeptide Y (NPY) modulates nociception in the spinal cord, but little is known about its mechanisms of release. We measured NPY release in situ using the internalization of its Y1 receptor in dorsal horn neurons. Y1 receptor immunoreactivity was normally localized to the cell surface, but addition of NPY to spinal cord slices increased the number of neurons with Y1 internalization in a biphasic fashion (ECs of 1 nM and 1 μM). Depolarization with KCl, capsaicin, or the protein kinase A activator 6-benzoyl-cAMP also induced Y1 receptor internalization, presumably by releasing NPY. NMDA receptor activation in the presence of BVT948, an inhibitor of protein tyrosine phosphatases, also released NPY. Electrical stimulation of the dorsal horn frequency-dependently induced NPY release; and this was decreased by the Y1 antagonist BIBO3304, the Nav channel blocker lidocaine, or the Cav2 channel blocker ω-conotoxin MVIIC. Dorsal root immersion in capsaicin, but not its electrical stimulation, also induced NPY release. This was blocked by CNQX, suggesting that part of the NPY released by capsaicin was from dorsal horn neurons receiving synapses from primary afferents and not from the afferent themselves. Mechanical stimulation in vivo, with rub or clamp of the hindpaw, elicited robust Y1 receptor internalization in rats with spared nerve injury but not sham surgery. In summary, NPY is released from dorsal horn interneurons or primary afferent terminals by electrical stimulation and by activation of TRPV1, PKA or NMDA receptors in. Furthermore, NPY release evoked by noxious and tactile stimuli increases after peripheral nerve injury.

Neuronal hyperactivity in the spinal dorsal horn can amplify nociceptive input in diabetic neuropathic pain. The glutamate N-methyl-D-aspartate and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (NMDA receptors and AMPA receptors, respectively) are involved in spinal nociceptive transmission. However, it is unclear whether painful diabetic neuropathy is associated with changes in the activity of synaptic NMDA receptors and AMPA receptors in spinal dorsal horn neurons. AMPA receptors lacking GluA2 are Ca2+ permeable (CP-AMPA receptors), and their currents display characteristic inward rectification. In this study, we showed that evoked excitatory postsynaptic currents (EPSCs) exhibited inward rectification in spinal dorsal neurons in diabetic rats induced by streptozotocin. Interestingly, presynaptic and postsynaptic NMDA receptor activity in the spinal dorsal horn was similar in diabetic and control rats. In the dorsal spinal cord, the membrane GluA2 protein level was significantly less in diabetic than in control rats, whereas the cytosolic GluA2 level was greater in diabetic than in control rats. In contrast, the GluA1 subunit levels in the plasma membrane and cytosol did not differ between the two groups. Blocking CP-AMPA receptors significantly reduced the amplitude of EPSCs of dorsal horn neurons in diabetic but not in control rats. Furthermore, blocking spinal CP-AMPA receptors reduced pain hypersensitivity in diabetic rats but had no effect on nociception in control rats. Our study suggests that diabetic neuropathy augments CP-AMPA receptor activity in the spinal dorsal horn by causing intracellular retention of GluA2 and impairing GluA2 membrane trafficking. Increased prevalence of spinal CP-AMPA receptors sustains diabetic neuropathic pain. SIGNIFICANCE STATEMENT: This study demonstrates that the prevalence of synaptic calcium permeable-AMPA receptors is increased in the spinal dorsal horn, which mediates pain hypersensitivity in diabetic neuropathy. Thus, calcium permeable-AMPA receptors play an important role in glutamatergic synaptic plasticity in the spinal cord in painful diabetic neuropathy. This new knowledge improves our understanding of the mechanisms involved in central sensitization associated with diabetic neuropathic pain and suggest that calcium permeable-AMPA receptors are an alternative therapeutic target for treating this chronic pain condition.

Neuropathic pain is chronic pain that follows nerve injury, mediated in the brain by elevated levels of the inflammatory protein tumor necrosis factor-alpha (TNF). We have shown that peripheral nerve injury increases TNF in the hippocampus/pain perception region, which regulates neuropathic pain symptoms. In this study we assessed pain sensation and perception subsequent to specific targeting of brain-TNF (via TNF antibody) administered through a novel subcutaneous perispinal route. Neuropathic pain was induced in Sprague-Dawley rats via chronic constriction injury (CCI), and thermal hyperalgesia was monitored for 10 days post-surgery. On day 8 following CCI and sensory pain behavior testing, rats were randomized to receive perispinal injection of TNF antibody or control IgG isotype antibody. Pain perception was assessed using conditioned place preference (CPP) to the analgesic, amitriptyline. CCI-rats receiving the perispinal injection of TNF antibody had significantly decreased CCI-induced thermal hyperalgesia the following day, and did not form an amitriptyline-induced CPP, whereas CCI-rats receiving perispinal IgG antibody experienced pain alleviation only in conjunction with i.p. amitriptyline and did form an amitriptyline-induced CPP. The specific targeting of brain TNF via perispinal delivery alleviates thermal hyperalgesia and positively influences the affective component of pain. Perspective This study presents a novel route of drug administration to target central TNF for treatment of neuropathic pain. Targeting central TNF through perispinal drug delivery could potentially be a more efficient and sustained method to treat patients with neuropathic pain.

The amygdala plays a critical role in emotional-affective aspects of behaviors and pain modulation. The central nucleus of amygdala (CeA) serves major output functions, and neuroplasticity in the CeA is linked to pain-related behaviors in different models. Activation of G-coupled group II metabotropic glutamate receptors (mGluRs), which consist of mGluR2 and mGluR3, can decrease neurotransmitter release and regulate synaptic plasticity. Group II mGluRs have emerged as targets for neuropsychiatric disorders and can inhibit pain-related processing and behaviors. Surprisingly, site and mechanism of antinociceptive actions of systemically applied group II mGluR agonists are not clear. Our previous work showed that group II mGluR activation in the amygdala inhibits pain-related CeA activity, but behavioral and spinal consequences remain to be determined. Here we studied the contribution of group II mGluRs in the amygdala to the antinociceptive effects of a systemically applied group II mGluR agonist (LY379268) on behavior and spinal dorsal horn neuronal activity, using the kaolin/carrageenan-induced knee joint arthritis pain model. Audible and ultrasonic vocalizations (emotional responses) and mechanical reflex thresholds were measured in adult rats with and without arthritis (5-6 h postinduction). Extracellular single-unit recordings were made from spinal dorsal horn wide dynamic range neurons of anesthetized (isoflurane) rats with and without arthritis (5-6 h postinduction). Systemic (intraperitoneal) application of a group II mGluR agonist (LY379268) decreased behaviors and activity of spinal neurons in the arthritis pain model but not under normal conditions. Stereotaxic administration of LY379268 into the CeA mimicked the effects of systemic application. Conversely, stereotaxic administration of a group II mGluR antagonist (LY341495) into the CeA reversed the effects of systemic application of LY379268 on behaviors and dorsal horn neuronal activity in arthritic rats. The data show for the first time that the amygdala is the critical site of action for the antinociceptive behavioral and spinal neuronal effects of systemically applied group II mGluR agonists.