Accepted

Chronic pain has been shown to depend on nociceptive sensitization in the spinal cord, and while multiple mechanisms involved in the initiation of plastic changes have been established, the molecular targets which maintain spinal nociceptive sensitization are still largely unknown. Building upon the established neurobiology underlying the maintenance of LTP in the hippocampus, this present study investigated the contributions of spinal atypical PKC isoforms PKCι/λ and PKM and their downstream targets (p62/GluA1 and NSF/GluA2 interactions, respectively) to the maintenance of spinal nociceptive sensitization in male and female rats. Pharmacological inhibition of atypical PKCs by ZIP reversed established allodynia produced by repeated intramuscular (i.m.) acidic saline injections in male animals only, replicating previously demonstrated sex differences. Inhibition of both PKCι/λ and downstream substrates p62/GluA1 resulted in male-specific reversals of i.m. acidic saline-induced allodynia, while female animals continued to display allodynia. Inhibition of NSF/GluA2, the downstream target to PKM, reversed allodynia induced by i.m. acidic saline in both sexes. Neither PKCι/λ, p62/GluA1 or NSF/GluA2 inhibition had any effect on formalin response for either sex.This study provides novel behavioural evidence for the male-specific role of PKCι/λ and downstream target p62/GluA1, highlighting the potential influence of ongoing afferent input. The sexually divergent pathways underlying persistent pain are shown here to converge at the interaction between NSF and the GluR2 subunit of the AMPA receptor. Though this interaction is thought to be downstream of PKM in males, these findings and previous work suggest that females may rely on a factor independent of atypical PKCs for the maintenance of spinal nociceptive sensitization.

Clinical evidence indicates that major depression is a common comorbidity of chronic pain, including neuropathic pain. However, the cellular basis for chronic pain-mediated major depression remains unclear. High-mobility group box 1 protein (HMGB1) has a key role in innate immune responses and appears to be have a role in mediating diverse disorders, including neuropathic pain and depression. The current study aimed to characterize neuropathic pain-induced changes in affect over time and to determine whether HMGB1 has a role in neuropathic pain-induced changes in affect. Neuropathic pain was induced by partial sciatic nerve ligation (PSNL) in mice. Anxiodepressive-like behaviors in mice were evaluated over 10 weeks, in the social interaction, forced swim, and novelty suppressed feeding tests. Mice developed anxiodepressive-like behavior 6 to 8 weeks after induction of neuropathy. Accompanying anxiodepressive-like behavior, increased HMGB1 protein and microglia activation were observed in frontal cortex at 8 weeks after PSNL. Intracerebroventricular administration of rHMGB1 in naïve mice induced anxiodepressive-like behavior and microglia activation. Blockage of HMGB1 in PSNL mice with glycyrrhizic acid (GZA) or anti-HMGB1 antibody reduced microglia activation and anxiodepressive-like behavior. These results indicate that PSNL-induced anxiodepressive-like behavior is likely mediated by HMGB1. Furthermore, the data indicate that inhibition of HMGB1-dependent microglia activation could be a strategy for the treatment of depression associated with neuropathic pain.

The primary sensory cortex processes competitive sensory inputs. Ablation of these competitive inputs induces neuroplastic changes in local cortical circuits. However, information concerning cortical plasticity induced by a disturbance of competitive nociceptive inputs is limited. Nociceptive information from the maxillary and mandibular molar pulps converges at the border between the ventral secondary somatosensory cortex (S2) and insular oral region (IOR); therefore, S2/IOR is a suitable target for examining the cortical changes induced by a disturbance of noxious inputs, which often causes neuropathic pain and allodynia. We focused on the plastic changes in S2/IOR excitation in a model of rats subjected to inferior alveolar nerve transection (IANX). Our optical imaging using a voltage-sensitive dye (VSD) revealed that the maxillary molar pulp stimulation-induced excitatory propagation was expanded one to two weeks after IANX at the macroscopic level. At the cellular level, based on Ca imaging using two-photon microscopy, the amplitude of the Ca responses and the number of responding neurons in S2/IOR increased in both excitatory and inhibitory neurons. The laser scanning photostimulation (LSPS) revealed that Layer II/III pyramidal and GABAergic fast-spiking neurons in S2/IOR received larger excitatory inputs from Layer IV in the IANX models, which supports the findings obtained by the macroscopic and microscopic optical imaging. Furthermore, the inhibitory postsynaptic inputs to the pyramidal neurons were decreased in the IANX models, suggesting suppression of inhibitory synaptic transmission onto excitatory neurons. These results suggest that IANX induces plastic changes in S2/IOR by changing the local excitatory and inhibitory circuits.

We developed a mouse model for central post-stroke pain (CPSP), a centrally-originated neuropathic pain (NeuP). In this mode, mice were first injected with Rose Bengal, followed by photo-irradiation of left middle cerebral artery (MCA) to generate thrombosis. Although the MCA thrombosis was soon dissolved, the reduced blood flow remained for more than 24 h due to subsequent occlusion of microvessels. This photochemically induced thrombosis (PIT) model showed a hypersensitivity to the electrical stimulation of both sides of paw, but did not show any abnormal pain in popular thermal or mechanical nociception tests. When tissue-type plasminogen activator (tPA) was injected 6 h after the PIT stress, tPA-dependent hypersensitivity to the electrical paw stimulation and stable thermal and mechanical hyperalgesia on both sides for more than 17 or 18 days after the PIT treatment. These hyperalgesic effects were abolished in lysophosphatidic acid receptor 1 (LPA)- and lysophosphatidic acid receptor 3 (LPA)-deficient mice. When Ki-16425, an LPA and LPA antagonist was treated twice daily for 6 days consecutively, the thermal and mechanical hyperalgesia at day 17 and 18 were significantly reversed. The liquid chromatography-mass spectrometry (LC-MS/MS) analysis revealed that there is a significant increase in several species of LPA molecules in somatosensory S-I and medial dorsal thalamus (MD), but not in striatum or ventroposterior thalamus. All these results suggest that LPA and LPA signaling play key roles in the development and maintenance of CPSP.

Galanin and galanin receptors (GalRs) play important roles in the transmission and modulation of nociceptive information. Our previous research has shown that the expression of GalR1 is upregulated and that GalR1 activation in the nucleus accumbens (NAc) of rats with neuropathic pain has an antinociceptive effect. However, the antinociceptive role of NAc galanin in neuralgia remains unclear. The present study aimed to explore the antinociceptive effect induced by galanin in rats with neuropathic pain and the underlying mechanism. The results showed that the intra-NAc injection of galanin induced a dose-dependent increase in hindpaw withdrawal latency (HWL) to noxious thermal and mechanical stimulation in mononeuropathic rats and that this effect was stronger than that in intact rats. The intra-NAc injection of the non-selective GalR antagonist galantide reduced HWL in the rats with neuropathic pain, but there was no influence of galantide on HWL in intact rats. Moreover, galanin expression in the NAc was upregulated after sciatic nerve ligation. All of these results demonstrate that galanin plays a role in antinociception via binding to GalRs in the NAc of rats and that endogenous galanin is involved in the antinociception after peripheral nerve injury.

Recently studies have focused on the anti-hyperalgesic 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-medium sized, capsaicin-sensitive, dorsal root ganglion (DRG) neurons isolated from 3-4 week-old rats. RT-PCR experiments and immunofluorescence analysis revealed A3AR expression in DRG neurons. Patch-clamp experiments demonstrated that two 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 of Ca influx via N-type voltage-dependent Ca channels (VDCCs) 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 upon 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.