Animal Studies

Increased activity and excitability (sensitisation) of a series of molecules including the transient receptor potential ion channel, vanilloid subfamily, member 1 (TRPV1) in pain-sensing (nociceptive) primary sensory neurons are pivotal for developing pathological pain experiences in tissue injuries. TRPV1 sensitisation is induced and maintained by two major mechanisms; post-translational and transcriptional changes in TRPV1 induced by inflammatory mediators produced and accumulated in injured tissues, and TRPV1 activation-induced feed-forward signalling. The latter mechanism includes synthesis of TRPV1 agonists within minutes, and upregulation of various receptors functionally linked to TRPV1 within a few hours, in nociceptive primary sensory neurons. Here, we report that a novel mechanism, which contributes to TRPV1 activation-induced TRPV1-sensitisation within ~ 30 min in at least ~ 30% of TRPV1-expressing cultured murine primary sensory neurons, is mediated through upregulation in cyclooxygenase 2 (COX2) expression and increased synthesis of a series of COX2 products. These findings highlight the importance of feed-forward signalling in sensitisation, and the value of inhibiting COX2 activity to control pain, in nociceptive primary sensory neurons in tissue injuries.

Rat somatosensory neurons express junctional protein, junctophilin-4 (JPH4) JPH4 is necessary for the formation of store operated Ca entry (SOCE) complex at the junctions between plasma membrane and endoplasmic reticulum in these neurons. Knockdown of JPH4 impairs endoplasmic reticulum Ca store refill and junctional Ca signalling in sensory neurons. In vivo knockdown of JPH4 in the dorsal root ganglion (DRG) sensory neurons significantly attenuated experimentally-induced inflammatory pain in rats. Junctional nanodomain Ca signalling maintained by JPH4 is an important contributor to the inflammatory pain mechanisms.

Dopamine D1 receptor (D1DR) and D2 receptor (D2DR) are closely associated with pain modulation, but their exact effects on neuropathic pain and the underlying mechanisms remain to be identified. Our research revealed that intrathecal administration of D1DR and D2DR antagonists inhibited D1-D2DR complex formation and ameliorated mechanical and thermal hypersensitivity in chronic constriction injury (CCI) rats. The D1-D2DR complex was formed in the rat spinal cord, and the antinociceptive effects of D1DR and D2DR antagonists could be reversed by D1DR, D2DR, and D1-D2DR agonists. Gαq, PLC, and IP3 inhibitors also alleviated CCI-induced neuropathic pain. D1DR, D2DR, and D1-D2DR complex agonists all increased the intracellular calcium concentration in primary cultured spinal neurons, and this increase could be reversed by D1DR, D2DR antagonists and Gαq, IP3, PLC inhibitors. D1DR and D2DR antagonists significantly reduced the expression of p-PKC γ, p-CaMKII, p-CREB, and p-MAPKs. Levo-corydalmine (l-CDL), a monomeric compound in Corydalis yanhusuo W.T. Wang, was found to obviously suppress the formation of the spinal D1-D2DR complex to alleviate neuropathic pain in CCI rats and to decrease the intracellular calcium concentration in spinal neurons. l-CDL-induced inhibition of p-PKC γ, p-MAPKs, p-CREB, and p-CaMKII was also reversed by D1DR, D2DR, and D1-D2DR complex agonists. In conclusion, these results indicate that D1DR and D2DR form a complex and in turn couple with the Gαq protein to increase neuronal excitability via PKC γ, CaMKII, MAPK, and CREB signaling in the spinal cords of CCI rats; thus, they may serve as potential drug targets for neuropathic pain therapy.

Anxiety disorders cause distress and are commonly found to be comorbid with chronic pain. Both are difficult-to-treat conditions for which alternative treatment options are being pursued. This study aimed to evaluate the effects of transcranial direct current stimulation (tDCS), treadmill exercise, or both, on anxiety-like behavior and associated growth factors and inflammatory markers in the hippocampus and sciatic nerve of rats with neuropathic pain. Male Wistar rats (n = 216) were subjected to sham-surgery or sciatic nerve constriction for pain induction. Fourteen days following neuropathic pain establishment, either bimodal tDCS, treadmill exercise, or a combination of both was used for 20 minutes a day for 8 consecutive days. The elevated plus-maze test was used to assess anxiety-like behavior and locomotor activity during the early (24 h) or late (7 days) phase after the end of treatment. BDNF, TNF-ɑ, and IL-10 levels in the hippocampus, and BDNF, NGF, and IL-10 levels in the sciatic nerve were assessed 48 h or 7 days after the end of treatment. Rats from the pain groups developed an anxiety-like state. Both tDCS and treadmill exercise provided ethological and neurochemical alterations induced by pain in the early and/or late phase, and a modest synergic effect between tDCS and exercise was observed. These results indicate that non-invasive neuromodulatory approaches can attenuate both anxiety-like status and locomotor activity and alter the biochemical profile in the hippocampus and sciatic nerve of rats with neuropathic pain and that combined interventions may be considered as a treatment option.

The loss of descending inhibitory control is thought critical to the development of chronic pain but what causes this loss in function is not well understood. We have investigated the dynamic contribution of prelimbic cortical neuronal projections to the periaqueductal grey (PrL-P) to the development of neuropathic pain in rats using combined opto- and chemo-genetic approaches. We found PrL-P neurons to exert a tonic inhibitory control on thermal withdrawal thresholds in uninjured animals. Following nerve injury, ongoing activity in PrL-P neurons masked latent hypersensitivity and improved affective state. However, this function is lost as the development of sensory hypersensitivity emerges. Despite this loss of tonic control, opto-activation of PrL-P neurons at late post-injury timepoints could restore the anti-allodynic effects by inhibition of spinal nociceptive processing. We suggest that the loss of cortical drive to the descending pain modulatory system underpins the expression of neuropathic sensitisation after nerve injury.

In the anterior lobe of the pituitary gland (AP), non-endocrine cells regulate hormone secretion by endocrine cells. However, the functions of non-endocrine cells in the AP during chronic pain are largely unclear. Here, we show that macrophages, but not folliculostellate (FS) cells, were selectively increased in the AP in the complete Freund's adjuvant (CFA)-induced chronic inflammatory pain model in rats. In addition, IL-1β expression was increased in the AP, and the IL-1β-immunopositive cells were identified as macrophages. On the other hand, increased macrophage density and IL-1β expression were not detected in a neuropathic pain model induced by partial sciatic nerve ligation (PSL). Furthermore, we found c-Fos expression specifically in the somatotrophs under the chronic inflammatory pain condition. Because IL-1β promotes growth hormone (GH) synthesis and release, our results suggest that AP macrophage contributes to GH release through IL-1βduring chronic inflammatory pain.　.