CB1 receptor-dependent desensitisation of TRPV1 channels contributes to the analgesic effect of dipyrone in sensitised primary sensory neurons

Dual effect of anandamide on spinal nociceptive transmission in control and inflammatory conditions

Anandamide (AEA) is an important modulator of nociception in the spinal dorsal horn, acting presynaptically through Cannabinoid (CB1) and Transient receptor potential vanilloid (TRPV1) receptors. The role of AEA (1 µM, 10 µM, and 30 µM) application on the modulation of nociceptive synaptic transmission under control and inflammatory conditions was studied by recording miniature excitatory postsynaptic currents (mEPSCs) from neurons in spinal cord slices. Inhibition of the CB1 receptors by PF514273, TRPV1 by SB366791, and the fatty acid amide hydrolase (FAAH) by URB597 was used. Under naïve conditions, the AEA application did not affect the mEPSCs frequency (1.43±0.12 Hz) when all the recorded neurons were considered. The mEPSC frequency increased (180.0±39.2%) only when AEA (30 µM) was applied with PF514273 and URB597. Analysis showed that one sub-population of neurons had synaptic input inhibited (39.1% of neurons), the second excited (43.5%), whereas 8.7% showed a mixed effect and 8.7% did not respond to the AEA. With inflammation, the AEA effect was highly inhibitory (72.7%), while the excitation was negligible (9.1%), and 18.2% were not modulated. After inflammation, more neurons (45.0%) responded even to low AEA by mEPSC frequency increase with PF514273/URB597 present. AEA-induced dual (excitatory/inhibitory) effects at the 1st nociceptive synapse should be considered when developing analgesics targeting the endocannabinoid system. These findings contrast the clear inhibitory effects of the AEA precursor 20:4-NAPE application described previously and suggest that modulation of endogenous AEA production may be more favorable for analgesic treatments.

Inhibition of synaptic transmission by anandamide precursor 20:4-NAPE is mediated by TRPV1 receptors under inflammatory conditions

Transient receptor potential ion channel, vanilloid subfamily, type 1 (TRPV1) cation channel, and cannabinoid receptor 1 (CB₁) are essential in the modulation of nociceptive signaling in the spinal cord dorsal horn that underlies different pathological pain states. TRPV1 and CB₁ receptors share the endogenous agonist anandamide (AEA), produced from N-arachidonoylphosphatidylethanolamine (20:4-NAPE). We investigated the effect of the anandamide precursor 20:4-NAPE on synaptic activity in naive and inflammatory conditions. Patch-clamp recordings of miniature excitatory postsynaptic currents (mEPSCs) from superficial dorsal horn neurons in rat acute spinal cord slices were used. Peripheral inflammation was induced by subcutaneous injection of carrageenan. Under naive conditions, mEPSCs frequency (0.96 ± 0.11 Hz) was significantly decreased after 20 μM 20:4-NAPE application (55.3 ± 7.4%). This 20:4-NAPE-induced inhibition was blocked by anandamide-synthesizing enzyme N-acyl phosphatidylethanolamine phospholipase D (NAPE-PLD) inhibitor LEI-401. In addition, the inhibition was prevented by the CB₁ receptor antagonist PF 514273 (0.2 μM) but not by the TRPV1 receptor antagonist SB 366791 (10 μM). Under inflammatory conditions, 20:4-NAPE (20 μM) also exhibited a significant inhibitory effect (74.5 ± 8.9%) on the mEPSCs frequency that was prevented by the TRPV1 receptor antagonist SB 366791 but not by PF 514273 application. Our results show that 20:4-NAPE application has a significant modulatory effect on spinal cord nociceptive signaling that is mediated by both TRPV1 and CB₁ presynaptic receptors, whereas peripheral inflammation changes the underlying mechanism. The switch between TRPV1 and CB₁ receptor activation by the AEA precursor 20:4-NAPE during inflammation may play an important role in nociceptive processing, hence the development of pathological pain.

The Clinical Pharmacology and Therapeutic Evaluation of Non-Steroidal Anti-Inflammatory Drugs in Adult Horses

This review firstly examines the underlying pathophysiology of pain and inflammation associated with orthopedic disease and endotoxemia. Then, it reviews the clinical pharmacology (pharmacokinetics and pharmacodynamics) of both conventional and non-conventional NSAIDs in the adult horse, and finally provides an overview of different modalities to evaluate the therapeutic efficacy of NSAIDs in research.

Kinin B2 and B1 Receptors Activation Sensitize the TRPA1 Channel Contributing to Anastrozole-Induced Pain Symptoms

Aromatase inhibitors (AIs) cause symptoms of musculoskeletal pain, and some mechanisms have been proposed to explain them. However, signaling pathways downstream from kinin B2 (B2R) and B1 (B1R) receptor activation and their possible sensitizing of the Transient Receptor Potential Ankyrin 1 (TRPA1) remain unknown. The interaction between the kinin receptor and the TRPA1 channel in male C57BL/6 mice treated with anastrozole (an AI) was evaluated. PLC/PKC and PKA inhibitors were used to evaluate the signaling pathways downstream from B2R and B1R activation and their effect on TRPA1 sensitization. Anastrozole caused mechanical allodynia and muscle strength loss in mice. B2R (Bradykinin), B1R (DABk), or TRPA1 (AITC) agonists induced overt nociceptive behavior and enhanced and prolonged the painful parameters in anastrozole-treated mice. All painful symptoms were reduced by B2R (Icatibant), B1R (DALBk), or TRPA1 (A967079) antagonists. We observed the interaction between B2R, B1R, and the TRPA1 channel in anastrozole-induced musculoskeletal pain, which was dependent on the activation of the PLC/PKC and PKA signaling pathways. TRPA1 seems to be sensitized by mechanisms dependent on the activation of PLC/PKC, and PKA due to kinin receptors stimulation in anastrozole-treated animals. Thus, regulating this signaling pathway could contribute to alleviating AIs-related pain symptoms, patients’ adherence to therapy, and disease control.

Activation of GPR18 by Resolvin D2 Relieves Pain and Improves Bladder Function in Cyclophosphamide-Induced Cystitis Through Inhibiting TRPV1

Purpose

Hyperalgesia and bladder overactivity are two main symptoms of interstitial cystitis/bladder pain syndrome (IC/BPS). Cannabinoid receptors participate in the modulation of pain and bladder function. GPR18, a member of the cannabinoid receptor family, also participates in the regulation of pain and bladder function, but its underlying mechanisms are unknown. In this work, we sought to study the role of GPR18 in IC/BPS.

Methods

A rat model of IC/BPS was established with cyclophosphamide (CYP). Paw withdrawal threshold (PWT) measurement and cystometry were used to evaluate pain and bladder function, respectively. RT-PCR, Western blotting and immunofluorescence were used to assess the expression and distribution of GPR18. The role of GPR18 in pain and bladder function was studied by intrathecal injection of resolvin D2 (RvD2, a GPR18 agonist) and O-1918 (a GPR18 antagonist). Calcium imaging was used to study the relationship between GPR18 and TRPV1.

Results

A rat model of IC/BPS, which exhibited a decreased PWT and micturition interval, was successfully established with CYP. The mRNA and protein expression of GPR18 was reduced in the bladder and dorsal root ganglia (DRG) in rats with CYP-induced cystitis. Intrathecal injection of RvD2 increased the PWT and micturition interval. However, O-1918 blocked the therapeutic effect of RvD2. GPR18 was present in bladder afferent nerves and colocalized with TRPV1 in DRG, and RvD2 decreased capsaicin-induced calcium influx in DRG.

Conclusion

Activation of GPR18 by RvD2 alleviated hyperalgesia and improved bladder function, possibly by inhibiting TRPV1 in rats with CYP-induced cystitis.

CB1 receptor-dependent desensitisation of TRPV1 channels contributes to the analgesic effect of dipyrone in sensitised primary sensory neurons

BACKGROUND AND PURPOSE

While dipyrone is a widely used analgesic, its mechanism of action is not completely understood. Recently, we have reported that the dipyrone metabolite 4-aminoantipyrine (4-AA) reduces PGE₂ -induced pain-related behaviour through cannabinoid CB₁ receptors. Here, we ascertained, in naive and PGE₂ -induced "inflamed" conditions, both in vivo and in vitro, the molecular mechanisms involved in the 4-AA-induced analgesic effects.

EXPERIMENTAL APPROACH

The effect of local administration of 4-AA (160 μg per paw) on capsaicin (0.12 μg per paw) injection-induced pain-related behaviour and 4-AA's effect on 500-nM capsaicin-induced changes in intracellular calcium concentration ([Ca²⁺ ]_i ) in cultured primary sensory neurons were assessed in vivo and in vitro, respectively.

KEY RESULTS

4-AA reduced capsaicin-induced nociceptive behaviour in naive and inflamed conditions through CB₁ receptors. 4-AA (100 μM) reduced capsaicin-induced increase in [Ca²⁺ ]_i in a CB₁ receptor-dependent manner, when PGE₂ was not present. Following PGE₂ application, 4-AA (1-50 μM) increased the [Ca²⁺ ]_i . Although 4-AA activated both TRPV1 and TRPA1 channels, increased [Ca²⁺ ]_i was mediated through TRPV1 channels. Activation of TRPV1 channels resulted in their desensitisation. Blocking CB₁ receptors reduced both the excitatory and desensitising effects of 4-AA.

CONCLUSION AND IMPLICATIONS

CB₁ receptor-mediated inhibition of TRPV1 channels and TRPV1-mediated Ca²⁺ -influx- and CB₁ receptor-dependent desensitisation of TRPV1 channels contribute to the anti-nociceptive effect of 4-AA in naive and inflamed conditions respectively. Agonists active at both CB₁ receptors and TRPV1 channels might be useful as analgesics, particularly in inflammatory conditions.