Role of ventrolateral orbital cortex muscarinic and nicotinic receptors in modulation of capsaicin-induced orofacial pain-related behaviors in rats

Cobratoxin Alleviates Cancer-Induced Bone Pain in Rats via Inhibiting CaMKII Signaling Pathway after Acting on M4 Muscarinic Cholinergic Receptors

Cancer-induced bone pain (CIBP) is a common pain in clinics, which can reduce the quality of life and increase the mortality of patients, but the treatment of CIBP is limited. This study was designed to investigate the analgesic effect of α-cobratoxin on CIBP and further to explore the molecular target and potential signal pathway. As shown by the mechanical allodynia test in a CIBP rat model, administration of α-cobratoxin produced significant analgesia in a dose-dependent manner, and the analgesic effects were blocked by pretreatment with an intrathecal injection of M4 mAChR-siRNA or intraperitoneal injection of tropicamide, an antagonist of M4 muscarinic cholinergic receptor. Whole-cell patch-clamp recording showed that α-cobratoxin can decrease the spontaneous firing and spontaneous excitatory postsynaptic currents of SDH neurons in CIBP rats. In primary lumber SDH neurons, intracellular calcium measurement revealed that α-cobratoxin decreased intracellular calcium concentration, and immunofluorescence demonstrated that M4 muscarinic cholinergic receptor and CaMKII/CREB were co-expressed. In the CIBP model and primary SDH neurons, Western blot showed that the levels of p-CaMKII and p-CREB were increased by α-cobratoxin and the effect of α-cobratoxin was antagonized by M4 mAChR-siRNA. The quantitative polymerase chain reaction (qPCR) results showed that α-cobratoxin downregulated the expression of proinflammatory cytokines through M4 muscarinic cholinergic receptor in SDH. These results suggest that α-cobratoxin may activate M4 muscarinic cholinergic receptor, triggering the inhibition of SDH neuronal excitability via CaMKII signaling pathway, thereby resulting in antagonistic effects in the CIBP rat model.

An orbitofrontal cortex to midbrain projection modulates hypersensitivity after peripheral nerve injury

Neuropathic pain is a debilitating condition that is often refractory to treatment. The network of neural substrates for pain transmission and control within the brain is complex and remains poorly understood. Through a combination of neuronal tracing, optogenetics, chemogenetics, electrophysiological recordings, and behavioral assessment, we demonstrate that activation of layer 5 pyramidal neurons in the ventrolateral orbitofrontal cortex (vlOFC) attenuates mechanical and thermal hypersensitivity and cold allodynia in mice with neuropathic pain induced by spared nerve injury (SNI). These vlOFC output neurons project to the posterior ventrolateral periaqueductal gray (vlPAG) region and receive inputs from the ventromedial thalamus (VM). Specific optogenetic and chemogenetic activation of the vlOFC-vlPAG and the VM-vlOFC circuits inhibits hypersensitivity associated with neuropathy. Thus, we reveal a modulatory role of the vlOFC and its projections to the vlPAG circuit in the processing of hypersensitive nociception.

The pronociceptive role of 5-HT6 receptors in ventrolateral orbital cortex in a rat formalin test model

Recent studies have shown the 5-HT₆ receptors are expressed in regions which are important in pain processing such as the cortex, amygdala, thalamus, PAG, spinal cord and dorsal root ganglia (DRG), suggesting a putative role of 5-HT₆ receptors in pain modulation. The ventrolateral orbital cortex (VLO) is part of an endogenous analgesic system, consisting of the spinal cord - thalamic nucleus submedius (Sm) - VLO - periaqueductal gray (PAG) - spinal cord loop. The present study assessed the possible role of 5-HT₆ receptors in the VLO in formalin-induced inflammatory pain model. Firstly we found that microinjection of selective 5-HT₆ receptor agonists EMD-386088 (5 μg in 0.5 μl) and WAY-208466 (8 μg in 0.5 μl) both augmented 5% formalin-induced nociceptive behavior. Microinjection of selective 5-HT₆ receptor antagonist SB-258585 (1,2 and 4 μg in 0.5 μl) significantly reduced formalin-induced flinching. Besides, the pronociceptive effects of EMD-386088 and WAY-208466 were dramatically reduced by SB-258585, implicating 5-HT₆ receptor mechanisms in mediating these responses. In addition, the pronociceptive effect of EMD-386088 was also prevented by the adenylate cyclase (AC) inhibitor SQ-22536 (2 nmol in 0.5 μl) and the protein kinase A (PKA) inhibitor H89 (10 nmol in 0.5 μl), respectively. We further confirmed the above results with quantification of spinal c-fos expression. Taken together, our results suggested that 5-HT₆ receptors play a pronociceptive role in the VLO in the rat formalin test due to its activation of AC - PKA pathway. Therefore, cerebral cortical 5-HT₆ receptors could be a new target to develop analgesic drugs.