Activity of protein kinase C controls the efficacy of cannabinoid receptor type 1 in the medial prefrontal cortex after neuropathic pain

Many patients with chronic pain conditions suffer from depression. In our previous report, we found that afferent noxious inputs after painful nerve injury compromise activity-dependent endocannabinoid (eCB) signaling in the medial prefrontal cortex (mPFC), resulting in depression. Since protein kinase C (PKC) and cAMP/protein kinase A (PKA) pathways can regulate eCB receptor type-1 (CB1R)-mediated synaptic transmission, we explored the possible roles of PKC and PKA in causing reduced eCB signaling in neuropathic pain (spared nerve injury, SNI). Four weeks after SNI, rats developed both depression and hypersensitivity of the plantar skin to punctate mechanical stimulation. Radioligand-based assay of CB1R binding in the harvested mPFC tissue showed no change after SNI in both male and female rats. Slice electrophysiological recordings from the mPFC showed that SNI did not alter the effects of a PKA agonist (forskolin) or blocker (H-89) on evoked inhibitory postsynaptic currents (eIPSCs). However, the PKC blocker chelerythrine (CHEL) fully prevented the inhibitory effect of the CB1R agonist (WIN-55212-2) on eIPSCs, whereas PKC activation with phorbol 12-myristate 13-acetate reversed the reduced effect of WIN-55212-2 on eIPSC and miniature IPSC after SNI. Together, these data suggest that PKC activity controls the efficacy of CB1Rs in mPFC after neuropathic pain.