Regulation of presynaptic CB1 receptors in persistent inflammation

Abstract

Opioids are the golden‐standard for pain treatment and therapeutics targeting the cannabinoid system are on the rise; yet the molecular mechanisms by which these systems are regulated is not fully understood. Opioids and cannabinoids can elicit analgesia through activation of mu‐opioid receptors (MOR) or cannabinoid 1 (CB1) receptors, respectively, in the ventrolateral region of the periaqueductal gray (vlPAG). Both receptors suppress GABA release from the presynaptic terminal under basal conditions. Our studies examine how persistent inflammation affects presynaptic MOR and CB1 receptor signaling in the vlPAG. Adult male and female Sprague Dawley rats (postnatal day 30–60) are naïve or injected with Complete Freund’s Adjuvent (CFA) into one hindpaw to induce inflammation for 5–7 days. Using electrophysiology to measure miniature inhibitory postsynaptic currents (mIPSCs), we find that CB1‐mediated inhibition of GABA mIPSC frequency is significantly reduced in inflamed rats compared to naïve rats. MOR‐mediated inhibition of mIPSC frequency is not altered. This effect is consistent in both males and females. There are no effects of inflammation on mIPSC amplitude. Previous work has shown that neuropathic pain, chronic THC exposure, or persistent inflammation reduces CB1 receptor protein levels while mRNA levels are unchanged suggesting that CB1 receptors may be regulated via local protein synthesis. We find that protein synthesis inhibitors, anisomycin (20 μM) or cyclohexamide (80 μM) reduce presynaptic CB1 receptor‐mediated inhibitionof GABA release in the vlPAG but do not affect presynaptic MOR function. These results provide evidence that presynaptic CB1 and MOR are regulated differentially under basal conditions and in persistent inflammation. These data have important implications for the future treatment of pain with pharmaceuticals targeted at either CB1 or MOR receptors.Support or Funding InformationNIDA T32 Training Grant; NIH NIDA R01DA042565