Cannabinoid administration suppresses pain by acting at spinal, supraspinal and peripheral levels. Intrinsic analgesic pathways also exploit endocannabinoids; however, the underlying neurobiological substrates of endocannabinoid-mediated analgesia have remained largely unknown. Compelling evidence shows that, upon exposure to a painful environmental stressor, an endocannabinoid molecule called 2-arachidonoylglycerol (2-AG) is mobilized in the lumbar spinal cord in temporal correlation with stress-induced antinociception. We therefore characterized the precise molecular architecture of 2-AG signaling and its involvement in nociception in the rodent spinal cord. Nonradioactive in situ hybridization revealed that dorsal horn neurons widely expressed the mRNA of diacylglycerol lipase-alpha (DGL-α), the synthesizing enzyme of 2-AG. Peroxidase-based immunocytochemistry demonstrated high levels of DGL-α protein and CB1 cannabinoid receptor, a receptor for 2-AG, in the superficial dorsal horn, at the first site of modulation of the ascending pain pathway. High-resolution electron microscopy uncovered postsynaptic localization of DGL-α at nociceptive synapses formed by primary afferents, and revealed presynaptic positioning of CB 1 on excitatory axon terminals. Furthermore, DGL-α in postsynaptic elements receiving nociceptive input was colocalized with metabotropic glutamate receptor 5 (mGluR5), whose activation induces 2-AG biosynthesis. Finally, intrathecal activation of mGluR5 at the lumbar level evoked endocannabinoid-mediated stress-induced analgesia through the DGL-2-AG-CB1 pathway. Taken together, these findings suggest a key role for 2-AG-mediated retrograde suppression of nociceptive transmission at the spinal level. The striking positioning of the molecular players of 2-AG synthesis and action at nociceptive excitatory synapses suggests that pharmacological manipulation of spinal 2-AG levels may be an efficacious way to regulate pain sensation.
- Metabotropic glutamate receptor
- Rodent spinal cord
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