Diminished synaptic inhibition in the spinal dorsal horn is a major contributor to chronic pain. Pathways, which reduce synaptic inhibition in inflammatory and neuropathic pain states, have been identified, but central hyperalgesia and diminished dorsal horn synaptic inhibition also occur in the absence of inflammation or neuropathy, solely triggered by intense nociceptive (C–fiber) input to the spinal dorsal horn. We found that endocannabinoids produced upon strong nociceptive stimulation activated CB1 receptors on inhibitory dorsal horn neurons to reduce the synaptic release of GABA and glycine and thus rendered nociceptive neurons excitable by non-painful stimuli. Spinal endocannabinoids and CB1 receptors on inhibitory dorsal horn interneurons act as mediators of heterosynaptic pain sensitization and play an unexpected role in dorsal horn pain controlling circuits.

Acetaminophen (paracetamol) is a widely used analgesic and antipyretic drug with only incompletely understood mechanisms of action. Previous work, using models of acute nociceptive pain, indicated that analgesia by acetaminophen involves an indirect activation of CB receptors by the acetaminophen metabolite and endocannabinoid reuptake inhibitor AM 404. However, the contribution of the cannabinoid system to antihyperalgesia against inflammatory pain, the main indication of acetaminophen, and the precise site of the relevant CB receptors have remained elusive. Here, we analyzed acetaminophen analgesia in mice of either sex with inflammatory pain and found that acetaminophen exerted a dose-dependent antihyperalgesic action, which was mimicked by intrathecally injected AM 404. Both compounds lost their antihyperalgesic activity in mice, confirming the involvement of the cannabinoid system. Consistent with a mechanism downstream of proinflammatory prostaglandin formation, acetaminophen also reversed hyperalgesia induced by intrathecal prostaglandin E To distinguish between a peripheral/spinal and a supraspinal action, we administered acetaminophen and AM 404 to mice, which lack CB receptors from the peripheral nervous system and the spinal cord. These mice exhibited unchanged antihyperalgesia indicating a supraspinal site of action. Accordingly, local injection of the CB receptor antagonist rimonabant into the rostral ventromedial medulla blocked acetaminophen-induced antihyperalgesia, while local rostral ventromedial medulla injection of AM 404 reduced hyperalgesia in wild-type mice but not in mice. Our results indicate that the cannabinoid system contributes not only to acetaminophen analgesia against acute pain but also against inflammatory pain, and suggest that the relevant CB receptors reside in the rostral ventromedial medulla. Acetaminophen is a widely used analgesic drug with multiple but only incompletely understood mechanisms of action, including a facilitation of endogenous cannabinoid signaling via one of its metabolites. Our present data indicate that enhanced cannabinoid signaling is also responsible for the analgesic effects of acetaminophen against inflammatory pain. Local injections of the acetaminophen metabolite AM 404 and of cannabinoid receptor antagonists as well as data from tissue-specific CB receptor-deficient mice suggest the rostral ventromedial medulla as an important site of the cannabinoid-mediated analgesia by acetaminophen.

Key points• Synaptic plasticity between primary nociceptors and second order dorsal horn neurons serves key roles in pain and analgesia • A contribution of NMDA receptors to long-term potentiation and long-term depression at these synapses has been demonstrated before, but much less is known about a possible role of endocannabinoids and cannabinoid (CB) 1 receptors.• Here we show that CB 1 receptors residing on the spinal terminals of primary nociceptors critically contribute to an NMDA receptor-independent form of long-term depression at these synapses, which requires simultaneous pre-and postsynaptic activity.• A similar long-lasting depression of nociceptive signal transmission can also be obtained with application of CB 1 receptor agonists in the presence of presynaptic stimulation alone.• These findings identify a previously unknown form of long-term depression at spinal nociceptor synapses, which may be important for our understanding of pain-related neural plasticity and analgesic actions of CB 1 receptor agonists.Abstract Neuroplastic changes at the spinal synapses between primary nociceptors and second order dorsal horn neurons play key roles in pain and analgesia. NMDA receptor-dependent forms of long-term plasticity have been studied extensively at these synapses, but little is known about possible contributions of the endocannabinoid system. Here, we addressed the role of cannabinoid (CB) 1 receptors in activity-dependent plasticity at these synapses. We report that conditional low-frequency stimulation of high-threshold primary sensory nerve fibres paired with depolarisation of the postsynaptic neuron evoked robust long-term depression (LTD) of excitatory synaptic transmission by about 40% in the vast majority (90%) of recordings made in wild-type mice. When recordings were made from global or nociceptor-specific CB 1 receptor-deficient mice (CB 1 −/− mice and sns-CB 1 −/− mice), the portion of neurons exhibiting LTD was strongly reduced to about 25%. Accordingly, LTD was prevented to a similar extent by the CB 1 receptor antagonist AM 251 and mimicked by pharmacological activation of CB 1 receptors. In a subset of neurons with EPSCs of particularly high stimulation thresholds, we furthermore found that the absence of CB 1 receptors in CB 1 −/− and sns-CB 1 −/− mice converted the response to the paired conditioning stimulation protocol from LTD to long-term potentiation (LTP). Our results identify CB 1 receptor-dependent LTD as a form of synaptic plasticity previously unknown in spinal nociceptors. They furthermore suggest that prevention of LTP may be a second hitherto unknown function of CB 1 receptors in primary nociceptors. Both findings may have important Ako Kato and Pradeep Punnakkal contributed equally to this work.