As cancer progresses, pain is increasingly associated with destruction of tissue, and severe pain occurs with bone destruction. Derivatives of Cannabis sativa (cannabinoids) are potent analgesics, and endogenous cannabinoids share this property. Anandamide (AEA) and 2-arachidonoylglycerol (2AG) are two endocannabinoids that are synthesized on demand from membrane phospholipids, and they may play fundamental roles in modulating our sensitivity to noxious stimuli. Indeed, we have generated evidence that increased degradation of AEA in skin is associated with mechanical hyperalgesia in a murine model of bone cancer pain. Cannabinoid-1 receptors mediate the inhibitory effects of AEA on somatosensory neurons, and expression of these receptors is increased in dorsal root ganglion neurons ipsilateral to tumors in tumor bearing mice. Activation of the increased pool of receptors by endogenous AEA can be manipulated by decreasing AEA degradation, resulting in prolonged anti-hyperalgesia in tumor-bearing mice. These data provide evidence to support the hypothesis that the local inhibition of endocannabinoid degradation in the periphery reduces mechanical hypersensitivity in a murine model of bone cancer pain through CB1 and CB2 receptor-dependent mechanisms. Studies outlined in this proposal will test this hypothesis further and address specific aims that explore 2AG metabolism in the same model of bone cancer pain.
Specific aim 1 addresses whether inhibition of 2AG degradation is more effective than inhibition of AEA degradation in attenuating mechanical hypersensitivity in tumor-bearing mice. Monoglyceride lipase (MGL) degrades 2AG in vivo.
Specific aim 2 examines the sources of MGL and cannabinoid receptors that underlie the anti- hyperalgesic effect of MGL inhibition in the periphery and whether expression of these proteins changes in tumor-bearing mice.
Specific aim 3 examines the capacities of cells relevant to cutaneous mechanosensation to synthesize AEA and 2AG and whether the capacity for synthesis is changed in the cancer condition.
Specific aim 4 addresses whether an MGL inhibitor acts directly on sensory neurons to reduce a depolarization-evoked calcium signal. Together with parallel data generated for AEA signaling, the data generated under specific aims 1-4 will provide extensive information on the cellular mechanisms through which inhibitors of AEA and 2AG degradation reduce mechanical hyperalgesia in cancer pain. The data will provide a rationale for the manipulation of endocannabinoids as a novel therapeutic strategy in the treatment of cancer pain.
Pain occurs as the spread of cancer cells destroys tissue, and bone pain is among the most severe. Novel strategies are needed to treat cancer pain because of limitations to the use of opioids. Cannabinoids are potent analgesics, and manipulation of the metabolism of endogenous cannabinoids may provide an effective strategy for reducing mechanical hypersensitivity in patients where destruction of bone is involved.
