Consequences of receptor cross talk on inflammation and algesia

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We established that neurons present in dorsal root ganglia (DRG), similar to leukocytes, express a wide variety of receptors for cytokines, chemokines, opioids, anandamide and other neuropeptides. We previously showed that prior exposure to chemokines such as MIP1α results in PKC mediated desensitization of the chemotactic response to opioids by opioid receptors, and thus potentially enhances pain. This decrease in the analgesic effect of opioids was evident from the enhanced tail flick assay of rats administered MIP1α or RANTES prior to an analgesic opioid into the PAG of the CNS. We then extended these earlier studies by showing that prior administration of chemokines Asensitized and primed the calcium flux of capsaicin or anandamide stimulated vanilloid (TRPV1) algesic receptor on DRG neurons. This response also increased pain as shown by the enhancement of paw withdrawal in response to the intrathecal administration of the chemokine prior to capsaicin in vivo. This sensitization of the vanilloid receptor was also PKC dependent. Consequently, proinflammatory chemokines can increase pain both by suppressing opioid and enhancing vanilloid receptor responses. Based on these studies, we predicted that the anti-inflammatory effects of adenosine, which also interacts with GiPCR, might have effects on chemokine receptors. Indeed our studies showed that prior addition of adenosine results in suppressing the in vitro chemotactic response of leukocytes to a variety of chemokines. Furthermore, prior in vivo injection of adenosine inhibited the in vivo influx of leukocytes into a murine air pouch by about 90%. This cross-desensitization of chemokine receptors by adenosine A2a receptors was PKA dependent. The role of adenosine as an immunosuppressive effector molecule also has been reported to mediate the cell contact dependent effects of Tregs and to interfere with host resistance to tumors. Thus, studies of adenosine effects are relevant to tumor biology and immunology. These studies therefore reveal novel pathways of receptor mediated intercommunication of inflammatory as well as painful stimuli. Means of interfering with these PKC and PKA dependent signals and the pathophysiological relevance of this receptor cross-talk to inflammation and pain need to be further evaluated. Our current project focuses on neuroimmune interactions contributing to pain sensation in cancer patients funded by an INIP postdoctoral IRA financial grant from NIAID and NCI. As previously shown, chemokine receptor cross-talk suppresses analgesic opioid receptors, but enhances algesic transient receptor potential channel (TRP) receptors, thus resulting in painful inflammation. a) In collaboration with Dr. Jeffrey Cohen, NIAID we have investigated this in a cotton rat herpes virus infection model for chemotherapy induced Herpes Zoster. Herpes infection of dorsal root ganglia results in extremely painful inflammatory responses along nerve tracts. It has been reported that VZV infection produces TLR ligands and we have found that peripheral neurons present in dorsal root ganglia express TLR3, 7 and 9 which when stimulated express mRNA for many cytokines and chemokines. In addition, TLR ligand stimulation of neurons upregulate the expression of TLRs and TRPV1. Furthermore, preincubation of neurons for 16 hours with the TLR ligands, enhances the calcium flux induced by capsaicin stimulation of TRPV1. Consequently, products of the herpes virus interacting with these TLR's can either directly or indirectly, by inducing chemokines, enhance the response of TRPV1 pain receptors, providing one possible basis for herpes Zoster neuralgesia in immunosuppressed cancer patients. These studies in a pain model indicate that the peripheral pain reported by many cancer patients may be addressed by effective regulation of neuroimmune molecules.