Project description:Background: We have previously used the rat 4 day Complete Freund's Adjuvant (CFA) model to screen compounds with potential to reduce osteoarthritic pain. The aim of this study was to identify genes altered in this model of osteoarthritic pain and use this information to infer analgesic potential of compounds based on their own gene expression profiles using the Connectivity Map approach. Results: Using microarrays, we identified differentially expressed genes in L4 and L5 dorsal root ganglia (DRG) from rats that had received intraplantar CFA for 4 days compared to matched, untreated control animals. Analysis of these data indicated that the two groups were distinguishable by differences in genes important in immune responses, nerve growth and regeneration. This list of differentially expressed genes defined a âCFA signatureâ. We used the Connectivity Map approach to identify pharmacologic agents in the Broad Institute Build02 database that had gene expression signatures that were inversely related (ânegatively connectedâ) with our CFA signature. To test the predictive nature of the Connectivity Map methodology, we tested phenoxybenzamine (an alpha adrenergic receptor antagonist) â one of the most negatively connected compounds identified in this database - for analgesic activity in the CFA model. Our results indicate that at 10mg/kg, phenoxybenzamine demonstrated analgesia comparable to that of Naproxen in this model. Conclusion: Evaluation of phenoxybenzamine-induced analgesia in the current study lends support to the utility of the Connectivity Map approach for identifying compounds with analgesic properties in the CFA model. A naive (control) group of rats (n = 6) and a group of rats injected with CFA (n = 6) were used for gene expression profiling experiments. One animal from the control group did not yield sufficient amount of RNA for microarray and thus was omitted from further processing. In total, 5 microarrays each from the 5 animals in the control group, and 6 microarrays each from the 6 animals in the CFA group were analyzed.

Project description:For over a millennium, mind-body interactions have fascinated scientists and doctors for their abilities to shape human perceptions of the external world 1,2. Placebo effects are striking demonstrations of mind-body interactions in which, in the absence of any treatment, a positive expectation of pain relief can reduce or even abolish the experience of pain 3–6. However, despite widespread recognition of the strength of placebo effects and their impact on everyday human experience and clinical trials for new analgesics, the neural circuit basis of the placebo effect has remained a mystery. Here, we show that analgesia from the expectation of pain relief is mediated by a distinct population of rostral anterior cingulate cortex (rACC) neurons that project to the pontine nuclei (rACC→Pn), a pair of brainstem pre-cerebellar nuclei with no established function in pain processing. To do this, we created a behavioral assay that models placebo analgesia by conditioning mice to expect pain relief when moving from a chamber with a heated floor to a second chamber. In this assay, an expectation of pain relief induces an analgesic effect that, like placebo analgesia in humans, is mediated by endogenous opioids. Calcium imaging of neural activity in freely moving mice and electrophysiological studies in cingulate cortical brain slices showed that expectations of pain relief boost the activity of rACC→Pn neurons and potentiate neurotransmission in this pathway. Transcriptomic studies of Pn neurons revealed an unusual abundance of opioid receptors in these cells, further suggesting a role in pain modulation. Selective inhibition of either the rACC→Pn pathway or of opioid-receptor-expressing Pn neurons disrupted placebo analgesia and decreased pain thresholds. Finally, a subset of cerebellar Purkinje cells exhibits activity patterns resembling those of rACC→Pn neurons during pain relief expectation, providing cellular-level evidence of a role for the cerebellum in cognitive pain modulation. Altogether, these findings elucidate longstanding mysteries surrounding the placebo effect by identifying a specific neural pathway that mediates expectation-based pain relief. This discovery opens the possibility of targeting this novel pathway with drugs or neurostimulation methods to treat pain. More broadly, our studies provide a framework for investigating the neural circuit basis of other mind-body interactions beyond those involving pain, and point to prefrontocortical-cerebellar communication as a potential basis for such effects.

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Evaluation of phenoxybenzamine in the CFA model of pain following gene expression studies and connectivity mapping

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