Chronic pain is a major health care burden in the US, affecting approximately 100 million people at an annual cost of $635 billion. This proposal will examine modulation in pain signaling (nociception) as a form of learning and memory. Acquisition of a nociceptive memory represents a form of sensitization-type learning involving either an increased response to nociceptive stimuli (hyperalgesia) or having non-nociceptive stimuli elicit a painful response (allodynia). Habituation, on the other hand, is a potential mechanism to erase a nociceptive memory and may represent an unrecognized process contributing to forms of analgesic modulation such as gate-control of pain or stress-induced analgesia. This approach has benefits not only in understanding how pain- induced sensitization develops, but also in potentially using learning-based approaches to reduce pain, such as using habituation to reduce activity in nociceptive circuits. We have identified a modulatory process, specifically endocannabinoids acting through Transient Receptor Potential Vanilloid (TRPV) channels, that contributes to both pain-related sensitization and habituation of pain- evoked behaviors. Endocannabinoids are lipid neurotransmitters that activate either metabotropic cannabinoid receptors (CB1 and CB2) or TRPV channels (in which case endocannabinoids are sometimes referred to as endovanilloids). While there is considerable interest in cannabinoid-based therapies, their effectiveness in treating chronic pain is questionable. This is, in part, because endocannabinoids can exert both anti-nociceptive and pro-nociceptive effects. Endocannabinoids can depress nociceptive synapses (an anti-nociceptive effect) and we have evidence that this modulatory process contributes the ability of repetitive non-painful stimuli to reduce responses to painful stimuli, a form of transfer of habituation. Endocannabinoids also mediate synaptic disinhibition/potentiation of non-nociceptive synapses, a pro-nociceptive effect that may contribute to pain- induced sensitization. In this study, we will examine the role of endocannabinoids and TRPV signaling during nociception-related sensitization and habituation using Hirudo verbana (the medicinal leech). This approach takes advantage of the well-characterized nervous system of Hirudo in which we have already identified distinct synapses that undergo either endocannabinoid/TRPV-mediated depression or disinhibition/potentiation. An additional advantage is that Hirudo lacks CB1 and CB2 receptors, so it is possible to isolate the role of endocannabinoids acting via TRPV.
In Aim 1, the cellular mechanisms mediating endocannabinoid/TRPV- induced potentiation of non-nociceptive synapses will be examined as well as how this plasticity contributes to sensitization to non-painful stimuli (allodynia).
In Aim 2, the role of endocannabinoid/TRPV-mediated depression of nociceptive synapses during habituation will be studied as well as whether this habituation can reverse injury- induced sensitization.
Aim 3 will address the question of what controls whether the anti-nociceptive or pro- nociceptive forms of endocannabinoid/TRPV modulation are initiated. This will focus on the potential roles of distinct sources of endocannabinoid transmitters and how different patterns of afferent activity can trigger either gene expression of proteins responsible for endocannabinoid synthesis or post-translational modification of these same proteins. These studies will have the following impact. First, using habituation approaches to ?erase? a nociceptive memory is a novel approach and one that could be used immediately in humans by modifying existing medical device therapies, e.g., transcutaneous electrical nerve stimulation (TENS) therapy, making them more effective. Second, the idea that endocannabinoids contribute to the neuromodulatory processes initiated by TENS therapies may provide an avenue for combining endocannabinoid-based drugs (which have failed to date) with this medical device-based therapy, producing an analgesic effect that neither approach can produce on its own. Finally, questions about the effectiveness of cannabinoid-based therapies is undoubtedly based on their ability to both increase and decrease activity by nociceptive circuits. Resolving the cellular mechanisms that mediate these opposing effects is critical if cannabinoid-based therapies are to be effective for treating pain.
Chronic pain is thought to involve changes in neural signaling similar to what is observed during learning. Therefore, a better understanding of learning-related changes in pain circuits may lead to improved treatment of chronic pain. This proposal will examine whether a class of neurotransmitters, known as endocannabinoids, mediate simple forms of learning (sensitization and habituation) that have an impact on how animals respond to noxious stimuli.
| Wang, Yanqing; Burrell, Brian D (2018) Endocannabinoid-mediated potentiation of nonnociceptive synapses contributes to behavioral sensitization. J Neurophysiol 119:641-651 |
| Hanson, Alex; Burrell, Brian D (2018) Are the persistent effects of ""gate control"" stimulation on nociception a form of generalization of habituation that is endocannabinoid-dependent? Neurobiol Learn Mem 155:361-370 |
| Burrell, Brian D (2017) Comparative biology of pain: What invertebrates can tell us about how nociception works. J Neurophysiol 117:1461-1473 |
