Midbrain cholinergic modulation of pain states

Chronic pain conditions plague more than 20% of adults in the United States, emphasizing the need for a more comprehensive understanding of pain control mechanisms and identification of better pain therapies. This need is amplified further by the current opioid crisis, which killed over 60,000 Americans in 2020. While opioids are remarkably effective treatments for acute and chronic pain, adverse side effects, including abuse liability and tolerance to the analgesic effects with repeated use, highlight the need for non-opioid pain therapies. This need motivates our investigations of pain modulation by acetylcholine (ACh) and its receptors. Ascending nociceptive signaling from periphery to central nervous system is modulated by the descending pain modulatory pathway, including the ventrolateral periaqueductal grey (vlPAG) and its projections to rostral ventromedial medulla (RVM). This pathway is a crucial site of action of opioids and endogenous pain control. While much is known about this circuitry, the impact of neuromodulators like ACh on this circuit, particularly under chronic pain conditions have not been investigated. Using a fluorescent ACh sensor, we will investigate how pain and other behaviorally relevant stimuli alter ACh release dynamics in the vlPAG. Using brain slice electrophysiology, we will identify the cellular mechanisms behind alterations in ACh release. And finally, we will attempt to reverse these maladaptive ACh dynamics using chemogenetics and pharmacological approaches. Our preliminary data show that cholinergic projections from the pedunculopontine tegmentum (PPTg), modulate excitability of vlPAG neurons. Under chronic pain conditions, we noted a decrease in ACh release in vlPAG and reduced neuronal activity in PPTg. We also found that M2 muscarinic AChRs are expressed on vlPAG neurons, and that pain increases the activity of these neurons. These observations lead us to hypothesize that chronic pain lowers cholinergic signaling from PPTg and reduces M2 activity in vlPAG, contributing to chronic pain symptoms. We further hypothesize that reversing these changes will relieve somatic and affective symptoms of chronic pain. We will use an ACh sensor with in vivo fiber photometry to assess the impact of acute and chronic pain on ACh release in the vlPAG. We will complement these assays using in vivo microdialysis for ACh. Brain slice electrophysiology will be used to explore the pain-induced changes in synaptic drive and intrinsic excitability of PPTg cholinergic neurons that project to vlPAG. In vivo imaging will be used to assess chronic pain induced changes in vlPAG neuronal activity. Then, using chemogenetics and M2 mAChR agonists we will attempt to reverse the pain-induced changes in excitability of vlPAG. Behavioral testing will confirm reversal of the somatic and affective components of chronic pain. Better understanding of these modulatory inputs to descending pain pathways will help identify novel targets for treating chronic pain.