Regular physical activity (exercise) can reduce pain in people with chronic musculoskeletal pain; whereas, unaccustomed exercise can exacerbate pain. This apparent dichotomy in pain response to physical activity is poorly understood, making exercise prescription for individuals with pain challenging. Dr. Sluka will present ongoing research examining the neurobiological mechanisms underlying the increased pain with an acute bout of exercise, and compare these to the mechanisms that decrease pain with regular physical activity using animal models of pain. All experiments were conducted after approval by the institutional review board, and in accordance with National Institutes of Health guidelines. We will show that there is a balance between inhibitory and excitatory pathways in brainstem sites that modulate pain so that regular physical activity increases overall inhibition to prevent the onset of chronic musculoskeletal pain. To induce chronic musculoskeletal pain, we show that repeated acid injections, or acid injections combined with fatigue, result in long-lasting widespread hyperalgesia. Regular physical activity, induced in mice with wheel running, prevents the hyperalgesia in these animal models of pain. We show increases in phosphorylation of the NR1 subunit of the NMDA receptor and the serotonin transporter (SERT) in the caudal raphe nuclei after induction of chronic musculoskeletal pain. Increasing NR1 expression in the RVM by viral delivery of cDNA to NR1 induces muscle hyperalgesia, while decreasing expression by viral delivery of an miRNA to NR1 prevents development of hyperalgesia. We further show increased phosphorylation of the NR1 subunit NMDA receptor, decreases in serotonin and increases in the serotonin transporter in the RVM, and increases in phosphorylation of the transcription factor CREB, in the amygdala, cingulate and insular cortices. Regular physical prevents these increases in p-NR1, SERT, and p-CREB in the RVM and cortex, and increases serotonin and 5-HT recpetors in RVM, and the analgesia produced by regular physical activity is prevented by blockade of opioid receptors in the RVM and periaqueductal gray. Modulation of opioid expression in physically active mice, knockouts or pharmacological blockade, prevents the activity induced decreases in SERT but not p-NR1. Thus, a complicated network involving the cortex and the brainstem that involves a balance between multiple inhibitory and excitatory systems mediates the hyperalgesia induced by an acute bout of exercise in physically inactive mice, and the ability of regular physical activity to prevent chronic pain.