Neuroanatomical Mechanisms of Opioid Analgesia and Addiction in the Anterior Cingulate and Primary Somatosensory Cortex

Opioids have been widely used as an effective form of analgesic treatment in clinical pharmacology. However, due to the highly potent and addictive nature of opioids, their increased consumption and misuse has led to a national public health crisis in the United States. As a result, opioid overuse can result in serious adverse effects such as hyperalgesia, withdrawal, emotional disturbances, and respiratory depression which can lead to death. Both the analgesic and harmful effects of opioids are mediated by the mu opioid receptor (MOR). Cortical regions, specifically the anterior cingulate (ACC) and primary somatosensory (SSp) cortex are known to control this plethora of cognitive and emotional behaviors to pain and addiction due to their enriched presence of neuropeptides and opioid receptors, such as the MOR. However, it is currently unknown which opioid receptor-expressing cells contribute differentially to pain relief and facets of addiction, such as reward. Based on single-cell RNA sequencing, the molecular identities of the neurons expressing genes for the mu, delta, and kappa opioid receptors were identified across the different cortical circuits. To determine the cortical and laminar expression pattern of the opioid receptor genes in the ACC and SSp, we created novel triple transgenic reporter mouse lines under the Cre-Lox and Flp-Frt system that express the red fluorophore tdTomato when a cell co-expresses a gene for the opioid receptor and another neuronal gene of interest. Cell types were validated using immunohistochemistry. Interestingly, we observed higher Oprm1 expression in the SSp compared to the ACC, specifically in layers II/III and V with further experiments showing these cells projecting to the thalamus and intracortical regions. This work elucidating the anatomical location of opioid receptor-expressing neurons will increase our understanding on the identity of cells mediating these behavioral effects in the brain. Furthermore, our mouse models provide genetic access to each cell type, permitting further experimentations on the behavioral contribution of each cell type to reward and antinociception.