Opioid receptor architecture for the modulation of brainstem functions

Opioids produce profound and diverse effects on a range of behaviors, many driven by brainstem activity; however, the presence of opioid and opioid-like receptors at this level has been poorly studied outside of nociceptive structures and components of respiratory circuitry. While previous studies identified expression of µ, δ, κ, and nociceptin opioid and opioid-like receptors in the brainstem, patterns have not been fully delineated, and neither has receptor coexpression nor the behavioral implications of their expression in most structures. We aimed to elucidate expression patterns for all four receptors across somatosensory-motor, auditory, and respiratory brainstem circuits; identify recurring themes to provide insight into the mechanisms by which exogenous opioids affect broader brainstem circuits; and characterize the function of endogenous opioids in subcortical processing and behavior modulation. Using a fluorescent reporter mouse line for each receptor, we created a comprehensive atlas of brainstem receptor distribution and identified novel expression patterns in modality-specific circuits. Each receptor showed unique expression patterns across the brainstem with minimal correlation between receptors. Orofacial somatosensory-motor circuits expressed all four receptors, though generally in distinct regions, suggesting differential opiate modulation of afferent and efferent trigeminal signaling. Within the auditory circuits, receptors segregated along the vertical and horizontal processing pathways with minimal colocalization. Finally, the respiratory circuit strongly expressed the µ opioid receptor in multiple crucial structures with minimal presence of the other three receptors. We further assessed the functional significance of these expression patterns, using the respiratory circuitry as an example, by characterizing respiratory responses to selective opioid agonists, finding that each agonist caused unique alterations in breathing pattern and/or breath shape. Together, these results establish a comprehensive atlas of opioid and opioid-like receptor expression throughout the brainstem, laying the essential groundwork for further evaluation of opioid neuromodulation across the broad spectrum of behaviors.