Role of Opioidergic Neurons in the Solitary Nucleus in Respiration and Pain Circuits

A bidirectional relationship exists between pain and respiration. For example, controlled, slow breathing has been recognized for centuries as a strategy to reduce pain and continues to be recommended clinically for patients with chronic pain and women in labor. Conversely, changes in respiration are often used as an indicator in clinical assessments of pain in non-verbal patients and infants. Without mechanistic understanding, few strategies to enhance or predict these relationships between breathing and pain exist. Nor do strategies to dissociate the interaction between breathing and pain, which are needed to stem an epidemic of opioid-induced respiratory depression and death. The nucleus tractus solitarius (NTS) is likely a key brainstem site that links respiration and pain. The NTS is a major relay for sensory signals from the lungs, plays a role in the transmission and perception of pain, and has reciprocal connections with other regions with known roles in respiration and pain such as the parabrachial nucleus (PBN) and preBötzinger complex (preBötC). The NTS contains subnuclei and transcriptionally heterogeneous cell-types that are functionally diverse. We hypothesize that NTS neurons that express the endogenous opioid ligands enkephalin ( PENK ) and/or β-endorphin ( POMC ) form circuits that functionally integrate breathing and pain by linking key brainstem sites involved in respiration and nociception. To test this hypothesis, we used a dual recombinase dual transgenic strategy to achieve cell-type and projection specificity. PENK Cre/Cre or POMC Cre/Cre mice were crossed with Rosa26 CreONFlpON-tdTomato (i.e., Ai65) mice, which express tdTomato only when both Cre and Flp are present. In offspring, separate groups of mice were injected with retroAAV that induces FlpO expression (retroAAV-FlpO) into either the PBN, preBötC, or rostral ventral medulla (RVM), an important site for descending pain modulation. Thus, tdTomato labeling is restricted to neurons that express PENK or POMC (Cre) and have synapses within the injected region (FlpO). We found that the PBN and preBötC receive substantial synaptic inputs from enkephalinergic ( PENK ) neurons in the NTS. Ongoing and future work will utilize optical manipulation and recording techniques combined with respiratory and nociceptive assays to determine how and when these opioidergic NTS circuits affect breathing and pain. R01 HL1660317 & R00 HL145004. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.