Despite rising opioid overdose rates and the central role of respiratory failure in mortality, the neural mechanisms producing opioid-induced respiratory depression (OIRD) and apnea remain poorly defined. Mu-opioid receptors (MOR) are distributed across the brainstem breathing network, including the dorsolateral pons (Kölliker-Fuse, KF) and ventrolateral medulla (Bötzinger and preBötzinger complexes; BötC/preBötC). Our lab recently showed that opioids selectively suppress an excitatory KF→medulla circuit, likely disrupting the balance of neurotransmission required for normal breathing. However, opioid-sensitive neurons account for only about half of medullary projecting dorsolateral pontine neurons. We propose that the activity of opioid-insensitive neurons, which continue to fire during fentanyl overdose, is a critical and underappreciated contributor to OIRD. Opioid-insensitive pontine neurons project to medullary respiratory centers without targeting excitatory neurons, suggesting preferential input to inhibitory neurons. We therefore hypothesized that opioid-insensitive KF neurons contribute to OIRD via glutamatergic drive onto inhibitory neurons in the BötC and preBötC. To test this, we used adult mice and cell-type- and projection-specific approaches across cellular, circuit, and in vivo levels. First, we combined optogenetic control of KF terminals with ex vivo slice recordings from fluorescently labeled BötC/preBötC inhibitory neurons while bath-applying the endogenous opioid Met-Enkephalin (ME). Interestingly, we found that glutamate release from KF→BötC/preBötC synapses was inhibited by ME, whereas most inhibitory neurons themselves were not directly hyperpolarized by ME. Second, using intersectional genetic strategies and whole-body plethysmography, we selectively stimulated opioid-insensitive KF neurons projecting to BötC/preBötC in vivo and found that activation of this population may induce apneas by prolonging expiration. Together, these experiments test whether persistent expiratory signaling from KF recruits inhibitory medullary neurons to maintain apnea and highlight non-MOR targets to rebalance respiratory control in overdose. Research supported by NIH-R01HL174547 (ESL) and NIH-T32DA060142 (LEM) This abstract was presented at the American Physiology Summit 2026 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.
Martinetti et al. (Fri,) studied this question.