Optogenetic manipulations of preBӧtC Oprm1 expressing neurons reveal dual mechanisms of opioid‐induced respiratory depression

Abstract

The analgesic utility of opioid-based drugs is limited by the risk of adverse and life-threatening side effects, including respiratory depression. Opioid-induced respiratory depression (OIRD) is characterized by a pronounced decrease in the frequency and regularity of inspiratory efforts. The inspiratory rhythm originates from the preBötzinger Complex (preBӧtC), a network of interconnected neurons located bilaterally in the ventral medulla. At the network level, the frequency and regularity of the inspiratory rhythm is primarily determined by the inter-burst interval (IBI) which is regulated, in part, by pre-inspiratory spiking activity of excitatory preBötC neurons. To shed light on the cellular- and network-level consequences of μ-opioid receptor (MOR) activation in the preBötC, we utilized Oprm1Cre mice to optogenetically identify and manipulate MOR expressing neurons. We found that ~50% of functionally identified preBӧtC neurons express Oprm1, and Oprm1 expression is distributed evenly among neurons with pre-inspiratory, inspiratory, expiratory, and tonic discharge identities. Patch clamp recordings revealed that MOR activation reduces spiking of Oprm1+ pre-inspiratory neurons preferentially between inspiratory bursts during the percolation phase of the inspiratory rhythm, with a more modest suppression of spiking activity during inspiratory bursts. These changes at the single-cell level were manifested at the level of the preBӧtC network as a decrease in the total integrated spiking activity during the IBI and an increase in the probability of burst failures. However, in both rhythmic brainstem slices and anesthetized adult mice, optogenetic hyperpolarization of preBӧtC Oprm1+ neurons to mimic the changes in spiking activity induced by MOR activation did not phenocopy OIRD. In addition to changes in spiking activity, we found that MOR activation reduces the efficacy of excitatory synaptic transmission from Oprm1+ neurons to their post-synaptic targets by ~50%. This synaptic effect of MOR activation limits the ability of Oprm1+ preBӧtC neurons to drive the inspiratory rhythm. Based on these results in vitro and in vivo, we constructed a computational network in silico to model the effects of MOR activation on preBötC rhythmogenesis. Using this in silico model, we included subpopulation of Oprm1+ model neurons and tested the functional consequences of 1) hyperpolarization to suppress spiking vs. 2) suppression of synaptic transmission. By manipulating these mechanisms independently or in combination, we find that these dual mechanisms of opioid action act synergistically to make a normally robust inspiratory rhythm generating network particularly prone to collapse when challenged with exogenous opioids.