Scaling down synaptic strength results in changes to the pattern of fictive breathing in a model of the brainstem respiratory network: medullary and pontine sources
Authors: Hayes John , Olsen Wendy , Morris Kendall , Bolser Donald 
Published: May 2023

Opioids remain a primary vehicle for pain management prescribed by clinicians. However, opioid-induced respiratory depression (OIRD) can cause apneas and be fatal at elevated doses. Two sites of action for these opioids have been suggested in the brainstem, one in the pons and the other in the preBötzinger Complex of the medulla. Additionally, there is electrophysiological evidence suggesting that an inward-rectifying K + current is opened upon μ-opioid receptor (μOR) activation causing neuronal hyperpolarizations. Activation of μORs may cause depletion of glutamate release from excitatory neurons. Our previous simulation efforts have demonstrated the plausibility of presynaptic actions of opioids among inspiratory neurons of the medulla and pons in depressing inspiratory drive or eliciting apnea within specific and singular synaptic connections. Here, we tested the effects of diminishing the strength of excitatory synaptic currents among several different classes of respiratory neurons in a large-scale model of the brainstem respiratory network and whether it would replicate OIRD. We simulated the simultaneous actions of μOR on multiple presynaptic sites among the medullary inspiratory network. We began by scaling down the synaptic conductances of target populations of medullary inspiratory neurons. The core of the model is made up of neurons that produce the fundamental inspiratory rhythm (“I-Driver” neurons), as well as neurons that normally have an inspiratory decrementing (“I-Dec”) or inspiratory augmenting (“I-Aug”) firing pattern and various expiratory phase firing patterns (“E-Dec,” “E-Aug,” “E-Tonic”). I-Driver neurons are analogous to medullary preBötzinger Complex neurons that putatively express μORs. In our model, these neurons project back onto other I-Driver neurons to promote recurrent excitation but also project onto I-Aug and I-Dec populations. Decreasing the synaptic strength from I-Driver neurons to these populations collectively slows fictive breathing until extended apnea occurs. To further understand how this apnea develops we systematically scaled down the synaptic strengths of I-Driver to each of the other specific populations and found that the excitatory connection from I-Driver to I-Dec populations is most influential in producing apnea. However, we also found that excitatory drive from tonic, non-respiratory modulated (NRM) neurons in the pons to the medulla could influence this pattern by grading down synaptic strength. In addition to any direct hyperpolarizing effects of opioids on μOR-expressing neurons, these data support the plausibility that OIRD could arise from changes due to glutamate release. This work was supported by the National Institutes of Health grant 1R01HL155721-01, 1R01HL163008, and T32HL134621. This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.