Brainstem mechanisms of opioid-induced respiratory depression

Project Summary Opioid overdoses continue to rise at an alarming rate in the US. Respiratory failure is the cause of death from opioid overdose. Yet, the mechanisms by which respiratory depression and cessation of breathing occur are poorly understood. Emerging evidence from our lab and others implicates mu opioid receptors expressed

throughout the brainstem control of breathing network, including areas in the dorsolateral pons and the ventrolateral medulla. Recently, we have found that opioids selectively suppress an excitatory pontomedullary circuit, which could disrupt the delicately balanced neurotransmission that is required for proper breathing.

However, opioid-sensitive neurons only account for approximately half of the pontomedullary respiratory neurons. We propose that to fully understand the effect of opioids on breathing we must also consider the activity of neurons that are not inhibited by opioids and continue to function unopposed. We know from

published and preliminary data that non-opioid-sensitive pontine neurons project to medullary respiratory centers, but do not synapse onto excitatory neurons. In addition, most non-opioid-sensitive pontine neurons are expiratory and continue to fire during opioid overdose. Thus, we hypothesize that expiratory, non-opioid-

sensitive pontine neurons contribute to breathing failure via tonic glutamatergic projections to inhibitory neurons in the ventrolateral medulla. We will test this hypothesis in adult mice with fully developed respiratory circuitry using cell-type and projection specific approaches at the cellular, circuit and in vivo level in three

specific aims. Aim 1 will determine the synaptic target of non-MOR-expressing dorsolateral pontine neurons using optogenetics and ex vivo recordings of fluorescently labeled inhibitory neurons in the ventrolateral medulla. Aim 2 will determine the respiratory phenotype of opioid-insensitive, glutamatergic, medullary

projecting dorsolateral pontine neurons using single unit recording of optically-tagged neurons in a unique arterially perfused mouse preparation that maintains an intact brainstem and “in vivo-like” respiratory cycle. Aim 3 will use intersectional genetics to optically stimulate or inhibit medullary projecting, non-opioid-sensitive

dorsolateral pontine neurons in arterially perfused preparations and in vivo to determine the ability of these neurons to promote or prevent apnea. Together, this R01 project will contribute to our long-term goal to generate a more complete model of the brain’s control of breathing network under different influences, such as

opioids, and can be expanded upon in future studies based on this research.