Opioid Suppression of an Excitatory Pontomedullary Respiratory Circuit by Convergent Mechanisms

Abstract

Opioids depress breathing by inhibition of inter-connected respiratory nuclei in the pons and medulla. Mu opioid receptor (MOR) agonists directly hyperpolarize a population of neurons in the dorsolateral pons, particularly the Kölliker-Fuse (KF) nucleus, that are key mediators of opioid-induced respiratory depression. However, the projection target and synaptic connections of MOR-expressing KF neurons is unknown. Here, we used retrograde labeling and brain slice electrophysiology to determine that MOR-expressing KF neurons project to respiratory nuclei in the ventrolateral medulla, including the pre-Bötzinger complex (preBötC) and rostral ventral respiratory group (rVRG). These medullary projecting, MOR-expressing dorsolateral pontine neurons express FoxP2 and are distinct from calcitonin gene-related peptide-expressing lateral parabrachial neurons. Furthermore, dorsolateral pontine neurons release glutamate onto excitatory preBötC and rVRG neurons via monosynaptic projections, which is inhibited by presynaptic opioid receptors. The excitatory preBötC and rVRG neurons receiving MOR-sensitive glutamatergic synaptic input from the dorsolateral pons are themselves hyperpolarized by opioids. Thus, opioids can synergistically inhibit this excitatory pontomedullary respiratory circuit by three distinct mechanisms—somatodendritic MORs on dorsolateral pontine and ventrolateral medullary neurons and presynaptic MORs on dorsolateral pontine neuron terminals in the ventrolateral medulla—all of which could contribute to opioid-induced respiratory depression.

Fentanyl effects on respiratory neuron activity in the dorsolateral pons

“…μ-Opioid receptor expression in the lateral parabrachial area is extremely abundant ( 13 , 41 , 42 ). Lateral parabrachial and KF neurons are hyperpolarized by μ-opioid receptor activation of G protein-coupled potassium conductance ( 11 , 43 ), including neurons that project to the preBötC and rostral ventral respiratory group (rVRG) ( 44 ). Thus, we suspect that inspiratory and a subset of IEPS neurons that are completely silent during fentanyl-induced apnea could be hyperpolarized by activation of postsynaptic μ-opioid receptors but cannot rule out presynaptic mechanisms.…”

“…The dorsolateral pons contains segregated populations of glutamatergic and GABAergic neurons ( 47 ). Glutamatergic dorsolateral pontine neurons project to respiratory-related areas of the ventrolateral medulla ( 47 ) and express μ-opioid receptors ( 41 , 44 ). We predict that the opioid-sensitive inspiratory neurons are glutamatergic, but this remains to be determined.…”

“…First, medullary propriobulbar inspiratory neurons are inhibited by fentanyl ( 22 ). Second, opioids inhibit dorsolateral pontine neurons ( 11 ) that provide excitatory input on preBötC and rVRG neurons ( 44 ). Finally, activation of opioid receptors in the dorsolateral pons eliminates the ramping phrenic output pattern and causes apneusis ( 11 ).…”