Opioid suppression of an excitatory pontomedullary respiratory circuit by convergent mechanisms.
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 Kolliker-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-Botzinger complex (preBotC) 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 preBotC and rVRG neurons via monosynaptic projections, which is inhibited by presynaptic opioid receptors. The excitatory preBotC 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.