Roles of GABAergic and Glutamatergic Prebötzinger Complex Circuits in Opioid‐Induced Respiratory Depression

Abstract

Introduction. Opioids are widely prescribed analgesic medication but are highly addictive and can be lethal with overdose. Respiratory depression is the primary cause of death by opioid overdose, and it limits the ability to use opioid analgesics safely and effectively. Respiratory depression by opioids is mediated by the inhibition of neural circuits in the brainstem, such as the preBötzinger Complex (preBötC), a small region of the medulla involved in respiratory rhythm generation. The generation of respiratory rhythm requires the activation of excitatory and inhibitory neurons in the preBötC. Glutamate is a main excitatory neurotransmitter, and preBötC glutamatergic neurons are involved in generating respiratory rhythm. γ‐aminobutyric acid (GABA) is a main inhibitory neurotransmitter, and GABAergic neurons are found in the preBötC. Interestingly, GABAergic neurons often co‐express µ‐opioid receptors, and disinhibition of GABAergic neurons by opioid drugs is involved in opioid addiction and euphoria. Here, we aim to determine the roles of excitatory glutamatergic and inhibitory GABAergic neurons in opioid‐induced respiratory depression.MethodsTo control glutamatergic and GABAergic neurons, we expressed opsin channels in cells expressing the vesicular glutamate transporter 2 (vGLUT2) and the vesicular GABA transporter (vGAT), respectively. Using a cre‐lox recombination approach, we injected into the preBötC cre‐dependent adeno‐associated viruses to locally express the inhibitory archaerhodopsin or the excitatory channelrhodopsin in vGLUT2 or vGAT‐expressing cells. Following a 4‐week incubation period, we applied 554 nm or 470 nm lights through optical fibres focused onto the preBötC while recording diaphragm and genioglossal muscle activities in anesthetized mice. Following systemic injection of fentanyl (5µg/kg), we inhibited or activated glutamatergic or GABAergic cells, respectively.ResultsOur preliminary data indicate that inhibition of vGLUT2 cells severely depressed respiratory rate, mimicking the respiratory depression observed with the opioid fentanyl. We are currently performing optical inhibition of vGAT preBötC cells in anesthetized mice and determining their role in respiratory depression by fentanyl.DiscussionWe will identify the role of GABAergic and glutamatergic preBötC cells in coordinating respiratory rhythm in preBötC cells and whether GABAergic cells contribute to respiratory depression by opioid drugs. Understanding the role of the populations of cells involved in respiratory depression by opioid drugs is critical to identify new molecular targets to reduce respiratory depression and provide safe and reliable pain killers without the risks associated with opioid pain killers.