Tac1‐expressing cells in the pre‐Bötzinger complex are potential targets to prevent opioid‐induced respiratory depression

Abstract

RationalOpioids are extensively used for their analgesic properties but present a variety of unwanted side effects, including tolerance, dependence and respiratory depression. The analgesic effect of opioids is due to activation of µ‐opioid receptors (MOR) in the central nervous system and no treatments are currently available to prevent respiratory depression without reducing their analgesic properties. The neural circuits and mechanisms regulating respiratory depression, sedation, and analgesia by opioids often overlap, therefore making challenging the identification of the mechanisms regulating respiratory depression. Neurons expressing tachykinin precursor 1 peptide (Tac1) located in the pre‐Bötzinger complex (preBötC) also co‐express neurokinin‐1 receptors (NK1R) and MORs. NK1R neurons are preferentially inhibited by opioids and play an essential role in mediating opioid‐induced respiratory depression.ObjectiveHere, we tested the hypothesis that optogenetic activation of Tac1‐expressing preBötC cells in freely behaving mice will modulate breathing and prevent respiratory depression by opioids.MethodsUsing a Cre‐loxP recombination approach, we injected in Tac1 Cre recombinase mice the adeno‐associated virus containing the gene cassette of the excitatory channelrhodopsin‐2 ChETA flanked between loxP sites. A 200 µm optical fiber was then fixed above the preBötC for laser stimulation with blue light (wavelength: 480 nm). After a four weeks period to allow ChETA expression in targeted cells, the mouse was placed in a plethysmographic chamber and breathing was measured while laser stimulations were performed under baseline conditions (30 Hz). Following baseline measurements, the clinically relevant µ‐opioid drug fentanyl was administered by intraperitoneal injection (0.3 mg/Kg). About 5 minutes after fentanyl injection, targeted preBötC cells were activated using laser stimulations (30 Hz).ResultsData show that stimulation of Tac1‐expressing cells increased respiratory frequency under baseline conditions. Fentanyl depressed breathing in 5 minutes and stimulation of Tac1 cells prevented opioid‐induced respiratory depression; an effect that was reversible.ConclusionTac1‐expressing preBötC cells may constitute a target to prevent depression of breathing by opioids. These results will help identify the cells mediating opioid‐induced respiratory depression, an essential step toward the development of therapeutic targets to reduce the risk of opioids overdose and associated mortality.