Propofol inhibits norepinephrine release in vivo at presynaptic varicosities of locus coeruleus neurones in zebrafish larvae.

Abstract

Understanding the mechanisms by which general anaesthetics induce loss of consciousness remains one of the important scientific challenges in medicine and neuroscience. The locus coeruleus-norepinephrine (LC-NE) system plays important roles in general anaesthesia. Both i.v. and inhalation anaesthetics can reduce NE signalling by suppressing the excitatory synaptic inputs and intrinsic excitability of LC-NE neurones. However, it remains unknown whether anaesthetics can directly target presynaptic sites on LC-NE neurone axons to impair NE release. In vivo whole-cell recording and chemogenetics were used to control the activity of LC-NE neurones in intact larval zebrafish, while in vivo time-lapse imaging was used to examine NE release and Ca²⁺ influx at individual presynaptic varicosities of the recorded neurones. The G protein-coupled receptor (GPCR) activation-based NE sensor GRAB_NE2m and the genetically encoded Ca²⁺ sensor GCaMP6s were used to detect NE release and Ca²⁺ levels, respectively. Propofol directly inhibited NE release from individual presynaptic varicosities of LC-NE neurones, because GRAB_NE2m signals induced by precisely controlled activity of LC-NE neurones were markedly reduced after propofol application. Propofol significantly suppressed presynaptic varicosity Ca²⁺ activities of LC-NE neurones. These effects were diminished by co-application of picrotoxin, an antagonist of gamma-aminobutyric acid type A receptors, to the area where the varicosities are located. The degree of propofol-induced inhibition varied across individual varicosities but was relatively homogeneous among different brain areas. Propofol can directly target the locus coeruleus norepinephrine neurone axons to suppress norepinephrine release at presynaptic varicosities.