Abstract
The mechanisms through which general anesthetics induce loss of consciousness remain unclear. Previous studies have suggested that dorsal raphe nucleus serotonergic (DRN(5-HT)) neurons are involved in inhalational anesthesia, but the underlying neuronal and synaptic mechanisms are not well understood. In this study, we investigated the role of DRN(5-HT) neurons in propofol-induced anesthesia in larval zebrafish (sex undetermined at this developmental stage) using a combination of in vivo single-cell calcium imaging, two-photon laser ablation, optogenetic activation, in vivo glutamate imaging, and in vivo whole-cell recording. We found that calcium activity of DRN(5-HT) neurons reversibly decreased during propofol perfusion. Ablation of DRN(5-HT) neurons prolonged emergence from 30 µM propofol anesthesia, while induction times were not affected under concentrations of 1, 3, and 30 µM. Additionally, optogenetic activation of DRN(5-HT) neurons strongly promoted emergence from propofol anesthesia. Propofol application to DRN(5-HT) neurons suppressed both spontaneous and current injection-evoked spike firing, abolished spontaneous excitatory postsynaptic currents, and decreased membrane input resistance. Presynaptic glutamate release events in DRN(5-HT) neurons were also abolished by propofol. Furthermore, the hyperpolarization of DRN(5-HT) neurons caused by propofol was abolished by picrotoxin, a GABA(A) receptor antagonist, which shortened emergence time from propofol anesthesia when locally applied to the DRN. Our results reveal that DRN(5-HT) neurons in zebrafish are involved in the emergence from propofol anesthesia by inhibiting presynaptic excitatory glutamate inputs and inducing GABA(A) receptor-mediated hyperpolarization.