The Serotonergic Dorsal Raphe Promotes Emergence from Propofol Anesthesia in Zebrafish.

The mechanisms through which general anesthetics induce loss of consciousness remain unclear. Previous studies have suggested that dorsal raphe nucleus serotonergic (DRN5-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 DRN5-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 DRN5-HT neurons reversibly decreased during propofol perfusion. Ablation of DRN5-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 DRN5-HT neurons strongly promoted emergence from propofol anesthesia. Propofol application to DRN5-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 DRN5-HT neurons were also abolished by propofol. Furthermore, the hyperpolarization of DRN5-HT neurons caused by propofol was abolished by picrotoxin, a GABAA receptor antagonist, which shortened emergence time from propofol anesthesia when locally applied to the DRN. Our results reveal that DRN5-HT neurons in zebrafish are involved in the emergence from propofol anesthesia by inhibiting presynaptic excitatory glutamate inputs and inducing GABAA receptor-mediated hyperpolarization.