Optogenetic activation of liver-innervating vagal sensory neurons increases anxiety-like behavior in mice

The liver plays a central role in energy balance, glucose homeostasis, and lipid metabolism through neural and humoral pathways. Intriguingly, impaired hepatic lipid metabolism has been also associated with an increased risk of anxiety and depression in rodents and humans. However, the mechanisms by which it affects mood behaviors via neural pathways remain poorly understood. This study investigated whether activation of the liver-brain axis can modulate anxiety-like behavior in mice. Advillin (Avil)^CreERT2; channelrhodopsin-tdTomato mice and wireless optogenetics were used to selectively stimulate Avil-positive vagal sensory neurons that innervate the liver in freely moving mice. Acute optogenetic stimulation of their nerves in the liver activated neurons in the nodose ganglia, the dorsal motor nucleus of the vagus, and the nucleus of the solitary tract (NTS). Behavioral assessments revealed that acute optogenetic stimulation of these liver-innervating vagal sensory nerves increased anxiety-like behavior in male and female mice during open field, elevated plus maze, and light/dark box tests. Retrograde viral tracing revealed that neurons in the NTS sent projections to the locus coeruleus (LC), and optogenetic stimulation of liver-innervating vagal sensory nerves resulted in significant activation of norepinephrine-expressing neurons in the LC. Chemogenetic inhibition of LC norepinephrine (NE) neurons completely abolished the anxiogenic effect of stimulating Slc6a2‑positive vagal sensory neurons, demonstrating that LC NE neuron activity is essential for this behavioral response. Therefore, these findings reveal a novel liver - NTS - LC circuit that plays a role in the regulation of anxiety-like behavior through vagal sensory neurons. Unlike the traditional top-down neuronal circuits associated with the liver, this newly identified liver-brain axis is essential for regulating not only systemic energy homeostasis but also emotional behaviors.