Regulation of wakefulness by GABAergic dorsal raphe nucleus-ventral tegmental area pathway.

The dorsal raphe nucleus (DRN) has previously been proved to be involved in the regulation of the sleep-wake behavior. DRN contains several neuron types, such as 5-HTergic and GABAergic neurons. GABAergic neurons, which are the second largest cell subtype in the DRN, participate in a variety of neurophysiological functions. However, their role in sleep-wake regulation and the underlying neural circuitry remains unclear. Herein, we used fiber photometry and synchronous electroencephalogram (EEG)/electromyography (EMG) recording to demonstrate that DRN GABAergic neurons exhibit high activities during wakefulness and low activities during NREM sleep. Short-term optogenetic activation of DRN GABAergic neurons reduced the latency of NREM-to-wake transition and increased the probability of wakefulness, while long-term optogenetic activation of these neurons significantly increased the amount of wakefulness. Chemogenetic activation of DRN GABAergic neurons increased wakefulness for almost 2 h and maintained long-lasting arousal. In addition, inhibition of DRN GABAergic neurons with chemogenetics caused a reduction in the amount of wakefulness. Finally, similar to the effects of activating the soma of DRN GABAergic neurons, optogenetic stimulation of their terminals in the ventral tegmental area (VTA) induced instant arousal and promoted wakefulness. Taken together, our results illustrated that DRN GABAergic neurons are vital to the induction and maintenance of wakefulness, which promote wakefulness through the GABAergic DRN-VTA pathway.

Control of Behavioral Arousal and Defense by a Glutamatergic Midbrain-Amygdala Pathway in Mice.

In response to external threatening signals, animals evolve a series of defensive behaviors that depend on heightened arousal. It is believed that arousal and defensive behaviors are coordinately regulated by specific neurocircuits in the central nervous system. The ventral tegmental area (VTA) is a key structure located in the ventral midbrain of mice. The activity of VTA glutamatergic neurons has recently been shown to be closely related to sleep-wake behavior. However, the specific role of VTA glutamatergic neurons in sleep-wake regulation, associated physiological functions, and underlying neural circuits remain unclear. In the current study, using an optogenetic approach and synchronous polysomnographic recording, we demonstrated that selective activation of VTA glutamatergic neurons induced immediate transition from sleep to wakefulness and obviously increased the amount of wakefulness in mice. Furthermore, optogenetic activation of VTA glutamatergic neurons induced multiple defensive behaviors, including burrowing, fleeing, avoidance and hiding. Finally, viral-mediated anterograde activation revealed that projections from the VTA to the central nucleus of the amygdala (CeA) mediated the wake- and defense-promoting effects of VTA glutamatergic neurons. Collectively, our results illustrate that the glutamatergic VTA is a key neural substrate regulating wakefulness and defensive behaviors that controls these behaviors through its projection into the CeA. We further discuss the possibility that the glutamatergic VTA-CeA pathway may be involved in psychiatric diseases featuring with excessive defense.

A glutamatergic basal forebrain to midbrain circuit mediates wakefulness and defensive behavior.

Defensive behavior, a group of responses that evolved due to threatening stimuli, is crucial for animal survival in the natural environment. For defensive measures to be timely and successful, a high arousal state and immediate sleep-to-wakefulness transition are required. Recently, the glutamatergic basal forebrain (BF) has been implicated in sleep-wake regulation; however, the associated physiological functions and underlying neural circuits remain unknown. Here, using in vivo fiber photometry, we found that BF glutamatergic neuron is activated by various threatening stimuli, including predator odor, looming threat, sound, and tail suspension. Optogenetic activation of BF glutamatergic neurons induced a series of context-dependent defensive behaviors in mice, including escape, fleeing, avoidance, and hiding. Similar to the effects of activated BF glutamatergic cell body, photoactivation of BF glutamatergic terminals in the ventral tegmental area (VTA) strongly drove defensive behaviors in mice. Using synchronous electroencephalogram (EEG)/electromyogram (EMG) recording, we showed that photoactivation of the glutamatergic BF-VTA pathway produced an immediate transition from sleep to wakefulness and significantly increased wakefulness. Collectively, our results clearly demonstrated that the glutamatergic BF is a key neural substrate involved in wakefulness and defensive behaviors, and encodes these behaviors through glutamatergic BF-VTA pathway. Overexcitation of the glutamatergic BF-VTA pathway may be implicated in clinical psychiatric diseases characterized by exaggerated defensive responses, such as autism spectrum disorders.