Optogenetic-mediated release of histamine reveals distal and autoregulatory mechanisms for controlling arousal.

Histaminergic neurons in the tuberomammillary nucleus (TMN) are an important component of the ascending arousal system and may form part of a "flip-flop switch" hypothesized to regulate sleep and wakefulness. Anatomical studies have shown that the wake-active TMN and sleep-active ventrolateral preoptic nucleus (VLPO) are reciprocally connected, suggesting that each region can inhibit its counterpart when active. In this study, we determined how histamine affects the two branches of this circuit.