Abstract

Neurons in the ventrolateral preoptic (VLPO) area are responsible for initiating and maintaining sleep. During wakefulness, the sleep-promoting VLPO galanin (VLPOGal) neurons are thought to be under inhibition by arousal signals, but the mechanisms by which this might occur are not fully understood. Herein, we investigated using in vivo and in vitro recordings how the locus coeruleus (LC) and noradrenaline (NA), regulate VLPO neurons. In the VLPO, we expressed a G-protein coupled receptor-activation-based NA sensor (GRAB_NA) and recorded changes in NA levels across the sleep-wake cycle in mice. We found that the NA signal in VLPO is higher in wakefulness than in NREM and REM sleep. We then investigated the effects of NA in VLPOGal neurons in brain slices. We found that NA directly inhibits the VLPOGal neurons by post-synaptic alpha-2 adrenergic receptors and indirectly inhibits them by increasing the GABAergic synaptic input. Optogenetic stimulation of local VLPO GABAergic neurons produced opto-evoked IPSCs in VLPOGal neurons, indicating a local inhibitory circuit controlling the sleep-active VLPOGal neurons. NA increased the amplitude of these opto-evoked IPSCs via alpha-1 receptors, demonstrating that NA inhibits VLPOGal neurons at least in part through the activation of the local GABAergic circuit. Next, we found that 1) stimulation of the LCNA→VLPOGal input evokes release of NA which directly inhibits, by alpha-2 adrenergic receptors, the VLPOGal neurons and 2) orexin can activate LC terminals that express Ox1R, and so presynaptically facilitate NA release. During wakefulness NA released from LC is enhanced by orexin contributing to inhibiting VLPOGal neurons. Finally, we found that the release of NA in VLPO, measured with GRAB_NA, is reduced in mice lacking orexin neurons compared to the controls. These results support the hypothesis that, in narcolepsy, the lack of orexin signaling may lead to a reduction in NA release in VLPO, resulting in diminished inhibition of VLPOGal neuron activity. This could likely contribute to the inability of these patients to sustain alertness and the frequent lapses in sleep that are common in narcolepsy.