Histamine is central in sleep-wake regulation. First, I mapped the distribution of histamine producing neurons in the adult brain using genetic approaches. Second, to further explore the histaminergic system, I found that a local circadian clock regulates the expression of histidine decarboxylase (HDC), the enzyme producing histamine in hypothalamic neurons. The level of this enzyme varies with time of day and is up-regulated by sleep deprivation. I disrupted this local clock by using HDC-Cre recombinase by deleting BMAL1, the transciption factor central to circadian rhythms, selectively in histaminergic neurons, generating HDC-Bmal1 mice. Hdc gene expression in HDC-Bmal1 mice showed a disrupted 24-hour rhythm. This greatly affected natural sleep and reduced recovery sleep after sleep deprivation.
Third, the HDC-neurons contain GABA. To understand the role of this GABA, I used different AAVs carrying shRNAs to deliver into the brain to knock down vesicular GABA transporter (vGAT) in histaminergic neurons. Reducing vGAT in HDC-neurons increased general activity and wakefulness in mice; moreover, these GABA in HDC-neurons contributed to recovery sleep after sleep deprivation. To further investigate the mechanism, we conducted an optogenetic method by delivering Channelrhodopsins (ChR2) into the HDC-neurons. We found that photostimulating tuberomamillary nucleus (TMN) fibers in neocortex and striatum triggered GABA release. Thus the decrease of ambient GABA might contribute to the phenotype that we observed in HDC-vGAT knock down mice.
In summary, I identified a local “histaminergic clock” that regulates HDC levels, and is necessary for maintaining appropriate sleep-wake cycle architecture as well as sleep homeostasis. I also found GABA produced by HDC-neurons is necessary for regulating the normal behavioral state.