Reassessing the Role of Histaminergic Tuberomammillary Neurons in Arousal Control

The histaminergic neurons of the tuberomammillary nucleus (TMN^HDC) of the posterior hypothalamus have long been implicated in promoting arousal. More recently, a role for GABAergic signaling by the TMN^HDC neurons in arousal control has been proposed. Here, we investigated the effects of selective chronic disruption of GABA synthesis (via genetic deletion of the GABA synthesis enzyme, glutamic acid decarboxylase 67) or GABAergic transmission (via genetic deletion of the vesicular GABA transporter (VGAT)) in the TMN^HDC neurons on sleep-wake in male mice. We also examined the effects of acute chemogenetic activation and optogenetic inhibition of TMN^HDC neurons upon arousal in male mice. Unexpectedly, we found that neither disruption of GABA synthesis nor GABAergic transmission altered hourly sleep-wake quantities, perhaps because very few TMN^HDC neurons coexpressed VGAT. Acute chemogenetic activation of TMN^HDC neurons did not increase arousal levels above baseline but did enhance vigilance when the mice were exposed to a behavioral cage change challenge. Similarly, acute optogenetic inhibition had little effect upon baseline levels of arousal. In conclusion, we could not identify a role for GABA release by TMN^HDC neurons in arousal control. Further, if TMN^HDC neurons do release GABA, the mechanism by which they do so remains unclear. Our findings support the view that TMN^HDC neurons may be important for enhancing arousal under certain conditions, such as exposure to a novel environment, but play only a minor role in behavioral and EEG arousal under baseline conditions.SIGNIFICANCE STATEMENT The histaminergic neurons of the tuberomammillary nucleus of the hypothalamus (TMN^HDC) have long been thought to promote arousal. Additionally, TMN^HDC neurons may counter-regulate the wake-promoting effects of histamine through co-release of the inhibitory neurotransmitter, GABA. Here, we show that impairing GABA signaling from TMN^HDC neurons does not impact sleep-wake amounts and that few TMN^HDC neurons contain the vesicular GABA transporter, which is presumably required to release GABA. We further show that acute activation or inhibition of TMN^HDC neurons has limited effects upon baseline arousal levels and that activation enhances vigilance during a behavioral challenge. Counter to general belief, our findings support the view that TMN^HDC neurons are neither necessary nor sufficient for the initiation and maintenance of arousal under baseline conditions.