This project seeks to study a population of glutamatergic neurons in the supramammillary region of the hypothalamus (SUM), which may be a previously unrecognized but important part of the ascending arousal system. Earlier studies have shown that SUM neurons diffusely innervate the cerebral cortex, and that lesions in this area cause profound somnolence not explained by involving nearby orexin or histaminergic neurons. Our preliminary observations indicate that the SUM neurons have massive projections to many components of the arousal system, and that they may be necessary to maintain a normal waking state. We will test this hypothesis by first examining the projections of the glutamatergic SUM neurons, using a conditional adeno- associated viral vector (AAV) containing the gene for GFP which is only produced in Cre-expressing neurons, in mice expressing Cre recombinase under the vesicular glutamate 2 (Vglut2) promoter. We next will selectively lesion the SUM glutamatergic neurons, and study the projections of their remaining non- glutamatergic neighbors, by using a conditional AAV that expresses mCherry in Cre- neurons, and the diphtheria toxin A (lethal) subunit in Cre+ neurons, and recording subsequent wake-sleep behavior. To determine whether the Vglut2+ neurons in the SUM may use other neurotransmitters than glutamate to cause arousal, we will then use AAV-Cre-2A-Venus to delete the Vglut2 gene in the SUM in Vlgut2-flox/flox mice, and express the fluorescent protein Venus in the same cells. We will measure the effect on wake-sleep behavior and correlate that with which target areas contain Venus+ axons, in which Vglut2 has been deleted. We will similarly study the role of GABA in some neurons in the SUM using Vgat-flox/flox mice. Finally, we will examine the acute effects on wake-sleep of either inhibiting or stimulating the Vglut2 neurons in the SUM, to determine the role of compensation in chronic deletion studies. We will inject Vglut2-Cre mice with either a conditional hM3-mCherry vector, which expresses a mutant M3 muscarinic receptor in Cre+ neurons that is activated by clozapine-N-oxide and stimulates neurons; or a conditional ivermectin vector, which expresses YFP fused with the ivermectin receptor, an invertebrate chloride channel that is activated by the antibiotic drug ivermectin, which inhibits neurons. We will then study the acute effects on wake-sleep of either activating or inhibiting the SUM Vglut2+ neurons, and correlate this with the innervation of SUM targets by axons labeled with the fluorescent proteins. This work will characterize the targets of the SUM glutamatergic neurons, and the role of those projections in mediating acute and chronic effects on sleep and wakefulness. These findings will test the hypothesis that the SUM neurons may plan an even more important role than the nearby orexin or histaminergic cell groups in maintaining a normal waking state.
The cerebral cortex is normally kept awake by a network of nerve cell groups located in lower parts of the brain called the ascending arousal system. In this proposal, we will examine the connections and functions of a cell group in the hypothalamus called the supramammillary area (SUM) that we propose may use the excitatory neurotransmitter glutamate to play a key role in regulating sleep and wakefulness. Damage to these neurons may cause prolonged sleepiness or even coma after stroke or brain injury, and learning how to manipulate the SUM glutamate neurons may promote recovery from brain injury or be used to treat diseases that cause daytime sleepiness, such as narcolepsy.
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| Abbott, Stephen B G; Saper, Clifford B (2017) Median preoptic glutamatergic neurons promote thermoregulatory heat loss and water consumption in mice. J Physiol 595:6569-6583 |
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