The basal forebrain (BF) is critically involved in maintaining wakefulness and an activated electroencephalogram (EEG). The mechanisms and substrates by which the BF regulates behavioral and EEG wake remains however poorly understood. A review of the literature reveals that the vast majority of studies on BF circuitry have focused on the cholinergic BF system in physiological and pathophysiological contexts, despite the facts that 1) lesions of the cholinergic BF system produce limited changes in EEG or behavioral wake; and 2) the BF also contains at least other two neuronal populations - GABAergic and glutamatergic neurons - that roughly intermingle with the cholinergic neurons. While it has been appreciated for some time that other BF neurotransmitter systems may interact or work in parallel with cholinergic neurons in the control of sleep and wake, the contribution of GABAergic BF neurons, and in particular their network output, to these processes remains largely unexplored. We recently found that selective activation of BF GABAergic, but not cholinergic or glutamatergic, neurons unexpectedly produces sustained wakefulness. In this proposal we seek to determine the in vitro (cellular) and in vivo (system) mechanisms and substrates by which BF GABAergic neurons support wakefulness. We hypothesize that the influence of BF GABAergic population in behavioral arousal is mediated by activation of the lateral hypothalamus and specifically through disinhibition of orexin/hypocretin neurons. Our model, which derives directly from our preliminary data, predicts that BF GABAergic neurons regulate wakefulness through two outputs: one branch of the circuit inhibits sleep regulatory neurons in the preoptic hypothalamus whereas the second branch disinhibits orexin neurons through local GABAergic neurons. We further propose that these networks are sensitive to homeostatic sleep pressure and that adenosine might act on these different nodes, in a site- and receptor- dependent manner. The current proposal thus seeks to identify, first in vitro and then in vivo, the postsynaptic targets of BF GABAergic input that are necessary for promoting wakefulness. To test our hypotheses we will first employ in Specific Aims 1 and 2 an in vitro circuit mapping paradigm that combines optogenetic- and AAV- based channelrhodopsin2 (ChR2)-assisted circuit mapping (CRACM) and retrograde microspheres to determine how these putative subcortical circuits support wakefulness. We will then determine how adenosine modulates these circuits. Finally in Specific Aim 3 will we place discrete injections of a cre-enabled viral vector system into the BF and a cre-enabled toxin into one of three efferent (post-synaptic) targets of these BF neurons? Doing so will allow us to activate BF GABAergic neurons in behaving mice in which specific synaptic neuronal targets of this BF input have been disrupted. In turn this will reveal which BF GABAergic efferent outputs are necessary for supporting wakefulness in behaving animals. We expect the results from this collaborative work to provide important and novel insights into the brain circuitry supporting wakefulness.
The basal forebrain and hypothalamus are two interconnected structures that play a critical role in maintaining a waking brain. This project seeks to define the pathways and transmitters by which the basal forebrain and hypothalamus contribute to behavioral and cortical arousal. Successful completion of the proposed studies may contribute to the development of new therapeutic and interventional strategies for a wide range of sleep and arousal disorders, including sleep apnea, hypersomnia and insomnia.
