Hypocretin-1 and GABA in the Pontine Reticular Formation Interact to Promote Wakefulness.

Abstract

Hypocretin-1 and hypocretin-2 are wake-promoting neuropeptides that are exclusively synthesized in the lateral hypothalamus. Hypocretin deficiency underlies the sleep disorder narcolepsy, which is characterized by excessive daytime sleepiness and disrupted nighttime sleep. In addition to hypocretin deficiency, narcoleptic patients display a reduction in GABA levels in the medial prefrontal cortex as compared to patients without narcolepsy, suggesting that GABA also plays a role in the pathophysiology of narcolepsy. The pontine reticular nucleus, oral part (PnO) is a key sleep-wake regulating brain region that is targeted by hypocretins and GABA. Microinjection of hypocretin-1 into the PnO increases wakefulness. Administration of hypocretin-1 to the PnO causes a concentration-dependent increase in GABA levels, and GABA in the PnO promotes wakefulness. The mechanisms by which hypocretin-1 increases wakefulness are not well understood. Therefore, the goal of my thesis research was to elucidate the mechanisms by which hypocretin-1 in the PnO causes an increase in wakefulness. I tested the hypothesis that hypocretin receptors in the PnO mediate the hypocretin-1-induced increase in wakefulness. Microinjection of hypocretin-1 into the PnO caused a concentration-dependent increase in wakefulness. Next I used the hypocretin receptor-1 antagonist SB-334867 and showed that the increase in wakefulness caused by hypocretin-1 was mediated by hypocretin receptors. I then tested whether the increase in wakefulness caused by hypocretin is dependent on GABAergic transmission. Indeed, I was able to show that the increase in wakefulness caused by administering hypocretin-1 to the PnO was blocked by the GABAA receptor antagonist bicuculline. My thesis research findings are the first to demonstrate that hypocretin-1 in the PnO promotes wakefulness by activating hypocretin receptors and GABAA receptors. Treatments for narcolepsy target the symptoms and not the pathophysiology of the disease, hypocretin deficiency. Because hypocretin-1 does not penetrate the blood-brain barrier it is important to develop drugs that target the downstream effectors of hypocretin. My thesis findings further our understanding of how hypocretin increases wakefulness and contribute to the rationale design of more selective drugs to treat narcolepsy.