Hypocretin (orexin) is a neuropeptide synthesized by lateral hypothalamic neurons that project widely throughout the CMS, including to regions of the brain that have been linked to the addictive actions of opioids and nicotine. Hypocretin plays a key role in activation of many brain circuits related to attention and arousal. Absence of hypocretin or its receptor results in narcolepsy, a neurological disease characterized by continued bouts of intense sleepiness during the day, both in animals and humans. Hypocretin neurons also play a role in addiction to opiates. Despite a growing interest in the role of these neurons in substance abuse, almost nothing is known about the cellular physiology of the responses of hypocretin cells to opiates. To facilitate the study of hypocretin neuron neurophysiology, transgenic mice that express GFP selectively in hypocretin neurons will be used. Whole cell patch clamp recording will be used to understand the role of dynorphin, an opioid neuropeptide cosynthesized with hypocretin. The hypothesis that hypocretin neurons are inhibited by dynorphin will be tested. Electron microscopy and immunocytochemistry will be used to study the subcellular localization of the dynorphin neuropeptide, and converging electrophysiological experiments will be used to address the hypothesis that dendrites and axons contain and release the opioid peptide, and that identified neurons postsynaptic to hypocretin axons show diverging responses to hypocretin and dynorphin. We will test the hypothesis that dynorphin receptors desensitize before hypocretin receptors, freeing postsynaptic neurons from opioid inhibition. Long term effects of the opiate morphine will be studied in brain slices after chronic morphine exposure in vitro and in vivo. The most common addictive substance is nicotine, found in tobacco. Nicotine increases general arousal, similar to the actions of hypocretin neurons. The hypothesis that nicotine has a direct action on hypocretin cells, and that long-term exposure will alter the physiology of these cells, will be tested. These experiments will provide a better understanding of the role hypocretin neurons play in the common lack of arousal and lethargic state shown by opiate users, and the arousal response to nicotine. Hypocretin neurons may thus serve as a future target for treatment of addictive drugs that alter the arousal state. Understanding the role of the endogenous opioid in hypocretin cells should also facilitate the understanding and treatment of narcolepsy.
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