In 2000, our group and Mignot's group identified the cause of human narcolepsy with cataplexy as a loss of hypocretin (Hcrt or orexin) cells. We found a 90% loss of Hcrt neurons in narcolepsy with cataplexy. In 2007, we identified a 60% Hcrt cell loss linked to the sleepiness of Parkinson's disease. Most studies of neuronal systems with identified transmitters have operated under the assumption that the number of cells in each group can be reduced with pathology or age, but cannot be substantially increased. We recently received a normal human brain that contained 114,000 Hcrt cells, 50% greater than the mean number and 37% greater than the highest number that we have seen in controls. Further investigation revealed that this individual was a former heroin addict. This suggested that opiate administration might permanently increase the number of Hcrt expressing cells, and that this alteration might be related to the addictive process. In pilot studies, we found that morphine administration increased the number of Hcrt cells in wild type mice, as it apparently had in the human addict. We propose to determine the magnitude and time course of the change in Hcrt cell number with morphine administration, and the extent to which this increase is permanent. We will observe the effect of morphine on Hcrt levels in the cerebrospinal fluid of normal and narcoleptic dogs. We will determine the effect of morphine administration in Hcrt KO mice on hypothalamic Hcrt cells (identified with Narp staining) to determine if Hcrt is necessary for the increase in the number of these cells after morphine. We will record unit activity of Hcrt cells in freely moving rats to determine the physiological response of Hcrt cells to morphine in naive and addicted rats. We will determine if the increased Hcrt cell population in mice after long-term morphine administration shows the same pattern of Fos expression in waking tasks as we have reported in drug naive mice. We will make the first measurements of Hcrt-1, melanin concentrating hormone, and dopamine release after morphine administration in humans who have been implanted with microdialysis probes for diagnostic purposes and given morphine for pain relief. We will determine if morphine administration that increases the number of Hcrt cells, reverses the symptoms of narcolepsy, using a newly developed mouse narcolepsy model, the DTA-orexin mouse. This model allows selective, controlled deletion of Hcrt cells in mature mice. We have new, exciting data showing that a period of morphine administration can completely reverse Hcrt cell loss in these mice, indicating that it should also be possible to achieve this reversal in human narcoleptics. Using this model, we will also determine if morphine conditioned place preference is reversed by induced degeneration of Hcrt cells. Establishing that the properties of Hcrt cells can be altered by opiates, will lay the foundation for a new treatment, and a possible cure, for human narcolepsy. It will also lead to a better understanding of opiate dependence and depression. The discovery of plasticity of neuronal transmitter phenotype has important implications for both clinical and basic neuroscience.
Recently, we found a marked increase in the number of hypocretin neurons in a human heroin addict, and that morphine can induce such an increase in mice. Using a unique new mouse model of narcolepsy, we will test the ability of morphine to permanently reverse narcolepsy, by increasing the number of hypocretin cells back to normal. Understanding the role of hypocretin in addiction and narcolepsy will have profound implications for understanding and treating these disorders as well as Parkinson's disease and depression.
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