Role of hypocretin in opiate addiction and withdrawal

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Project Summary/Abstract Major causes of the opiate crisis are the addictive nature of opiates and the negative physiological and psychological effects of withdrawal that lead to relapse. These effects persist for months or years after the last dose is taken. In recent work we have found what may be the largest change in neuronal morphology in the brain of human heroin addicts; an average 54% increase in the number of neurons producing detectable levels of hypocretin (Hcrt, orexin), and an average 32% shrinkage in the volume of hypocretin neurons. Adjacent melanin concentrating hormone neurons do not change in number or size. We saw a similar long- lasting increase in the number of detected hypocretin cells and a shrinkage of these neurons, accompanied by elevated levels of brain hypocretin, in wild-type mice after longterm, but not short term, morphine administration. The increase in hypocretin cell number is not due to neurogenesis, but rather to increased production of hypocretin in neurons that do not produce detectable levels of these peptides under baseline conditions. Our pilot data, using a newly developed transgenic mouse in which hypocretin neurons can be selectively deleted (DTA-Hcrt mice), shows that removal of hypocretin neurons greatly reduces withdrawal symptoms and the expression of the molecular markers Fos and DeltaFosB in addiction related regions. These preliminary findings suggest that a blockade of hypocretin receptors, a decrease in of the activity of hypocretin neurons, or a reduction of elevated hypocretin neuronal number back to baseline levels would aid in withdrawal and prevent relapse in human opiate addicts. We propose to identify the opiate receptors responsible for the changes in hypocretin cell number and size. We will record from hypocretin neurons in freely moving animals using our chronic unit recording technique to determine, for the first time, how the response of these cells to systemic morphine changes with addiction and withdrawal. We will determine how the newly identifiable hypocretin producing cells induced by opiate administration differ from the baseline population in distribution, projection, and co-transmitters. We will determine the changes in the receptor and membrane physiology of hypocretin neurons produced by opiates using in vitro techniques. In our prior work we found that human narcolepsy is caused by an average 90% loss of hypocretin cells. Of great interest, is that human narcoleptics rarely abuse or escalate dosage of therapeutic drugs, even though the amphetamines, methylphenidate and gamma hydroxy-butyrate that they take daily to reverse symptoms are frequently abused in the non-narcoleptic population. The increased activity of hypocretin neurons that we find in mice after morphine administration, the greatly increased number of hypocretin cells in human addicts, and the greatly decreased number of hypocretin cells in narcoleptics, who are resistant to drug addiction, suggests that hypocretin neurons have a major role in maintaining opiate addiction and relapse. Manipulation of hypocretin neurons may be the key to facilitating withdrawal in addicts.