OPIATE WITHDRAWAL-INDUCED NEGATIVE REINFORCEMENT AND DESCENDING FACILITATION

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The positive reinforcement provided by opiates have been considered as primary factors in promoting drug abuse and dependence. However, opiates also produce a host of "negative" effects which can manifest both during opiate administration and which are particularly expressed during withdrawal. Withdrawal from opiates is unpleasant and may provide negative reinforcement and contribute to drug dependence. Thus, both positive and negative reinforcement are likely to contribute to the compulsive use of opioids, a critical feature of "opioid addiction". The mechanisms by which opiates produce negative effects are unknown. Our preliminary data indicate that blockade of descending facilitation in the rostral ventromedial medulla (RVM) prevents expression of somatic and autonomic signs of opiate withdrawal indicating that facilitatory outflow from the RVM is critical in mediating both nociceptive (i.e., opiate-induced hyperalgesia) and many nonnociceptive features of the withdrawal syndrome. We hypothesize that opiate-induced descending facilitation from the RVM and subsequent upregulation of spinal dynorphin are essential for the expression of many of the negative symptoms which comprise withdrawal from opiates. Mechanisms which underlie the expression of the withdrawal syndrome thus also represent mechanisms likely promote negative reinforcement which may contribute to opiate dependence, the continued use and abuse of opiates and drug seeking behavior. This hypothesis will be explored with four Specific Aims: First, we will characterize pronociceptive transmitters in the RVM in which can mediate naloxone-precipitated withdrawal. Second, we will determine whether prevention of descending facilitation from the RVM prevents opiate-induced physical dependence (naloxone-induced or spontaneous withdrawal). Third, we will determine whether blockade of the pronociceptive actions of spinal dynorphin, or the bradykinin B2 receptor will prevent withdrawal. Fourth, we will determine if blockade of descending facilitation blocks withdrawal-induced aversion/negative reinforcement. These studies assess mechanisms which may contribute to negative reinforcement, drug dependence and abuse.