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The Journal of Neuroscience, February 1, 2002, 22(3):1034-1041

Chronic Morphine Treatment Modulates the Extracellular Levels of Endogenous Enkephalins in Rat Brain Structures Involved in Opiate Dependence: A Microdialysis Study

Magdalena Mas Nieto¹, Jodie Wilson¹, Annie Cupo², Bernard P. Roques¹, and Florence Noble

¹ Département de Pharmacochimie Moléculaire et Structurale Institut National de la Santé et de la Recherche Médicale U266, Centre National de la Recherche Scientifique Unité Mixte de Recherche 8600, Université René Descartes, Unité de Formation et de Recherche des Sciences Pharmaceutiques et Biologiques, 75270 Paris Cedex 06, France, and ² Institut de Pharmacologie Moléculaire et Cellulaire, Centre National de la Recherche Scientifique Unité Propre de Recherche 411, Université de Nice-Sophia-Antipolis, 0650 Valbonne, France

The endogenous opioid system is often assumed to play a role in vulnerability to drug abuse. However, controversial results have been reported regarding the levels of enkephalins or preproenkephalin in neurons of rodent brains after opiate administration. The present study was performed to determine the extracellular levels of enkephalins and its physiological antagonist cholecystokinin (CCK), using in vivo microdialysis in freely moving rats after morphine-induced physical dependence or positive place conditioning. A large increase (340%) of Met-enkephalin was observed in the periaqueductal gray matter, a structure involved in morphine withdrawal syndrome, in morphine-dependent rats. No change in CCK immunoreactivity occurred in these conditions. Moreover, using the conditioning place preference paradigm, we observed for the first time opposite changes of enkephalin outflow in the nucleus accumbens (NAc). Thus, an increase in enkephalin levels was observed in rats placed in the drug-associated compartment and a decrease in the saline-paired side. These changes in opioid peptides in the NAc may reflect an "emotional state" of the animals in relation to the expectation of drug reward (reinforcing effects of morphine). Moreover, the lack of regulation in CCK outflow suggests that CCK-opioid interactions in morphine dependence involve probably post-receptor events.

Key words: enkephalin; cholecystokinin; microdialysis; reward; morphine dependence; hippocampus; nucleus accumbens; periaqueductal gray matter

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Copyright © 2002 Society for Neuroscience  0270-6474/02/2231034-08$05.00/0

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