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The role of central noradrenergic systems in morphine tolerance development Klonoff, Pamela Susan


The role of noradrenaline (NA) in the behavioural and pharmacological effects of morphine was evaluated in rats. Animals received specific injections of 6-hydroxydopamine (6-OHDA) into the dorsal noradrenergic bundle (DB) resulting in selective depletion of telencephalic NA levels and increased levels of noradrenaline in the spinal cord and cerebellum. Employing changes in the hypoactive phase of morphine-induced locomotor activity as an index of tolerance development, it was observed that injection of 6-OHDA into the dorsal noradrenergic bundle resulted in a slower rate and a lesser degree of tolerance development to morphine. The effect of the DB-6-0HDA lesion on physical dependence was assessed by measuring naltrexone-induced withdrawal in lesioned and control animals who had received chronic morphine treatment. Results indicate that although NA is important in tolerance development, it does not mediate a dominant role in withdrawal, although behavioural evidence suggesting a secondary or modulatory role is presented. The interaction of amphetamine and morphine with the dopamine (DA) system was also assessed by studying the behavioural effects of amphetamine in animals following either acute or chronic morphine treatment. It was observed that amphetamine potentiated the spontaneous locomotor hyperactivity following both acute and chronic morphine treatment. The DB-6-OHDA lesion did not affect the locomotor potentiation of amphetamine in morphine pre-treated animals, and the hypothesis that another transmitter system mediates this effect, specifically DA, is discussed.

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