Morphine-Induced Place Preference: Interactions with Neurotransmitter Systems

This review gives an overview of our recent findings and developments in research on brain mechanisms of morphine reward from studies using the place preference conditioning paradigm. Intracranial place conditioning methodology has become a valuable and firmly established and very widely used tool in behavioural pharmacology and drug reward mechanisms. Several studies have established that morphine induces a conditioned preference for the place in which it has been administered in rats. Transmitter systems that have been investigated with respect to their involvement in morphine reward mechanisms include dopamine, GABA, acetylcholine, adrenalin and nitric oxide, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs. Although, considerable evidence suggested that the mesocorticolimbic DA system, which originates in the VTA and projects to the Nac, various limbic and cortical areas is a major neural substrate of the rewarding effects produced by morphine, but the role of other brain sites such as hippocampus and amygdala also exist. Overall, our intracranial place conditioning studies showed that there are a number of receptors, neuronal pathways and discrete central nervous system sites involved in the morphine reward mechanisms.