Neurons in part of the brain known as the basolateral amygdala mediate persistent behaviour induced by cocaine exposure, reports a study in the August 2007 issue of Nature Neuroscience. These results suggest that the amygdala may be involved in the inability of addicts to modify their behaviour in the face of adverse consequences.
Goeffrey Schoenbaum and colleagues trained rats to respond to one odour to receive a reward and to avoid responding to another, which led to a punishment. When the association between odour and outcome was reversed, animals exposed to cocaine before learning had particular difficulty learning to stop responding to the previously rewarded cue. The authors also recorded from the basolateral amygdala of the rats' brains during this task and identified neurons that responded separately to either odour, once the animal had learned which odour predicted which outcome. When the association between odour and outcome was reversed, the responses of most neurons switched between the two. In contrast, in rats that had been exposed to cocaine before learning, very few of the neurons switched their responses.
In another experiment, the authors lesioned the basolateral amygdala of both control and cocaine-exposed rats before learning and reversal. While the lesions did not affect the acquisition of the odour-outcome associations, they did abolish the reversal deficit in cocaine-exposed animals. This further supports the idea that the basolateral amygdala inhibits the ability of drug exposed rats to adapt their behaviour to changing contingencies in the environment.
Geoffrey Schoenbaum (University of Maryland School of Medicine, Baltimore, MD, USA)
Abstract available online.
(C) Nature Neuroscience press release.
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