Chronic morphine exposure alters the dendritic morphology of pyramidal neurons in visual cortex of rats.

Repeated treatment of psychotropic drugs produces changes in brain and behavior that far outlast their initial neuropharmacological effects. The nature of persistent drug-induced neural plasticity is of interest because it is thought to contribute to the development of drug dependency and addiction. To determine if chronic morphine treatment alters the morphology of visual cortical neurons, we statistically examined the dendrites of layer III pyramidal neurons in the primary visual cortex of both morphine-treated and saline-control rats. Compared with control rats, the pyramidal cells of morphine-treated animals showed a significant decrease in the total dendritic length (24%) and a significant reduction (27%) in the dendritic spine density of dendritic arborization at the level of the second branch order. Our results suggest that some of the persistent neurobehavioral consequences and cognitive impairment resulting from repeated exposure to morphine may involve a reorganization of synaptic connectivity in visual cortical neurons.