The Effects of Morphine Self-Administration on Cortical Pyramidal Cell Structure in Addiction-Prone Lewis Rats

Abstract

The consumption of drugs of abuse provokes sensitization, the development of tolerance, dependency, and eventually addiction. It is thought that these events are partially a consequence of drug-induced alterations in the organization of neuronal circuits in specific areas of the brain. In the present study, we have used intracellular injections of lucifer yellow to examine the alterations that may occur in cortical pyramidal neurons of addiction-prone Lewis rats following 15 days of self-administration of morphine. Specifically, the effects of morphine on the structure, size and branching complexity of the basal dendrites, and spine density were determined in the basal dendritic arbors of layer III pyramidal neurons in both the prelimbic and motor cortex. We found that following morphine self-administration, there was a reduction in the size and branching complexity of the dendritic arbors of pyramidal cells in the motor cortex. In contrast, prelimbic pyramidal neurons from these morphine-treated animals had larger and longer basal dendritic arbors. Furthermore, the spine density on pyramidal neurons was higher in both cortical regions of morphine self-administered rats. These results suggest that at least part of the behavioral changes produced by repeated opiate administration may be attributed to alterations in pyramidal cell structure.