Introduction: Chronic exposure to morphine impairs performance in tasks which needs sensory processing. Using single unit recordings we investigate the effect of chronic morphine exposure on the firing properties of neurons in layers IV and V of the whisker related area of rat somatosensory cortex.
Methods: In urethane anesthetized animals, neuronal activity was recorded in response to principal and adjacent whisker deflections either stimulated independently or in a conditioning test paradigm. A conditioning test ratio (CTR) was calculated for assessing the inhibitory receptive field.
Results: In layer IV, chronic morphine treatment did not change the spontaneous discharge activity. On responses to principal and adjacent whisker deflections did not show any significant changes following chronic morphine exposure. Considering Off responses, the magnitude of neuronal activity to adjacent whisker deflection decreased while its response latency increased. In addition, there was a significant increase in the latency of off responses to principal whisker deflection. CTR did not change significantly following morphine exposure. In layer V neurons on the other hand, did not show any significant changes in their spontaneous activity or their evoked responses following morphine exposure.
Conclusion: Our results suggest that chronic morphine exposure has a modulatory effect on response properties of neurons in layers IV.