"Increased" sensory stimulation leads to changes in energy-related enzymes in the brain

The facial whiskers of mice project through several synapses to anatomically distinct structures called barrels in the contralateral cerebral cortex. With appropriate illumination, individual barrels can be recognized and dissected from unfixed, freeze-dried tissue sections taken parallel to the plane of layer IV. The tissue then can be analyzed using quantitative microhistochemical techniques to determine the level of various substances of biological importance (W.D. Dietrich, D. Durham, O. H. Lowry, and T. A. Woolsey (1981) J. Neurosci. 1: 929-935). The present paper describes results obtained in this way from adult mice subjected to a chronic "sensory deprivation" by repeatedly clipping all of the whisker hairs on one side of the face and during the recovery from this deprivation in which the whisker hairs were allowed to grow back. Sensory deprivation for 60 days leads to significant changes in the levels of the three energy-related enzymes studied--citrate synthase, malate dehydrogenase, and glycogen phosphorylase. surprisingly, during clipping, the enzyme levels in the barrels of the contralateral cortex are essentially normal, whereas enzyme levels in the barrels of the ipsilateral cortex are increased significantly. Specifically, activities expressed as a percentage of levels in normal animals were: citrate synthase, 135%; malate dehydrogenase, 130%; and glycogen phosphorylase, 170%. Forty-five days after the deprivation is reversed, the levels return to normal. These significant changes occurred in adult mice several synapses away from the sensory periphery. The data are in contrast to our earlier results in which damage to the primary afferents reduced the levels of the enzymes citrate synthase and malate dehydrogenase contralateral to the manipulation. A possible explanation for the enzymatic changes observed in the cortex ipsilateral to the clipped whiskers is an increased utilization of the intact sensory periphery by the animals; this has some behavioral support.