The effect of motor learning on protein expression and functional hyperemia in the cerebral cortex

The refinement of motor movements to achieve a goal, such as learning to reach for and grasp an object (skilled reach), is defined as motor learning. In general, there are three types of brain modifications that have been associated with skilled reach motor learning. (1) Protein expression must change to allow related cellular structure and functional modifications to occur. Although general protein synthesis is known to be important for motor learning, few specific proteins have been identified. (2) Synaptogenesis and somatotopic map reorganization occur in the forelimb cortex contralateral to the limb being used. These changes are not apparent in the hindlimb motor cortex. While astrocyte growth has also been reported in the cerebellum during motor learning, it is unknown whether cortical astrocytes display similar changes. (3) The level of cerebral blood flow is altered. It remains to be determined if cerebral blood flow adaptations are positively or negatively correlated with motor learning.;The first goal of this project was to utilize mass spectrometry to identify candidate proteins within the caudal forelimb cortex that were up-regulated during motor learning. To accomplish this, we developed a system where rats were trained to reach through a slot in order to grasp a food pellet. Animals trained in this task were designated as the skilled reach group. Improvements in grasping success indicated motor learning. A second group was trained to reach through a hole to press a lever (lever press), which elicited a food pellet reward that could be eaten directly without grasping. Because lever press rats were not required to improve at grasping the food pellet, motor learning did not take place. We found slight changes in protein expression in the contralateral caudal forelimb cortex compared to the ipsilateral caudal forelimb cortex. However, these changes did not fit our chosen criteria of two-fold or higher up-regulation in five or more samples. Instead, we identified several proteins that matched our criteria when comparing skilled reach to lever press rats when we did not take into account contralateral or ipsilateral brain classification.;Second, we investigated expression patterns of glial fibrillary acidic protein (GFAP), a marker for astrocytes, and growth associated protein 43 (GAP-43), a marker for neural sprouting, in the caudal forelimb cortex and the hindlimb cortex of skilled reach and lever press groups using Western blot. We used expression levels of these proteins as readouts for astrocyte growth and synaptogenesis, respectively. There were no significant changes in protein expression between the contralateral and ipsilateral cortices, supporting our mass spectrometry findings. However, we initially found an up-regulation of each protein in the hindlimb cortex of skilled reach rats suggesting the hindlimb cortex is necessary to support skilled reach motor learning. To investigate these protein expression patterns further, we examined GFAP and GAP-43 in untrained controls. These investigations revealed that GFAP and GAP-43 were down regulated in the hindlimb cortex of lever press animals but no differences were detected between skilled reach and control animals. We interpret these results as indicating that these proteins are down regulated in the hindlimb cortex during repetitive motor activity. Additionally, skilled reach and lever press groups both displayed higher GFAP expression in the caudal forelimb cortex compared to untrained controls. Therefore, the repetitive reaching activity common to skilled reach and lever press groups may induce changes in protein expression associated with astrocyte modifications.;Lastly, we modified our animal training system to include laser Doppler flowmetry to measure blood flow during skilled reach. Our goal was to determine whether functional hyperemic magnitude, the increase in flow elicited from brain activation, or baseline blood flow in the caudal forelimb cortex contralateral to the trained limb was correlated to motor learning. We discovered a positive correlation between grasping success rate and functional hyperemia. Baseline blood flow was not correlated with success rate, but did show an overall increase on the last day of training compared to the first day of training. Our blood flow data suggests that the degree of training induced changes in functional hyperemia may depend upon the level of success achieved, while baseline blood flow could be related to motor activity.;These findings help in understanding the intricacies of brain adaptation to motor learning. Our hope is that future studies will build upon these findings in an effort to determine causal or contributing factors to pathologies that affect motor function. We also hope that the introduction of a model for measuring cerebral blood flow during skilled reach will assist others in further elucidating how cerebral blood flow is controlled in animals performing skilled tasks.