Exercise induces angiogenesis but does not alter movement representations within rat motor cortex

Abstract

The effects of exercise on the topography of movement representations and blood vessel density within the rat forelimb motor cortex was examined. Adult male rats were allocated to either a Voluntary eXercise (VX) or Inactive Condition (IC). VX animals were housed for 30 days with unlimited access to running wheels while IC animals were housed in standard laboratory cages. VX animals exhibited a progressive increase in the distance traveled per day and ran an average of 58.3 km across the 30-day training period. Microelectrode stimulation was used to derive high resolution maps of the forelimb representations within the motor cortex of animals from both conditions. No significant differences in the area of either distal (wrist/digit) or proximal (elbow/shoulder) movement representations were found between VX and IC animals. However, VX animals did have a significantly greater density of blood vessels within layer V of the forelimb motor cortex. These results demonstrate that increases in forelimb motor activity sufficient to induce cortical angiogenesis does not alter the topography of forelimb movement representations within forelimb motor cortex.

Introduction

The existence of a motor map within mammalian motor cortex has been known for over a century [12]. Early experiments demonstrated that although general map topography was consistent between animals, the size and organization of individual maps was unstable [38]. The capacity for motor map reorganization has now been demonstrated to occur in response to a variety of manipulations including changes in sensory input [8], [18], administration of GABA antagonists [20] and repetitive electrical stimulation [29]. It has been hypothesized that motor map plasticity exists to support the acquisition of novel motor behaviors [37]. Specifically, the development of skilled action patterns causes an expansion of representations corresponding to trained movements into areas occupied by untrained representations [11], [22], [31], [34]. Further, this plasticity also appears to be learning-dependent and does not occur in response to the simple repetition of existing, unskilled movement patterns [23], [22], [34].

Although the topography of movement representations within motor cortex may reflect the capacity for skilled motor behavior [24], [30], [34], there is evidence that training-dependent increases in motor endurance may also influence cortical function. Extensive exercise training has been shown not only to increase local glucose utilization [40] but also the capacity for glucose utilization within motor cortex [26]. In addition, persistent changes in the density of various neurotransmitter receptors [9], [10], [27] and prolonged increases in neurotransmitter release [7], [16], [32] have been reported within frontal cortex following exercise training. An increase in norepinephrine [32] and NE receptors [30] is consistent with an increase in cortical vasculature given that most NE afferents to the cortex terminate within the vicinity of microvessels [17]. Further, release of various neurotrophic factors [28] and the density of FLK-1 receptors within the cortex [5] have also been shown to increase with exercise. The persistent increase in activity and concomitant neurochemical changes observed within motor cortex following exercise training likely reflect changes in cortical vasculature and physiology. The present experiment examined how exercise affected both the topography of movement representations and the density of blood vessels within the rat forelimb motor cortex.