Activity-dependent plasticity of astrocytic morphologies in the globus pallidus

K. Tatsumi, H. Okuda, S. Morita, and A. Wanaka
Department of Anatomy and Neuroscience, Nara Medical University, Kashihara, Nara

The basal ganglia control activities of motor cortex. Recently we localized Olig2-expressing astrocytes in the basal ganglionic nuclei including the globus pallidus (GP). Based on the "tripartite synapse theory", we speculated that these Olig2+ astrocytes might modulate motor functions. To test the hypothesis, we first focused on the relationship between astrocytic morphologies and voluntary exercise, using olig2-CreER/ ROSA-EGFP-GAP43 double transgenic mice that express membrane-targeted EGFP in the olig2-positive cells. Mice were divided into three groups; first group of mice were kept in a cage with a locked running wheel to limit the exercise (Control group) and second group of mice were given a free running wheel (Runner group) for three weeks. Some mice in the Runner group were further housed with locked running wheel for additional three weeks (Rest-group). The astrocytic morphologies of the Runner group mice appeared more elaborated than those of the control group mice. The more complex the astrocytic process arborization became after voluntary running, the more EGFP fluorescence per area increased. Taking advantage of this phenomenon, we measured average fluorescence intensity of an astrocyte and extrapolated the complexity of the astrocytic morphology from the intensity. The EGFP-fluorescence intensities of astrocytes in the GP are significantly greater in the Runner group than the Control group. Interestingly, fluorescence intensities of the Rest group were significantly lower than those of the Runner group, suggesting that the morphological changes may be reversible in response to motor activities. Furthermore, Immunoelectron microscopy revealed that fine processes of olig2+ astrocytes frequently surrounded synapses and constituted 'tripartite synapses'.