Brain tissue comprises a complex milieu of cell types, including a multitude of neuronal, as well as non-neuronal, cell types. Astrocytes are one of the most prominent non-neuronal cell classes and can constitute close to half of all cells in some brain regions and species. Astrocytes are intimately linked with neurons, anatomically, metabolically and physiologically. They are positioned and equipped to both respond to neural activity, and provide feed-back to neurons. An extensive body of work in vitro has demonstrated the ability of astrocytes to modulate neural activity on multiple time scales, raising the possibility that astrocytes have substantial interactions with neural networks in vivo. Previous work has demonstrated that cortical astrocytes respond to sensory stimulation and have receptive field properties similar to those of nearby neurons. Furthermore, blocking astrocyte activity leads increased visual responses in neighboring neurons, suggesting the possibility that astrocytes regulate neural responses. In order to understand the role of astrocytes in cortical network activity, it will be necessary to carefully describe the relationship between evoked neuronal activity and astrocyte activity. Several lines of evidence suggest that this relationship is not linear; astrocytes are more sharply tuned, and more sensitive to depth of anesthesia. To examine this issue in detail, we have measured the contrast-response functions, contrast invariance of orientation tuning, and contrast adaptation in layer 2/3 visual cortical neurons and astrocytes in the ferret using two-photon imaging of bulk-loaded calcium indicators in vivo. We find that the contrast-response functions of neighboring neurons are highly heterogeneous, and those of astrocytes are less so. In general, astrocytes have higher contrast thresholds and steeper contrast-response functions than neurons. As with neurons, orientation tuning in astrocytes is fairly contrast-invariant. Interestingly contrast adaptation is more pronounced in astrocytes than in neurons. Together, these results suggest a complex interaction between the levels of neural activity and astrocyte activity.

Society for Neuroscience Abstract, 2009.