Somatotopic astrocytic activity in the somatosensory cortex

Astrocytes play a crucial role in maintaining neuronal function and monitoring their activity. According to neuronal activity maps, the body is represented topographically in the somatosensory cortex. In rats, neighboring cortical areas receive forelimb (FL) and hindlimb (HL) sensory inputs. Whether astrocytic activity is also restricted to the cortical area receiving the respective peripheral sensory inputs is not known. Using wide field optical imaging we measured changes in the concentration of astrocytic calcium within the FL and HL sensorimotor cortex in response to peripheral sensory inputs. Mapping the calcium signals upon electrical stimulation of the forepaw and hindpaw we found activity largely restricted to the FL and HL area, respectively. In comparison to neuronal activity the time course of the astrocytic calcium activity was considerably slower. The signal took 6 s to peak after the onset of a 2 Hz and 2 s long electrical stimulation of the hindpaw and 8 s for a 4 s stimulation. The astrocytic signals were delayed relative to cerebral blood flow measured using laser speckle imaging. The intensity of both the astrocytic and neuronal signals in the HL sensorimotor cortex declined with increase in stimulation frequency. Moreover, blocking neuronal input by tetrodotoxin abolished astrocytic calcium signals. We suggest that the topographical representation of the body is not only true for cortical neurons but also for astrocytes. The maps and the frequency-dependent activations reflect strong reciprocal neuroglial communication and provide a new experimental approach to explore the role of astrocytes in health and disease.