Abstracts

Rationale: Neuronal and glial activity are thought to coordinate regional blood flow through a complex sequence of events. During normal sensory processing, these events appear to be coupled in space and time. Using a 4-AP injection into rat cerebral cortex and simultaneous voltage sensitive and intrinsic signal (IS) wide-field imaging, we have previously shown that ictal events are coupled in space, but not time, with perfusion. Although glia are a key component in neurovascular coupling, wide-field imaging of glial waves during seizures has not been investigated.Methods: Using calcium dyes, either OGB-1, which can be filtered to image glia (< 1 Hz) or neuronal activity (> 1Hz), or the calcium dye Rhod-2, which stains only astrocytes, along with IS and local field potentials, we were able to measure each compartment of the neurovascular unit through a wide area of in vivo rat neocortex during seizure onset and evolution.Results: A clear glial wave which began focally and spread across the cortex occurred simultaneous with each ictal event. However, glial waves propagated 43% further (4.3±1.3 mm) than CBV changes (3.0 ± 1.0 mm, t-test, p=0.0013). Despite widely varying seizure durations (10-70s, 43.5±17.6 s), the duration of astrocytic activation remained more constant (10-40s, 23.6±6.5 s) and did not significantly correlate with the duration of the seizures (n = 25 seizures from 4 rats, r = 0.28, p = 0.18). The hemodynamic change, on the other hand, lasted longer than both the astrocytic and neuronal activity in all trials (54.5±17.7 s, p <0.01). Moreover, glial waves were significantly delayed in onset compared with hemodynamic waves (2.4±1.1 s versus 0.8±1.0s respectively, p<0.01). Our results suggest that during ictal events, each compartment in the neurovascular unit displays a unique spatiotemporal onset and evolution.Conclusions: Although clearly coupled in a macro-global scale i.e. similar number of events in a similar region of cortex, they are uncoupled on a micro-detailed scale. In spite of rapidly propagating multidirectional subthreshold neuronal waves of varying duration, glial activity is characterized by a more homogeneous slowly propagating wave that extends well beyond the limits of the neuronal or perfusion changes, having a more constant predictable duration. These results call into question the putative essential role of astrocytes in ictal neurovascular coupling.