Abstracts

Rationale: To date, little is known about the spatiotemporal dynamics of epileptic waves in anatomically distinct brain structures and how such dynamics may be altered by neuromodulators. With the advent of fast voltage sensitive dye (VSD) imaging it is now possible to define key spatiotemporal mechanisms that may underlie the pathological neuronal activity. In the current study, using electrophysiology and VSD imaging we describe the dynamics of spontaneous epileptiform bursts and high frequency oscillations (HFOs) in anatomically distinct structures in vitro. Methods: Concurrent VSD (Di-4ANNEPS) imaging and extracellular or whole-cell recordings were performed in the juvenile rat (P21-P30) brain slices (350μM) from transverse hippocampus, coronal primary visual (VC) and tangential somatosensory (SS, layers II-III) cortices. Epileptiform activity was evoked with 4-Aminopyridine (100mM, 4-AP). For optical imaging we used Micam Ultima camera (100x100 pixels; sampling rate of 0.5-1kHz). β-adrenergic receptor agonist isoproterenol (ISO, 10mM) was used to assess the role of this neuromodulator on spontaneous burst activity. Optical signal properties, such as epileptiform activity origin, speed of propagation, and spatiotemporal correlations were analyzed. To compute spatial and temporal correlations we used time average over characteristic periods, average over fixed regions in the images, and/or principal component analysis. Results: All three structures exhibited short duration (70-500msec) bursts and HFOs termed ripples (20-200ms duration; 80-600Hz). In the hippocampal slices, most of the bursts originated from dentate gyrus/CA3 border and propagated down the tri-synaptic pathway toward CA1. Out of 349 optical bursts analyzed in VC, 46.7% originated in layers II/III, 11.5% in layer IV, 9% in layer V, 25% in layer VI. The velocity of the bursts was slower in the VC (0.05m/sec) than in the hippocampus (0.1m/sec). In VC, addition of ISO to 4-AP confined the spatial spread and decreased duration of bursts and seizure events but increased the frequency of their occurrence. Well-organized bursts were only seldom observed in the SS slices. However, HFOs observed in SS cortex exhibited well organized complex wave dynamics (n=37 slices; 80 ripples). HFO pattern initiated as stationary ‘hubs’ of excitation that subsequently formed inhibitory surround, followed by propagating wave-like fronts that terminated as spatially ‘patchy’ temporal oscillations. The velocity of HFO propagation was 0.4m/sec and the spectral analysis showed that most power in the signal ranged from 150-250Hz, suggesting that these HFOs are “fast ripples” (FR). Conclusions: Spatially and temporally complex dynamics of spontaneous epileptiform activity were found in distinct anatomical structures. Varying levels of spatiotemporal correlations during characteristic periods of HFO activity suggested that FRs may be partitioned into initiation, body, and terminations stages. Effects of ISO on epileptiform dynamics stress the important role that neuromodulators play in control of pathological neuronal network activation patterns.