Abstracts

Rationale: The traditional models of ictal propagation involve the concept of an initiation site and a progressive outward march of activation. Using voltage sensitive dyes, we have shown widespread propagation of excitatory subthreshold membrane potential change from the onset of the seizure, however, suprathreshold activation (action potentials) were not recorded. Methods: We now used bulk loading of Oregon Green 488 BAPTA-1, a calcium dye, to image the propagation of calcium spikes during this same model, namely 4-aminopyridine (4-AP, 15mM, 0.5µl) induced focal neocortical seizures. Adult SD rats were anaesthetized with 1.5-2.5% isoflurane in 30% N2-70% O2. A craniotomy window was created over the left hemisphere. Calcium dye was injected into the cortex at multiple sites, avoiding major blood vessels. The brain was alternatively illuminated with 530nm (intrinsic optical imaging for local blood volume change) and 470nm (excitation for Calcium dye) at 200 frames/sec. The reflection and florescence change was recorded with a CCD camera in Imager 3001. Results: Interictal events exhibit widespread and transient simultaneous calcium increases. Ictal events, on the other hand, rapidly propagate to a similarly widespread area at the onset of the seizure. As the seizure evolves, a rapid oscillation of intracellular calcium occurs initially over a widespread area which then contracts over the course of the seizure. The local blood volume change shows a similar monophasic increase in the whole field of view and does not oscillate like the calcium waves. Local blood volume increases last much longer than the calcium signal recorded from the same location. Conclusions: Our data showed that the calcium oscillations and blood volume changes are two separate but parallel behaviors of neuronal tissue during epileptiform activity further illustrating dynamic neurovascular coupling during the ictal event.