Abstracts

Rationale: Epileptiform activity propagates to different brain areas, and mirror epileptic foci on the contralateral hemisphere can be sustained over long periods. However, before a noticeable mirror focus is formed, the early involvement of contralateral cortex (CC) during primary epileptogenesis remains largely unknown. Under in vivo conditions where long range connections are preserved, we found that initiation sites of interictal spikes (ISs) exhibit spatial clustering dependent on the activity history [J Neurophysiol. 2004;91(2):934-45]. This implies that remote areas may be involved in the spatial organization of ISs. ISs may play fundamental roles during the early stages of epileptogenesis, and their spatial consolidation may promote long-term changes that give rise to seizure as disease progresses. Methods: We use voltage-sensitive dye (VSD) imaging to map IS initiation sites in rat visual cortex in vivo. Surgical and imaging methods were described in detail in our methodology paper [J Neurophysiol. 2007;98(1):502-12]. Craniotomy was created over both hemispheres, and one hemisphere was stained with VSD RH-1691 solution. Each detector of a 464-channel photodiode array received light from a cortical area ~160 μm diameter. ISs were induced on the imaging hemisphere with bicuculline (1mM, epidural). For each IS the timing of the VSD signal (λ~700 nm) reaching 50% peak amplitude was measured, and the detector with earliest half-peak amplitude time is defined as the initiation site. In some experiments, CC was either inactivated by muscimol (3mM, epidural) or received point electrical stimulation (10 μA; 0.5 Hz) prior to inducing the ISs. Results: CC facilitates the spatial consolidation of IS initiation sites. During the first 5 min after the earliest IS, initiation sites are widely distributed over the imaging field (mean spatial entropy ~3 bits). If CC is unaltered, then within 10 min (~450 ISs) the initiation sites began to consolidate into a single dominant site which initiated >80% of ISs throughout the remaining time (80 min; ~3,000 ISs; n=12 animals; mean entropy ~0.5 bits). Consolidation is dependent on the activity of CC in the following ways: 1) CC must be active during the consolidation process. If CC is inactivated by muscimol, the consolidation can be completely blocked (n=5; mean spatial entropy ~3.5 bits; 160 min). 2) CC is necessary to sustain the dominant site at least during the first hour of ISs emergence. If CC inactivation is timed 30-50 min after consolidation, the developed dominant site can be obliterated (n=3). 3) Moderate CC stimulation is sufficient to generate a dominant IS initiation site before bicuculline treatment (n=2), but this effect is blocked when muscimol is applied (n=1). Conclusions: If the IS-generating area is connected with another area of normal excitation/inhibition balance, a dominant initiation site should develop. Accumulated synaptic reorganization following IS consolidation may be important for epileptogenesis. Manipulating the dominant sites may become a new strategy for intervention in epilepsy.