Voltage Imaging of Epileptiform Activity in Slices From Rat Piriform Cortex: Onset and Propagation

Abstract

The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.

Sustained and Accelerating Activity at Two Discrete Sites Generate Epileptiform Discharges in Slices of Piriform Cortex

“…Pharmacological, physiological, and anatomical analysis in the En revealed recurrent excitatory synaptic activity within the dominant multipolar cell population, suggesting that the hypothesis of synaptically mediated positive feedback, developed as a mechanism for the latent period in hippocampus (Ayala et al, 1973;Johnston and Brown, 1986;Traub et al, 1989;Traub and Miles, 1991), also may be applicable to the PC (Hoffman and Haberly, 1993). Using voltage imaging techniques (Grinvald et al, 1988;Wu and Cohen, 1993), we examined the spatiotemporal pattern of electrical activity in slices of PC and observed the accelerating build-up of activity at the onset site described previously (Hoffman and Haberly, 1993;Demir et al, 1998). In addition, a second form of abnormal activity was observed during the latent period, which has not been described previously.…”

“…Slices through PC and En of adult male rats were cut at a thickness of 350 m in a near-coronal plane perpendicular to the brain surface Haberly 1991, 1993;Demir et al, 1998). The physiological solution for slice preparation and recording was (in mM): 124 NaC l, 5 KC l, 26 NaHC O 3 , 1.2 K H 2 PO 4 , 2.4 CaCl 2 , 1.3 MgSO 4 , and 10 glucose bubbled with 95% O 2 /5% C O 2 .…”

“…Important landmarks present in all slices are the three layers of the PC, the underlying En, and the external capsule, which separates PC from caudate putamen. Most of the slices studied here were taken from a portion of PC corresponding to intermediate slices of Demir et al (1998) [anteroposterior, Ϫ1.80 to Ϫ0.70 mm (Paxinos and Watson, 1986)]. This stereotaxic level encompasses the boundary between agranular insula and anterior perirhinal cortex [terminology of C echetto and Sapir (1987), Burwell et al (1995), andMcIntyre et al (1996)], so that PC in these slices is bounded laterally by the junctional region between these two cytoarchitecturally distinct forms of neocortex.…”

“…Latent periods of up to 150 msec have been observed to precede epileptiform discharges in slices of rat piriform cortex (PC) Haberly, 1989, 1996;Demir et al, 1998). In contrast to the hippocampal slice, discharges in PC slices begin at a well defined location that can include the dorsal-most part of endopiriform nucleus (En) (Hoffman and Haberly, 1991 and adjoining layer VI of neocortex (Demir et al, 1998).…”

“…In contrast to the hippocampal slice, discharges in PC slices begin at a well defined location that can include the dorsal-most part of endopiriform nucleus (En) (Hoffman and Haberly, 1991 and adjoining layer VI of neocortex (Demir et al, 1998). This localization of onset gives the PC slice a potential advantage in studying the genesis of epileptiform activity.…”