Characteristics of Plateau Activity During the Latent Period Prior to Epileptiform Discharges in Slices From Rat Piriform Cortex

Abstract

The deep piriform region has an unusually high seizure susceptibility. Voltage imaging previously located the sites of epileptiform discharge onset in slices of rat piriform cortex and revealed the spatiotemporal pattern of development of two types of electrical activity during the latent period prior to discharge onset. A ramplike depolarization (onset activity) appears at the site of discharge onset. Onset activity is preceded by a sustained low-amplitude depolarization (plateau activity) at another site, which shows little if any overlap with the site of onset. Because synaptic blockade at either of these two sites blocks discharges, it was proposed that both forms of latent period activity are necessary for the generation of epileptiform discharges and that the onset and plateau sites work together in the amplification of electrical activity. The capacity for amplification was examined here by studying subthreshold responses in slices of piriform cortex using two different in vitro models of epilepsy. Under some conditions electrically evoked responses showed a nonlinear dependence on stimulus current, suggesting amplification by strong polysynaptic excitatory responses. The sites of plateau and onset activity were mapped for different in vitro models of epilepsy and different sites of stimulation. These experiments showed that the site of plateau activity expanded into deep layers of neighboring neocortex in parallel with expansions of the onset site into neocortex. These results provide further evidence that interactions between the sites of onset and plateau activity play an important role in the initiation of epileptiform discharges. The site of plateau activity showed little variation with different stimulation sites in the piriform cortex, but when stimulation was applied in the endopiriform nucleus (in the sites of onset of plateau activity), plateau activity had a lower amplitude and became distributed over a much wider area. These results indicate that in the initiation of epileptiform discharges, the location of the circuit that generates plateau activity is not rigidly defined but can exhibit flexibility.

Initiation and Propagation of Neuronal Coactivation in the Developing Hippocampus

“…Optical recording with VSDs measures membrane depolarization during neuronal population activity, which allows accurate visualization of initiation sites and propagating patterns of waves without distortion of current density distribution (source/sink pairs) in cortical tissue (Bao and Wu 2003;Jin et al 2002). This technique has been used to visualize epileptiform and oscillatory activities in hippocampal and neocortical slices, and propagating waves are a common spatiotemporal characteristic in these activities (Albowitz and Kuhnt 1995;Bao and Wu 2003;Demir et al 1998Demir et al , 2000Huang et al 2004;Miyakawa et al 2003;Tsau et al 1998;Wu et al 1999bWu et al , 2001. In this study, we found that GDP activity in developing hippocampus appears as propagating waves and that the propagation patterns are strongly correlated to anatomical structure in the CA3 and CA1 areas.…”

“…6). Such "point initiation" was also seen in the initiation of epileptiform events observed in disinhibited neocortex (Ma et al 2004;Tsau et al 1998Tsau et al , 1999, whereas "area initiation" (or "plateau activity" Demir et al 2000) has been observed in rat piriform cortical slices. The synaptic mechanism for GDP initiation is different from that of epileptiform activity (Khalilov et al 1999).…”

A network that performs brute-force conversion of a temporal sequence to a spatial pattern: relevance to odor recognition

“…Another possible site for conversion of the temporal pattern to a SP is the endopiriform nucleus, which is deep to piriform cortex and is sometimes called layer 4 of piriform cortex (Shepherd, 1998). The endopiriform nucleus is a major locus of epilepsy (Demir et al, 2000), a condition that occurs when persistent firing becomes pathological. This suggests that the mechanisms of persistent firing required by our model are present in this structure.…”

Inhibitory Interneurons in the Piriform Cortex

“…The PC, like the hippocampus, is strongly epileptogenic 56 . Unsurprisingly, inhibition has been shown to limit excessive excitability in the PC in several models of epilepsy 57–59 . For instance, in an in vivo kindling model, it was shown that a precisely timed inhibitory post‐synaptic potential is critical for limiting excessive excitation in PC pyramidal cells 60 …”

“…56 Unsurprisingly, inhibition has been shown to limit excessive excitability in the PC in several models of epilepsy. [57][58][59] For instance, in an in vivo kindling model, it was shown that a precisely timed inhibitory post-synaptic potential is critical for limiting excessive excitation in PC pyramidal cells. 60 In conclusion, the present brief review of inhibitory interneurons in the PC has shown that this field is in many respects a terra incognita.…”