Multiple-unit recordings during slow field-potential shifts in low-[Ca2+]0 solutions in rat hippocampal and cortical...

Multiple-unit and field-potential recordings in low-[Ca2+]0 solutions were used to study epileptiform bursts generated in hippocampal region CA1 and medial entorhinal cortex, a cortical region that is not as densely packed or highly laminated as the hippocampus. As expected in CA1, multiple-unit activity appeared as large spikes that corresponded one-to-one with population spikes in the field-potential recordings. During the negative field-potential shifts that lacked large population spikes, the multiple-unit recordings showed an increase in baseline activity. Initiation of the negative field-potential shift always coincided with increased multiple-unit activity. Slices displaying a post-burst positive overshoot showed a corresponding decrease in multiple-unit activity. In addition to the large ictal-like events in CA1, small-amplitude field-potential shifts were also observed, these events were associated with increases in baseline spike activity in the multiple-unit recording. These small-amplitude field-potential shifts appeared to precede the occurrence of the ictal-like events, but they decreased in frequency during low-[Ca2+]0 exposure. Recordings in normal artificial cerebrospinal fluid (nominally 1.3 mM [Ca2+]0) showed rhythmic, multiple-unit bursts of action potentials and corresponding negative small-amplitude field-potential shifts in the medial entorhinal cortex of immature rats (two-to three-weeks old), but not of adult rats. Rhythmic, spontaneous bursts of activity in low-[Ca2+]0 solution were found in both immature and adult medial entorhinal cortex, and were similar in amplitude to the small field-potential events generated in CA1. The probability of burst generation was higher in the immature than the adult medial entorhinal cortex, and the bursts in the immature cortex had more robust multiple-unit activity and an increased burst frequency compared with adult. These results indicate that the medial entorhinal cortex can also generate spontaneous synchronous bursts of activity in low-[Ca2+]0 solutions, and they suggest that the increased susceptibility of medial entorhinal cortex from immature versus adult rats to generate intense bursts of electrical activity does not require active chemical synaptic transmission. The various forms of epileptiform activity in low-[Ca2+]0 solutions probably arise from different contributions of electrical and ionic mechanisms of synchronization in these neuronal populations. The data suggest the hypothesis that ionic mechanisms (i.e. changes in [K+]0) may synchronize neurons in cortical regions (e.g. entorhinal cortex) that are not as densely packed and highly laminated as the hippocampus and dentate gyrus. The data also support the hypothesis that these mechanisms contribute significantly to the increased seizure susceptibility of the immature brain.