Gamma oscillation underlies hyperthermia-induced epileptiform-like spikes in immature rat hippocampal slices

Abstract

Background: Recently a hyperthermic rat hippocampal slice model system has been used to investigate febrile seizure pathophysiology. Our previous data indicates that heating immature rat hippocampal slices from 34 to 41 degrees C in an interface chamber induced epileptiform-like population spikes accompanied by a spreading depression (SD). This may serve as an in vitro model of febrile seizures.

Results: In this study, we further investigate cellular mechanisms of hyperthermia-induced initial population spike activity. We hypothesized that GABA(A) receptor-mediated 30-100 Hz gamma oscillations underlie some aspects of the hyperthermic population spike activity. In 24 rat hippocampal slices, the hyperthermic population spike activity occurred at an average frequency of 45.9 +/- 14.9 Hz (Mean +/- SE, range = 21-79 Hz, n = 24), which does not differ significantly from the frequency of post-tetanic gamma oscillations (47.1 +/- 14.9 Hz, n = 34) in the same system. High intensity tetanic stimulation induces hippocampal neuronal discharges followed by a slow SD that has the magnitude and time course of the SD, which resembles hyperthermic responses. Both post-tetanic gamma oscillations and hyperthermic population spike activity can be blocked completely by a specific GABA(A) receptor blocker, bicuculline (5-20 microM). Bath-apply kynurenic acid (7 mM) blocks synaptic transmission, but fails to prevent hyperthermic population spikes, while intracellular diffusion of QX-314 (30 mM) abolishes spikes and produces a smooth depolarization in intracellular recording.

Conclusion: These results suggest that the GABA(A) receptor-governed gamma oscillations underlie the hyperthermic population spike activity in immature hippocampal slices.

Figure 1

Hyperthermia-induced γ oscillation. (A): Heating hippocampal slice to 38.2°C induced epileptiform-like spikes, followed by a slow spreading depression (SD) recorded by intracellular (V_in) and extracellular (V_ex) electrodes simultaneously. (B): Expanded time scale from (A) to show initial epileptiform-like spikes. (C): Distribution of frequency of initial epileptiform spikes in 24 experimental cases. (D): Gaussian-fitting shows a γ band frequency (30–80 Hz) distribution of hyperthermia-induced epileptiform spikes.

Figure 2

Comparison of hyperthermic and post-tetanic γ oscillations. (A): a. Post-tetanic stimulation (100 Hz, 20 trains, 2 mA, 100 μs) induced γ oscillation at 34°C. b. Heating the same recorded slice to 38°C induced an initial γ oscillation accompanied by a slow SD. c. Expanded time scale shows the similar frequency ranges of post-tetanic and hyperthermic oscillations. (B): Comparison of frequency power by power spectrum analysis between Aa and Ab. (C) There is no significant difference of average frequency between post-tetanic and hyperthermic oscillations.

Figure 3

Post-tetanic stimulation at high intensity induced γ oscillations followed by a slow SD. (A): Post-tetanic stimulation at 2.25 mA elicited a rudimentary γ oscillation. (B): Post-tetanic stimulation at 3.25 mA induced a typical γ oscillation with large amplitude and long duration. (C): Post-tetanic stimulation at 4.25 mA induced a γ oscillation followed by a slow SD. Traces A-C are recorded from the same slice, which is a typical representative of five experiments.

Figure 4

The GABA_A receptor antagonist, bicuculline (BMI), blocks post-tetanic γ oscillations. (A): a. BMI (5 μM) induced multiple field potential spikes to a single stimulation, b and c. BMI reversibly blocked post-tetanic γ oscillations. (B) BMI (20 μM) blocked high intensity (4 mA) tetanic stimulation-induced γ oscillation, but not the slow SD. The traces in (A) were recorded from the same neuron (n = 6 experiments) and in (B) from another neuron (n = 4 experiments).

Figure 5

(A): BMI (10 μM) blocked post-tetanic γ oscillations. (B): Persistence of spontaneous spikes and bursts after BMI block of γ oscillation. (C): In the presence of BMI, heating the slice to 38.8°C induced a slow SD without initial γ oscillations. Traces A-C were recorded from the same neuron (n = 6 experiments).

Figure 6

(A): Kynurenic acid (Kyn, 7 mM) blocked evoked synaptic transmission, but failed to prevent γ oscillations and SDs elicited by heating the slice. Traces illustrate four slice recordings. (B): QX-314 blocks hyperthermia-induced γ oscillation in the intracellular recording from three slices.