Coexistence of gamma and high-frequency oscillations in rat medial entorhinal cortex in vitro

Abstract

High frequency oscillations (> 80-90 Hz) occur in neocortex and hippocampus in vivo where they are associated with specific behavioural states and more classical EEG frequency bands. In the hippocampus in vitro these oscillations can occur in the absence of pyramidal neuronal somatodendritic compartments and are temporally correlated with on-going, persistent gamma frequency oscillations. Their occurrence in the hippocampus is dependent on gap-junctional communication and it has been suggested that these high frequency oscillations originate as collective behaviour in populations of electrically coupled principal cell axonal compartments. Here we demonstrate that the superficial layers of medial entorhinal cortex can also generate high frequency oscillations associated with gamma rhythms. During persistent gamma frequency oscillations high frequency oscillations occur with a high bispectral coherence with the field gamma activity. Bursts of high frequency oscillations are temporally correlated with both the onset of compound excitatory postsynaptic potentials in fast-spiking interneurones and spikelet potentials in both pyramidal and stellate principal neurones. Both the gamma frequency and high frequency oscillations were attenuated by the gap junction blocker carbenoxolone. These data suggest that high frequency oscillations may represent the substrate for phasic drive to interneurones during persistent gamma oscillations in the medial entorhinal cortex.

Figure 1. Both gamma frequency and high frequency field potential oscillations are sensitive to carbenoxolone

A, effects of carbenoxolone, 0.2 mm, on gamma frequency field potential oscillations in superficial medial entorhinal cortex. a, upper trace shows gamma oscillation in the presence of 400 nm kainate. Lower trace shows reduced amplitude of gamma oscillation after 60 min bath application of carbenoxolone. Scale bars 0.2 mV, 200 ms. b, pooled power spectra (n = 8) taken from 60 s epochs of field gamma oscillation in the presence of kainate alone (black line) or kainate plus carbenoxolone (red line). B, effects of carbenoxolone on high frequency oscillations. a, upper traces show another example of population gamma frequency oscillations attenuated by bath application of 0.2 mm carbenoxolone. Lower traces show the high-pass filtered (> 150 Hz) versions of the upper traces demonstrating the gamma-frequency modulated high frequency activity is attenuated by carbenoxolone. Scale bars 0.2 mV, 100 ms. b, example traces of high-pass filtered data on expanded time base, with corresponding autocorrelograms. Scale bars 10 ms, 20 μV. Spectrograms of high-pass filtered field potential data in the absence (left hand panel) and presence (right hand panel) of carbenoxolone. Note the packets of high-frequency activity occurring at gamma frequency in control are abolished by carbenoxolone.

Figure 2. Bispectral analysis showing dependence between high frequency and gamma frequency field potential oscillations

A, relationship between gamma frequency and high frequency oscillations in medial entorhinal layer I. Upper example trace shows 0.5 s epoch of field potential data from LI; below that is the corresponding high-pass filtered trace (> 150 Hz). Bispectral coefficient shown for frequencies 5 Hz to 250 Hz; values above 3.3 (the lowest value contour) indicate a significant higher order interaction between frequency components. B, relationship between gamma frequency and high frequency oscillations in medial entorhinal layer II–III. Upper trace shows 0.5 s epoch of field potential data. Note the phase reversal compared to A. Lower trace shows the corresponding high-pass filtered trace (> 150 Hz). Graph sows the bispectral coefficient as in A. C, relationship between gamma and higher frequencies for deep medial entorhinal field potentials. Data represented as in A and B. Scale bars 0.1 mV (unfiltered field), 0.05 mV (high-pass filtered fields), 100 ms.

Figure 3. Superficial pyramidal cells show spikelets temporally correlated to high frequency field oscillations during field gamma oscillations

A, spiking behaviour in a LIII pyramidal cell during kainate-induced gamma frequency population oscillations. a, example trace of a pyramidal cell (membrane potential −61 V) showing sparse action potential generation but more regular (ca 8 Hz) spikelet generation. Inset shows 1st order differential of a full spike to show relationship between full spike and spikelet (scale bars, inset, 40 V s⁻¹, 5 ms). b, the same pyramid hyperpolarized to −70 mV and then to −85 mV to show characteristic membrane voltage dependence of spikelets. Scale bars 10 mV, 0.5 s (a), 2 s (b). B, expanded time scale traces showing concurrently recorded field potential oscillation and spikelets in a pyramidal cell. Upper trace is the unfiltered field potential, with the middle trace the corresponding high-pass filtered recording (> 150 Hz). Spikelets occurred during the high frequency bursts of activity seen in the field. Inset shows spikelet-triggered average (n = 20) of the corresponding field potential 20 ms either side of the pyramidal cell spikelets. Note the temporal relationship between the spikelet and the field. Scale bars, 0.1 mV (unfiltered trace), 0.02 mV (filtered field), 5 mV (pyramidal cell), 25 ms.

Figure 4. Superficial fast-spiking interneurones receive compound EPSPs temporally correlated with high frequency field potential oscillations

Interneurones received compound EPSPs coincident with bursts of high-frequency activity in the concurrently recorded field potential. A, example 1 s trace of fast-spiking interneurone activity during kainate-induced gamma frequency field potential oscillations. Scale bars 10 mV, 0.1 s. B, concurrently recorded field potential and interneuronal EPSP traces (from −70 mV membrane potential). Upper traces show unfiltered data (400 ms epochs). Lower traces show corresponding high-pass filtered versions of the above traces. Scale bars: 5 mV (unfiltered), 1 mV (filtered) for EPSPs, 0.1 mV (unfiltered), 0.02 mV (filtered) for field traces, 100 ms. Ca, averaged interneuronal EPSPs and corresponding field potential recording (n = 10) showing compound nature of EPSP is preserved with averaging, along with temporal correlation between EPSP components and field oscillations. Scale bars 4 mV (EPSP), 5 μV (field), 10 ms. b, average cross-correlogram of high-pass filtered EPSP and field recordings (n = 10, epoch = 3 s).