Chronometric readout from a memory trace: gamma-frequency field stimulation recruits timed recurrent activity in the rat CA3 network

Abstract

Synchronous population activity is prevalent in neurones of the central nervous system and experimentally captured as oscillatory electric fields, the frequency of which can represent the state of the neural circuit, e.g. theta (approximately 5 Hz) and gamma (approximately 40 Hz). Such field oscillations, however, are not merely a result of coherent neuronal activity. They may also play active roles in information processing in the brain. In this study, we observed that, in cultured hippocampal slices, CA3 pyramidal cells responded to single-pulse stimuli with monosynaptic and polysynaptic potentials and firing spikes which occurred after variable latencies. The variability of the spike latencies was greatly reduced in the presence of weak electric field oscillations, especially the oscillation in the gamma-band frequency range, that per se induced only small fluctuations in the subthreshold membrane potential, and this effect was inhibited by blockade of NMDA receptor activity. Furthermore, the latency of the firing spikes changed if the stimulus was applied at a different phase of the imposed gamma oscillations. These results may suggest that the background field oscillations serve as an extracellular time reference and assure accurate and stable decoding of a memory trace present in cortical feedback networks.

Figure 1. Spike jitters increase linearly with their latencies in the CA3 recurrent circuit

A, intracellular responses of a CA3 pyramidal cell following a single-pulse stimulus (arrow) applied to the CA3 stratum pyramidale. The traces of the boxed region are magnified in the inset. B, voltage map of intracellular responses. Horizontal sweeps represent 20 successive trials, out of which the odd 10 traces are shown in panel A. A stimulus was given every 15 s at time 0. C, linear correlations between the mean latency of the first spikes and its standard deviation. Each point represents each neurone and the line is the best fit with linear regression (r = 0.89, d.f. = 27, P < 0.0001).

Figure 2. External field oscillators ensure stable propagation of activity and chronometric spike output

A, representative field waveforms and their FFT power spectra obtained from the CA3 region in the presence of 20 μm carbachol (Carbachol) or oscillating electric fields at sinusoidal 40 Hz (Ext gamma). B, representative voltage maps in the absence (Stimulus only) and presence of extracellular 40-Hz oscillations (Stimulus with gamma). Each trace in the inset shows a representative intracellular recording in each case. Stimulation was applied at phase 90 deg of gamma. C, stimulus phase-dependent spike timing. The ordinate indicates the mean latency of the first spike evoked by a single-pulse stimulus that was applied at either 0, 90, 180, or 270 deg in gamma cycle. Left traces show representative late responses (spikes truncated). In the both panels, the broken lines indicate the mean latency of the first spike in the absence of gamma oscillations, referred to here as ‘stimulus only’. *P < 0.05, **P < 0.01 versus stimulus only; Fisher's protected least significant difference following one-way ANOVA (n = 5 cells). D, stimulus phase-dependent initial EPSP responses. The ordinate indicates the amplitude of the initial EPSPs evoked by a single-pulse stimulus. The dashed line indicates the mean amplitude of monosynaptic response in the absence of gamma oscillations. *P < 0.05, **P < 0.01 versus stimulus only; Fisher's protected least significant difference following one-way ANOVA (n = 5 cells). E, signal propagation with small spike jitters. The mean latency and standard deviation of the first spike latency were independent of each other. The filled circles represent the data of phase 270 deg, the filled squares 90 deg, and the open circles 0 and 180 deg. The dashed line indicates a correlation of stimulus only (derived from Fig. 1C).

Figure 3. Gamma frequency-specific amplification of subthreshold membrane potential

A, subthreshold responses and their FFT power spectra of a QX314-loaded CA3 pyramidal neurone to extracellular gamma oscillations (Gamma only), a single-pulse stimulus (Stimulus only), and a combination of both (Stimulus with gamma). Stimulation was applied at phase 90 deg of gamma. The salient 40-Hz peak in the FFT spectrum indicates a supralinear enhancement of gamma field oscillations. B, onset timings of EPSP occurrence relative to the gamma phase. Data were obtained from Fig. 3A (Stimulus with gamma). Recurrent synaptic inputs were phase-locked. C, carbachol induces gamma-frequency recurrent inputs. The responses (insets) evoked by a single stimulus applied in the presence of 20 μm carbachol were FFT-transformed. D, frequency-dependent enhancement of field oscillations. Typical stimulus-evoked responses (left) and their FFT power spectra (right) to an external oscillator with its frequency ranging form 25 to 60 Hz. E, summary of the frequency preference in the oscillation enhancement. The degrees of amplification are expressed in decibels. Each line shows each cell (n = 7).

Figure 4. Gamma-rhythmic recurrent inputs are network driven, NMDA receptor dependent

A, representative subthreshold responses of a CA3 pyramidal neurone to a single-pulse stimulus with 40-Hz field oscillations in the absence (Control) and presence of 50 μm d,l-AP5 or 20 μm CNQX. B, NMDA and non-NMDA receptor antagonists block the enhancement of gamma oscillations. The areas under the curve of the 40-Hz peak (37–43 Hz range) were measured in the FFT power spectra. **P < 0.01 versus gamma only, ##P < 0.01 versus control: Tukey's multiple range test after one-way ANOVA (n = 8–23 cells).