GABAergic inhibition shapes interictal dynamics in awake epileptic mice

International audienceEpilepsy is characterized by recurrent seizures and brief, synchronous bursts called interictal spikes that are present in-between seizures and observed as transient events in EEG signals. While GABAergic transmission is known to play an important role in shaping healthy brain activity, the role of inhibition in these pathological epileptic dynamics remains unclear. Examining the microcircuits that participate in interictal spikes is thus an important first step towards addressing this issue, as the function of these transient synchronizations in either promoting or prohibiting seizures is currently under debate. To identify the microcircuits recruited in spontaneous interictal spikes in the absence of any proconvulsive drug or anaesthetic agent, we combine a chronic model of epilepsy with in vivo two-photon calcium imaging and multiunit extracellular recordings to map cellular recruitment within large populations of CA1 neurons in mice free to run on a self-paced treadmill. We show that GABAergic neurons, as opposed to their glutamatergic counterparts, are preferentially recruited during spontaneous interictal activity in the CA1 region of the epileptic mouse hippocampus. Although the specific cellular dynamics of interictal spikes are found to be highly variable, they are consistently associated with the activation of GABAergic neurons, resulting in a perisomatic inhibitory restraint that reduces neuronal spiking in the principal cell layer. Given the role of GABAergic neurons in shaping brain activity during normal cognitive function, their aberrant unbalanced recruitment during these transient events could have important downstream effects with clinical implications

Awake hippocampal reactivations project onto orthogonal neuronal assemblies

International audienceThe chained activation of neuronal assemblies is thought to support major cognitive processes,including memory. In the hippocampus, this is observed during population bursts oftenassociated with sharp-wave ripples, in the form of an ordered reactivation of neurons. However,the organization and lifetime of these assemblies remain unknown.We used calcium imagingto map patterns of synchronous neuronal activation in the CA1 region of awakemice during runson a treadmill.The patterns were composed of the recurring activation of anatomicallyintermingled, but functionally orthogonal, assemblies.These assemblies reactivated discretetemporal segments of neuronal sequences observed during runs and could be stable acrossconsecutive days. A binding of these assemblies into longer chains revealed temporallyordered replay

Internal representation of hippocampal neuronal population spans a time-distance continuum

International audienceThe hippocampus plays a critical role in episodic memory: the sequential representation of visited places and experienced events. This function is mirrored by hippocampal activity that self organizes into sequences of neuronal activation that integrate spatio-temporal information. What are the underlying mechanisms of such integration is still unknown. Single cell activity was recently shown to combine time and distance information; however, it remains unknown whether a degree of tuning between space and time can be defined at the network level. Here, combining daily calcium imaging of CA1 sequence dynamics in running head-fixed mice and network modeling, we show that CA1 network activity tends to represent a specific combination of space and time at any given moment, and that the degree of tuning can shift within a continuum from one day to the next. Our computational model shows that this shift in tuning can happen under the control of the external drive power. We propose that extrinsic global inputs shape the nature of spatio-temporal integration in the hippocampus at the population level depending on the task at hand, a hypothesis which may guide future experimental studies

Hippocampal units maintain coupling precision to the local field potential during alternating network states.

<p><b>A</b>: Local field potential recordings and single unit activity (symbolized by vertical ticks) in CA1 and CA3 during the three experimental steps of the <i>CCh</i> experiments: baseline recording (SPW 1, left panel) followed by carbachol-induced gamma oscillations (Gamma, middle panel) and re-established SPW by wash-in of atropine (SPW 2, right panel). Two representative units are indicated by colored ticks in each region. Note that the individual units could be observed in all three phases of the experiment. <b>B</b>: Event cross-correlograms of field-potentials and firing time points of the units depicted in A. During SPW 1 and SPW 2, unit firing is correlated to the ripple-oscillation troughs in CA1 (peak intervals at ∼5 ms, corresponding to ripple cycle length); during the gamma episode they are correlated to the local gamma oscillation troughs (peak intervals at ∼30 ms). <b>C</b>: Mean firing rates of units remain constant during gamma oscillations. <b>D</b>: Firing phases of units to ripple troughs remain stable after an intermittent episode of gamma oscillations. The ripple cycle is described by a circular scale of 360° and the ripple trough is set to 0°. Each unit's preferred firing phase angle to the ripple contributes as one data point. Mean angle of the sum of all units is represented by the red arrow. The length of the red arrow is proportional to the length of the vector corresponding to the mean angle.</p

SPW-coupled firing is specifically enhanced after carbachol induced gamma oscillations in both CA1 and CA3.

<p><b>A</b>: Each line represents one unit and the modulation of its SPW-R-related firing rate after carbachol induced gamma oscillations. Red lines illustrate potentiated units, black lines depict suppressed units. Note the dominance of enhanced units in the <i>CCh</i> experiments in contrast to ongoing SPW-R recordings. <b>B</b>: Increase of SPW-R-coupled firing (see Methods section for plasticity coefficient) was significantly stronger in the <i>CCh</i> experiments compared to ongoing SPW-R-recordings and the <i>atropine</i> control, p<0.0001 for CA1, p<0.001 for CA3; ANOVA with post-hoc test between groups). <b>C</b>: Modulation in firing outside SPW-R was calculated as a coefficient (see <i>c2</i> in Methods Section) and compared between groups. In CA1 of <i>CCh</i> experiments firing outside SPW decreased in contrast to ongoing SPW-recordings and <i>atropine</i> control (p<0.001, ANOVA and post-hoc tests). In CA3 firing outside SPWs was slightly increased in all groups. However, groups did not differ statistically (p>0.05, ANOVA).</p

Different network states and corresponding unit activity can be analyzed in the in vitro model.

<p><b>A</b>: Schematics of recording conditions. Tetrode recordings are performed in the pyramidal cell layers of CA3 and CA1. During sharp wave-ripple oscillations (SPW-R), field potentials are generated by the recurrent network of CA3 (green) and transmitted via the Schaffer collaterals to selectively activate cell assemblies in CA1 (red). Examples of field potential waveforms are shown in the respective colors. <b>B</b>: Example traces of field potentials recorded in CA1 during SPW-R (left panel) and gamma oscillations (right panel). Raw field potential (upper trace) and the high pass filtered (0.5–10 kHz) potential in the four tetrode channels (lower traces). Filtering reveals high-frequency multiunit activity on SPW-R and gamma cycles. Firing of two individual units extracted by waveform analysis is highlighted in orange and blue. Time points of firing relative to the field potential are visualized by vertical ‘ticks’. The middle panel illustrates the detection criterion for unit events: horizontal histograms of all data points show largely normal distribution around zero. Events are detected if they exceed 4.5 times the standard deviation as indicated by the dashed red line. <b>C</b>: Wavelet spectrograms of the example traces from B. Sharp waves are superimposed by high-frequency ripple oscillations (∼200 Hz, left panel). The carbachol-induced gamma state consists of a continuous oscillation at ∼30 Hz.</p

Intermittent carbachol induced gamma oscillations lead to changes in SPW-R-waveforms reflecting modulation of local neuronal assembly structure.

<p><b>A</b>: Example of 2456 (left panel, baseline episode of 1000 s) and 7237 (right panel, post gamma episode of 2000 s) individual SPW-R events from one experiment sorted onto the respective reference SOM. Traces left and right to the SOM depict an enlarged view of the waveforms at the corner positions. Grey lines indicate individual events, yellow lines show mean waveforms which are also shown in the respective map unit. Note the shift in SPW-R-waveforms after intermittent gamma oscillations. <b>B</b>: Differences of reference maps and partial data maps after <i>CCh</i> (red), ongoing <i>SPW-R</i> oscillations (blue), and <i>atropine</i> application (green) are quantified by their mean distance to the reference episode. The shift to more different waveforms was significantly more pronounced after gamma oscillations than in the other two experimental groups (p<0.05; ANOVA with post-hoc test between groups). <b>C</b>: similar to B but baseline SPWs are normalized to the median SPW-amplitude during baseline recording and SPWs from the second recording episode are normalized to the median amplitude during that recording period. Again, gamma-induced modulation in SPW-R waveforms significantly exceeds changes in ongoing SPW-R recordings and <i>atropine</i> control (p<0.05; ANOVA with post-hoc test between groups).</p

Potentiation of unit assemblies is reflected by plastic changes on the network level.

<p><b>A</b>: Increase of amplitudes was significantly stronger after an intermittent carbachol induced gamma state than after ongoing SPW-R-oscillations. However, the increase in amplitude was not significantly different from values derived from the <i>atropine</i> control (p<0.05, <i>CCh</i> vs. <i>SPW-R</i>: p<0.01, <i>CCh</i> vs. <i>atropine</i>: p>0.05, ANOVA with post-hoc test between groups). <b>C</b>: Population spike amplitude in CA1 str. pyramidale (upper panel) and the slope of field-EPSPs in CA1 str. radiatum (lower panel) are potentiated after gamma oscillations. Intakes depict representative waveforms before (black) and after gamma (red).</p