Publications

Focal cortical epilepsies are frequently refractory to available anticonvulsant drug therapies. One key factor contributing to this state is the limited availability of animal models that allow to reliably study focal cortical seizures and how they recruit surrounding brain areas in vivo. In this study, we selectively expressed the inhibitory chemogenetic receptor, hM4D, in GABAergic neurons in focal cortical areas using viral gene transfer. GABAergic silencing using Clozapine-N-Oxide (CNO) demonstrated reliable induction of local epileptiform events in the electroencephalogram signal of awake freely moving mice. Anesthetized mice experiments showed consistent induction of focal epileptiform-events in both the barrel cortex (BC) and the medial prefrontal cortex (mPFC), accompanied by high-frequency oscillations, a known characteristic of human seizures. Epileptiform-events showed propagation indication with favored propagation pathways: from the BC on 1 hemisphere to its counterpart and from the BC to the mPFC, but not vice-versa. Lastly, sensory whisker-pad stimulation evoked BC epileptiform events post-CNO, highlighting the potential use of this model in studying sensory-evoked seizures. Combined, our results show that targeted chemogenetic inhibition of GABAergic neurons using hM4D can serve as a novel, versatile, and reliable model of focal cortical epileptic activity suitable for systematically studying cortical ictogenesis in different cortical areas.

Interhemispheric correlation between homotopic areas is a major hallmark of cortical physiology and is believed to emerge through the corpus callosum. However, how interhemispheric correlations and corpus callosum activity are affected by behavioral states remains unknown. We performed laminar extracellular and intracellular recordings simultaneously from both barrel cortices in awake mice. We find robust interhemispheric correlations of both spiking and synaptic activities that are reduced during whisking compared to quiet wakefulness. Accordingly, optogenetic inactivation of one hemisphere reveals that interhemispheric coupling occurs only during quiet wakefulness, and chemogenetic inactivation of callosal terminals reduces interhemispheric correlation especially during quiet wakefulness. Moreover, in contrast to the generally elevated firing rate observed during whisking epochs, we find a marked decrease in the activity of imaged callosal fibers. Our results indicate that the reduction in interhemispheric coupling and correlations during active behavior reflects the specific reduction in the activity of callosal neurons.Interhemispheric correlations are mediated by the corpus callosum, an extensive fiber bundle connecting the cortical hemispheres. The authors show that interhemispheric correlations between the somatosensory cortices of awake mice are reduced during whisking as a result of lower callosal activity.

Deep brain nuclei, such as the amygdala, nucleus basalis, and locus coeruleus, play a crucial role in cognition and behavior. Nonetheless, acutely recording electrical activity from these structures in head-fixed awake rodents has been very challenging due to the fact that head-fixed preparations are not designed for stereotactic accuracy. We overcome this issue by designing the DeepTarget, a system for stereotactic head fixation and recording, which allows for accurately directing recording electrodes or other probes into any desired location in the brain. We then validated it by performing intracellular recordings from optogenetically tagged amygdalar neurons followed by histological reconstruction, which revealed that it is accurate and precise to within similar to 100 mu m. Moreover, in another group of mice we were able to target both the mammillothalamic tract and subthalamic nucleus. This approach can be adapted to any type of extracellular electrode, fiber optic, or other probe in cases where high accuracy is needed in awake, bead-fixed rodents.NEW & NOTEWORTHY Accurate targeting of recording electrodes in awake head-restrained rodents is currently beyond our reach. We developed a device for stereotactic implantation of a custom head bar and a recording system that together allow the accurate and precise targeting of any brain structure, including deep and small nuclei. We demonstrated this by performing histology and intracellular recordings in the amygdala of awake mice. The system enables the targeting of any probe to any location in the awake brain.

The nucleus basalis (NB) projects cholinergic axons to the cortex, where they play a major role in arousal, attention, and learning. Cholinergic inputs shift cortical dynamics from synchronous to asynchronous and improve the signal-to-noise ratio (SNR) of sensory responses. However, the underlying mechanisms of these changes remain unclear. Using simultaneous extracellular and whole-cell patch recordings in layer 4 of the mouse barrel cortex, we show that electrical or optogenetic activation of the cholinergic system has a differential effect on ongoing and sensory evoked activities. Cholinergic activation profoundly reduced the large spontaneous fluctuations in membrane potential and decorrelated ongoing activity. However, NB stimulation had no effect on the response to whisker stimulation or on signal correlations. These effects of cholinergic activation provide a unified explanation for the increased SNR of sensory response and for the reduction in noise correlations and explain the shift into the desynchronized cortical state, which are the hallmarks of arousal and attention.

Local field potentials (LFPs) are an important measure of brain activity and have been used to address various mechanistic and behavioral questions. We revealed a prominent whisker-evoked LFP signal in the olfactory bulb and investigated its physiology. This signal, dependent on barrel cortex activation and highly correlated with its local activity, represented a pure volume conduction signal that was sourced back to the activity in the ventro-lateral orbitofrontal cortex, located a few millimeters away. Thus, we suggest that special care should be taken when acquiring and interpreting LFP data. Local field potential (LFP) recordings are an important yet still obscure tool in neuroscience. In this issue, Parabucki and Lampl show volume conduction of LFP signals from cortex to olfactory bulb in mice, emphasizing that LFP can be misleading under certain circumstances.

Advanced statistical methods have enabled trial-by-trial inference of the underlying excitatory and inhibitory synaptic conductances (SCs) of membrane-potential recordings. Simultaneous inference of both excitatory and inhibitory SCs sheds light on the neural circuits underlying the neural activity and advances our understanding of neural information processing. Conventional Bayesian methods can infer excitatory and inhibitory SCs based on a single trial of observed membrane potential. However, if multiple recorded trials are available, this typically leads to suboptimal estimation because they neglect common statistics (of synaptic inputs (SIs)) across trials. Here, we establish a new expectation maximization (EM) algorithm that improves such single-trial Bayesian methods by exploiting multiple recorded trials to extract common SI statistics across the trials. In this paper, the proposed EM algorithm is embedded in parallel Kalman filters or particle filters for multiple recorded trials to integrate their outputs to iteratively update the common SI statistics. These statistics are then used to infer the excitatory and inhibitory SCs of individual trials. We demonstrate the superior performance of multiple-trial Kalman filtering (MtKF) and particle filtering (MtPF) relative to that of the corresponding single-trial methods. While relative estimation error of excitatory and inhibitory SCs is known to depend on the level of current injection into a cell, our numerical simulations using MtKF show that both excitatory and inhibitory SCs are reliably inferred using an optimal level of current injection. Finally, we validate the robustness and applicability of our technique through simulation studies, and we apply MtKF to in vivo data recorded from the rat barrel cortex.

Adaptation allows neurons to respond to a wide range of stimulus intensities. However, it also leads to ambiguity as the representation of the external world depends on the context. We recorded neurons from Wistar rats' brainstem nuclei belonging to two major somatosensory pathways (lemniscal and paralemniscal) and explored the way in which they encode noisy stimuli under different contexts. We found that although their unadapted intensity-response curves are very similar, the adapted curves of the two pathways are distinctively different as they are optimized for encoding different intensity ranges. Lemniscal neurons most faithfully encoded stimuli when the background intensity was high, whereas paralemniscal cells best encoded stimuli under low intensity context. Intracellular recordings indicate that these differences emerge already at the synaptic level. We suggest that the two pathways synergistically improve the ability of this system to encode a wide range of intensities during natural stimulation, potentially reducing the inherent ambiguity of adaptive coding.

The ability of the brain to adapt to environmental demands implies that neurons can change throughout life. The extent to which single neurons actually change remains largely unstudied, however. To evaluate how functional properties of single neurons change over time, we devised a way to perform in vivo time-lapse electrophysiological recordings from the exact same neuron. We monitored the contralateral and ipsilateral sensory-evoked spiking activity of individual L2/3 neurons from the somatosensory cortex of mice. At the end of the first recording session, we electroporated the neuron with a DNA plasmid to drive GFP expression. Then, 2 wk later, we visually guided a recording electrode in vivo to the GFP-expressing neuron for the second time. We found that contralateral and ipsilateral evoked responses (i.e., probability to respond, latency, and preference), and spontaneous activity of individual L2/3 pyramidal neurons are stable under control conditions, but that this stability could be rapidly disrupted. Contralateral whisker deprivation induced robust changes in sensoryevoked response profiles of single neurons. Our experiments provide a framework for studying the stability and plasticity of single neurons over long time scales using electrophysiology.

In contrast to neurons of the lateral geniculate nucleus (LGN), neurons in the primary visual cortex (V1) are selective for the direction of visual motion. Cortical direction selectivity could emerge from the spatiotemporal configuration of inputs from thalamic cells, from intracortical inhibitory interactions, or from a combination of thalamic and intracortical interactions. To distinguish between these possibilities, we studied the effect of adaptation (prolonged visual stimulation) on the direction selectivity of intracellularly recorded cortical neurons. It is known that adaptation selectively reduces the responses of cortical neurons, while largely sparing the afferent LGN input. Adaptation can therefore be used as a tool to dissect the relative contribution of afferent and intracortical interactions to the generation of direction selectivity. In both simple and complex cells, adaptation caused a hyperpolarization of the resting membrane potential (-2.5 mV, simple cells, -0.95 mV complex cells). In simple cells, adaptation in either direction only slightly reduced the visually evoked depolarization; this reduction was similar for preferred and null directions. In complex cells, adaptation strongly reduced visual responses in a direction-dependent manner: the reduction was largest when the stimulus direction matched that of the adapting motion. As a result, adaptation caused changes in the direction selectivity of complex cells: direction selectivity was reduced after preferred direction adaptation and increased after null direction adaptation. Because adaptation in the null direction enhanced direction selectivity rather than reduced it, it seems unlikely that inhibition from the null direction is the primary mechanism for creating direction selectivity.

In this issue of Neuron, Busse et al. describe the population response to superimposed visual stimuli while Sit et al. examine the spatiotemporal evolution of cortical activation in response to small visual stimuli. Surprisingly, these two studies of V1 report that a single gain control model accounts for their results.

Sustained stimulation of sensory organs results in adaptation of the neuronal response along the sensory pathway. Whether or not cortical adaptation affects equally excitation and inhibition is poorly understood. We examined this question using patch recordings of neurons in the barrel cortex of anesthetized rats while repetitively stimulating the principal whisker. We found that inhibition adapts more than excitation, causing the balance between them to shift toward excitation. A comparison of the latency of thalamic firing and evoked excitation and inhibition in the cortex strongly suggests that adaptation of inhibition results mostly from depression of inhibitory synapses rather than adaptation in the firing of inhibitory cells. The differential adaptation of the evoked conductances that shifts the balance toward excitation may act as a gain mechanism which enhances the subthreshold response during sustained stimulation, despite a large reduction in excitation.

In vitro studies of inferior olive neurons demonstrate that they are intrinsically active, generating periodic spatiotemporal patterns. These self-generated patterns of activity extend the role of olivary neurons beyond that of a deliverer of teaching or error signals. However, autorhythmicity or patterned activity of complex spikes in the cerebellar cortex was observed in only a few studies. This discrepancy between the self-generated rhythmicity in the inferior olive observed in vitro and the sporadic reports on rhythmicity of complex spikes can be reconciled by recording intracellularly from inferior olive neurons in situ. To this end, we recorded intracellularly from olivary neurons of anesthetized rats. We demonstrate that, in vivo, olivary neurons show both slow and fast rhythmic processes. The slow process (0.2-2 Hz) is expressed as rhythmic transitions from quiescent periods to periods of fast rhythm, manifested as subthreshold oscillations of 6-12 Hz. Spikes, if they occur, are locked to the depolarized phase of these subthreshold oscillations and, therefore, hold and transfer rhythmic information. The transient nature of these oscillatory epochs accounts for the difficulties to uncover them by prolonged recordings of complex spikes activity in the cerebellar cortex.