Witnessing another person’s suffering elicits vicarious brain activity in areas that are active when we ourselves are in pain. Whether this activity influences prosocial behavior remains the subject of debate. Here participants witnessed a confederate express pain through a reaction of the swatted hand or through a facial expression, and could decide to reduce that pain by donating money. Participants donate more money on trials in which the confederate expressed more pain. Electroencephalography shows that activity of the somatosensory cortex I (SI) hand region explains variance in donation. Transcranial magnetic stimulation (TMS) shows that altering this activity interferes with the pain–donation coupling only when pain is expressed by the hand. High-definition transcranial direct current stimulation (HD-tDCS) shows that altering SI activity also interferes with pain perception. These experiments show that vicarious somatosensory activations contribute to prosocial decision-making and suggest that they do so by helping to transform observed reactions of affected body-parts into accurate perceptions of pain that are necessary for decision-making.

Photoactivatable drugs targeting ligand-gated ion channels open up new opportunities for light-guided therapeutic interventions. Photoactivable toxins targeting ion channels have the potential to control excitable cell activities with low invasiveness and high spatiotemporal precision. As proof-of-concept, we develop HwTxIV-Nvoc, a UV light-cleavable and photoactivatable peptide that targets voltage-gated sodium (NaV) channels and validate its activity in vitro in HEK293 cells, ex vivo in brain slices and in vivo on mice neuromuscular junctions. We find that HwTxIV-Nvoc enables precise spatiotemporal control of neuronal NaV channel function under all conditions tested. By creating multiple photoactivatable toxins, we demonstrate the broad applicability of this toxin-photoactivation technology.

In neocortical layer‐5 pyramidal neurons, the action potential (AP) is generated in the axon initial segment (AIS) when the membrane potential (Vm) reaches the threshold for activation of the voltage‐gated Na+ channels (VGNCs) Nav1.2 and Nav1.6. Yet, whereas these VGNCs are known to differ in spatial distribution along the AIS and in biophysical properties, our understanding of the functional differences between the two channels remains elusive. Here, using ultrafast Na+, Vm and Ca2+ imaging in combination with partial block of Nav1.2 by the peptide G1G4‐huwentoxin‐IV, we demonstrate an exclusive role of Nav1.2 in shaping the generating AP. Precisely, we show that selective block of ∼30% of Nav1.2 widens the AP in the distal part of the AIS and we demonstrate that this effect is due to a loss of activation of BK Ca2+‐activated K+ channels (CAKCs). Indeed, Ca2+ influx via Nav1.2 activates BK CAKCs, determining the amplitude and the early phase of repolarization of the AP in the AIS. By using control experiments using 4,9‐anhydrotetrodotoxin, a moderately selective inhibitor of Nav1.6, we concluded that the Ca2+ influx shaping the early phase of the AP is exclusive of Nav1.2. Hence, we mimicked this result with a neuron model in which the role of the different ion channels tested reproduced the experimental evidence. The exclusive role of Nav1.2 reported here is important for understanding the physiology and pathology of neuronal excitability. Key points We optically analysed the action potential generated in the axon initial segment of mouse layer‐5 neocortical pyramidal neurons and its associated Na+ and Ca2+ currents using ultrafast imaging techniques. We found that partial selective block of the voltage‐gated Na+ channel Nav1.2, produced by a recently developed peptide, widens the shape of the action potential in the distal part of the axon initial segment. We demonstrate that this effect is due to a reduction of the Ca2+ influx through Nav1.2 that activates BK Ca2+‐activated K+ channels. To validate our conclusions, we generated a neuron model that reproduces the ensemble of our experimental results. The present results indicate a specific role of Nav1.2 in the axon initial segment for shaping of the action potential during its generation.

In neocortical layer-5 pyramidal neurons, the action potential (AP) is generated in the axon initial segment (AIS) when the membrane potential (Vm) reaches the threshold for activation of two diverse voltage-gated Na+ channels, that differ in spatial distribution and biophysical properties. Yet, the understanding of specific functional differences between these two channels remains elusive. Here, using ultrafast Na+, Vm and Ca2+ imaging in combination with partial block of Nav1.2 by a recent peptide, we demonstrate an exclusive role of Nav1.2 in shaping the generating AP. Precisely, Ca2+ influx via Nav1.2 activates BK Ca2+-activated K+ channels determining the amplitude and the early phase of repolarisation of the AP in the AIS. We mimicked this result with a NEURON model where the role of the different ion channels tested reproduced the experimental evidence. The exclusive role of Nav1.2 reported here is important for understanding the physiology and pathology of neuronal excitability.

Ultrafast Ca 2+ imaging using low-affinity fluorescent indicators allows the precise measurement of the kinetics of fast Ca 2+ currents mediated by voltage-gated Ca 2+ channels. Thus far, only a few indicators provided fluorescence transients with sufficient signal-to-noise ratio necessary to achieve this measurement, with Oregon Green BAPTA-5N exhibiting the best performance. Here we evaluated the performance of the low-affinity Ca 2+ indicator Cal-520FF to record fast Ca 2+ signals and to measure the kinetics of Ca 2+ currents. Compared to Oregon Green BAPTA-5N and to Fluo4FF, Cal-520FF offers a superior signal-to-noise-ratio providing the optimal characteristics for this important type of biophysical measurement. This ability is the result of a relatively high fluorescence at zero Ca 2+ , necessary to detect enough photons at short exposure windows, and a high dynamic range leading to large fluorescence transients associated with short Ca 2+ influx periods. We conclude that Cal-520FF is at present the optimal commercial low-affinity Ca 2+ indicator for ultrafast Ca 2+ imaging applications.

Innate defensive responses such as freezing or escape are essential for animal survival. Mice show defensive behaviour to stimuli sweeping overhead, like a bird cruising the sky. Here, we tested this in young male mice and found that mice reduced their defensive freezing after sessions with a stimulus passing overhead repeatedly. This habituation is stimulus-specific, as mice freeze again to a novel shape. We found no evidence for head-centred stimulus location-specific habituation. The mice generalized over a range of sizes and shapes, but distinguished objects when they differed in both size and shape. Innate visual defensive responses are thus strongly influenced by previous experience as mice learn to ignore specific stimuli.

Monitoring Na + influx in the axon initial segment (AIS) at high spatial and temporal resolution is fundamental to understanding the generation of an action potential (AP). Here, we present protocols to obtain this measurement, focusing on the AIS of layer 5 (L5) somatosensory cortex pyramidal neurons in mouse brain slices. We first outline how to prepare slices for this application, how to select and patch neurons, and how to optimize the image acquisition. Specifically, we describe the preparation of optimal slices, patching and loading of L5 pyramidal neurons with the Na + indicator ING-2, and Na + imaging at 100 μs temporal resolution with a pixel resolution of half a micron. Then, we present a data analysis strategy in order to extract information on the kinetics of activated voltage-gated Na + channels by determining the change in Na + by compensating for bleaching and calculating the time derivative of the resulting fit. In sum, this approach can be widely applied when investigating the function of Na + channels during initiation of an AP and propagation under physiological or pathological conditions in neuronal subtypes.

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