Understanding how behavior modulates neuronal integration is a fundamental goal in neuroscience. We combined voltage imaging with optogenetics to reveal how excitatory (E) and inhibitory (I) inputs modulate spiking output, subthreshold dynamics, and gain in genetically defined CA1 neurons. We imaged pyramidal cells (PCs), vasoactive intestinal peptide (VIP), somatostatin (SST), and parvalbumin (PV) interneurons (INs) and found that locomotion reduced firing in PCs and VIP INs while increasing activity in SST and PV INs. Prolonged optical depolarization revealed that inhibitory inputs substantially contribute to intracellular theta oscillations in PCs and VIP cells. Firing rate-laser intensity (F-I) curves revealed distinct gain modulation across cell types, with a divisive gain reduction in PC bursting during locomotion, while simple spikes are unaffected. A two-compartment model suggested that this effect results from a balanced increase in E/I input to the soma and dendrite. These findings reveal how behavior coordinates E/I signaling to modulate hippocampal computations.

Advances in optical techniques and two-photon (2P) sensitive genetic voltage indicators (GEVIs) enabled in-depth voltage imaging at single-spike and single-cell resolution. These results were achieved using ASAP-type sensors, while rhodopsin-based GEVIs were mainly used with one-photon (1P) illumination. Here, we demonstrate compatibility of rhodopsin-based GEVIs with 2P illumination. We rationally engineer a fully genetically encoded, rhodopsin-based GEVI, just another voltage indicating sensor (Jarvis), and demonstrate its utility under 1P and 2P illumination. We further show 2P usability of the fluorescence resonance energy transfer (FRET)-opsin GEVIs pAce and Voltron2. Comparing 2P scanless with fast 2P scanning illumination revealed that responses are resolved with both approaches, but FRET-opsin GEVIs show improved signal-to-noise ratio (SNR) with low irradiance, inherent to scanless illumination. Utilizing Jarvis and pAce, we establish high-SNR action potential detection at kilohertz imaging rates in mouse hippocampal slices, zebrafish larvae, and the cortex of awake mice, demonstrating high-contrast action potential detection under 2P illumination with rhodopsin-based GEVIs in vitro and in vivo.

The hippocampus plays a crucial role in consolidating episodic memories from diverse experiences that encompass spatial, temporal, and novel information. This study analyzed the spike patterns of hippocampal place cells in the CA3 and CA1 areas of male rats that sequentially foraged in five rooms, including familiar and novel rooms, followed by a rest period. Across the five rooms, both CA3 and CA1 place cells showed overlapping spatial representations. In a post-experience rest period, both CA3 and CA1 place cells increased baseline spike rates depending on the temporal distance from when the cells had place fields. In addition, CA3 place cells that encoded novel environments showed stronger sharp wave ripple reactivation. Coordinated reactivation of CA1 place cell ensembles that encoded temporally distant environments was eliminated. These results suggest that, following sequential experiences in multiple environments, increases in sharp wave ripple-induced spikes of hippocampal neurons more specifically process novelty-related aspects of memory, while global increases in baseline spike rates process temporal distance-related aspects. This study investigated how the hippocampus processes and stores memories from a series of experiences in different environments. While rats experienced familiar and novel rooms, both CA3 and CA1 neurons exhibited overlapping maps. In a post-experience rest period, these place cells increased baseline spike rates depending on the temporal distance from when the cells had place fields, suggesting processing of temporal distance-related aspects of memory. In addition, CA3 place cells that encoded novel environments specifically showed stronger reactivation during sharp wave ripples, suggesting processing of novelty-related aspects. These differential activation patterns reveal how the hippocampus integrates spatial, temporal, and novelty information from multiple experiences.

Hebbian synaptic plasticity is currently the main framework to relate neuronal activity, network structure, and learning and memory. However, recent experimental and computational modeling studies have revealed a new form of synaptic plasticity termed behavioral timescale synaptic plasticity (BTSP). It is triggered by dendritic plateau potentials associated with somatic burst firing, causes large changes in synaptic strength in a single shot, and operates on the timescale of seconds. Here we review the recent advances in our understanding of the circuit, cellular, and molecular mechanisms of BTSP, its prevalence in the brain, its role in shaping neuronal representations, and the emerging ideas regarding its contribution to different forms of learning.

How positive and negative affective stimuli interact in the brain to influence behavioral outcomes remains poorly understood. Here, we show that recall of a positively valenced reward-associated cue (reward-conditioned stimulus, CS+) can prevent or reverse fear generalization in mice. Modification of generalized fear by recall of a CS+ is dependent on the midbrain dopamine system and the regulation of discriminatory fear encoding by the central amygdala (CeA). Precisely timed, transient elevations in dopamine are necessary to reverse fear generalization and nondiscriminatory fear encoding in the CeA. Recall of a positive association is also effective at enhancing the extinction of a conditioned fear response in a dopamine-dependent manner. These data demonstrate that recall of a positive experience can be an effective means to suppress generalized fear and show that dopamine projections to the CeA are an important neural substrate for this phenomenon.

The ventral pallidum (VP) lies at the intersection of basal ganglia and basal forebrain circuitry, possessing attributes of both major subcortical systems. Basal forebrain cholinergic neurons are rapidly recruited by reinforcement feedback and project to cortical and subcortical forebrain targets; in contrast, striatal cholinergic cells are local interneurons exhibiting classical 'pause-burst' responses to rewards. However, VP cholinergic neurons (VPCNs) are less characterized, and it is unclear whether basal forebrain and striatal type cholinergic neurons mix in the VP. Therefore, we performed anterograde and mono-transsynaptic retrograde labeling, in vitro acute slice recordings and bulk calcium recordings of VPCNs in mice of either sex. We found that VPCNs broadly interact with the mesocorticolimbic circuit that processes rewards and punishments, targeting the basolateral amygdala, the medial prefrontal cortex and the lateral habenula, while receiving inputs from the nucleus accumbens, hypothalamus, central amygdala, bed nucleus of stria terminalis and the ventral tegmental area. Bulk calcium recordings revealed that VPCNs responded to rewards, punishments and reward-predicting cues. Acute slice recordings showed that most VPCNs resembled the bursting type of basal forebrain cholinergic neurons (BFCNs), while a few of them were of the regular rhythmic type, which differentiated most VPCNs from striatal cholinergic interneurons. These results were confirmed by in vivo electrophysiological recordings of putative VPCNs. We conclude that VPCNs show burst firing and specialized connectivity to relay aversive and appetitive stimuli to the reinforcement circuitry, possibly implicated in mood disorders and addiction. The ventral pallidum is a special brain area, being part of both the basal ganglia system implicated in goal-directed behavior and the basal forebrain system implicated in learning and attention. It houses, among others, neurons that release the neurotransmitter acetylcholine. While these cholinergic neurons have distinct characteristics in other regions of the basal ganglia and basal forebrain, it is unclear whether those in the ventral pallidum resemble one or the other or both. Here we demonstrate that they are closer to basal forebrain cholinergic neurons both anatomically and functionally, especially resembling a burst-firing subtype thereof. In accordance, we found that they convey information about aversive and appetitive stimuli to the reinforcement circuitry, possibly implicated in mood disorders and addiction.

Knowledge of how vocal communication signals are represented in the auditory system is crucial for understanding the perceptual basis of vocal communication. Using male and female zebra finches, we identified a series of differentially expressed markers that helped define distinct (caudal, rostral, dorsal and ventral) domains within the caudomedial nidopallium (NCM), a high-order cortical auditory area known for its song-selective responses. Using expression analysis of the activity-inducible gene , we found that the number of activated neurons is more stimulus dependent in NCM than in the auditory midbrain or the caudomedial mesopallium, and that information on the density and spatial distribution of responsive neurons in NCM is sufficient to discriminate responses to conspecific song from other stimuli. We observed stronger activation of dorsal NCM, higher selectivity of caudal NCM towards conspecific song, and strong activation of the inhibitory network of rostral NCM by non-conspecific song stimuli. Song auditory representation in NCM was dependent on acoustic features, with the spatial organization of responsive cells particularly sensitive to both spectral and temporal components. We also obtained evidence of broadly distributed song-selective neuronal ensembles and that individual NCM neurons participate in the representation of conspecific songs, implying independent activation and molecular induction responses. We conclude that some basic aspects of the cortical response to complex auditory stimuli are topographically organized, a finding that has been elusive in other systems. These findings advance our knowledge of the functional organization of a key song-processing cortical area, providing novel insights into the auditory representation of conspecific vocal communication signals. Understanding how vocal signals are processed and represented in the brain is fundamental to the study of animal communication. Songbirds provide a powerful model for investigating these processes due to their rich vocal behavior and well-characterized neural circuits. Through analysis of differentially expressed markers and mapping of activity-induced gene expression, we have uncovered how different domains and neuronal populations within a high-order auditory cortical area respond to acoustic features of song and other stimuli. Besides providing in-depth knowledge of the functional organization of a key avian brain area, these findings provide insights into how acoustic features of complex learned vocal signals are processed and represented in cortical circuits, including evidence of how basic aspects of this representation can be topographically organized.

Effort is costly: given a choice, we tend to avoid it. However, in many cases, effort adds value to the ensuing rewards. From ants to humans, individuals prefer rewards that had been harder to achieve. This counterintuitive process may promote reward seeking even in resource-poor environments, thus enhancing evolutionary fitness. Despite its ubiquity, the neural mechanisms supporting this behavioural effect are poorly understood. Here we show that effort amplifies the dopamine response to an otherwise identical reward, and this amplification depends on local modulation of dopamine axons by acetylcholine. High-effort rewards evoke rapid acetylcholine release from local interneurons in the nucleus accumbens. Acetylcholine then binds to nicotinic receptors on dopamine axon terminals to augment dopamine release when reward is delivered. Blocking the cholinergic modulation blunts dopamine release selectively in high-effort contexts, impairing effortful behaviour while leaving low-effort reward consumption intact. These results reconcile in vitro studies, which have long demonstrated that acetylcholine can trigger dopamine release directly through dopamine axons, with in vivo studies that failed to observe such modulation, but did not examine high-effort contexts. Our findings uncover a mechanism that drives effortful behaviour through context-dependent local interactions between acetylcholine and dopamine axons.