Rafael Yuste

The integration of synaptic inputs is a fundamental function of neurons. In the traditional model, excitatory inputs are summed at the soma to generate action potentials. However, how synaptic inputs are integrated by dendrites in vivo remains poorly explored. We used intravital two-photon dendritic imaging with a genetically encoded voltage indicator (accelerated sensor of action potentials 5) together with somatic whole-cell patch clamp recordings to investigate how synaptic depolarizations are transferred to the soma in pyramidal neurons of the mouse somatosensory cortex. We studied the integration of synaptic inputs under spontaneous and sensory-evoked conditions, as well as following electrical and optogenetic stimulation. In all cases, while multiple inputs evoked measurable depolarizations in the cell body, isolated synaptic potentials were strongly attenuated. Our results suggest that isolated synaptic inputs have a minimal contribution to somatic depolarization, whereas coincident inputs within short temporal windows are more effective, indicating a regime of dendritic integration that favors coincident or clustered neuronal activity in cortical networks.

Neuronal ensembles, defined as groups of coactive neurons, are physiological modules of the cerebral cortex. Calcium imaging and optogenetics have enabled mapping and manipulating ensembles with single-cell resolution in mouse visual cortex, providing evidence of their importance. Ensembles dominate cortical activity and are generated endogenously or by sensory stimulation. Ensembles are imprinted by activating neurons synchronously and can be reactivated by “pattern completion” trigger cells. Intrinsic excitability mediates ensemble coactivation and reactivation, while UP states shield ongoing ensembles from external inputs. Neurons can belong to different ensembles, forming a combinatorial system that encodes visual stimuli accurately and stably. Ensembles contain pyramidal neurons and interneurons and inhibited “offsemble” cells. Cross inhibition makes ensembles orthogonal to one another, while astrocytic activation increases ensemble occurrence. Ensembles can last for weeks, providing a substrate for long-term information storage, and they also capture the recent history of stimulus presentation, implementing short-term memory. Optogenetic manipulation of ensembles demonstrates that they are necessary and sufficient for visual discrimination and perceptual states. Ensembles are altered in mouse models of epilepsy, schizophrenia, Alzheimer’s disease, autism spectrum disorders, and medically induced loss of consciousness. An ensemble model of the cortex is proposed in which ensembles are functional units that activate each other via trigger cells and silence nondesired ensembles by cross-inhibition. This generates a map of orthogonal attractor states, forming a computationally powerful memory and processing system. Ensembles are likely involved in many brain diseases, so manipulating them could offer avenues for new therapeutics.

Dendritic spines, small protrusions covering the dendrites of most neurons, are fundamental elements of synaptic connectivity, yet their network-level organization remains poorly understood. Here we leverage the large-scale MICrONS volumetric electron microscopy dataset of mouse primary visual cortex to explore the connectivity logic of dendritic spines across multiple spatial scales. Our analysis provides structural support for the “connectivity and diversity” hypothesis, showing that, for both excitatory and inhibitory neurons, dendritic spine density correlates with the number and the diversity of presynaptic partners. We further show that the preference for excitatory axons to form synapses on spines increases with distance from the soma. In addition, we find that spines serve as postsynaptic targets for high-output neurons (“out hubs”). We finally uncover an input/output correlation, in which neurons that share their input also preferentially target spines through their axonal output. These correlations persist across network sizes, consistent with a scale-invariant structural organization of cortical connectivity. We hypothesize that dendritic spines provide a structural substrate for network-level cooperation, by enabling distributed cortical activity to propagate through parallel synchronous chains, potentially amplified by nonlinear voltage responses at the spine and cellular level. These findings suggest that dendritic spines are key structural mechanisms for neuronal ensembles, attractor dynamics, and pattern completion.

Compulsive use of social media, when it becomes problematic, shares behavioral similarities with substance use disorders. Moreover, preliminary imaging studies have reported structural and functional brain deficits that overlap with those seen in drug addiction, supporting an addictive dimensional component underlying problematic use of social media. Current evidence is consistent with the hypothesis that excessive or maladaptive reliance on social media can trigger or exacerbate symptoms of depression and anxiety, particularly among young people, further perpetuating its use and expanding its potential adverse effects. To better understand the potential negative public health outcomes from social media, there is a pressing need for increased oversight of algorithms and business models, coupled with rigorous research to better identify the at-risk populations and understand its consequences. This could help develop evidence-based prevention and treatment interventions for those who may develop problematic use of social media.

Synaptic connectivity defines groups of neurons that engage in correlated activity during specific functional tasks. These co-active groups of neurons form ensembles, the operational units involved in, for example, sensory perception, motor coordination and memory (then called an engram). Traditionally, ensemble formation has been thought to occur via strengthening of synaptic connections via long-term potentiation (LTP) as a plasticity mechanism. This synaptic theory of memory arises from the learning rules formulated by Hebb and is consistent with many experimental observations. Here, we propose, as an alternative, that the intrinsic excitability of neurons and its plasticity constitute a second, non-synaptic mechanism that could be important for the initial formation of ensembles. Indeed, enhanced neural excitability is widely observed in multiple brain areas subsequent to behavioral learning. In cortical structures and the amygdala, excitability changes are often reported as transient, even though they can last tens of minutes to a few days. Perhaps it is for this reason that they have been traditionally considered as modulatory, merely supporting ensemble formation by facilitating LTP induction, without further involvement in memory function (memory allocation hypothesis). We here suggest−based on two lines of evidence—that beyond modulating LTP allocation, enhanced excitability plays a more fundamental role in learning. First, enhanced excitability constitutes a signature of active ensembles and, due to it, subthreshold synaptic connections become suprathreshold in the absence of synaptic plasticity (iceberg model). Second, enhanced excitability promotes the propagation of dendritic potentials toward the soma and allows for enhanced coupling of EPSP amplitude (LTP) to the spike output (and thus ensemble participation). This permissive gate model describes a need for permanently increased excitability, which seems at odds with its traditional consideration as a short-lived mechanism. We propose that longer modifications in excitability are made possible by a low threshold for intrinsic plasticity induction, suggesting that excitability might be on/off-modulated at short intervals. Consistent with this, in cerebellar Purkinje cells, excitability lasts days to weeks, which shows that in some circuits the duration of the phenomenon is not a limiting factor in the first place. In our model, synaptic plasticity defines the information content received by neurons through the connectivity network that they are embedded in. However, the plasticity of cell-autonomous excitability could dynamically regulate the ensemble participation of individual neurons as well as the overall activity state of an ensemble.

Genetically encoded calcium indicators (GECIs) such as GCaMP are invaluable tools in neuroscience to monitor neuronal activity using optical imaging. The viral transduction of GECIs is commonly used to target expression to specific brain regions, can be conveniently used with any mouse strain of interest without the need for prior crossing with a GECI mouse line, and avoids potential hazards due to the chronic expression of GECIs during development. A key requirement for monitoring neuronal activity with an indicator is that the indicator itself minimally affects activity. Here, using common adeno-associated viral (AAV) transduction procedures, we describe spatially confined aberrant Ca²⁺ microwaves slowly travelling through the hippocampus following expression of GCaMP6, GCaMP7, or R-CaMP1.07 driven by the synapsin promoter with AAV-dependent gene transfer in a titre-dependent fashion. Ca²⁺ microwaves developed in hippocampal CA1 and CA3, but not dentate gyrus nor neocortex, were typically first observed at 4 wk after viral transduction, and persisted up to at least 8 wk. The phenomenon was robust and observed across laboratories with various experimenters and setups. Our results indicate that aberrant hippocampal Ca²⁺ microwaves depend on the promoter and viral titre of the GECI, density of expression, as well as the targeted brain region. We used an alternative viral transduction method of GCaMP which avoids this artefact. The results show that commonly used Ca²⁺-indicator AAV transduction procedures can produce artefactual Ca²⁺ responses. Our aim is to raise awareness in the field of these artefactual transduction-induced Ca²⁺ microwaves, and we provide a potential solution.