Neuronal ensembles, coactive groups of neurons found in spontaneous and evoked cortical activity, are causally related to memories and perception, but it still unknown how stable or flexible they are over time. We used two-photon multiplane calcium imaging to track over weeks the activity of the same pyramidal neurons in layer 2/3 of the visual cortex from awake mice and recorded their spontaneous and visually evoked responses. Less than half of the neurons were commonly active across any two imaging sessions. These 'common neurons' formed stable ensembles lasting weeks, but some ensembles were also transient and appeared only in one single session. Stable ensembles preserved ~68 % of their neurons up to 46 days, our longest imaged period, and these 'core' cells had stronger functional connectivity. Our results demonstrate that neuronal ensembles can last for weeks and could, in principle, serve as a substrate for long-lasting representation of perceptual states or memories.

Neuronal ensembles are coactive groups of cortical neurons, found in spontaneous and evoked activity, that can mediate perception and behavior. To understand the mechanisms that lead to the formation of ensembles, we co-activated layer 2/3 pyramidal neurons in brain slices from mouse visual cortex, in animals of both sexes, replicating in vitro an optogenetic protocol to generate ensembles in vivo. Using whole-cell and perforated patch-clamp pair recordings we find that, after optogenetic or electrical stimulation, coactivated neurons increase their correlated activity, a hallmark of ensemble formation. Coactivated neurons showed small biphasic changes in presynaptic plasticity, with an initial depression followed by a potentiation after a recovery period. Optogenetic and electrical stimulation also induced significant increases in frequency and amplitude of spontaneous EPSPs, even after single-cell stimulation. In addition, we observed unexpected strong and persistent increases in neuronal excitability after stimulation, with increases in membrane resistance and reductions in spike threshold. A pharmacological agent that blocks changes in membrane resistance can revert this effect. These significant increases in excitability may partly generate the observed biphasic synaptic plasticity. We propose that cell-intrinsic changes in excitability are involved in the formation of neuronal ensembles. We propose an 'iceberg' model, by which increased neuronal excitability makes subthreshold connections suprathreshold, enhancing the effect of already existing synapses, and generating a new neuronal ensemble.

Coactive neuronal ensembles are found in spontaneous and evoked cortical activity and are thought to participate in the internal representation of memories, perceptions, and mental states. In mouse visual cortex, ensembles can be optogenetically imprinted and are causally related to visual percepts, but it is still unknown how stable they are over time. Using two-photon volumetric microscopy, we performed calcium imaging over several weeks of the same neuronal populations in layer 2/3 of visual cortex of awake mice, tracking over time the activity of the same neurons in response to visual stimuli and under spontaneous activity. Only a small number of neurons remained active across days. Analyzing them, we found both stable ensembles, lasting up to 46 days, and transient ones, observed during only one imaging session. The majority of ensembles in visually-evoked activity were stable, whereas in spontaneous activity similar numbers of stable and transient ensembles were found. Among stable ensembles, more than 60 % of neurons still belonged to the same ensemble even after several weeks. These core ensemble cells had stronger functional connectivity than neurons that stopped belonging to the ensemble. Our results demonstrate that spontaneous and evoked neuronal ensembles can last weeks, providing a neuronal mechanism for the long-lasting representation of perceptual states or memories.

Spontaneously hypertensive rats (SHR) are widely used as model to investigate the pathophysiological mechanisms of essential hypertension. Catecholamine plasma levels are elevated in SHR, suggesting alterations of the sympathoadrenal axis. The residual hypertension in sympathectomized SHR is reduced after demedullation, suggesting a dysfunction of the adrenal medulla. Intact adrenal glands exposed to acetylcholine or high K+ release more catecholamine in SHR than in normotensive Wistar Kyoto (WKY) rats, and adrenal chromaffin cells (CCs) from SHR secrete more catecholamines than CCs from WKY rats. Since Ca2+ entry through voltage-gated Ca2+ channels (VGCC) triggers exocytosis, alterations in the functional properties of these channels might underlie the enhanced catecholamine release in SHR. This study compares the electrophysiological properties of VGCC from CCs in acute adrenal slices from WKY rats and SHR at an early stage of hypertension. No significant differences were found in the macroscopic Ca2+ currents (current density, I–V curve, voltage dependence of activation and inactivation, kinetics) between CCs of SHR and WKY rats, suggesting that Ca2+ entry through VGCC is not significantly different between these strains, at least at early stages of hypertension. Ca2+ buffering, sequestration and extrusion mechanisms, as well as Ca2+ release from intracellular stores, must now be evaluated to determine if alterations in their function can explain the enhanced catecholamine secretion reported in CCs from SHR.

Neuronal ensembles are coactive groups of cortical neurons, found in both spontaneous and evoked activity, which can mediate perception and behavior (Cossart et al., 2003; Buzsáki, 2010; Carrillo-Reid et al., 2019; Marshel et al., 2019). To understand the mechanism that lead to the formation of neuronal ensembles, we generated optogenetically artificial photo-ensembles in layer 2/3 pyramidal neurons in brain slices of mouse visual cortex from both sexes, replicating an optogenetic protocol to generate ensembles in vivo by simultaneous coactivation of neurons (Carrillo-Reid et al. 2016). Using whole-cell voltage-clamp recordings from individual neurons and connected pairs, we find that synaptic properties of photostimulated were surprisingly unaffected, without any signs of Hebbian plasticity. However, extracellular recordings revealed that photostimulation induced strong increases in spontaneous action potential activity. Using perforated patch clamp recordings, we find increases in neuronal excitability, accompanied by increases in membrane resistance and a reduction in spike threshold. We conclude that the formation of neuronal ensemble by photostimulation is mediated by cell-intrinsic changes in excitability, rather than by Hebbian synaptic plasticity or changes in local synaptic connectivity. We propose an “iceberg” model, by which increased neuronal excitability makes subthreshold connections become suprathreshold, increasing the functional effect of already existing synapses and generating a new neuronal ensemble.