Resilience of network activity in preconditioned neurons exposed to 'stroke-in-a-dish' insults.

Exposing cultured cortical neurons to stimulatory agents - the K+ channel blocker 4-aminopyridine (4-ap), and the GABAA receptor antagonist bicuculline (bic) - for 48 h induces down-regulated synaptic scaling, and preconditions neurons to withstand subsequent otherwise lethal 'stroke-in-a-dish' insults; however, the degree to which usual neuronal function remains is unknown. As a result, multi-electrode array and patch-clamp electrophysiological techniques were employed to characterize hallmarks of spontaneous synaptic activity over a 12-day preconditioning/insult experiment. Spiking frequency increased 8-fold immediately upon 4-ap/bic treatment but declined within the 48 h treatment window to sub-baseline levels that persisted long after washout. Preconditioning resulted in key markers of network activity - spiking frequency, bursting and avalanches - being impervious to an insult. Surprisingly, preconditioning resulted in higher peak NMDA mEPSC amplitudes, resulting in a decrease in the ratio of AMPA:NMDA mEPSC currents, suggesting a relative increase in synaptic NMDA receptors. An investigation of a broad mRNA panel of excitatory and inhibitory signaling mediators indicated preconditioning rapidly up-regulated GABA synthesis (GAD67) and BDNF, followed by up-regulation of neuronal activity-regulated pentraxin and down-regulation of presynaptic glutamate release (VGLUT1). Preconditioning also enhanced surface expression of GLT-1, which persisted following an insult. Overall, preconditioning resulted in a reduced spiking frequency which was impervious to subsequent exposure to 'stroke-in-a-dish' insults, a phenotype initiated predominantly by up-regulation of inhibitory neurotransmission, a lower neuronal postsynaptic AMPA: NMDA receptor ratio, and trafficking of GLT-1 to astrocyte plasma membranes.

A community-based transcriptomics classification and nomenclature of neocortical cell types.

To understand the function of cortical circuits, it is necessary to catalog their cellular diversity. Past attempts to do so using anatomical, physiological or molecular features of cortical cells have not resulted in a unified taxonomy of neuronal or glial cell types, partly due to limited data. Single-cell transcriptomics is enabling, for the first time, systematic high-throughput measurements of cortical cells and generation of datasets that hold the promise of being complete, accurate and permanent. Statistical analyses of these data reveal clusters that often correspond to cell types previously defined by morphological or physiological criteria and that appear conserved across cortical areas and species. To capitalize on these new methods, we propose the adoption of a transcriptome-based taxonomy of cell types for mammalian neocortex. This classification should be hierarchical and use a standardized nomenclature. It should be based on a probabilistic definition of a cell type and incorporate data from different approaches, developmental stages and species. A community-based classification and data aggregation model, such as a knowledge graph, could provide a common foundation for the study of cortical circuits. This community-based classification, nomenclature and data aggregation could serve as an example for cell type atlases in other parts of the body.

Neuronal hypertrophy dampens neuronal intrinsic excitability and stress responsiveness during chronic stress.

KEY POINTS: The hypothalamic-pituitary-adrenal (HPA) axis habituates to repeated stress exposure. We studied hypothalamic corticotropin-releasing hormone (CRH) neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. The intrinsic excitability of CRH neurons decreased after repeated stress in a time course that coincided with the development of HPA axis habituation. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load and dampened membrane depolarization in response to the influx of positive charge. We report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for HPA axis habituation. ABSTRACT: Encountering a stressor immediately activates the hypothalamic-pituitary-adrenal (HPA) axis, but this stereotypic stress response also undergoes experience-dependent adaptation. Despite the biological and clinical importance, how the brain adjusts stress responsiveness in the long term remains poorly understood. We studied hypothalamic corticotropin-releasing hormone neurons that form the apex of the HPA axis in a mouse model of stress habituation using repeated restraint. Using patch-clamp electrophysiology in acute slices, we found that the intrinsic excitability of these neurons substantially decreased after daily repeated stress in a time course that coincided with their loss of stress responsiveness in vivo. This intrinsic excitability plasticity co-developed with an expansion of surface membrane area, which increased a passive electric load, and dampened membrane depolarization in response to the influx of positive charge. Multiphoton imaging and electron microscopy revealed that repeated stress augmented ruffling of the plasma membrane, suggesting an ultrastructural plasticity that may efficiently accommodate the membrane area expansion. Overall, we report a novel structure-function relationship for intrinsic excitability plasticity as a neural correlate for adaptation of the neuroendocrine stress response.

Changes in the Proportion of Inhibitory Interneuron Types from Sensory to Executive Areas of the Primate Neocortex: Implications for the Origins of Working Memory Representations.

Neuronal spiking activity encoding working memory (WM) is robust in primate association cortices but weak or absent in early sensory cortices. This may be linked to changes in the proportion of neuronal types across areas that influence circuits' ability to generate recurrent excitation. We recorded neuronal activity from areas middle temporal (MT), medial superior temporal (MST), and the lateral prefrontal cortex (LPFC) of monkeys performing a WM task and classified neurons as narrow (NS) and broad spiking (BS). The ratio NS/BS decreased from MT > MST > LPFC. We analyzed the Allen Institute database of ex vivo mice/human intracellular recordings to interpret our data. Our analysis suggests that NS neurons correspond to parvalbumin (PV) or somatostatin (SST) interneurons while BS neurons are pyramidal (P) cells or vasoactive intestinal peptide (VIP) interneurons. We labeled neurons in monkey tissue sections of MT/MST and LPFC and found that the proportion of PV in cortical layers 2/3 decreased, while the proportion of CR cells increased from MT/MST to LPFC. Assuming that primate CR/CB/PV cells perform similar computations as mice VIP/SST/PV cells, our results suggest that changes in the proportion of CR and PV neurons in layers 2/3 cells may favor the emergence of activity encoding WM in association areas.

Non-monotonic accumulation of spike time variance during membrane potential oscillations.

A spike-phase neural code has been proposed as a mechanism to encode stimuli based on the precise timing of spikes relative to the phase of membrane potential oscillations. This form of coding has been reported in both in vivo and in vitro experiments across several regions of the brain, yet there are concerns that such precise timing may be compromised by an effect referred to as variance accumulation, wherein spike timing variance increases over the phase of an oscillation. Here, we provide a straightforward explanation of this effect based on the theoretical spike time variance. The proposed theory is consistent with recordings of mitral neurons. It shows that spike time variance can increase in a nonlinear fashion with spike number, in a way that is dependent upon the frequency and amplitude of the oscillation. Further, non-monotonic accumulation of variance can arise from different combinations of oscillation parameters. Nonlinear accumulation sometimes leads to lower variance than that of a mean rate-matched homogeneous Poisson process, particularly for spikes that occur in later phases of oscillation. However, such an advantage is limited to a narrow range of oscillation amplitudes and frequencies. These results suggest fundamental constraints on spike-phase coding, and reveal how certain spikes in a sequence may exhibit increased firing time precision relative to their neighbors.

Burst predicting neurons survive an in vitro glutamate injury model of cerebral ischemia.

Neuronal activity in vitro exhibits network bursts characterized by brief periods of increased spike rates. Recent work shows that a subpopulation of neurons reliably predicts the occurrence of network bursts. Here, we examined the role of burst predictors in cultures undergoing an in vitro model of cerebral ischemia. Dissociated primary cortical neurons were plated on multielectrode arrays and spontaneous activity was recorded at 17 days in vitro (DIV). This activity was characterized by neuronal avalanches where burst statistics followed a power law. We identified burst predictors as channels that consistently fired immediately prior to network bursts. The timing of these predictors relative to bursts followed a skewed distribution that differed sharply from a null model based on branching ratio. A portion of cultures were subjected to an excitotoxic insult (DIV 18). Propidium iodine and fluorescence imaging confirmed cell death in these cultures. While the insult did not alter the distribution of avalanches, it resulted in alterations in overall spike rates. Burst predictors, however, maintained baseline levels of activity. The resilience of burst predictors following excitotoxic insult suggests a key role of these units in maintaining network activity following injury, with implications for the selective effects of ischemia in the brain.