Ikuko T. Smith

Dendritic spikes enhance stimulus selectivity in cortical neurons in vivo

Neuronal dendrites are electrically excitable: they can generate regenerative events such as dendritic spikes in response to sufficiently strong synaptic input. Although such events have been observed in many neuronal types, it is not well understood how active dendrites contribute to the tuning of neuronal output in vivo. Here we show that dendritic spikes increase the selectivity of neuronal responses to the orientation of a visual stimulus (orientation tuning). We performed direct patch-clamp recordings from the dendrites of pyramidal neurons in the primary visual cortex of lightly anaesthetized and awake mice, during sensory processing. Visual stimulation triggered regenerative local dendritic spikes that were distinct from back-propagating action potentials. These events were orientation tuned and were suppressed by either hyperpolarization of membrane potential or intracellular blockade of NMDA (N-methyl-d-aspartate) receptors. Both of these manipulations also decreased the selectivity of subthreshold orientation tuning measured at the soma, thus linking dendritic regenerative events to somatic orientation tuning. Together, our results suggest that dendritic spikes that are triggered by visual input contribute to a fundamental cortical computation: enhancing orientation selectivity in the visual cortex. Thus, dendritic excitability is an essential component of behaviourally relevant computations in neurons.

Wide field-of-view, multi-region, two-photon imaging of neuronal activity in the mammalian brain

Two-photon calcium imaging provides an optical readout of neuronal activity in populations of neurons with subcellular resolution. However, conventional two-photon imaging systems are limited in their field of view to ∼1 mm2, precluding the visualization of multiple cortical areas simultaneously. Here, we demonstrate a two-photon microscope with an expanded field of view (>9.5 mm2) for rapidly reconfigurable simultaneous scanning of widely separated populations of neurons. We custom designed and assembled an optimized scan engine, objective, and two independently positionable, temporally multiplexed excitation pathways. We used this new microscope to measure activity correlations between two cortical visual areas in mice during visual processing.

A Preferentially Segregated Recycling Vesicle Pool of Limited Size Supports Neurotransmission in Native Central Synapses

Summary At small central synapses, efficient turnover of vesicles is crucial for stimulus-driven transmission, but how the structure of this recycling pool relates to its functional role remains unclear. Here we characterize the organizational principles of functional vesicles at native hippocampal synapses with nanoscale resolution using fluorescent dye labeling and electron microscopy. We show that the recycling pool broadly scales with the magnitude of the total vesicle pool, but its average size is small (∼45 vesicles), highly variable, and regulated by CDK5/calcineurin activity. Spatial analysis demonstrates that recycling vesicles are preferentially arranged near the active zone and this segregation is abolished by actin stabilization, slowing the rate of activity-driven exocytosis. Our approach reveals a similarly biased recycling pool distribution at synapses in visual cortex activated by sensory stimulation in vivo. We suggest that in small native central synapses, efficient release of a limited pool of vesicles relies on their favored spatial positioning within the terminal.

The metamorphosis of the developing cerebellar microcircuit.

Research highlights ► The developing cerebellar circuit exhibits transient synaptic elements. ► Network activity and synaptic plasticity are shaped by such transient circuit elements. ► Transient circuit features may be pivotal in the development of cerebellar circuits. ► The cerebellum is a highly attractive model system for the study of circuit development.

Stream-dependent development of higher visual cortical areas

Multiple cortical areas contribute to visual processing in mice. However, the functional organization and development of higher visual areas are unclear. Here we used intrinsic signal optical imaging and two-photon calcium imaging to map visual responses in adult and developing mice. We found that visually driven activity was well correlated among higher visual areas within two distinct subnetworks resembling the dorsal and ventral visual streams. Visual response magnitude in dorsal stream areas slowly increased over the first 2 weeks of visual experience. By contrast, ventral stream areas exhibited strong responses shortly after eye opening. Neurons in a dorsal stream area showed little change in their tuning sharpness to oriented gratings while those in a ventral stream area increased stimulus selectivity and expanded their receptive fields significantly. Together, these findings provide a functional basis for grouping subnetworks of mouse visual areas and revealed stream differences in the development of receptive field properties.

Wide field-of-view, twin-region two-photon imaging across extended cortical networks

We demonstrate a two-photon imaging system with corrected optics including a custom objective that provides cellular resolution across a 3.5 mm field of view (9.6 mm2). Temporally multiplexed excitation pathways can be independently repositioned in XY and Z to simultaneously image regions within the expanded field of view. We used this new imaging system to measure activity correlations between neurons in different cortical areas in awake mice.Two-photon calcium imaging can provide an optical readout of spiking activity with cellular resolution, revealing activity correlations and population dynamics within an individual cortical area. However, many conventional two-photon imaging systems maintain cellular resolution only over a field of view (FOV) ~500 μm wide. This FOV precludes the simultaneous visualization of multiple cortical areas, which can be spread over a millimeter or more. Thus it has not typically been possible to use two-photon calcium imaging to measure neural activity dynamics and correlations between cortical areas. Here, we demonstrate a new two-photon imaging system that is designed to image neural activity in multiple cortical areas simultaneously. Low aberration scan optics support an expanded FOV with cellular resolution that is 1.4 to 1.8 mm wide using a commercially available objective, and 3.5 mm wide using a custom objective. Two independently positionable multiplexed excitation pathways simultaneously image any two regions within the expanded FOV. We use this new imaging system to measure activity correlations between two cortical visual areas in mice.

Bcl-xL Is Essential for the Survival and Function of Differentiated Neurons in the Cortex That Control Complex Behaviors

Apoptosis plays an essential role during brain development, yet the precise mechanism by which this pathway is regulated in the brain remains unknown. In particular, mammalian cells are known to express multiple anti-apoptotic Bcl-2 family proteins. However, the cells of the developing brain could also exist in a primed state in which the loss of a single anti-apoptotic Bcl-2 family protein is sufficient to trigger apoptosis. Here, we examined the critical role of Bcl-xL, an anti-apoptotic protein, during brain development. Using conditional knock-out mice in which Bcl-xL is deleted in neural progenitor cells (Bcl-xLEmx1–Cre), we show that the loss of Bcl-xL is not sufficient to trigger apoptosis in these proliferating progenitors. In contrast, specific populations of postmitotic neurons derived from these progenitors, including upper layer cortical neurons and the CA1–CA3 regions of the hippocampus, were acutely dependent on Bcl-xL. Consistent with this finding, deletion of Bcl-xL selectively in the postmitotic neurons in the brain (Bcl-xLNex–Cre) also resulted in similar patterns of apoptosis. This Bcl-xL deficiency-induced neuronal death was a consequence of activation of the apoptotic pathway, because the cell death was rescued with codeletion of the proapoptotic proteins Bax and Bak. Importantly, the loss of these Bcl-xL-dependent neurons led to severe neurobehavioral abnormalities, including deficits in motor learning, hyperactivity, and increased risk-taking and self-injurious behaviors. Together, our results identify a population of neurons in the developing brain that are acutely dependent on Bcl-xL during the peak period of synaptic connectivity that are important for the establishment of higher-order complex behaviors. SIGNIFICANCE STATEMENT Although Bcl-xL is known to inhibit apoptosis, exactly which cells in the brain are dependent on Bcl-xL has remained unclear because of the embryonic lethality of mice globally deleted for Bcl-xL. Here, we conditionally deleted Bcl-xL in the brain and found that this did not result in widespread apoptosis in the proliferating progenitors. Instead, Bcl-xL deficiency induced apoptosis in a select population of differentiated neurons predominantly in the early postnatal stages. Importantly, these Bcl-xL-dependent neurons are not essential for survival of the organism but instead regulate complex behaviors. Our results show that the selective loss of these Bcl-xL-dependent neurons results in mice exhibiting severe neurobehavioral abnormalities, including self-injurious and risk-taking behaviors, hyperactivity, and learning and memory defects.

Getting it through your thick skull

Neural activity up to 3 mm deep in mouse brain can now be inhibited optogenetically through the intact cranium with a red-shifted opsin called Jaws.

Mice use robust and common strategies to discriminate natural scenes

Mice use vision to navigate and avoid predators in natural environments. However, their visual systems are compact compared to other mammals, and it is unclear how well mice can discriminate ethologically relevant scenes. Here, we examined natural scene discrimination in mice using an automated touch-screen system. We estimated the discrimination difficulty using the computational metric structural similarity (SSIM), and constructed psychometric curves. However, the performance of each mouse was better predicted by the mean performance of other mice than SSIM. This high inter-mouse agreement indicates that mice use common and robust strategies to discriminate natural scenes. We tested several other image metrics to find an alternative to SSIM for predicting discrimination performance. We found that a simple, primary visual cortex (V1)-inspired model predicted mouse performance with fidelity approaching the inter-mouse agreement. The model involved convolving the images with Gabor filters, and its performance varied with the orientation of the Gabor filter. This orientation dependence was driven by the stimuli, rather than an innate biological feature. Together, these results indicate that mice are adept at discriminating natural scenes, and their performance is well predicted by simple models of V1 processing.

Erratam to Synapse-specific control of experience-dependent plasticity by presynaptic NMDA receptors[Neuron, 83, (2014), 879-893]

In the original version of this paper, the first two columns in Figure 2D inadvertently included the wrong data sets, although the corresponding results, statistical analyses, and figure legend were correct. The data in Figure 2D has now been corrected online.