Christopher D. Harvey

Intracellular dynamics of hippocampal place cells during virtual navigation

Hippocampal place cells encode spatial information in rate and temporal codes. To examine the mechanisms underlying hippocampal coding, here we measured the intracellular dynamics of place cells by combining in vivo whole-cell recordings with a virtual-reality system. Head-restrained mice, running on a spherical treadmill, interacted with a computer-generated visual environment to perform spatial behaviours. Robust place-cell activity was present during movement along a virtual linear track. From whole-cell recordings, we identified three subthreshold signatures of place fields: an asymmetric ramp-like depolarization of the baseline membrane potential, an increase in the amplitude of intracellular theta oscillations, and a phase precession of the intracellular theta oscillation relative to the extracellularly recorded theta rhythm. These intracellular dynamics underlie the primary features of place-cell rate and temporal codes. The virtual-reality system developed here will enable new experimental approaches to study the neural circuits underlying navigation.

Locally dynamic synaptic learning rules in pyramidal neuron dendrites

Long-term potentiation (LTP) of synaptic transmission underlies aspects of learning and memory. LTP is input-specific at the level of individual synapses, but neural network models predict interactions between plasticity at nearby synapses. Here we show in mouse hippocampal pyramidal cells that LTP at individual synapses reduces the threshold for potentiation at neighbouring synapses. After input-specific LTP induction by two-photon glutamate uncaging or by synaptic stimulation, subthreshold stimuli, which by themselves were too weak to trigger LTP, caused robust LTP and spine enlargement at neighbouring spines. Furthermore, LTP induction broadened the presynaptic–postsynaptic spike interval for spike-timing-dependent LTP within a dendritic neighbourhood. The reduction in the threshold for LTP induction lasted ∼10 min and spread over ∼10 µm of dendrite. These local interactions between neighbouring synapses support clustered plasticity models of memory storage and could allow for the binding of behaviourally linked information on the same dendritic branch.

Functional imaging of hippocampal place cells at cellular resolution during virtual navigation

Spatial navigation is often used as a behavioral task in studies of the neuronal circuits that underlie cognition, learning and memory in rodents. The combination of in vivo microscopy with genetically encoded indicators has provided an important new tool for studying neuronal circuits, but has been technically difficult to apply during navigation. Here we describe methods for imaging the activity of neurons in the CA1 region of the hippocampus with subcellular resolution in behaving mice. Neurons that expressed the genetically encoded calcium indicator GCaMP3 were imaged through a chronic hippocampal window. Head-restrained mice performed spatial behaviors in a setup combining a virtual reality system and a custom-built two-photon microscope. We optically identified populations of place cells and determined the correlation between the location of their place fields in the virtual environment and their anatomical location in the local circuit. The combination of virtual reality and high-resolution functional imaging should allow a new generation of studies to investigate neuronal circuit dynamics during behavior.

Choice-specific sequences in parietal cortex during a virtual-navigation decision task

The posterior parietal cortex (PPC) has an important role in many cognitive behaviours; however, the neural circuit dynamics underlying PPC function are not well understood. Here we optically imaged the spatial and temporal activity patterns of neuronal populations in mice performing a PPC-dependent task that combined a perceptual decision and memory-guided navigation in a virtual environment. Individual neurons had transient activation staggered relative to one another in time, forming a sequence of neuronal activation spanning the entire length of a task trial. Distinct sequences of neurons were triggered on trials with opposite behavioural choices and defined divergent, choice-specific trajectories through a state space of neuronal population activity. Cells participating in the different sequences and at distinct time points in the task were anatomically intermixed over microcircuit length scales (<100 micrometres). During working memory decision tasks, the PPC may therefore perform computations through sequence-based circuit dynamics, rather than long-lived stable states, implemented using anatomically intermingled microcircuits.

The Spread of Ras Activity Triggered by Activation of a Single Dendritic Spine

In neurons, individual dendritic spines isolate N-methyl-d-aspartate (NMDA) receptor–mediated calcium ion (Ca2+) accumulations from the dendrite and other spines. However, the extent to which spines compartmentalize signaling events downstream of Ca2+ influx is not known. We combined two-photon fluorescence lifetime imaging with two-photon glutamate uncaging to image the activity of the small guanosine triphosphatase Ras after NMDA receptor activation at individual spines. Induction of long-term potentiation (LTP) triggered robust Ca2+-dependent Ras activation in single spines that decayed in ∼5 minutes. Ras activity spread over ∼10 micrometers of dendrite and invaded neighboring spines by diffusion. The spread of Ras-dependent signaling was necessary for the local regulation of the threshold for LTP induction. Thus, Ca2+-dependent synaptic signals can spread to couple multiple synapses on short stretches of dendrite.

A genetically encoded fluorescent sensor of ERK activity.

The activity of the ERK has complex spatial and temporal dynamics that are important for the specificity of downstream effects. However, current biochemical techniques do not allow for the measurement of ERK signaling with fine spatiotemporal resolution. We developed a genetically encoded, FRET-based sensor of ERK activity (the extracellular signal-regulated kinase activity reporter, EKAR), optimized for signal-to-noise ratio and fluorescence lifetime imaging. EKAR selectively and reversibly reported ERK activation in HEK293 cells after epidermal growth factor stimulation. EKAR signals were correlated with ERK phosphorylation, required ERK activity, and did not report the activities of JNK or p38. EKAR reported ERK activation in the dendrites and nucleus of hippocampal pyramidal neurons in brain slices after theta-burst stimuli or trains of back-propagating action potentials. EKAR therefore permits the measurement of spatiotemporal ERK signaling dynamics in living cells, including in neuronal compartments in intact tissues.

Supersensitive Ras activation in dendrites and spines revealed by two-photon fluorescence lifetime imaging

To understand the biochemical signals regulated by neural activity, it is necessary to measure protein-protein interactions and enzymatic activity in neuronal microcompartments such as axons, dendrites and their spines. We combined two-photon excitation laser scanning with fluorescence lifetime imaging to measure fluorescence resonance energy transfer at high resolutions in brain slices. We also developed sensitive fluorescent protein–based sensors for the activation of the small GTPase protein Ras with slow (FRas) and fast (FRas-F) kinetics. Using FRas-F, we found in CA1 hippocampal neurons that trains of back-propagating action potentials rapidly and reversibly activated Ras in dendrites and spines. The relationship between firing rate and Ras activation was highly nonlinear (Hill coefficient ∼5). This steep dependence was caused by a highly cooperative interaction between calcium ions (Ca2+) and Ras activators. The Ras pathway therefore functions as a supersensitive threshold detector for neural activity and Ca2+ concentration.

Recurrent Network Models of Sequence Generation and Memory

Sequential activation of neurons is a common feature of network activity during a variety of behaviors, including working memory and decision making. Previous network models for sequences and memory emphasized specialized architectures in which a principled mechanism is pre-wired into their connectivity. Here we demonstrate that, starting from random connectivity and modifying a small fraction of connections, a largely disordered recurrent network can produce sequences and implement working memory efficiently. We use this process, called Partial In-Network Training (PINning), to model and match cellular resolution imaging data from the posterior parietal cortex during a virtual memory-guided two-alternative forced-choice task. Analysis of the connectivity reveals that sequences propagate by the cooperation between recurrent synaptic interactions and external inputs, rather than through feedforward or asymmetric connections. Together our results suggest that neural sequences may emerge through learning from largely unstructured network architectures.

Regulation of Alternative Splicing by SRrp86 and Its Interacting Proteins

ABSTRACT SRrp86 is a unique member of the SR protein superfamily containing one RNA recognition motif and two serine-arginine (SR)-rich domains separated by an unusual glutamic acid-lysine (EK)-rich region. Previously, we showed that SRrp86 could regulate alternative splicing by both positively and negatively modulating the activity of other SR proteins and that the unique EK domain could inhibit both constitutive and alternative splicing. These functions were most consistent with the model in which SRrp86 functions by interacting with and thereby modulating the activity of target proteins. To identify the specific proteins that interact with SRrp86, we used a yeast two-hybrid library screen and immunoprecipitation coupled to mass spectrometry. We show that SRrp86 interacts with all of the core SR proteins, as well as a subset of other splicing regulatory proteins, including SAF-B, hnRNP G, YB-1, and p72. In contrast to previous results that showed activation of SRp20 by SRrp86, we now show that SAF-B, hnRNP G, and 9G8 all antagonize the activity of SRrp86. Overall, we conclude that not only does SRrp86 regulate SR protein activity but that it is, in turn, regulated by other splicing factors to control alternative splice site selection.

History-dependent variability in population dynamics during evidence accumulation in cortex

We studied how the posterior parietal cortex combines new information with ongoing activity dynamics as mice accumulate evidence during a virtual navigation task. Using new methods to analyze population activity on single trials, we found that activity transitioned rapidly between different sets of active neurons. Each event in a trial, whether an evidence cue or a behavioral choice, caused seconds-long modifications to the probabilities that govern how one activity pattern transitions to the next, forming a short-term memory. A sequence of evidence cues triggered a chain of these modifications resulting in a signal for accumulated evidence. Multiple distinguishable activity patterns were possible for the same accumulated evidence because representations of ongoing events were influenced by previous within- and across-trial events. Therefore, evidence accumulation need not require the explicit competition between groups of neurons, as in winner-take-all models, but could instead emerge implicitly from general dynamical properties that instantiate short-term memory.