Large scale, simultaneous, chronic neural recordings from multiple brain areas.

Understanding how brain activity is related to animal behavior requires measuring multi-area interactions on multiple timescales. However, methods to perform chronic, simultaneous recordings of neural activity from many brain areas are lacking. Here, we introduce a novel approach for independent chronic probe implantation that enables flexible, simultaneous interrogation of neural activity from many brain regions during head restrained or freely moving behavior. The approach, that we called indie (independent dovetail implants for electrophysiology), enables repeated retrieval and reimplantation of probes. The chronic implantation approach can be combined with other modalities such as skull clearing for cortex wide access and optogenetics with optic fibers. Using this approach, we implanted 6 probes chronically in one hemisphere of the mouse brain. The implant is lightweight, allows flexible targeting with different angles, and offers enhanced stability. Our approach broadens the applications of chronic recording while retaining its main advantages over acute recordings (superior stability, longitudinal monitoring of activity and freely moving interrogations) and provides an appealing venue to study processes not accessible by acute methods, such as the neural substrate of learning across multiple areas.

Engaged decision-makers align spontaneous movements to stereotyped task demands.

Neural activity during sensory-guided decision-making is strongly modulated by animal movements. Although the impact of movements on neural activity is now well-documented, the relationship between these movements and behavioral performance remains unclear. To understand this relationship, we first tested whether the magnitude of animal movements (assessed with posture analysis of 28 individual body parts) was correlated with performance on a perceptual decision-making task. No strong relationship was present, suggesting that task performance is not affected by the magnitude of movements. We then tested if performance instead depends on movement timing and trajectory. We partitioned the movements into two groups: task-aligned movements that were well predicted by task events (such as the onset of the sensory stimulus or choice) and task independent movement (TIM) that occurred independently of task events. TIM had a reliable, inverse correlation with performance in head-restrained mice and freely moving rats. This argues that certain movements, defined by their timing and trajectories relative to task events, might indicate periods of engagement or disengagement in the task. To confirm this, we compared TIM to the latent behavioral states recovered by a hidden Markov model with Bernoulli generalized linear model observations (GLM-HMM) and found these, again, to be inversely correlated. Finally, we examined the impact of these behavioral states on neural activity measured with widefield calcium imaging. The engaged state was associated with widespread increased activity, particularly during the delay period. However, a linear encoding model could account for more overall variance in neural activity in the disengaged state. Our analyses demonstrate that this is likely because uninstructed movements had a greater impact on neural activity during disengagement. Taken together, these findings suggest that TIM is informative about the internal state of engagement, and that movements and state together have a major impact on neural activity.

Voltage imaging reveals that hippocampal interneurons tune memory-encoding pyramidal sequences.

Hippocampal spiking sequences encode and link behavioral information across time. How inhibition sculpts these sequences remains unknown. We performed longitudinal voltage imaging of CA1 parvalbumin- and somatostatin-expressing interneurons in mice during an odor-cued working memory task, before and after training. During this task, pyramidal odor-specific sequences encode the cue throughout a delay period. In contrast, most interneurons encoded odor delivery, but not odor identity, nor delay time. Population inhibition was stable across days, with constant field turnover, though some cells retained odor-responses for days. At odor onset, a brief, synchronous burst of parvalbumin cells was followed by widespread membrane hyperpolarization and then rebound theta-paced spiking, synchronized across cells. Two-photon calcium imaging revealed that most pyramidal cells were suppressed throughout the odor. Positive pyramidal odor-responses coincided with interneuronal rebound spiking; otherwise, they had weak odor-selectivity. Therefore, inhibition increases the signal-to-noise ratio of cue representations, which is crucial for entraining downstream targets.

Dysfunction of parvalbumin neurons in the cerebellar nuclei produces an action tremor.

Essential tremor is a common brain disorder affecting millions of people, yet the neuronal mechanisms underlying this prevalent disease remain elusive. Here, we showed that conditional deletion of synaptotagmin-2, the fastest Ca2+ sensor for synaptic neurotransmitter release, from parvalbumin neurons in mice caused an action tremor syndrome resembling the core symptom of essential tremor patients. Combining brain region-specific and cell type-specific genetic manipulation methods, we found that deletion of synaptotagmin-2 from excitatory parvalbumin-positive neurons in cerebellar nuclei was sufficient to generate an action tremor. The synaptotagmin-2 deletion converted synchronous into asynchronous neurotransmitter release in projections from cerebellar nuclei neurons onto gigantocellular reticular nucleus neurons, which might produce an action tremor by causing signal oscillations during movement. The tremor was rescued by completely blocking synaptic transmission with tetanus toxin in cerebellar nuclei, which also reversed the tremor phenotype in the traditional harmaline-induced essential tremor model. Using a promising animal model for action tremor, our results thus characterized a synaptic circuit mechanism that may underlie the prevalent essential tremor disorder.

A central amygdala to zona incerta projection is required for acquisition and remote recall of conditioned fear memory.

The formation and retrieval of conditioned fear memories critically depend on the amygdala. Here we identify an inhibitory projection from somatostatin-positive neurons in the central amygdala to parvalbumin-positive neurons in the zona incerta that is required for both recent and remote fear memories. Thus, the amygdala inhibitory input to parvalbumin-positive neurons in the zona incerta, a nucleus not previously implicated in fear memory, is an essential component of the fear memory circuitry.