Our goal is to understand how neural circuits generate sensory perception and behavior. To address this question we use a combination of molecular genetic, in vivo imaging, computational and behavioral approaches, and we focus our efforts on determining fundamental functional properties of neural networks in the mouse olfactory system.
A central question in neuroscience is how sensory stimuli are detected and processed by neural circuits in the brain to generate sensory perception and behavior. Our laboratory has recently developed new molecular genetic and viral approaches that allow us to target and manipulate defined neural cell types in the olfactory cortex of mice. Furthermore, we have identified, using in vivo two-photon microscopy, electrophysiological recordings and computational approaches, fundamental principles of odor information coding in cortical neural networks. These recent advances open up new opportunities to explore how diverse neural cell types contribute to odor information coding in the cortex, and how this information is transmitted to downstream target areas involved in sensory integration, cognition, and motor control. Finally, we are interested in how learning and experience alter olfactory neural network functions and behavior.
The olfactory system of mice provides a simple, tractable model system of outstanding ethological importance. Furthermore, olfactory neural circuits are particularly vulnerable to aging and neurodegenerative disease, thus representing a highly relevant experimental model system for clinical and translational neuroscience research.
Odor coding in cortical neural circuits
Olfactory perception and behaviors critically depend on our ability to detect and discriminate odorant molecules, identify changes in the composition of odorant mixtures, and accurately track dynamic fluctuations in odor concentration. Using in vivo 2-photon calcium imaging and population coding analyses, we have recently shown that odor identity can accurately be decoded from ensembles of neurons in the mouse piriform cortex (Roland et al., 2017). We have further proposed a model that distinct perceptual features of odors, such as odor identity, intensity, or valence, are encoded in independent subnetworks of neurons in the olfactory cortex.
An important question is whether odor information encoded in piriform ensembles is selectively transmitted to different downstream brain target, and how information routing is shaped by learning and experience. We address these questions by recording neuronal activity in the piriform cortex of awake, behaving mice using gradient-index (GRIN) lens technology and 2-photon and mini-endoscope microscopy. We monitor the activity of specific subsets of neurons by labelling neurons based on their molecular identity or projection targets, using intersectional viral/genetic gene targeting.
Two-photon imaging of piriform cortex in awake, behaving mice. We inject a jGCaMP7f-encoding adeno-associated virus into piriform cortex and implant a GRIN lens above the injection site, as shown schematically on the left. This allows us to chronically image the activity of hundreds of neurons across multiple z-planes using two-photon microscopy. One representative plane is shown in the video on the right (~5X actual speed, discontinuous recording, motion corrected).
Neuronal circuit assembly
A defining characteristic of the mammalian cortex is its enormous diversity of cell types. However, the molecular mechanisms underlying neuronal lineage specification and circuit assembly of the olfactory (piriform) cortex, an evolutionarily old, allocortical structure, remain poorly understood. Our lab has recently identified genes selectively expressed in different layers of piriform cortex (Diodato et al., 2016). Neuronal tracing experiments revealed that these layer-specific piriform genes mark different subclasses of neurons that project to distinct target areas. Interestingly, these molecular signatures of connectivity are maintained in mutant mice in which neural positioning is scrambled.
Our results highlight the importance of intrinsic genetic programs that specify a neuron’s molecular identity and connectivity. Using single cell RNA sequencing, whole cleared brain preparations combined with light sheet microscopy, computational modeling, and genetic manipulations in mice, we investigate cortical neuronal circuit development. Our experiments aim at uncovering key gene regulatory network mechanisms for cortical neuronal lineage specification and circuit evolution.
Single nuclei RNA sequencing of adult mouse piriform cortex. Louvain clustering method, which shows gene expression profiles of 461 piriform nuclei, identifies 7 clusters of distinct cell types. Each dot representing one nucleus is colored by cluster membership and labeled post hoc according to known cell type-specific markers.
Light sheet microscopy image of cleared mouse brain at post natal day 7 (P7). We employ cleared whole-brain preparations (IDISCO+) combined with immunohistochemistry for candidate genes. Resulting expression is imaged using light sheet microscopy. The figure above shows expression of the transcription factor CTIP2 (in red).
Olfactory learning and behavior
Odor memories are exceptionally robust and essential for animal survival. The olfactory cortex has long been hypothesized to encode odor memories, yet the cellular substrates for olfactory learning and memory remained unknown. Using intersectional, cFos-based genetic manipulations (‘‘Fos tagging’’), we have shown that olfactory fear conditioning activates sparse and distributed ensembles of neurons in the mouse piriform cortex (Meissner-Bernard et al., 2019). Chemogenetic silencing of these Fos-tagged piriform ensembles selectively interferes with odor fear memory retrieval but does not compromise basic odor detection and discrimination. Furthermore, chemogenetic reactivation of piriform neurons that were Fos tagged during olfactory fear conditioning causes a decrease in exploratory behavior, mimicking odor-evoked fear memory recall. Together, our experiments identify specific ensembles of piriform neurons as critical components of an olfactory fear memory trace and provide new experimental avenues for investigating memory formation, retrieval, and memory dysfunction in the olfactory system.