A distinct cortical code for socially learned threat.

Animals can learn about sources of danger while minimizing their own risk by observing how others respond to threats. However, the distinct neural mechanisms by which threats are learned through social observation (known as observational fear learning1-4 (OFL)) to generate behavioural responses specific to such threats remain poorly understood. The dorsomedial prefrontal cortex (dmPFC) performs several key functions that may underlie OFL, including processing of social information and disambiguation of threat cues5-11. Here we show that dmPFC is recruited and required for OFL in mice. Using cellular-resolution microendoscopic calcium imaging, we demonstrate that dmPFC neurons code for observational fear and do so in a manner that is distinct from direct experience. We find that dmPFC neuronal activity predicts upcoming switches between freezing and moving state elicited by threat. By combining neuronal circuit mapping, calcium imaging, electrophysiological recordings and optogenetics, we show that dmPFC projections to the midbrain periaqueductal grey (PAG) constrain observer freezing, and that amygdalar and hippocampal inputs to dmPFC opposingly modulate observer freezing. Together our findings reveal that dmPFC neurons compute a distinct code for observational fear and coordinate long-range neural circuits to select behavioural responses.

Symmetry in frontal but not motor and somatosensory corticocortical and corticostriatal circuitry.

Neocortex and striatum are topographically organized by cortical areas representing sensory and motor functions, where primary cortical areas are generally used as models for other cortical regions. But different cortical areas are specialized for distinct purposes, with sensory and motor areas lateralized for touch and motor control, respectively. Frontal areas are involved in decision making, where lateralization of function may be less important. This study contrasted the topographic precision of ipsilateral and contralateral projections from cortex based on the injection site location. While sensory cortical areas had strongly topographic outputs to ipsilateral cortex and striatum, they were weaker and not as topographically strong to contralateral targets. Motor cortex had somewhat stronger projections, but still relatively weak contralateral topography. In contrast, frontal cortical areas had high degrees of topographic similarity for both ipsilateral and contralateral projections to cortex and striatum. This contralateral connectivity reflects on the pathways in which corticostriatal computations might integrate input outside closed basal ganglia loops, enabling the two hemispheres to act as a single unit and converge on one result in motor planning and decision making.

Basic Neuroanatomical Methods.

This unit covers some basic procedures that are common to a wide range of neuroanatomical protocols for brain tissue. Procedures are provided for preparation of unfixed fresh brain tissue as well as for perfusion fixation of animals to obtain fixed neural tissue. A variety of methods for sectioning are described, including frozen sectioning using a cryostat or microtome and sectioning with a vibratome. The choice of sectioning method depends on how the brain has been prepared and what histochemical method is to be used. A fluorescent immunohistochemical method to localize endogenous molecules as well as induced markers such as green fluorescent protein and red fluorescent protein is also provided. Additionally, three post-sectioning procedures are described: defatting of slide-mounted sections, fluorescent Nissl staining, and thionin staining of sections. Finally, support protocols are provided, describing a method for maintaining the correct order of cut tissue, whether rostral to caudal or lateral to medial; a procedure for subbing slides with gelatin, which is necessary in some protocols in order for sections to adhere to slides; and preparation of custom 3D-printed 10- or 20-well tissue plates and trays for subsequent immunostaining. Published 2019. U.S. Government. Basic Protocol 1: Preparation of unfixed fresh-frozen brain tissue Basic Protocol 2: Perfusion fixation Basic Protocol 3: Cryostat sectioning of frozen brain tissue Basic Protocol 4: Sliding-microtome sectioning of fixed brain tissue Basic Protocol 5: Vibratome and Compresstome sectioning Support Protocol 1: Tissue collection in a 1-in-10 series Support Protocol 2: Preparation of gelatin-subbed microscope slides Support Protocol 3: Custom 3D-printed 10- and 20-well tissue plates Basic Protocol 6: Post-sectioning procedures I: Fluorescent immunohistochemical localization Basic Protocol 7: Post-sectioning procedures II: Defatting Basic Protocol 8: Post-sectioning procedures III: Nissl staining Basic Protocol 9: Post-sectioning procedures IV: Thionin staining.

Whole mouse brain reconstruction and registration to a reference atlas with standard histochemical processing of coronal sections.

Advances in molecular neuroanatomical tools have expanded the ability to map in detail connections of specific neuron subtypes in the context of behaviorally driven patterns of neuronal activity. Analysis of such data across the whole mouse brain, registered to a reference atlas, aids in understanding the functional organization of brain circuits related to behavior. A process is described to image mouse brain sections labeled with standard histochemical techniques, reconstruct those images into a whole brain image volume and register those images to the Allen Mouse Brain Common Coordinate Framework. Image analysis tools automate detection of cell bodies and quantification of axon density labeling in the structures in the annotated reference atlas. Examples of analysis are provided for mapping the axonal projections of layer-specific cortical neurons using Cre-dependent AAV vectors and for mapping inputs to such neurons using retrograde transsynaptic tracing with modified rabies viral vectors.

Author Correction: Topographic precision in sensory and motor corticostriatal projections varies across cell type and cortical area.

In the original version of this Article, support provided during initiation of the project was not fully acknowledged. The PDF and HTML versions of the Article have now been corrected to include support from Karel Svoboda, members of the Svoboda lab, and members of Janelia's Vivarium staff.

Topographic precision in sensory and motor corticostriatal projections varies across cell type and cortical area.

The striatum shows general topographic organization and regional differences in behavioral functions. How corticostriatal topography differs across cortical areas and cell types to support these distinct functions is unclear. This study contrasted corticostriatal projections from two layer 5 cell types, intratelencephalic (IT-type) and pyramidal tract (PT-type) neurons, using viral vectors expressing fluorescent reporters in Cre-driver mice. Corticostriatal projections from sensory and motor cortex are somatotopic, with a decreasing topographic specificity as injection sites move from sensory to motor and frontal areas. Topographic organization differs between IT-type and PT-type neurons, including injections in the same site, with IT-type neurons having higher topographic stereotypy than PT-type neurons. Furthermore, IT-type projections from interconnected cortical areas have stronger correlations in corticostriatal targeting than PT-type projections do. As predicted by a longstanding model, corticostriatal projections of interconnected cortical areas form parallel circuits in the basal ganglia.

Reversible inhibition of CREB/ATF transcription factors in region CA1 of the dorsal hippocampus disrupts hippocampus-dependent spatial memory.

CREB is critical for long-lasting synaptic and behavioral plasticity in invertebrates. Its role in the mammalian hippocampus is less clear. We have interfered with CREB family transcription factors in region CA1 of the dorsal hippocampus. This impairs learning in the Morris water maze, which specifically requires the dorsal hippocampus, but not context conditioning, which does not. The deficit is specific to long-term memory, as shown in an object recognition task. Several forms of late-phase LTP are normal, but forskolin-induced and dopamine-regulated potentiation are disrupted. These experiments represent the first targeting of the dorsal hippocampus in genetically modified mice and confirm a role for CREB in hippocampus-dependent learning. Nevertheless, they suggest that some experimental forms of plasticity bypass the requirement for CREB.