The mouse dorsal peduncular cortex encodes fear memory

Dorsal-Caudal and Ventral Hippocampi Target Different Cell Populations in the Medial Frontal Cortex in Rodents

Direct projections from the ventral hippocampus (vHPC) to the medial frontal cortex (MFC) play crucial roles in memory and emotional regulation. Using anterograde transsynaptic tracing and ex vivo electrophysiology in male mice, we document a previously unexplored pathway that parallels the established vHPC-MFC connectivity. This pathway connects the dorsal-caudal hippocampus (dcHPC) to specific subregions of the ventral MFC (vMFC), in particular the dorsal peduncular cortex. Notably, this pathway exerts a strong inhibitory influence on vMFC by targeting a substantial proportion of inhibitory neurons. Retrograde transsynaptic tracing in male rats indicated that vMFC subregions project disynaptically back to vHPC. These results, altogether, suggest the existence of a remarkable functional circuit connecting distinct functional areas: the cognition-related dcHPC with the emotion-related vMFC and vHPC. These findings further provide valuable insights in the cognitive and emotional abnormalities associated with the HPC-MFC connectivity in neurological and psychiatric disorders.

Neural Networks of the Mouse Primary Visceromotor Cortex

The medial prefrontal cortex (MPF) regulates emotions, stress responses, and goal-directed behaviors like attention and decision-making. However, the precise mechanisms underlying MPF function remain poorly understood, largely due to an incomplete characterization of its neural circuitry. Leveraging neuroanatomical, neurophysiological, and behavioral techniques, we present a detailed wiring diagram of the MPF, with a particular focus on the dorsal peduncular area (DP), an underexplored MPF area implicated in psychological stress, fear conditioning, anxiety, depression, and opioid addiction. Our analysis identifies the deep (DPd) and superficial (DPs) layers of the DP, together with the infralimbic area (ILA), as key components of the primary visceromotor cortex, that generate monosynaptic projections to regulate neuroendocrine, sympathetic, and parasympathetic functions in distinct, yet coordinated ways. Further, we demonstrate that the DP serves as a unique network hub for unidirectional cortical information flow, that integrates diverse cortical inputs and modulates social behavior. Based on the mesoscale connectome of entire MPF, we propose a unified MPF network model that regulates different aspects of motor actions associated with goal-directed behavior. This study provides novel insights into the complex role of the MPF in orchestrating physiological and behavioral responses to environmental stimuli in mammals.

An infralimbic cortex engram encoded during learning attenuates fear generalization

Generalization allows previous experience to adaptively guide behavior when conditions change. The infralimbic (IL) subregion of the ventromedial prefrontal cortex plays a known role in generalization processes, although mechanisms remain unclear. A basic physical unit of memory storage and expression in the brain is a sparse, distributed group of neurons known as an engram. Here, we set out to determine whether an engram established during learning contributes to generalized responses in IL. Generalization was tested in male and female mice by presenting a novel, ambiguous, tone generalization stimulus following Pavlovian defensive (fear) conditioning. The first experiment was designed to test a global role for IL in generalization using chemogenetic manipulations. Results show IL regulates defensive behavior in response to ambiguous stimuli. IL silencing led to a switch in defensive state, from vigilant scanning to generalized freezing, while IL stimulation reduced freezing in favor of scanning. Leveraging activity-dependent "tagging" technology (ArcCreERT2 x eYFP system), an engram, preferentially located in IL Layer 2/3, was associated with the generalization stimulus. Remarkably, in the identical discrete location, fewer reactivated neurons were associated with the generalization stimulus at the remote timepoint (30 days) following learning. When an IL engram established during learning was selectively chemogenetically silenced, freezing increased. Conversely, IL engram stimulation reduced freezing, suggesting attenuated fear generalization. Overall, these data identify a crucial role for IL in suppressing generalized conditioned responses. Further, an IL engram formed during learning functions to later attenuate a conditioned response in the presence of ambiguous threat stimuli.Significance statement Generalization refers to the ability for organisms to use previous experience to guide behavior when environmental conditions change. Despite the immense importance of generalization in adaptive behavior, the precise brain mechanisms remain unknown. Here we identified a small population of neurons, known as an engram, in a discrete region of the frontal cortex that was associated with the expression of generalization related to a threatening situation. When these cells were turned off, generalization increased. When they were turned on, generalization decreased. Considering that over-generalization of threatening stimuli is a known fundamental dimension of both anxiety and post-traumatic stress disorders, these findings have implications not only for our understanding of intrinsic generalization processes but also highly prevalent clinical disorders.