Bi-directional encoding of context-based odors and behavioral states by the nucleus of the lateral olfactory tract

Altered odor perception in Dlgap2 mutant mice, a mouse model of autism spectrum disorder

Olfactory dysfunction has been observed in patients with Autism Spectrum Disorder (ASD). A microdeletion at the 8p23 terminal regions of chromosome 8p23 was identified in a Taiwanese patient with ASD, suggesting a potential association with mutations in the DLGAP2 gene. DLGAP2 is expressed in the olfactory bulb in rodents. The current study investigated olfactory phenotypes of Dlgap2 mutant mice. The results indicated that odor detection capabilities were comparable between wild-type (WT) and Dlgap2 mutant mice. However, homozygous mutant (Homo) mice showed less interest in sniffing odors of banana and almond but greater sniffing activity in response to bedding from unfamiliar cages. Notably, exposure to banana odor elicited significant c-fos expression in most olfaction-related brain regions of WT mice, while Homo mice did not show much increase in c-fos levels in major olfactory areas, which may correlate with their diminished sniffing behavior. Bedding stimuli induced pronounced c-fos expression in WT brains and some olfaction-related regions, including the olfactory bulb, amygdala, hypothalamus, and medial prefrontal cortex, in Homo mice. These mutants may still process olfactory signals from the bedding through a relatively narrow channel, which might elicit their interest, leading to increased sniffing behaviors that may compensate for their olfactory deficits. The DLGAP2 protein was absent in the olfactory bulb of Homo mice, and the levels of PSD95 and CaMKIIβ were also affected, indicating alterations in synaptic transmission and signaling within the olfactory system. This study evaluated olfactory perception in a mouse model of ASD, which may advance diagnostic and therapeutic strategies.

Synaptic Connectivity and Electrophysiological Properties of the Nucleus of the Lateral Olfactory Tract

The sense of smell is tightly linked to emotions, a link that is thought to rely on the direct synaptic connections between the olfactory bulb (OB) and nuclei of the amygdala. However, there are multiple pathways projecting olfactory information to the amygdala, and their unique functions are unknown. The pathway via the nucleus of the lateral olfactory tract (NLOT) that receives input from olfactory regions and projects to the basolateral amygdala (BLA) is among them. NLOT has been very little studied, and consequentially its function is unknown. Furthermore, formulation of informed hypotheses about NLOT function is at this stage limited by the lack of knowledge about its connectivity and physiological properties. Here, we used virus-based tracing methods to systematically reveal inputs into NLOT, as well as NLOT projection targets in mice of both sexes. We found that the NLOT is interconnected with several olfactory brain regions and with the BLA. Some of these connections were reciprocal, and some showed unique interhemispheric patterns. We tested the excitable properties of NLOT neurons and the properties of each of the major synaptic inputs. We found that the NLOT receives powerful input from the piriform cortex, tenia tecta, and the BLA but only very weak input from the OB. When input crosses threshold, NLOT neurons respond with calcium-dependent bursts of action potentials. We hypothesize that this integration of olfactory and amygdalar inputs serves behaviors that combine smell and emotion.

An intra-oral flavor detection task in freely moving mice

Flavor plays a critical role in the pleasure of food. Flavor research has mainly focused on human subjects and revealed that many brain regions are involved in flavor perception. However, animal models for elucidating the mechanisms of neural circuits are lacking. Herein, we demonstrate the use of a novel behavioral task in which mice are capable of flavor detection. When the olfactory pathways of the mice were blocked, they could not perform the task. However, behavioral accuracy was not affected when the gustatory pathway was blocked by benzocaine. These results indicate that the mice performed this detection task mainly based on the olfaction. We conclude that this novel task can contribute to research on the neural mechanisms of flavor perception.

Neuronal types in the mouse amygdala and their transcriptional response to fear conditioning

The amygdala is a brain region primarily associated with emotional response. The use of genetic markers and single-cell transcriptomics can provide insights into behavior-associated cell state changes. Here we present a detailed cell-type taxonomy of the adult mouse amygdala during fear learning and memory consolidation. We perform single-cell RNA sequencing on naïve and fear-conditioned mice, identify 130 neuronal cell types and validate their spatial distributions. A subset of all neuronal types is transcriptionally responsive to fear learning and memory retrieval. The activated engram cells upregulate activity-response genes and coordinate the expression of genes associated with neurite outgrowth, synaptic signaling, plasticity and development. We identify known and previously undescribed candidate genes responsive to fear learning. Our molecular atlas may be used to generate hypotheses to unveil the neuron types and neural circuits regulating the emotional component of learning and memory.

Hippocampal CA1 Neurons Represent Positive Feedback During the Learning Process of an Associative Memory Task

The hippocampus is crucial for forming associations between environmental stimuli. However, it is unclear how neural activities of hippocampal neurons dynamically change during the learning process. To address this question, we developed an associative memory task for rats with auditory stimuli. In this task, the rats were required to associate tone pitches (high and low) and ports (right and left) to obtain a reward. We recorded the firing activity of neurons in rats hippocampal CA1 during the learning process of the task. As a result, many hippocampal CA1 neurons increased their firing rates when the rats received a reward after choosing either the left or right port. We referred to these cells as "reward-direction cells." Furthermore, the proportion of the reward-direction cells increased in the middle-stage of learning but decreased after the completion of learning. This result suggests that the activity of reward-direction cells might serve as "positive feedback" signal that facilitates the formation of associations between tone pitches and port choice.