Tamar Licht

Foraging animals must continually evaluate whether potential food sources are safe to consume. Such decisions rely on both innate preferences and learned associations formed through prior experience. Consumption of a food followed by malaise or sickness leads to avoidance of that food, reflecting learned associations between sensory cues that identify a food and its negative post-ingestive consequences. Although this form of learning has been studied extensively using taste cues, taste signals arise only after oral contact, at which point exposure to potential toxins has already occurred. Olfactory cues, in contrast, provide information about potential food sources prior to consumption, yet how odors acquire aversive value remains unclear. Here, using targeted chemogenetic perturbations, we identify the nucleus of the lateral olfactory tract (NLOT), which has direct bidirectional connectivity to olfactory regions and the basolateral amygdala, as a critical circuit element for odor-aversion learning. We find that the NLOT is required for conditioned odor aversion but is dispensable for other odor-guided behaviors and for fear conditioning driven by a non-olfactory cue. Our findings identify a selective role for the NLOT in linking olfactory cues to learned aversive outcomes, that supports odor-guided behavioral decisions. Significance statement: Animals rely on smell to evaluate food safety before consuming it, yet the brain circuits that link odor cues to learned aversive outcomes remain poorly understood. Here we identify the nucleus of the lateral olfactory tract (NLOT), a largely unstudied region at the interface between olfactory and limbic circuits, as a critical and selective node for conditioned odor aversion learning. Chemogenetic perturbations of the NLOT disrupted both the acquisition and expression of odor-malaise associations, while leaving odor detection, odor discrimination, and non-olfactory fear learning intact — revealing a previously unrecognized specialization within olfactory-amygdala circuits that supports odor-guided decisions about food safety.

Increasing scientific interest has been directed toward understanding sexual dimorphism in the brain. Although several brain structures exhibit masculine or feminine characteristics, strictly binary anatomical feature, comparable to those observed in genitalia, has been rarely identified. In this study, we identified a dense, sexually dimorphic cluster of neurons in the posterodorsal medial amygdala (MeApd), which we named DIMPLE (Dimorphic IEGs Medial Posterodorsal amygdala Labeled Ensemble) that exhibited a remarkable binary pattern of c-fos promoter activation. Using the TRAP2 (Targeted Recombination in Active Populations) transgenic mouse model, we found that it was consistently labeled in all females, regardless of age or sexual experience. In contrast, DIMPLE labeling was absent in adult virgin males but present both prior to weaning and following mating. Surgical removal of gonads (ovariectomy or orchiectomy) did not alter the labeling pattern of DIMPLE in either sex. Interestingly, a single intraperitoneal injection of prolactin, a hormone that increases in males after mating, induced DIMPLE labeling in virgin males. However, treatment with cabergoline, a potent inhibitor of prolactin secretion, did not prevent DIMPLE labeling in females or in postmating males. Given the established role of the MeApd in social and reproductive behaviors, we hypothesize that DIMPLE may support neural mechanisms underlying female-typical behavior and potentially contribute to postmating behavioral shifts in males.

Increasing scientific interest has been directed toward understanding sexual dimorphism in the brain. Although various brain structures exhibit masculine or feminine characteristics, no strictly binary anatomical feature, such as those seen in genitalia. has been identified. In this study, we identified a dense, sexually dimorphic cluster of neurons in the posterodorsal medial amygdala (MeApd), which we named DIMPLE, that exhibited a remarkable binary pattern of cFos activation. Using the TRAP2 (Targeted Recombination in Active Populations) transgenic mouse model, we found that it was consistently labeled in all females, regardless of age or sexual experience. In males, however, DIMPLE was not labeled in any of the adult virgins but was evident pre-weaning and following mating. Surgical removal of gonads (ovariectomy or orchiectomy) did not alter the labeling pattern of DIMPLE in either sex. Interestingly, a single intraperitoneal injection of prolactin, a hormone known to increase in males after mating, induced DIMPLE labeling in virgin males. However, treatment with cabergoline, a potent inhibitor of prolactin secretion, did not prevent DIMPLE labeling in females or in post-mating males. Given the established role of the MeApd in social and reproductive behaviors, we propose that DIMPLE may support neural mechanisms underlying female-typical behavior and potentially contribute to post-mating behavioral shifts in males. Abstract Figure: Graphical Summary of DIMPLE activity during puberty and sexual status.

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.

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 and nuclei of the amygdala. A small number of amygdaloid nuclei are the recipients of such direct input from the olfactory bulb and their unique functions are not known. Among them, the nucleus of the lateral olfactory tract (NLOT) is unique in its developmental history and gene expression. 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 pseudo-rabies tracing methods to systematically reveal monosynaptic inputs into NLOT, and adeno-associated viruses to reveal NLOT projection targets. We found that the NLOT is interconnected with several olfactory brain regions and with the basolateral amygdala. 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 piriform cortex, tenia tecta, and the basolateral amygdala, but only very weak input from the olfactory bulb. When input crosses threshold, NLOT neurons respond with calcium-dependent bursts of action potentials. This integration of olfactory and amygdalar inputs suggests that NLOT plays a role in behaviors that combine smell and emotion, possibly assigning emotional value to odors. Significance statement Despite the well-known functional links between olfaction and emotions, the physiological properties of these links remain largely understudied. One major pathway by which olfactory and emotional signals interact, is via the nucleus of the lateral olfactory tract (NLOT). NLOT has been little studied and its function is yet unclear. The lack of physiological information hinders informed hypotheses. Here, we characterize the synaptic and intrinsic properties of NLOT neurons. We show that the NLOT receives converging olfactory and amygdalar inputs, and that NLOT neurons respond to input with high-rate bursts of action potentials. This suggests that the NLOT, that harbors ∼2500 cells, encodes a low-dimensional signal that is of high importance. We hypothesize that the NLOT assigns emotional value to odors.

The olfactory bulb (OB) is a critical component of mammalian olfactory neuroanatomy. Beyond being the first and sole relay station for olfactory information to the rest of the brain, it also contains elaborate stereotypical circuitry that is considered essential for olfaction. Indeed, substantial lesions of the OB in rodents lead to anosmia. Here, we examined the circuitry that underlies olfaction in a mouse model with severe developmental degeneration of the OB. These mice could perform odor-guided tasks and even responded normally to innate olfactory cues. Despite the near total loss of the OB, piriform cortex in these mice responded to odors normally and its neural activity sufficed to decode odor identity. We analyzed the circuitry that supports olfactory function in these mice. We found that sensory neurons express the full repertoire of olfactory receptors and their axons project primarily to the rudimentary OB, but also ectopically, to olfactory cortical regions. Within the OB, the number of principal neurons was greatly reduced and the morphology of their dendrites was abnormal, extending over larger regions within the OB. Glomerular organization was lost. This study shows that olfactory functionality can be preserved despite reduced and aberrant circuitry that is missing many of the elements that are believed to be essential for olfaction, and may explain the retention of olfaction in humans with degenerated OBs.

More VEGF, more life—and health span: Advanced aging is celebrated but its ill effects, deterioration at the cell, tissue, and organ levels, are not. Grunewald et al . provide evidence for the vascular theory of aging, which reports that age-related decrease of vascular function is a driver of organismal aging at large (see the Perspective by Augustin and Kipnis). Vascular endothelial growth factor (VEGF) signaling insufficiency underlies this vascular insufficiency in aged mice. A modest compensatory increase in circulatory VEGF was sufficient to preserve a young-like vascular homeostasis, alleviate multiple adverse age-related processes, and ameliorate a host of age-associated pathologies in mice. —BAP

Adaptive sample pooling for SARS-CoV-2 testing based on positivity rates spares 76% of test reactions without compromising specificity or sensitivity.

Pooling multiple swab samples prior to RNA extraction and RT-PCR analysis was proposed as a strategy to reduce costs and increase throughput of SARS-CoV-2 tests. However, reports on practical large-scale group testing for SARS-CoV-2 have been scant. Key open questions concern reduced sensitivity due to sample dilution; the rate of false positives; the actual efficiency (number of tests saved by pooling) and the impact of infection rate in the population on assay performance. Here we report analysis of 133,816 samples collected at April-September 2020, tested by pooling for the presence of SARS-CoV-2. We spared 76% of RNA extraction and RT-PCR tests, despite the reality of frequently changing prevalence rate (0.5%-6%). Surprisingly, we observed pooling efficiency and sensitivity that exceed theoretical predictions, which resulted from non-random distribution of positive samples in pools. Overall, the findings strongly support the use of pooling for efficient large high throughput SARS-CoV-2 testing.