Parallel neural pathways control sodium consumption and taste valence.

The hedonic value of salt fundamentally changes depending on the internal state. High concentrations of salt induce innate aversion under sated states, whereas such aversive stimuli transform into appetitive ones under sodium depletion. Neural mechanisms underlying this state-dependent salt valence switch are poorly understood. Using transcriptomics state-to-cell-type mapping and neural manipulations, we show that positive and negative valences of salt are controlled by anatomically distinct neural circuits in the mammalian brain. The hindbrain interoceptive circuit regulates sodium-specific appetitive drive , whereas behavioral tolerance of aversive salts is encoded by a dedicated class of neurons in the forebrain lamina terminalis (LT) expressing prostaglandin E2 (PGE2) receptor, Ptger3. We show that these LT neurons regulate salt tolerance by selectively modulating aversive taste sensitivity, partly through a PGE2-Ptger3 axis. These results reveal the bimodal regulation of appetitive and tolerance signals toward salt, which together dictate the amount of sodium consumption under different internal states.

Recovery of missing single-cell RNA-sequencing data with optimized transcriptomic references.

Single-cell RNA-sequencing (scRNA-seq) is an indispensable tool for characterizing cellular diversity and generating hypotheses throughout biology. Droplet-based scRNA-seq datasets often lack expression data for genes that can be detected with other methods. Here we show that the observed sensitivity deficits stem from three sources: (1) poor annotation of 3' gene ends; (2) issues with intronic read incorporation; and (3) gene overlap-derived read loss. We show that missing gene expression data can be recovered by optimizing the reference transcriptome for scRNA-seq through recovering false intergenic reads, implementing a hybrid pre-mRNA mapping strategy and resolving gene overlaps. We demonstrate, with a diverse collection of mouse and human tissue data, that reference optimization can substantially improve cellular profiling resolution and reveal missing cell types and marker genes. Our findings argue that transcriptomic references need to be optimized for scRNA-seq analysis and warrant a reanalysis of previously published datasets and cell atlases.

A sex-specific thermogenic neurocircuit induced by predator smell recruiting cholecystokinin neurons in the dorsomedial hypothalamus.

Olfactory cues are vital for prey animals like rodents to perceive and evade predators. Stress-induced hyperthermia, via brown adipose tissue (BAT) thermogenesis, boosts physical performance and facilitates escape. However, many aspects of this response, including thermogenic control and sex-specific effects, remain enigmatic. Our study unveils that the predator odor trimethylthiazoline (TMT) elicits BAT thermogenesis, suppresses feeding, and drives glucocorticoid release in female mice. Chemogenetic stimulation of olfactory bulb (OB) mitral cells recapitulates the thermogenic output of this response and associated stress hormone corticosterone release in female mice. Neuronal projections from OB to medial amygdala (MeA) and dorsomedial hypothalamus (DMH) exhibit female-specific cFos activity toward odors. Cell sorting and single-cell RNA-sequencing of DMH identify cholecystokinin (CCK)-expressing neurons as recipients of predator odor cues. Chemogenetic manipulation and neuronal silencing of DMHCCK neurons further implicate these neurons in the propagation of predator odor-associated thermogenesis and food intake suppression, highlighting their role in female stress-induced hyperthermia.

Serotonergic fast lane from taste detection to preparatory digestive actions.

Nutrient detection through the taste system triggers various physiological changes in the body. In this issue of Neuron, Yao and Scott (2022) identify two distinct classes of serotonergic neurons in Drosophila that transform sweet and bitter taste signals into endocrine and digestive responses.

Sensory representation and detection mechanisms of gut osmolality change.

Ingested food and water stimulate sensory systems in the oropharyngeal and gastrointestinal areas before absorption1,2. These sensory signals modulate brain appetite circuits in a feed-forward manner3-5. Emerging evidence suggests that osmolality sensing in the gut rapidly inhibits thirst neurons upon water intake. Nevertheless, it remains unclear how peripheral sensory neurons detect visceral osmolality changes, and how they modulate thirst. Here we use optical and electrical recording combined with genetic approaches to visualize osmolality responses from sensory ganglion neurons. Gut hypotonic stimuli activate a dedicated vagal population distinct from mechanical-, hypertonic- or nutrient-sensitive neurons. We demonstrate that hypotonic responses are mediated by vagal afferents innervating the hepatic portal area (HPA), through which most water and nutrients are absorbed. Eliminating sensory inputs from this area selectively abolished hypotonic but not mechanical responses in vagal neurons. Recording from forebrain thirst neurons and behavioural analyses show that HPA-derived osmolality signals are required for feed-forward thirst satiation and drinking termination. Notably, HPA-innervating vagal afferents do not sense osmolality itself. Instead, these responses are mediated partly by vasoactive intestinal peptide secreted after water ingestion. Together, our results reveal visceral hypoosmolality as an important vagal sensory modality, and that intestinal osmolality change is translated into hormonal signals to regulate thirst circuit activity through the HPA pathway.

The cellular basis of distinct thirst modalities.

Fluid intake is an essential innate behaviour that is mainly caused by two distinct types of thirst1-3. Increased blood osmolality induces osmotic thirst that drives animals to consume pure water. Conversely, the loss of body fluid induces hypovolaemic thirst, in which animals seek both water and minerals (salts) to recover blood volume. Circumventricular organs in the lamina terminalis are critical sites for sensing both types of thirst-inducing stimulus4-6. However, how different thirst modalities are encoded in the brain remains unknown. Here we employed stimulus-to-cell-type mapping using single-cell RNA sequencing to identify the cellular substrates that underlie distinct types of thirst. These studies revealed diverse types of excitatory and inhibitory neuron in each circumventricular organ structure. We show that unique combinations of these neuron types are activated under osmotic and hypovolaemic stresses. These results elucidate the cellular logic that underlies distinct thirst modalities. Furthermore, optogenetic gain of function in thirst-modality-specific cell types recapitulated water-specific and non-specific fluid appetite caused by the two distinct dipsogenic stimuli. Together, these results show that thirst is a multimodal physiological state, and that different thirst states are mediated by specific neuron types in the mammalian brain.

Multimodal Analysis of Cell Types in a Hypothalamic Node Controlling Social Behavior.

The ventrolateral subdivision of the ventromedial hypothalamus (VMHvl) contains ∼4,000 neurons that project to multiple targets and control innate social behaviors including aggression and mounting. However, the number of cell types in VMHvl and their relationship to connectivity and behavioral function are unknown. We performed single-cell RNA sequencing using two independent platforms-SMART-seq (∼4,500 neurons) and 10x (∼78,000 neurons)-and investigated correspondence between transcriptomic identity and axonal projections or behavioral activation, respectively. Canonical correlation analysis (CCA) identified 17 transcriptomic types (T-types), including several sexually dimorphic clusters, the majority of which were validated by seqFISH. Immediate early gene analysis identified T-types exhibiting preferential responses to intruder males versus females but only rare examples of behavior-specific activation. Unexpectedly, many VMHvl T-types comprise a mixed population of neurons with different projection target preferences. Overall our analysis revealed that, surprisingly, few VMHvl T-types exhibit a clear correspondence with behavior-specific activation and connectivity.

Effect of Age on Substantia Nigra Hyper-echogenicity in Parkinson's Disease Patients and Healthy Controls.

Substantia nigra (SN) hyper-echogenicity (SN+) describes an enlargement (>90th percentile) of the area of echogenicity at the anatomic site of the SN in the midbrain detected by transcranial sonography. This ultrasound sign has proven to be a valuable marker supporting the clinical diagnosis of Parkinson's disease (PD). Although there is considerable variation in the extent of echogenic signals at the anatomic site of the SN among PD patients, previous work suggests that SN+ is a stable marker throughout the course of the disease. The present study focused on two aspects: (i) determining whether SN+ values differ between the sides, mirroring the asymmetric character of the disease; and (ii) determining whether age has an influence on SN echogenicity. This cross-sectional study included 300 PD patients and 200 healthy controls. SN+ was measured planimetrically by transcranial sonography. Echogenicity was analyzed separately for onset and non-onset sides, with onset side defined as the SN contralateral to the side of the body that first manifested PD-related motor impairment. Age of the patients and healthy controls at study time was used for correlation. We found that the onset SN+ contralateral to the side of initial motor symptoms was on average 17.6% larger than its counterpart. However, we also found that contrary to the control group, where an increase in age was associated with an increase in size of SN+, age of PD patients was associated with a decline in size of the onset SN+. Furthermore, SN measured at the onset side of PD patients correlated significantly with patient age and Hoehn and Yahr stage, a scale that grades PD severity, although this was not the case for the non-onset side. The present study indicates that changes in SN echogenicity have a different dynamic depending on the onset side of the disease. The age at study time had a significantly negative effect on the size of onset SN+, the effect on the non-onset side was non-significant. We conclude that for appropriate PD analysis, onset SN+ is a more important marker than the average of both sides of SN. Furthermore, we found that among healthy controls, the size of SN+ increases with age.

Four GABAergic interneurons impose feeding restraint in Drosophila.

Feeding is dynamically regulated by the palatability of the food source and the physiological needs of the animal. How consumption is controlled by external sensory cues and internal metabolic state remains under intense investigation. Here, we identify four GABAergic interneurons in the Drosophila brain that establish a central feeding threshold which is required to inhibit consumption. Inactivation of these cells results in indiscriminate and excessive intake of all compounds, independent of taste quality or nutritional state. Conversely, acute activation of these neurons suppresses consumption of water and nutrients. The output from these neurons is required to gate activity in motor neurons that control meal initiation and consumption. Thus, our study reveals a layer of inhibitory control in feeding circuits that is required to suppress a latent state of unrestricted and nonselective consumption.

Feeding regulation in Drosophila.

Neuromodulators play a key role in adjusting animal behavior based on environmental cues and internal needs. Here, we review the regulation of Drosophila feeding behavior to illustrate how neuromodulators achieve behavioral plasticity. Recent studies have made rapid progress in determining molecular and cellular mechanisms that translate the metabolic needs of the fly into changes in neuroendocrine and neuromodulatory states. These neuromodulators in turn promote or inhibit discrete feeding behavioral subprograms. This review highlights the links between physiological needs, neuromodulatory states, and feeding decisions.

Neuropeptide Y gene therapy decreases chronic spontaneous seizures in a rat model of temporal lobe epilepsy.

Temporal lobe epilepsy remains amongst the most common and drug refractory of neurological disorders. Gene therapy may provide a realistic therapeutic approach alternative to surgery for intractable focal epilepsies. To test this hypothesis, we applied here a gene therapy approach, using a recombinant adeno-associated viral (rAAV) vector expressing the human neuropeptide Y (NPY) gene, to a progressive and spontaneous seizure model of temporal lobe epilepsy induced by electrical stimulation of the temporal pole of the hippocampus, which replicates many features of the human condition. rAAV-NPY or a control vector lacking the expression cassette (rAAV-Empty) was delivered into the epileptic rat hippocampi at an early progressive stage of the disease. Chronic epileptic rats were video-EEG monitored to establish pre-injection baseline recordings of spontaneous seizures and the effect of rAAV-NPY versus rAAV-Empty vector injection. Both non-injected stimulated controls and rAAV-empty injected rats showed a similar progressive increase of spontaneous seizure frequency consistent with epileptogenesis. The delivery of rAAV-NPY in epileptic rat brain leads to a remarkable decrease in the progression of seizures as compared to both control groups and this effect was correlated with the NPY over-expression in the hippocampus. Moreover, spontaneous seizure frequency was significantly reduced in 40% of treated animals as compared to their pre-injection baseline. Our data show that this gene therapy strategy decreases spontaneous seizures and suppresses their progression in chronic epileptic rats, thus representing a promising new therapeutic strategy.