Male Drosophila Research Articles

Reference genes to study the sex-biased expression of genes regulating Drosophila metabolism

Sex is an important variable in biology. Notable differences have been observed between male and female Drosophila in regulation of metabolism, in response to nutritional challenges, and in phenotypes relevant for obesity and metabolic disorders. The differences between males and females can be expected to result from differences in gene expression. We observed that expression levels of reference genes commonly used for normalization of qRT-PCR results such as GAPDH, β-actin, and 18SrRNA, show prominent sexual dimorphism. Since this will impact relative expression and conclusions related to that, we performed a systematic analysis of candidate reference genes with the objective of identifying reference genes with stable expression in male and female Drosophila. These reference genes (LamCa, βTub60D and βTub97EF) were then used to assess sex-specific differences in expression of metabolism associated genes. Additionally, we evaluated the utility of these reference genes following a nutritional challenge and showed that LamCa and βtub97EF are stably expressed between sexes and under different nutritional conditions and are thus suitable as reference genes. Our results highlight the importance of evaluating the stability of reference genes when sex-specific differences in gene expression are studied, and identify structural genes as a category worth exploring as reference genes in other species. Finally, we also uncovered hitherto unknown sexually dimorphic expression of a number of metabolism-associated genes, information of interest to others working in the field of metabolic disorders.

Sensory perception of rivals has trait-dependent effects on plasticity in Drosophila melanogaster.

The social environment has myriad effects on individuals, altering reproduction, immune function, cognition, and aging. Phenotypic plasticity enables animals to respond to heterogeneous environments such as the social environment but requires that they assess those environments accurately. It has been suggested that combinations of sensory cues allow animals to respond rapidly and accurately to changeable environments, but it is unclear whether the same sensory inputs are required in all traits that respond to a particular environmental cue. Drosophila melanogaster males, in the presence of rival males, exhibit a consistent behavioral response by extending mating duration. However, exposure to a rival also results in a reduction in their lifespan, a phenomenon interpreted as a trade-off associated with sperm competition strategies. D. melanogaster perceive their rivals by using multiple sensory cues; interfering with at least two olfactory, auditory, or tactile cues eliminates the extension of mating duration. Here, we assessed whether these same cues were implicated in the lifespan reduction. Removal of combinations of auditory and olfactory cues removed the extended mating duration response to a rival, as previously found. However, we found that these manipulations did not alter the reduction in lifespan of males exposed to rivals or induce any changes in activity patterns, grooming, or male-male aggression. Therefore, our analysis suggests that lifespan reduction is not a cost associated with the behavioral responses to sperm competition. Moreover, this highlights the trait-specific nature of the mechanisms underlying plasticity in response to the same environmental conditions.

Biological aging of two innate behaviors of Drosophila melanogaster: Escape climbing versus courtship learning and memory.

Motor and cognitive aging can severely affect life quality of elderly people and burden health care systems. In search for diagnostic behavioral biomarkers, it has been suggested that walking speed can predict forms of cognitive decline, but in humans, it remains challenging to separate the effects of biological aging and lifestyle. We examined a possible association of motor and cognitive decline in Drosophila, a genetic model organism of healthy aging. Long term courtship memory is present in young male flies but absent already during mid life (4-8 weeks). By contrast, courtship learning index and short term memory (STM) are surprisingly robust and remain stable through mid (4-8 weeks) and healthy late life (>8 weeks), until courtship performance collapses suddenly at ~4.5 days prior to death. By contrast, climbing speed declines gradually during late life (>8 weeks). The collapse of courtship performance and short term memory close to the end of life occur later and progress with a different time course than the gradual late life decline in climbing speed. Thus, during healthy aging in male Drosophila, climbing and courtship motor behaviors decline differentially. Moreover, cognitive and motor performances decline at different time courses. Differential behavioral decline during aging may indicate different underlying causes, or alternatively, a common cause but different thresholds for defects in different behaviors.

Impairment of the Glial Phagolysosomal System Drives Prion-Like Propagation in a Drosophila Model of Huntington's Disease.

Protein misfolding, aggregation, and spread through the brain are primary drivers of neurodegenerative disease pathogenesis. Phagocytic glia are responsible for regulating the load of pathological proteins in the brain, but emerging evidence suggests that glia may also act as vectors for aggregate spread. Accumulation of protein aggregates could compromise the ability of glia to eliminate toxic materials from the brain by disrupting efficient degradation in the phagolysosomal system. A better understanding of phagocytic glial cell deficiencies in the disease state could help to identify novel therapeutic targets for multiple neurological disorders. Here, we report that mutant huntingtin (mHTT) aggregates impair glial responsiveness to injury and capacity to degrade neuronal debris in male and female adult Drosophila expressing the gene that causes Huntington's disease (HD). mHTT aggregate formation in neurons impairs engulfment and clearance of injured axons and causes accumulation of phagolysosomes in glia. Neuronal mHTT expression induces upregulation of key innate immunity and phagocytic genes, some of which were found to regulate mHTT aggregate burden in the brain. A forward genetic screen revealed Rab10 as a novel component of Draper-dependent phagocytosis that regulates mHTT aggregate transmission from neurons to glia. These data suggest that glial phagocytic defects enable engulfed mHTT aggregates to evade lysosomal degradation and acquire prion-like characteristics. Together, our findings uncover new mechanisms that enhance our understanding of the beneficial and harmful effects of phagocytic glia in HD and other neurodegenerative diseases.

Chronic trans fatty acid consumption shortens lifespan in male Drosophila melanogaster on a high-sugar and high-fat diet.

Aging entails the progressive decline in the body's self-regulation and functionality over time. Notably, obesity and aging exhibit parallel phenotypes, with obesity further accelerating the aging process across multiple dimensions and diminishing lifespan. In this study, we explored the impact of trans fatty acid (TFA) consumption on the overall health and lifespan of male Drosophila melanogaster under an isocaloric high-sugar and high-fat diet. Our results indicate that TFA intake results in a shortened lifespan, elevated body weight, and increased triglyceride levels in flies fed a high-sugar and high-fat diet with equivalent caloric intake. Additionally, TFA exposure induces oxidative stress, locomotor deficits, and damage to the intestinal barrier in flies. Collectively, chronic TFA consumption expedites the aging process and reduces the lifespan of male Drosophila melanogaster. These results contribute supplementary evidence regarding the adverse health effects associated with TFAs.

Evolution of Phenotypic Variance Provides Insights into the Genetic Basis of Adaptation.

Most traits are polygenic, and the contributing loci can be identified by genome-wide association studies. The genetic basis of adaptation (adaptive architecture) is, however, difficult to characterize. Here, we propose to study the adaptive architecture of traits by monitoring the evolution of their phenotypic variance during adaptation to a new environment in well-defined laboratory conditions. Extensive computer simulations show that the evolution of phenotypic variance in a replicated experimental evolution setting can distinguish between oligogenic and polygenic adaptive architectures. We compared gene expression variance in male Drosophila simulans before and after 100 generations of adaptation to a novel hot environment. The variance change in gene expression was indistinguishable for genes with and without a significant change in mean expression after 100 generations of evolution. We suggest that the majority of adaptive gene expression evolution can be explained by a polygenic architecture. We propose that tracking the evolution of phenotypic variance across generations can provide an approach to characterize the adaptive architecture.

Effects of an entomopathogenic fungus on the reproductive potential of Drosophila males.

While mortality is often the primary focus of pathogen virulence, non-lethal consequences, particularly for male reproductive fitness, are less understood; however, they are essential for understanding how sexual selection contributes to promoting resistance. We investigated how the fungal pathogen Metarhizium brunneum affects mating ability, fertility, and seminal fluid protein (SFP) expression of male Drosophila melanogaster paired with highly receptive virgin females in non-competitive settings. Depending on sex and dose, there was a 3-6-day incubation period after infection, followed by an abrupt onset of mortality. Meanwhile, the immune response was strongly induced already 38 h after infection and continued to increase as infection progressed. Latency to mate somewhat increased during the incubation period compared to sham-treated males, but even on Day 5 post infection >90% of infected males mated within 2 h. During the incubation period, M. brunneum infection reduced male reproductive potential (the number of offspring sired without mate limitation) by 11%, with no clear increase over time. Approaching the end of the incubation period, infected males had lower ability to convert number of mating opportunities into number of offspring. After repeated mating, infected males had lower SFP expression than sham controls, more so in males that mated with few mates 24 h earlier. Overall, despite strong activation of the immune response, males' mating ability and fertility remained surprisingly little affected by the fungal infection, even shortly before the onset of mortality. This suggests that the selection for resistance acts mainly through mortality, and the scope for fertility selection to enhance resistance in non-competing settings is rather limited.

Functional Role of C-terminal Domains in the MSL2 Protein of Drosophila melanogaster.

Dosage compensation complex (DCC), which consists of five proteins and two non-coding RNAs roX, specifically binds to the X chromosome in males, providing a higher level of gene expression necessary to compensate for the monosomy of the sex chromosome in male Drosophila compared to the two X chromosomes in females. The MSL2 protein contains the N-terminal RING domain, which acts as an E3 ligase in ubiquitination of proteins and is the only subunit of the complex expressed only in males. Functional role of the two C-terminal domains of the MSL2 protein, enriched with proline (P-domain) and basic amino acids (B-domain), was investigated. As a result, it was shown that the B-domain destabilizes the MSL2 protein, which is associated with the presence of two lysines ubiquitination of which is under control of the RING domain of MSL2. The unstructured proline-rich domain stimulates transcription of the roX2 gene, which is necessary for effective formation of the dosage compensation complex.

Genetic Variation in Male Aggression Is Influenced by Genotype of Prior Social Partners in Drosophila melanogaster.

AbstractSocial behaviors can be influenced by the genotypes of interacting individuals through indirect genetic effects (IGEs) and can also display developmental plasticity. We investigated how developmental IGEs, which describe the effects of a prior social partner's genotype on later behavior, can influence aggression in male Drosophila melanogaster. We predicted that developmental IGEs cannot be estimated by simply extending the effects of contextual IGEs over time and instead have their own unique effects on behavior. On day 1 of the experiment, we measured aggressive behavior in 15 genotypic pairings ( males). On day 2, each of the males was paired with a new opponent, and aggressive behavior was again measured. We found contextual IGEs on day 1 of the experiment and developmental IGEs on day 2 of the experiment: the influence of the day 1 partner's genotype on the focal individual's day 2 behavior depended on the genotypic identity of both the day 1 partner and the focal male. Importantly, the developmental IGEs in our system produced fundamentally different dynamics than the contextual IGEs, as the presence of IGEs was altered over time. These findings represent some of the first empirical evidence demonstrating developmental IGEs, a first step toward incorporating developmental IGEs into our understanding of behavioral evolution.

Fasting as a precursor to high-fat diet enhances mitochondrial resilience in Drosophila melanogaster.

Changes in diet type and nutrient availability can impose significant environmental stress on organisms, potentially compromising physiological functions and reproductive success. In nature, dramatic fluctuations in dietary resources are often observed and adjustments to restore cellular homeostasis are crucial to survive this type of stress. In this study, we exposed male Drosophila melanogaster to two modulated dietary treatments: one without a fasting period before exposure to a high-fat diet and the other with a 24-h fasting period. We then investigated mitochondrial metabolism and molecular responses to these treatments. Exposure to a high-fat diet without a preceding fasting period resulted in disrupted mitochondrial respiration, notably at the level of complex I. On the other hand, a short fasting period before the high-fat diet maintained mitochondrial respiration. Generally, transcript abundance of genes associated with mitophagy, heat-shock proteins, mitochondrial biogenesis, and nutrient sensing pathways increased either slightly or significantly following a fasting period and remained stable when flies were subsequently put on a high-fat diet, whereas a drastic decrease of almost all transcript abundances was observed for all these pathways when flies were exposed directly to a high-fat diet. Moreover, mitochondrial enzymatic activities showed less variation after the fasting period than the treatment without a fasting period. Overall, our study sheds light on the mechanistic protective effects of fasting prior to a high-fat diet and highlights the metabolic flexibility of Drosophila mitochondria in response to abrupt dietary changes and have implication for adaptation of species to their changing environment.

Processivity and specificity of histone acetylation by the male-specificlethal complex.

Acetylation of lysine 16 of histone H4 (H4K16ac) stands out among the histone modifications, because it decompacts the chromatin fiber. The metazoan acetyltransferase MOF (KAT8) regulates transcription through H4K16 acetylation. Antibody-based studies had yielded inconclusive results about the selectivity of MOF to acetylate the H4 N-terminus. We used targeted mass spectrometry to examine the activity of MOF in the male-specificlethal core (4-MSL) complex on nucleosome array substrates. This complex is part of the Dosage Compensation Complex (DCC) that activates X-chromosomal genes in male Drosophila. During short reaction times, MOF acetylated H4K16 efficiently and with excellent selectivity. Upon longer incubation, the enzyme progressively acetylated lysines 12, 8 and 5, leading to a mixture of oligo-acetylated H4. Mathematical modeling suggests that MOF recognizes and acetylates H4K16 with high selectivity, but remains substrate-bound and continues to acetylate more N-terminal H4 lysines in a processive manner. The 4-MSL complex lacks non-coding roX RNA, a critical component of the DCC. Remarkably, addition of RNA to the reaction non-specifically suppressed H4 oligo-acetylation in favor of specific H4K16 acetylation. Because RNA destabilizes the MSL-nucleosome interaction in vitro we speculate that RNA accelerates enzyme-substrate turn-over in vivo, thus limiting the processivity of MOF, thereby increasing specific H4K16 acetylation.

The genome sequence of the drosophilid fruit fly, Drosophila phalerata (Meigen, 1830).

We present a genome assembly from an individual male Drosophila phalerata (drosophilid fruit fly, Arthropoda; Insecta; Diptera; Drosophilidae). The genome sequence is 223.9 megabases in span. Most of the assembly is scaffolded into 7 chromosomal pseudomolecules, including the X and Y sex chromosomes. The mitochondrial genome has also been assembled and is 16.14 kilobases in length. Gene annotation of this assembly on Ensembl identified 18,973 protein coding genes.

Yo-Yo Dieting Delays Male Drosophila melanogaster Aging Through Enhanced Mitochondrial Function, Relative to Sustained High-Calorie Diet Feeding.

Weight regain subsequent to weight reduction resulting from dietary interventions represents a prevalent phenomenon recognized as "Yo-yo dieting." However, the impact of prolonged Yo-yo dieting on health, especially in relation to the aging process, remains poorly understood. This study aimed to investigate the influence of Yo-yo dieting on the aging process in male Drosophila melanogaster that have been exposed to a high-calorie (HC) diet. Fruit flies were fed with either a consistent HC diet or an alternating regimen of HC and low-calorie diets every 3 days (referred to as "Yo-yo dieting") for a total of 24 days. Biochemical assays were utilized to quantify levels of oxidative stress and activities of the mitochondrial respiratory chain complexes. The frozen section staining method was employed to assess the presence of lipid droplets, reactive oxygen species, cellular viability, and mitochondrial abundance in tissues. Additionally, we examined the expression of key regulators involved in mitochondrial dynamics and biogenic signaling pathways. Yo-yo dieting resulted in an extension of the fruit flies' lifespan, concomitant with reduced body weight, decreased body protein content, and lower triglyceride levels compared to continuous a HC diet feeding. Furthermore, Yo-yo dieting ameliorated impairments in motility and intestinal barrier function. Importantly, it improved mitochondrial function and upregulated the expression of essential mitochondrial fusion proteins, namely mitofusin 1 and mitofusin 2, optic atrophy 1, and peroxisome proliferator-activated receptor-γ coactivator-1α. Therefore, the practice of Yo-yo dieting extends the lifespan of fruit flies by modulating mitochondrial dynamics and the associated biogenic signaling pathways.

Fecundity is optimized by levels of nutrient signal-dependent expression of Dve and EcR in Drosophila male accessory gland

Steroid hormones play various physiological roles including metabolism and reproduction. Steroid hormones in insects are ecdysteroids, and the major form in Drosophila melanogaster is ecdysone. In Drosophila males, the accessory gland is responsive to nutrient-dependent regulation of fertility/fecundity. The accessory gland is composed of two types of binucleated epithelial cells: a main cell and a secondary cell (SC). The transcription factors Defective proventriculus (Dve), Abdominal-B, and Ecdysone receptors (EcRs) are strongly expressed in adult SCs. We show that this EcR expression is regulated by parallel pathways of nutrient signaling and the Dve activity. Induction of Dve expression is also dependent on nutrient signaling, and it becomes nutrient signal-independent during a restricted period of development. Forced dve expression during the restricted period significantly increased the number of SCs. Here, we provide evidence that the level of nutrient signal-dependent Dve expression during the restricted period determines the number of SCs, and that ecdysone signaling is also crucial to optimize male fecundity through nutrient signal-dependent survival and maturation of SCs.

A new hybrid incompatibility locus between Drosophila melanogaster and Drosophila sechellia.

Despite the fundamental importance of hybrid incompatibilities to the process of speciation, there are few cases where the evolution and genetic architecture of hybrid incompatibilities are understood. One of the longest studied hybrid incompatibilities causes F1 hybrid male inviability in crosses between Drosophila melanogaster females and males from the Drosophila simulans clade of species-Drosophila simulans, Drosophila mauritiana, and Drosophila sechellia. Here, we discover dramatic differences in the manifestation of this lethal hybrid incompatibility among the D. simulans clade of species. In particular, F1 hybrid males between D. melanogaster and D. sechellia are resistant to hybrid rescue through RNAi knockdown of an essential hybrid incompatibility gene. To understand the genetic basis of this inter-species difference in hybrid rescue, we developed a triple-hybrid mapping method. Our results show that 2 discrete large effect loci and many dispersed small effect changes across the genome underlie D. sechellia aversion to hybrid rescue. The large effect loci encompass a known incompatibility gene Lethal hybrid rescue (Lhr) and previously unknown factor, Sechellia aversion to hybrid rescue (Satyr). These results show that the genetic architecture of F1 hybrid male inviability is overlapping but not identical in the 3 inter-species crosses. Our results raise questions about whether new hybrid incompatibility genes can integrate into an existing hybrid incompatibility thus increasing in complexity over time, or if the continued evolution of genes can gradually strengthen an existing hybrid incompatibility.

The Effects of poly-GA and poly-PR C9orf72 Dipeptide Repeats on Sleep Patterns in Drosophila melanogaster.

C9orf72 is the most common familial gene associated with amyotrophic lateral sclerosis (ALS). Dipeptide repeats (DPRs) encoded by an expanded nucleotide repeat sequence in the C9orf72 gene were found in the sleep-related neurons of patients, indicating a role of DPRs in ALS-associated sleep disruptions. Poly-GA or poly-PR DPRs were expressed in male Drosophila melanogaster to study their effect on sleep . Poly-PR expression caused sleep disruptions while poly-GA expression did not. This study validates the use of Drosophila as an in vivo model system for exploring the roles of DPRs in perturbing the underlying molecular mechanisms in sleep regulation.

Among- and Within-Individual Variance in Metabolic Thermal Reaction Norms.

AbstractIn ectotherms, temperature has a strong effect on metabolic rate (MR), yet the extent to which the thermal sensitivity of MR varies among versus within individuals is largely unknown. This is of interest because significant among-individual variation is a prerequisite for the evolution of metabolic thermal sensitivity. Here, we estimated the repeatability (R) of the thermal sensitivity of MR in individual virgin, adult male Drosophila melanogaster () by taking repeated overnight measures of their MRs at two temperatures (~24°C and ~27°C). At the population level, thermal sensitivity decreased with locomotor activity, and older individuals showed a higher thermal sensitivity of MR than younger individuals. Taking these effects (and body mass) into account, we detected significant repeatability in both the centered intercept () and the slope () of the metabolic thermal reaction norms, which respectively represent average MR and thermal sensitivity of MR. Furthermore, individuals with a higher overall MR also displayed greater increases in MR as temperature increased from ~24°C to ~27°C (). Average MR and thermal sensitivity of MR were also positively correlated within individuals (). Our study represents a point of departure for future larger studies, in which more complex protocols (e.g., wider temperature range, breeding design) can be applied to quantify the causal components of variation in thermal sensitivity that are needed to make accurate predictions of adaptive responses to global warming.

KYNURENINE PATHWAY INHIBITORS COMBINED WITH NAD+ PRECURSORS INCREASES LONGEVITY GENE EXPRESSION IN DROSOPHILA

Abstract With rates of aging-related functional decline rising, the need for novel intervention strategies to extend lifespan has increased. In humans, the kynurenine pathway (KP) metabolizes tryptophan and produces nicotinamide adenine dinucleotide (NAD+). KP is correlated negatively with longevity and increased KP metabolite concentrations decrease the average and maximum lifespans in flies. However, genetic and pharmacological inhibition of tryptophan metabolism and dietary supplementation of NAD+ precursors increase the average and maximum lifespans in flies. To comprehensively explore the effects of KP manipulation, we fed DGRP_229 Drosophila males diets infused with ⍺-MT (KP inhibitor), nicotinamide (NAM, an NAD+ precursor), or the combination of NAM and ⍺-MT. We found that all treatments resulted in increased average and maximum lifespans, with the combination treatment displaying the greatest increase compared to the control of 30% (P< 0.0001). The combination treatment resulted in an average lifespan increase of 15% in relation to ⍺-MT alone (P< 0.0001) and 18% in comparison to NAM alone (P< 0.0001). To gain more insight into the biological mechanisms through which different treatments affect lifespan, we performed qRT-PCR on 11 longevity genes in Drosophila. We found that the combination treatment significantly increased the expression of gigas (Gig) (P=0.03), which interacts with Tsc1 to control cellular growth by regulating insulin and TOR signaling pathways, and sirtuin 1 (Sirt1) (P=0.03), an NAD+-dependent histone deacetylase. Our results demonstrate that the combined treatment of KP inhibition and NAD+ precursor elicits a transcription response similar to other lifespan-enhancing treatments and could be used as an alternative intervention method.

3-HK SUPPLEMENTATION INCREASES NICOTINATE PHOSPHORIBOSYLTRANSFERASE (NAPRT) EXPRESSION IN DROSOPHILA

Abstract The increasing prevalence of physical frailty and aging related diseases in older adults necessitates the development of novel biological strategies to slow age-related declines. In humans, tryptophan metabolism through the kynurenine pathway (KP), which produces the energy metabolite and cellular cofactor nicotinamide adenine dinucleotide (NAD+), has been linked to various aging-related diseases including metabolic syndrome, neurodegenerative diseases, and physical frailty. Elevated levels of KP metabolites, such as 3-hydroxykinurenine (3-HK), correlate negatively with lifespan and physical function. A profile of downstream KP metabolites in the serum of older adults showed that 3-HK was increased in frail compared to both non-frail and young subjects, and liver kynurenine/tryptophan ratio correlates negatively with maximum lifespan in twenty-six mammalian species. To comprehensively explore the effects of elevated cytotoxic KP metabolites, we fed DGRP_229 Drosophila males diet infused with 3-HK. We found that treatment with 3-HK reduced average lifespan by 17% (P< 0.0001) compared against control. To elucidate the biological mechanisms of 3-HK treatment, we performed qRT-PCR on 11 genes involved in tryptophan metabolism. We found that treatment with 3-HK significantly decreased the expression of nicotinate phosphoribosyltransferase (Naprt) (P=0.0022), which plays an important role in NAD+ biosynthesis and protects against oxidative stress. Our findings indicate that some of the detrimental effects observed after 3-HK supplementation may be mediated by changes in NAD+ biosynthesis and highlight the potential feasibility of using NAD+ supplementation as an intervention method.

The neuropeptide drosulfakinin enhances choosiness and protects males from the aging effects of social perception.

The motivation to reproduce is a potent natural drive, and the social behaviors that induce it can severely impact animal health and lifespan. Indeed, in Drosophila males, accelerated aging associated with reproduction arises not from the physical act of courtship or copulation but instead from the motivational drive to court and mate. To better understand the mechanisms underlying social effects on aging, we studied male choosiness for mates. We found that increased activity of insulin-producing cells (IPCs) of the fly brain potentiated choosiness without consistently affecting courtship activity. Surprisingly, this effect was not caused by insulins themselves, but instead by drosulfakinin (DSK), another neuropeptide produced in a subset of the IPCs, acting through one of the two DSK receptors, CCKLR-17D1. Activation of Dsk+ IPC neurons also decreased food consumption, while activation of Dsk+ neurons outside of IPCs affected neither choosiness nor feeding, suggesting an overlap between Dsk+neurons modulating choosiness and those influencing satiety. Broader activation of Dsk+ neurons (both within and outside of the IPCs) was required to rescue the detrimental effect of female pheromone exposure on male lifespan, as was the function of both DSK receptors. The same broad set of Dsk+ neurons was found to reinforce normally aversive feeding interactions, but only after exposure to female pheromones, suggesting that perception of the opposite sex gates rewarding properties of these neurons. We speculate that broad Dsk+ neuron activation is associated with states of satiety and social experience, which under stressful conditions is rewarding and beneficial for lifespan.