Seasonal changes in urinary odors and in responses to them by blind subterranean mole rats

Terror in the dirt: Sensory determinants of host seeking in soil-transmitted mammalian-parasitic nematodes

Infection with gastrointestinal parasitic nematodes is a major cause of chronic morbidity and economic burden around the world, particularly in low-resource settings. Some parasitic nematode species, including the human-parasitic threadworm Strongyloides stercoralis and human-parasitic hookworms in the genera Ancylostoma and Necator, feature a soil-dwelling infective larval stage that seeks out hosts for infection using a variety of host-emitted sensory cues. Here, we review our current understanding of the behavioral responses of soil-dwelling infective larvae to host-emitted sensory cues, and the molecular and cellular mechanisms that mediate these responses. We also discuss the development of methods for transgenesis and CRISPR/Cas9-mediated targeted mutagenesis in Strongyloides stercoralis and the closely related rat parasite Strongyloides ratti. These methods have established S. stercoralis and S. ratti as genetic model systems for gastrointestinal parasitic nematodes and are enabling more detailed investigations into the neural mechanisms that underlie the sensory-driven behaviors of this medically and economically important class of parasites.

The neuroendocrinology of sexual attraction

Sexual attraction has two components: Emission of sexually attractive stimuli and responsiveness to these stimuli. In rodents, olfactory stimuli are necessary but not sufficient for attraction. We argue that body odors are far superior to odors from excreta (urine, feces) as sexual attractants. Body odors are produced by sebaceous glands all over the body surface and in specialized glands. In primates, visual stimuli, for example the sexual skin, are more important than olfactory. The role of gonadal hormones for the production of and responsiveness to odorants is well established. Both the androgen and the estrogen receptor α are important in male as well as in female rodents. Also in primates, gonadal hormones are necessary for the responsiveness to sexual attractants. In males, the androgen receptor is sufficient for sustaining responsiveness. In female non-human primates, estrogens are needed, whereas androgens seem to contribute to responsiveness in women.

Photoperiodic regulation of behavior: Peromyscus as a model system

Winter and summer present vastly different challenges to animals living outside of the tropics. To survive and reproduce, individuals must anticipate seasonal environmental changes and adjust physiology and behavior accordingly. Photoperiod (day length) offers a relatively 'noise free' environmental signal that non-tropical animals use to tell the time of year, and whether winter is approaching or receding. In some cases, photoperiodic signals may be fine-tuned by other proximate cues such as food availability or temperature. The pineal hormone, melatonin, is a primary physiological transducer of the photoperiodic signal. It tracks night length and provokes changes in physiology and behavior at appropriate times of the year. Because of their wide latitudinal distribution, Peromyscus has been well studied in the context of photoperiodic regulation of physiology and behavior. Here, we discuss how photoperiodic signals are transduced by pineal melatonin, how melatonin acts on target tissues, and subsequent consequences for behavior. Using a life-history paradigm involving trade-offs between the immune and reproductive systems, specific emphasis is placed on aggression, metabolism, and cognition. We discuss future directions including examining the effects of light pollution on photoperiodism, genetic manipulations to test the role of specific genes in the photoperiodic response, and using Peromyscus to test evolutionary theories of aging.

Temperature-dependent changes in the host-seeking behaviors of parasitic nematodes

Background

Entomopathogenic nematodes (EPNs) are lethal parasites of insects that are of interest as biocontrol agents for insect pests and disease vectors. Although EPNs have been successfully commercialized for pest control, their efficacy in the field is often inconsistent for reasons that remain elusive. EPN infective juveniles (IJs) actively search for hosts to infect using a diverse array of host-emitted odorants. Here we investigate whether their host-seeking behavior is subject to context-dependent modulation.

Results

We find that EPN IJs exhibit extreme plasticity of olfactory behavior as a function of cultivation temperature. Many odorants that are attractive for IJs grown at lower temperatures are repulsive for IJs grown at higher temperatures and vice versa. Temperature-induced changes in olfactory preferences occur gradually over the course of days to weeks and are reversible. Similar changes in olfactory behavior occur in some EPNs as a function of IJ age. EPNs also show temperature-dependent changes in their host-seeking strategy: IJs cultured at lower temperatures appear to more actively cruise for hosts than IJs cultured at higher temperatures. Furthermore, we find that the skin-penetrating rat parasite Strongyloides ratti also shows temperature-dependent changes in olfactory behavior, demonstrating that such changes occur in mammalian-parasitic nematodes.

Conclusions

IJs are developmentally arrested and long-lived, often surviving in the environment through multiple seasonal temperature changes. Temperature-dependent modulation of behavior may enable IJs to optimize host seeking in response to changing environmental conditions, and may play a previously unrecognized role in shaping the interactions of both beneficial and harmful parasitic nematodes with their hosts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0259-0) contains supplementary material, which is available to authorized users.

Detrimental role of prolonged sleep deprivation on adult neurogenesis

Adult mammalian brains continuously generate new neurons, a phenomenon called adult neurogenesis. Both environmental stimuli and endogenous factors are important regulators of adult neurogenesis. Sleep has an important role in normal brain physiology and its disturbance causes very stressful conditions, which disrupt normal brain physiology. Recently, an influence of sleep in adult neurogenesis has been established, mainly based on sleep deprivation studies. This review provides an overview on how rhythms and sleep cycles regulate hippocampal and subventricular zone neurogenesis, discussing some potential underlying mechanisms. In addition, our review highlights some interacting points between sleep and adult neurogenesis in brain function, such as learning, memory, and mood states, and provides some insights on the effects of antidepressants and hypnotic drugs on adult neurogenesis.

Gestational methyl donor deficiency alters key proteins involved in neurosteroidogenesis in the olfactory bulbs of newborn female rats and is associated with impaired olfactory performance

Gestational methyl donor deficiency (MDD) leads to growth retardation as well as to cognitive and motor disorders in 21-d-old rat pups. These disorders are related to impaired neurogenesis in the cerebral neurogenic areas. Olfactory bulbs (OB), the main target of neuronal progenitors originating from the subventricular zone, play a critical role during the postnatal period by allowing the pups to identify maternal odour. We hypothesised that growth retardation could result from impaired suckling due to impaired olfactory discrimination through imbalanced apoptosis/neurogenesis in the OB. Since neurosteroidogenesis modulates neurogenesis in OB, in the present study, we investigated whether altered neurosteroidogenesis could explain some these effects. Pups born to dams fed a normal diet (n 24) and a MDD diet (n 27) were subjected to olfactory tests during the lactation and weaning periods (n 24 and 20, respectively). We studied the markers of apoptosis/neurogenesis and the expression levels of the key neurosteroidogenic enzyme aromatase, the cholesterol-transfer protein StAR (steroidogenic acute regulatory protein) and the ERα oestrogen receptor and the content of oestradiol in OB. The 21-d-old MDD female pups displayed lower body weight and impaired olfactory discrimination when compared with the control pups. MDD led to greater homocysteine accumulation and more pronounced apoptosis, along with impaired cell proliferation in the OB of female pups. The expression levels of aromatase, StAR and ERα as well as the content of oestradiol were lower in the OB of the MDD female pups than in those of the control female pups. In conclusion, gestational MDD may alter olfactory discrimination performances by affecting neurogenesis, apoptosis and neurosteroidogenesis in OB in a sex-dependent manner. It may be involved in growth retardation through impaired suckling.

Photoperiod mediated changes in olfactory bulb neurogenesis and olfactory behavior in male white-footed mice (Peromyscus leucopus)

Brain plasticity, in relation to new adult mammalian neurons generated in the subgranular zone of the hippocampus, has been well described. However, the functional outcome of new adult olfactory neurons born in the subventricular zone of the lateral ventricles is not clearly defined, as manipulating neurogenesis through various methods has given inconsistent and conflicting results in lab mice. Several small rodent species, including Peromyscus leucopus, display seasonal (photoperiodic) brain plasticity in brain volume, hippocampal function, and hippocampus-dependent behaviors; plasticity in the olfactory system of photoperiodic rodents remains largely uninvestigated. We exposed adult male P. leucopus to long day lengths (LD) and short day lengths (SD) for 10 to 15 weeks and then examined olfactory bulb cell proliferation and survival using the thymidine analog BrdU, olfactory bulb granule cell morphology using Golgi-Cox staining, and behavioral investigation of same-sex conspecific urine. SD mice did not differ from LD counterparts in granular cell morphology of the dendrites or in dendritic spine density. Although there were no differences due to photoperiod in habituation to water odor, SD mice rapidly habituated to male urine, whereas LD mice did not. In addition, short day induced changes in olfactory behavior were associated with increased neurogenesis in the caudal plexiform and granule cell layers of the olfactory bulb, an area known to preferentially respond to water-soluble odorants. Taken together, these data demonstrate that photoperiod, without altering olfactory bulb neuronal morphology, alters olfactory bulb neurogenesis and olfactory behavior in Peromyscus leucopus.

What is Xenohormesis?

Polyphenols such as resveratrol and quercetin, which are produced by stressed plants, activate sirtuin enzymes and extend the lifespan of fungi and animals, ostensibly by mimicking the beneficial effects of caloric restriction. This observation raises an interesting question: Why should foreign molecules that are non-nutritive and seemingly unrelated to any endogenous molecule modulate the same biochemical pathways that mediate the response to an energy deficit? A possible explanation is that the sirtuin enzymes have evolved to respond to plant stress molecules as indicators of an impending deterioration of the environment. This idea has become known as the Xenohormesis Hypothesis, the name stemming from a combination of the prefix xeno-(for stranger) with hormesis (a protective response induced by mild stress). Here we review the evidence for xenohormesis in a broader context, taking into account the diverse spectrum of phytochemicals to which animals are exposed. We also consider alternative hypotheses that may explain some of the beneficial effects of plant-based foods. We suggest that xenohormesis, defined as an adaptive response in the physiology of an organism to molecular cues that are neither nutritive nor direct stressors, most likely occurs at some level. Whether this can fully or partially account for the beneficial effects of resveratrol and other phytochemicals remains to be seen. However, there is already sufficient cause to re-evaluate the relationship between complex organisms, including humans and their food.

Neuroendocrine control of urine-marking behavior in male rats

Sexually experienced Wistar male rats were used to investigate (a) urine voiding in the presence of nearby estrous females and the control of such voiding by (b) steroid hormones and (c) peripheral nerves supplying the genitourinary system. The first experiment showed that males always have a low rate of urine voiding that is significantly increased when a receptive female is around. Thus, it is suggested that an airborne scent from the female stimulates the olfactory system of males, triggering urine emission to transmit sex-related messages, i.e., male rats display the well-known urine-marking behavior of mammals. The number of urine marks and sniffing to females decreased after castration, and were restored after exogenous treatment with testosterone or estradiol. The proposed hypothesis is that airborne scents from the female activate the aromatization process in nuclei of the olfactory pathway of the male, evoking a cascade of neuronal responses that finish in urine marking. Peripheral nerves supplying the genitourinary system are the viscerocutaneous branch of the pelvic nerve (Vc) and the hypogastric (Hg). Data showed that both nerves are important for the central control of urine storage and voiding. Transection of Vc almost blocked urine marking, while Hg lesion increased the number of marks. Thus, it is discussed that Vc is the most important nerve in charge of voiding the bladder, and that Hg is important for continence.

Individual odour similarities across species parallel phylogenetic relationships in the S. ehrenbergi superspecies of mole-rats

Research using habituation techniques has shown that rodents from the same kin group, population, or species share similarities in their individual odours that covary with shared genetic similarities between them, that is, the closer their genetic relatedness, the more similar their odours. We assessed similarities in individual odours across four sibling species of subterranean mole-rats from the Spalax ehrenbergi superspecies in Israel. Mole-rats were habituated to the urine odour of a same-sex individual from one species then tested with urine odours of individuals from two different species of the superspecies. Subjects treated urine odours of individuals from more closely genetically related species as similar compared with the odours of individuals from a less closely related species, showing that the covariance between odours and genes extends across species. These similarities in odour also paralleled genetic similarities determined by molecular analysis: odours of descendent species were perceived as similar to those of their closest ancestral species, suggesting that some qualities of the odour of the ancestral species persist in the descendent species. It is generally assumed that during speciation incipient species develop species-specific markers, including, for example, odour markers, to facilitate discrimination of conspecifics from close ancestral heterospecifics. Our findings indicate that similarities in odours across species are more salient than species-specific odour markers. Such findings may also have important implications for mechanisms of species recognition. Copyright 2000 The Association for the Study of Animal Behaviour.