Odors from proximal species reverse the stress-decreased neurogenesis via main olfactory processing

Male Stressed Mice Having Behavioral Control Exhibit Escalations in Dorsal Dentate Adult-Born Neurons and Spatial Memory

An escapable (ES)/inescapable stress (IS) paradigm was used to study whether behavioral control and repeated footshock stressors may affect adult neurogenesis and related cognitive function. Male stressed mice having behavioral control (ES) had a short-term escalation in dorsal dentate gyrus (DG) neurogenesis, while similarly stressed mice having no such control had unaltered neurogenesis as compared to control mice receiving no stressors. Paradoxically, ES and IS mice had comparable stress-induced corticosterone elevations throughout the stress regimen. Appetitive operant conditioning and forced running procedures were used to model learning and exercise effects in this escapable/inescapable paradigm. Further, conditioning and running procedures did not seem to affect the mice's corticosterone or short-term neurogenesis. ES and IS mice did not show noticeable long-term changes in their dorsal DG neurogenesis, gliogenesis, local neuronal density, apoptosis, autophagic flux, or heterotypic stress responses. ES mice were found to have a greater number of previously labeled and functionally integrated DG neurons as compared to IS and control mice 6 weeks after the conclusion of the stressor regimen. Likewise, ES mice outperformed IS and non-stressed control mice for the first two, but not the remaining two, trials in the object location task. Compared to non-stressed controls, temozolomide-treated ES and IS mice having a lower number of dorsal DG 6-week-old neurons display poor performance in their object location working memory. These results, taken together, prompt us to conclude that repeated stressors, albeit their corticosterone secretion-stimulating effect, do not necessary affect adult dorsal DG neurogenesis. Moreover, stressed animals having behavioral control may display adult neurogenesis escalation in the dorsal DG. Furthermore, the number of 6-week-old and functionally-integrated neurons in the dorsal DG seems to confer the quality of spatial location working memory. Finally, these 6-week-old, adult-born neurons seem to contribute spatial location memory in a use-dependent manner.

Social disruption-induced stress pre-exposure aggravates, while the presence of conspecifics diminishes, acetic acid-induced writhing

RATIONALE AND OBJECTIVE

This study was undertaken to assess the modulating effects of (1) pre-exposure to repeated social disruption and (2) group testing on writhing associated with visceral pain induced by intraperitoneal administration of acetic acid.

MATERIALS AND METHODS

Six consecutive days of social disruption were used to prime for stress, while group testing referred to 3 mouse cage-mates receiving the acetic acid-induced writhing test as a group.

RESULTS

Social disruption-induced stress-pre-exposed mice displayed a greater number acid-induced writhes compared to mice not receiving the pre-exposure. However, mice displayed fewer acid-induced writhes in a triad group vs. individually, suggesting group-mediated writhing-reducing effects. Likewise, group testing prevented the stress pre-exposure escalation in acid-induced writhes. Additional studies revealed that the stress-pre-exposed mice had increased expression in accumbal TRPV1 receptors. Systemic (0.25 mg/kg) and bilateral intra-accumbal (0.2 ng/0.2 µl/side) administration of SB366791, a TRPV1 receptor antagonist, reliably prevented the stress pre-exposure escalation in acid-induced writhing; SB366791 treatment alone did not affect acid-induced writhing, stress pre-exposure anxiety-like behavior, or the group testing effects. Furthermore, lower neuronal activation was found in the medial septal nucleus in group vs. individual tested mice. Intra-medial septum (0.2 µg/0.5 µl) infusion with bicuculline, a GABAA receptor antagonist, effectively prevented group-mediated writhing-reducing effects, but not individual acid-induced writhing effects.

CONCLUSIONS

These findings suggest that social disruption-induced stress pre-exposure may upregulate accumbal TRPV1 receptor expression and consequently aggravate acid-induced writhing. Group testing prevents such stress pre-exposure escalation of acid-induced writhing most likely by strengthening the GABAergic inhibition on local neural activity in the medial septum.

Smell and Stress Response in the Brain: Review of the Connection between Chemistry and Neuropharmacology

The stress response in the brain is not fully understood, although stress is one of the risk factors for developing mental disorders. On the other hand, the stimulation of the olfactory system can influence stress levels, and a certain smell has been empirically known to have a stress-suppressing effect, indeed. In this review, we first outline what stress is and previous studies on stress-responsive biomarkers (stress markers) in the brain. Subsequently, we confirm the olfactory system and review previous studies on the relationship between smell and stress response by species, such as humans, rats, and mice. Numerous studies demonstrated the stress-suppressing effects of aroma. There are also investigations showing the effects of odor that induce stress in experimental animals. In addition, we introduce recent studies on the effects of aroma of coffee beans and essential oils, such as lavender, cypress, α-pinene, and thyme linalool on the behavior and the expression of stress marker candidates in the brain. The transfer of volatile components into the brain is also discussed while using the results of thyme linalool as an example. These studies may provide a good opportunity to connect chemical research at the molecular level with neuropharmacological approaches in the future.

Hormonal Regulation of Mammalian Adult Neurogenesis: A Multifaceted Mechanism

Adult neurogenesis—resulting in adult-generated functioning, integrated neurons—is still one of the most captivating research areas of neuroplasticity. The addition of new neurons in adulthood follows a seemingly consistent multi-step process. These neurogenic stages include proliferation, differentiation, migration, maturation/survival, and integration of new neurons into the existing neuronal network. Most studies assessing the impact of exogenous (e.g., restraint stress) or endogenous (e.g., neurotrophins) factors on adult neurogenesis have focused on proliferation, survival, and neuronal differentiation. This review will discuss the multifaceted impact of hormones on these various stages of adult neurogenesis. Specifically, we will review the evidence for hormonal facilitation (via gonadal hormones), inhibition (via glucocorticoids), and neuroprotection (via recruitment of other neurochemicals such as neurotrophin and neuromodulators) on newly adult-generated neurons in the mammalian brain.

Relevance of number and physiological status of conspecifics in preventing stress-induced decreases in newly proliferated cells and neuroblasts

RATIONALE AND OBJECTIVE

The presence of three conspecifics prevents stress-induced decreases in newly proliferated cells and neuroblasts in mouse dentate gyrus (DG). In this study, we sought to determine how many conspecifics are required to exert these protective effects against stress. In addition, we manipulated the physiological status of those conspecifics in the context of their stress-buffering effects and used airborne oxytocin exposure as a substitute for the presence of conspecifics.

MATERIALS AND METHODS

Bromodeoxyuridine staining was used to indicate the newly proliferated cells and co-staining with doublecortin to reveal the proliferative neuroblasts.

RESULTS

Presentation of three intact and lipopolysaccharide-treated conspecifics prevented the stress-induced decreases in the number of newly proliferated cells and neuroblasts in DG. Presentation of one saline- or oxytocin (OT)-treated conspecific did not exert observable stress-buffering effects. In contrast, airborne oxytocin prevented the stress-induced decreases in DG cell proliferation and early neurogenesis, while pretreatment with L-371,257, a selective OT receptor antagonist, abolished the buffering effects of OT.

CONCLUSIONS

Physical interaction with the conspecifics and conspecifics' sickness, at best, play a minor role in mediating the buffering effects against stress-induced decreases in DG cell proliferation or early neurogenesis. Moreover, stress-buffering effects are negligible with the presence of only one conspecific. Finally, airborne OT produced stress-buffering effects possibly via its stimulation of OT receptors. Oxytocin merits further study as a substitute for the stress-buffering effects of companions.

Sex Differences in Stress and Group Housing Effects on the Number of Newly Proliferated Cells and Neuroblasts in Middle-Aged Dentate Gyrus

Sex differences in stress and coping responses have been frequently documented in aged people, while whether such differences in aged people may appear at the middle age are unknown. This study was undertaken to study the impact of acute stress and social interaction on early neurogenesis in the dentate gyrus (DG) and hippocampus-related memory in two sexes of middle-aged mice. The number of newly proliferated cells, neuroblasts in DG, the object recognition and location memory in 9-month-old male and female C57BL/6N mice were assessed under baseline conditions as well as following an acute stressor regimen and group housing. Three conspecific companions, serving as "the housing group," were used to model the social interaction throughout the stressor regimen. Males had lower numbers of newly proliferated cells and neuroblasts under baseline conditions as compared to females. The stressor regimen caused rapid decreases in the number of newly proliferated cells and neuroblasts in female DG but no obvious changes were observed in male DG. Group housing, regardless of companions' age, prevented the stress-induced decreases in the number of newly proliferated cells and neuroblasts in female DG. In contrast, the presence of young or age-matched companions potentiated the stress effect in males by decreasing the number of newly proliferated cells and neuroblasts. Finally, neither the stressor regimen nor group housing affected mouse performances in the object recognition and location memory in either sex. These findings, taken together, provide evidence to support a notion that middle-aged females appear to demonstrate more stress susceptibility on early neurogenesis in DG as compared to middle-aged males, although the hippocampus-related memory performances are comparable and not affected by stress in these males and females. Experiencing stress, middle-aged females are more prone to benefit from social interaction as compared to middle-aged males in this regard. We suggest, accordingly, that involving social interaction may afford a therapeutic advance in preventing stress-produced decreases in early neurogenesis in middle-aged females' DG.

Rottlerin impairs the formation and maintenance of psychostimulant-supported memory

RATIONALE AND OBJECTIVE

Since brain proteins such as protein kinase C (PKC), brain-derived neurotrophic factor (BDNF), and mammalian target of rapamycin (mTOR) are involved in the establishment and maintenance of psychostimulant memory, we sought to determine if systemic treatment with rottlerin, a natural compound affecting all these proteins, may modulate stimulant-supported memory.

MATERIALS AND METHODS

Stimulant-induced conditioned place preference (CPP) was used in modeling stimulant-supported memory.

RESULTS

Three cocaine (10 mg/kg; COC) or three methamphetamine (1 mg/kg; MA) conditioning trials reliably established the drug-induced CPP in male C57BL/6 mice. An intra-peritoneal rottlerin injection (5 mg/kg) at least 24 h prior to the first COC or first MA conditioning trial prevented the establishment of CPP. Following the establishment of the COC- or MA-induced CPP, saline conditioning trial was used to extinguish the CPP. Rottlerin (5 mg/kg, intra-peritoneal (i.p.)) administered 20 h prior to the first saline conditioning trial diminished subsequent drug- and stressor-primed reinstatement of the extinguished CPP. Rottlerin (5 mg/kg, i.p.) produced a fast-onset and long-lasting increase in hippocampal BDNF levels. However, treatment with a BDNF tropomyosin receptor kinase B (TrkB) receptor antagonist, K252a (5 μg/kg), did not affect rottlerin's suppressing effect on COC-induced CPP and treatment with 7,8-dihydroxyflavone (10 mg/kg x 6, 7,8-DHF), a selective TrkB agonist, prior to each conditioning trial did not affect COC-induced CPP.

CONCLUSION

These results suggest that systemic rottlerin treatment may impair the formation of COC- and MA-supported memory. Importantly, such a treatment may advance our understanding of the underlying mechanism through which extinction training resulted in the "forgetting" of the COC- and MA-supported memory.

Behavioral responses to odors from other species: introducing a complementary model of allelochemics involving vertebrates

It has long been known that the behavior of an animal can be affected by odors from another species. Such interspecific effects of odorous compounds (allelochemics) are usually characterized according to who benefits (emitter, receiver, or both) and the odors categorized accordingly (allomones, kairomones, and synomones, respectively), which has its origin in the definition of pheromones, i.e., intraspecific communication via volatile compounds. When considering vertebrates, however, interspecific odor-based effects exist which do not fit well in this paradigm. Three aspects in particular do not encompass all interspecific semiochemical effects: one relates to the innateness of the behavioral response, another to the origin of the odor, and the third to the intent of the message. In this review we focus on vertebrates, and present examples of behavioral responses of animals to odors from other species with specific reference to these three aspects. Searching for a more useful classification of allelochemical effects we examine the relationship between the valence of odors (attractive through to aversive), and the relative contributions of learned and unconditioned (innate) behavioral responses to odors from other species. We propose that these two factors (odor valence and learning) may offer an alternative way to describe the nature of interspecific olfactory effects involving vertebrates compared to the current focus on who benefits.

Sex differences and the modulating effects of gonadal hormones on basal and the stressor-decreased newly proliferative cells and neuroblasts in dentate gyrus

This study was undertaken to assess sex differences and the modulating effects of gonad intactness and the estrous phase on basal and the stressor-decreased cell proliferation and early differentiation in Balb/C mouse dentate gyrus (DG). Besides, we compared the stress-reversing effects exerted by the presence of male and female Balb/C mouse odors in stressed male and female mouse DG in this regard. Female mice had lower baselines in the number of newly proliferated cells and neuroblasts than male mice. Although the stressor induced decreases in the number of newly proliferative cells and neuroblasts in both male and female DG, an obvious decrease in neuronal lineage commitment was observed in female DG. Moreover, ovariectomy induced decreases in baselines in the number of proliferative cells and neuroblasts but did not affect the stressor-induced decrease in neuronal lineage commitment in female DG. Interestingly, pro-estrous mice exhibited the stressor-decreased neuronal lineage commitment, while estrous and diestrous mice did not display such a decrease. Furthermore, orchidectomy did not affect basal or the stressor-decreased newly proliferative cells or neuroblasts in male DG. Finally, male odors were less effective than female odors in abolishing the stressor-decreased neuronal lineage commitment in female mice, while male and female odors were comparable in reversing the stressor-decreased newly proliferated cells and neuroblasts in male mice. The protective effects of mouse odors' company in the stressed male mouse DG were associated with local BDNF and NGF replenishment. Taken together, sexual differences in baselines in the number of newly proliferative cells, neuroblasts, and the sensitivity to stress-altered neuronal lineage commitment in the DG could be, in part, due to gonadal hormone differences between the two sexes. Mouse odors may reverse stressor-decreased newly proliferative cells and neuroblasts in male, but not in female, mouse DG by restoring BDNF and NGF levels.

Companions reverse stressor-induced decreases in neurogenesis and cocaine conditioning possibly by restoring BDNF and NGF levels in dentate gyrus

The presence of companions can reverse the stressor-induced decrease in neurogenesis in mouse dentate gyrus (DG). In this study, we decided to study the underlying mechanisms of the companions' protective effect and to assess whether two DG neurogenesis-related memories, cocaine-induced conditioned place preference (CPP) and spatial memory, can be affected by our stressor and companions. Neurotrophin levels in DG were measured, in this regard, to reveal their roles in mediating the stressors' and companions' effect. We found that the stressor did not affect NT-3 but acutely decreased NGF and BDNF levels in DG. The presence of companions abolished these stressor-decreased NGF and BDNF levels. Neither the stressor nor the presence of companions affected TrkA, TrkB or TrkC expression in DG. Pre-exposure to the stressor rendered deficits in cocaine-induced CPP and spatial memory, while companions reversed the stressor-decreased cocaine-induced CPP. Intra-ventricular infusion with K252a, a mixed TrkA and TrkB antagonist, did not affect the protective effects of companions on local NGF, BDNF levels in DG, but abolished the companions' protective effects against the stressor-decreased DG neurogenesis and cocaine-induced CPP. Systemic pretreatment with 7,8-dihydroxyflavone (DHF), a selective TrkB agonist, did not affect baseline, the stressor-stimulated corticosterone (CORT) secretion or local NGF, BDNF levels in DG, but in part mimicked companions' protective effects. These results, taken together, indicate that stressor-decreased NGF and BDNF levels in DG could be involved in the stressor-decreased DG neurogenesis and cocaine conditioning. The presence of companions reverses the stressor-decreased DG neurogenesis and cocaine conditioning possibly by restoring BDNF and NGF levels in DG.