Sex differences in contextual pattern separation, neurogenesis, and functional connectivity within the limbic system

The impact of chronic intermittent hypoxia on enzymatic activity in memory-associated brain regions of male and female rats

BACKGROUND

Obstructive sleep apnea (OSA) is an intermittent hypoxia disorder associated with cognitive dysfunction, including learning and memory impairments. There is evidence that alterations in protease activity and neuronal activation are associated with cognitive dysfunction, are dependent on sex, and may be brain region-specific. However, the mechanisms mediating OSA-induced cognitive impairments are unclear. Therefore, we used a rat model of OSA, chronic intermittent hypoxia (CIH) to investigate protease activity (e.g., calpain and caspase-3) on spectrin, a cytoskeletal protein associated with neurotransmitter release, and neuronal activation (early growth response protein 1, EGR-1) in brain regions associated with learning and memory.

METHODS

Male and female Sprague Dawley rats were exposed to CIH or room air (normoxic) for 14 days. We quantified protease activity and cleaved spectrin products, along with EGR-1 protein expression in hippocampal subregions (CA1, CA3), cortical regions [entorhinal cortex (ETC), retrosplenial cortex (RSC), cerebellar cortex (CC)], and subcortical regions [raphe nucleus (RN), locus coeruleus (LC)] associated with learning and memory. Within each group, Pearson correlations of calpain activity, caspase-3 activity, and EGR-1 expression were performed between brain regions. Sex differences within normoxic and CIH correlations were examined.

RESULTS

CIH dysregulated calpain activity in male ETC, and female CA1 and RSC. CIH dysregulated caspase-3 activity in male RN, and female CA1 and RSC. CIH decreased calpain and caspase-3 cleavage products in male ETC. CIH decreased calpain-cleaved spectrin in male RSC but increased these products in female RSC. EGR-1 expression was decreased in male and female RN. Correlational analysis revealed CIH increased excitatory connections in males and increased inhibitory connections in females. EGR-1 expression in males shifted from negative to positive correlations.

CONCLUSIONS

Overall, these data indicate CIH dysregulates protease activity and impairs neuronal function in a brain region- and sex-dependent manner. This indicates that males and females exhibit sex-specific vulnerabilities to mild OSA. These findings concur with our previous behavioral studies that demonstrated memory impairment in CIH-exposed rats.

Dentate gyrus norepinephrine ramping facilitates aversive contextual processing

Dysregulation in aversive contextual processing is believed to affect several forms of psychopathology, including post-traumatic stress disorder (PTSD). The dentate gyrus (DG) is an important brain region in contextual discrimination and disambiguation of new experiences from prior memories. The DG also receives dense projections from the locus coeruleus (LC), the primary source of norepinephrine (NE) in the mammalian brain, which is active during stressful events. However, how noradrenergic dynamics impact DG-dependent function during contextual discrimination and pattern separation remains unclear. Here, we report that aversive contextual processing in mice is linked to linear elevations in tonic norepinephrine release dynamics within the DG and report that this engagement of prolonged norepinephrine release is sufficient to produce contextual disambiguation, even in the absence of a salient aversive stimulus. These findings suggest that spatiotemporal ramping characteristics of LC-NE release in the DG during stress likely serve an important role in driving contextual processing.

Alterations in local activity and whole-brain functional connectivity in human immunodeficiency virus-associated neurocognitive disorders: a resting-state functional magnetic resonance imaging study

Background

Approximately half of human immunodeficiency virus (HIV) patients experience HIV-associated neurocognitive disorders (HAND); however, the neurophysiological mechanisms underlying HAND remain unclear. This study aimed to evaluate changes in functional brain activity patterns during the early stages of HIV infection by comparing local and global indicators using resting-state functional magnetic resonance imaging (rs-fMRI).

Methods

A total of 165 people living with HIV (PLWH) but without neurocognitive disorders (PWND), 173 patients with asymptomatic neurocognitive impairment (ANI), and 100 matched healthy controls (HCs) were included in the study. A cross-sectional study of the participants was conducted. The metrics of functional segregation and integration were computed, using graph theory to explore differences across methodologies. Brain functional changes in the PWND and ANI groups were assessed, and correlations between the rs-fMRI metrics, clinical data, and cognitive function were examined.

Results

As cognitive function declined, changes reflected by regional homogeneity (ReHo) were primarily observed in the default mode network (DMN). In the DMN and visual network (VIS), amplitude of low-frequency fluctuation (ALFF) decreases were mainly observed in the parieto-occipital lobes, while increases were mainly observed in the limbic network (LIM). Reductions in fractional ALFF (fALFF) were mainly observed in the somatomotor network (SMN) and LIM, while increases were observed in the DMN and LIM. Unlike local indicators, global functional connectivity (FC) significantly decreased in both the PWND and ANI groups compared to the HC group. The ANI group showed partial increases in FC compared to the PWND group, with major changes observed in the DMN, VIS, and LIM. Notably, FC between the right insula and right supramarginal gyrus decreased significantly following HIV infection, while FC between the right caudate nucleus and the left middle frontal gyrus declined further in the ANI group. Graph theory further confirmed the significance of the DMN, and revealed changes in the eigenvector centrality mapping (ECM) values of the frontoparietal network (FPN) and dorsal attention network (DAN).

Conclusions

HIV patients exhibit complex changes in both local and global brain activity, regardless of cognitive impairment. Widespread abnormalities primarily involve the DMN, VIS, and LIM. Changes in FC along the fronto-striatal pathway may play a crucial role in the decline of cognitive function in individuals with HAND. Our findings provide new insights that may assist in the early detection of brain damage in the early stages of HIV infection. The use of multiple methodologies may offer a more comprehensive and effective approach, enabling the early detection of brain damage in HIV patients.

The impact of chronic intermittent hypoxia on enzymatic activity in memory-associated brain regions of male and female rats

Background

Obstructive sleep apnea (OSA) is an intermittent hypoxia disorder associated with cognitive dysfunction, including learning and memory impairments. There is evidence that alterations in protease activity and neuronal activation as associated with cognitive dysfunction, are dependent on sex, and may be brain region-specific. However, the mechanisms mediating OSA-induced cognitive impairments are unclear. Therefore, we used a rat model of OSA, chronic intermittent hypoxia (CIH), to investigate protease activity (e.g., calpain and caspase-3) and neuronal activation (early growth response protein 1, EGR-1) in brain regions associated with learning and memory. We used a rat model of OSA known as chronic intermittent hypoxia (CIH) to investigate protease activity (calpain and caspase-3) and neuronal activation (early growth response protein 1, EGR-1) in brain regions associated with learning and memory.

Methods

Male and female Sprague Dawley rats were exposed to CIH or room air (normoxic) for 14 days. We quantified protease activity and cleaved spectrin products, along with EGR-1 protein expression in hippocampal subregions (CA1, CA3), cortical regions [entorhinal cortex (ETC), retrosplenial cortex (RSC), cerebellar cortex (CC)], and subcortical regions [raphe nucleus (RN), locus coeruleus (LC)] associated with learning and memory. Within each group, Pearson correlations of calpain activity, caspase-3 activity, and EGR-1 expression were performed between brain regions. Sex differences within normoxic and CIH correlations were examined.

Results

CIH dysregulated calpain activity in male ETC and female CA1 and RSC. CIH dysregulated caspase-3 activity in male RN and female CA1 and RSC. CIH decreased calpain and caspase-3 cleavage products in male ETC. CIH decreased calpain-cleaved spectrin in male RSC but increased these products in female RSC. EGR-1 expression was decreased in male and female RN. Correlational analysis revealed CIH increased excitatory connections in males and increased inhibitory connections in females. EGR-1 expression in males shifted from negative to positive correlations.

Conclusions

Overall, these data show that CIH dysregulates protease activity and impairs neuronal function in a brain region- and sex-dependent manner. This indicates that males and females exhibit sex-specific vulnerabilities to mild OSA. These findings concur with our previous behavioral studies that demonstrated memory impairment in CIH-exposed rats.

Leveraging research into sex differences and steroid hormones to improve brain health

Sex differences, driven in part by steroid hormones, shape the structure and function of the brain throughout the lifespan and manifest across brain health and disease. The influence of steroid hormones on neuroplasticity, particularly in the adult hippocampus, differs between the sexes, which has important implications for disorders and diseases that compromise hippocampus integrity, such as depression and Alzheimer disease. This Review outlines the intricate relationship between steroid hormones and hippocampal neuroplasticity across the adult lifespan and explores how the unique physiology of male and female individuals can affect health and disease. Despite calls to include sex and gender in research, only 5% of neuroscience studies published in 2019 directly investigated the influence of sex. Drawing on insights from depression, Alzheimer disease and relevant hippocampal plasticity, this Review underscores the importance of considering sex and steroid hormones to achieve a comprehensive understanding of disease susceptibility and mechanisms. Such consideration will enable the discovery of personalized treatments, ultimately leading to improved health outcomes for all.

Dentate Gyrus Norepinephrine Ramping Facilitates Aversive Contextual Processing

Dysregulation in aversive contextual processing is believed to affect several forms of psychopathology, including post-traumatic stress disorder (PTSD). The dentate gyrus (DG) is an important brain region in contextual discrimination and disambiguation of new experiences from prior memories. The DG also receives dense projections from the locus coeruleus (LC), the primary source of norepinephrine (NE) in the mammalian brain, which is active during stressful events. However, how noradrenergic dynamics impact DG-dependent function during contextual discrimination and pattern separation remains unclear. Here, we report that aversive contextual processing in mice is linked to linear elevations in tonic norepinephrine release dynamics within the DG and report that this engagement of prolonged norepinephrine release is sufficient to produce contextual disambiguation, even in the absence of a salient aversive stimulus. These findings suggest that spatiotemporal ramping characteristics of LC-NE release in the DG during stress likely serve an important role in driving contextual processing.

Acute sleep loss impairs object but not spatial pattern separation in humans

Pattern separation allows us to form discrete representations of information in memory. Pattern separation can be measured in several domains including spatial and object-based discrimination. The brain area largely involved in this process is the dentate gyrus of the hippocampus, which has been shown to be particularly sensitive to the effects of sleep loss. However, methodology in rodent and human studies varies greatly making translational conclusions difficult. Therefore, the aim of the current study was to measure the effects of sleep deprivation on human hippocampal function, using well-validated spatial and object-based pattern separation tests. The effects of acute sleep loss were examined, as this method is frequently used in rodent research but not human studies. Results show that sleep loss impaired performance on the object-based version of the test, but not spatial pattern separation. The findings support the notion that these discrimination projections represent separate but complimentary hippocampal processes, and further elucidates how they may be discretely affected by acute sleep loss.

High estradiol reduces adult neurogenesis but strengthens functional connectivity within the hippocampus during spatial pattern separation in adult female rats

Adult neurogenesis in the dentate gyrus plays an important role for pattern separation, the process of separating similar inputs and forming distinct neural representations. Estradiol modulates neurogenesis and hippocampus function, but to date no examination of estradiol's effects on pattern separation have been conducted. Here, we examined estrogenic regulation of adult neurogenesis and functional connectivity in the hippocampus after the spatial pattern separation task in female rats. Ovariectomized Sprague-Dawley rats received daily injections of vehicle, 0.32 μg (Low) or 5 μg (High) of estradiol benzoate until the end of experiment. A single bromodeoxyuridine (BrdU) was injected one day after initiation of hormone or vehicle treatment and rats were tested in the delayed nonmatching to position spatial pattern separation task in the 8-arm radial maze for 12 days beginning two weeks after BrdU injection. Rats were perfused 90 min after the final trial and brain sections were immunohistochemically stained for BrdU/neuronal nuclei (NeuN) (new neurons), Ki67 (cell proliferation), and the immediate early gene, zif268 (activation). Results showed that high, but not low, estradiol reduced the density of BrdU/NeuN-ir cells and had significant inter-regional correlations of zif268-ir cell density in the hippocampus following pattern separation. Estradiol treatment did not influence pattern separation performance or strategy use. These results show that higher doses of estradiol can reduce neurogenesis but at the same time increases correlations of activity of neurons within the hippocampus during spatial pattern separation.

Toward Antifragility: Social Defeat Stress Enhances Learning and Memory in Young Mice Via Hippocampal Synaptosome Associated Protein 25

Social adversity not only causes severe psychological diseases but also may improve people's ability to learn and grow. However, the beneficial effects of social adversity are often ignored. In this study, we investigated whether and how social adversity affects learning and memory in a mouse social defeat stress (SDS) model. A total of 652 mice were placed in experimental groups of six to 23 mice each. SDS enhanced spatial, novelty, and fear memory with increased synaptosome associated protein 25 (SNAP-25) level and dendritic spine density in hippocampal neurons among young but not middle-aged mice. Chemogenetic inhibition of hippocampal CaMK2A+ neurons blocked SDS-induced enhancement of learning or memory. Knockdown of SNAP-25 or blockade of N-methyl-D-aspartate (NMDA) receptor subunit GluN2B in the hippocampus prevented SDS-induced learning memory enhancement in an emotion-independent manner. These findings suggest that social adversity promotes learning and memory ability in youths and provide a neurobiological foundation for biopsychological antifragility.

Young adult and aged female rats are vulnerable to amygdala-dependent, but not hippocampus-dependent, memory impairment following short-term high-fat diet

Global populations are increasingly consuming diets high in saturated fats and refined carbohydrates, and such diets have been well-associated with heightened inflammation and neurological dysfunction. Notably, older individuals are particularly vulnerable to the impact of unhealthy diet on cognition, even after a single meal, and pre-clinical rodent studies have demonstrated that short-term consumption of high-fat diet (HFD) induces marked increases in neuroinflammation and cognitive impairment. Unfortunately though, to date, most studies on the topic of nutrition and cognition, especially in aging, have been performed only in male rodents. This is especially concerning given that older females are more vulnerable to develop certain memory deficits and/or severe memory-related pathologies than males. Thus, the aim of the present study was to determine the extent to which short-term HFD consumption impacts memory function and neuroinflammation in female rats. Young adult (3 months) and aged (20-22 months) female rats were fed HFD for 3 days. Using contextual fear conditioning, we found that HFD had no effect on long-term contextual memory (hippocampus-dependent) at either age, but impaired long-term auditory-cued memory (amygdala-dependent) regardless of age. Gene expression of Il-1β was markedly dysregulated in the amygdala, but not hippocampus, of both young and aged rats after 3 days of HFD. Interestingly, modulation of IL-1 signaling via central administration of the IL-1 receptor antagonist (which we have previously demonstrated to be protective in males) had no impact on memory function following the HFD in females. Investigation of the memory-associated gene Pacap and its receptor Pac1r revealed differential effects of HFD on their expression in the hippocampus and amygdala. Specifically, HFD induced increased expression of Pacap and Pac1r in the hippocampus, whereas decreased Pacap was observed in the amygdala. Collectively, these data suggest that both young adult and aged female rats are vulnerable to amygdala-dependent (but not hippocampus-dependent) memory impairments following short-term HFD consumption, and identify potential mechanisms related to IL-1β and PACAP signaling in these differential effects. Notably, these findings are strikingly different than those previously reported in male rats using the same diet regimen and behavioral paradigms, and highlight the importance of examining potential sex differences in the context of neuroimmune-associated cognitive dysfunction.

Sex and BDNF Val66Met polymorphism matter for exercise-induced increase in neurogenesis and cognition in middle-aged mice

Females show greater benefits of exercise on cognition in both humans and rodents, which may be related to brain-derived neurotrophic factor (BDNF). A single nucleotide polymorphism (SNP), the Val66Met polymorphism, within the human BDNF gene, causes impaired activity-dependent secretion of neuronal BDNF and impairments to some forms of memory. We evaluated whether sex and BDNF genotype (Val66Met polymorphism (Met/Met) versus wild-type (Val/Val)) influenced the ability of voluntary running to enhance cognition and hippocampal neurogenesis in mice. Middle-aged C57BL/6J (13 months) mice were randomly assigned to either a control or an aerobic training (AT) group (running disk access). Mice were trained on the visual discrimination and reversal paradigm in a touchscreen-based technology to evaluate cognitive flexibility. BDNF Met/Met mice had fewer correct responses compared to BDNF Val/Val mice on both cognitive tasks. Female BDNF Val/Val mice showed greater cognitive flexibility compared to male mice regardless of AT. Despite running less than BDNF Val/Val mice, AT improved performance in both cognitive tasks in BDNF Met/Met mice. AT increased neurogenesis in the ventral hippocampus of BDNF Val/Val mice of both sexes and increased the proportion of mature type 3 doublecortin-expressing cells in the dorsal hippocampus of female mice only. Our results indicate AT improved cognitive performance in BDNF Met/Met mice and increased hippocampal neurogenesis in BDNF Val/Val mice in middle age. Furthermore, middle-aged female mice may benefit more from AT than males in terms of neuroplasticity, an effect that was influenced by the BDNF Val66Met polymorphism.

Network Neuroscience Untethered: Brain-Wide Immediate Early Gene Expression for the Analysis of Functional Connectivity in Freely Behaving Animals

Studying how spatially discrete neuroanatomical regions across the brain interact is critical to advancing our understanding of the brain. Traditional neuroimaging techniques have led to many important discoveries about the nature of these interactions, termed functional connectivity. However, in animal models these traditional neuroimaging techniques have generally been limited to anesthetized or head-fixed setups or examination of small subsets of neuroanatomical regions. Using the brain-wide expression density of immediate early genes (IEG), we can assess brain-wide functional connectivity underlying a wide variety of behavioural tasks in freely behaving animal models. Here, we provide an overview of the necessary steps required to perform IEG-based analyses of functional connectivity. We also outline important considerations when designing such experiments and demonstrate the implications of these considerations using an IEG-based network dataset generated for the purpose of this review.

Sex differences in inflammation in the hippocampus and amygdala across the lifespan in rats: associations with cognitive bias

Background

Cognitive symptoms of major depressive disorder, such as negative cognitive bias, are more prevalent in women than in men. Cognitive bias involves pattern separation which requires hippocampal neurogenesis and is modulated by inflammation in the brain. Previously, we found sex differences in the activation of the amygdala and the hippocampus in response to negative cognitive bias in rats that varied with age. Given the association of cognitive bias to neurogenesis and inflammation, we examined associations between cognitive bias, neurogenesis in the hippocampus, and cytokine and chemokine levels in the ventral hippocampus (HPC) and basolateral amygdala (BLA) of male and female rats across the lifespan.

Results

After cognitive bias testing, males had more IFN-γ, IL-1β, IL-4, IL-5, and IL-10 in the ventral HPC than females in adolescence. In young adulthood, females had more IFN-γ, IL-1β, IL-6, and IL-10 in the BLA than males. Middle-aged rats had more IL-13, TNF-α, and CXCL1 in both regions than younger groups. Adolescent male rats had higher hippocampal neurogenesis than adolescent females after cognitive bias testing and young rats that underwent cognitive bias testing had higher levels of hippocampal neurogenesis than controls. Neurogenesis in the dorsal hippocampus was negatively associated with negative cognitive bias in young adult males.

Conclusions

Overall, the association between negative cognitive bias, hippocampal neurogenesis, and inflammation in the brain differs by age and sex. Hippocampal neurogenesis and inflammation may play greater role in the cognitive bias of young males compared to a greater role of BLA inflammation in adult females. These findings lay the groundwork for the discovery of sex-specific novel therapeutics that target region-specific inflammation in the brain and hippocampal neurogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12979-022-00299-4.

• Adolescent male rats had more hippocampal inflammation than females after cognitive bias testing.

• Adult female rats had more basolateral amygdalar inflammation than males after cognitive bias testing.

• HPC neurogenesis was negatively associated to cognitive bias in young adult male rats.

ERPs in Children and Adolescents with Generalized Anxiety Disorder: Before and after an Intervention Program

According to DSM 5, generalized anxiety disorder (GAD) is characterized by excessive, uncontrollable worry about various topics that occupies the majority of the subject’s time for a period of at least six months. The aforementioned state causes distress and/or functional impairments. This paper presents the outcomes of a pilot study that evaluated the implementation of cognitive behavioral therapy (CBT) and CBT with an SSRIs intervention program. The participants comprised 16 children and adolescents with GAD (8 males and 8 females) matched with 16 typically developing peers (8 males and 8 females) aged from 10 to 16 years old (M = 12.56 SD = 2.18). Baseline assessment consisted of event related potentials (ERPs), which indicated that participants with GAD presented cognitive deficits in attention and memory, as they exhibited longer P300 latencies. Following treatment with the CBT program and/or medication, children and adolescents with GAD did not present statistically significantly longer P300 latencies and reaction times in comparison to the control group. Lastly, children and adolescents who followed the CBT program or the CBT program with psychopharmacological assistance did not reveal statistically significant differences in 13 out of 15 topographic brain areas and in reaction time.