Are the dorsal and ventral hippocampus functionally distinct structures?

The neural basis of affective empathy: What is known from rodents

Empathy is the cornerstone of social interactions between conspecies for human beings and other social animals. Human beings with empathy defects might either suffer unpleasant or failed social interactions as ASD patients, or even display antisocial behaviors. To find efficient cure for empathy defects, first of all, the neural mechanisms underpinning various empathy behaviors should be well studied and understood. And the research in the field of affective empathy thrives fast in recent years. It is necessary to review the important contributions in this field, especially for understanding the delicate neural mechanisms of diverse forms of affective empathy. Here, we have summarized the characteristics of various types of affective empathy. We also discuss the distinctions between empathy for pain and fear, as well as instinctive and experienced empathy. Our analysis further highlights the findings in the complex neural mechanisms and potential brain regions underlying different affective empathy behaviors. Above all, this work is expected to help enhance our comprehension of behavioral dynamics and neural basis of affective empathy along with its role in emotional regulation and social behavior.

Age-related synaptic signatures of brain and cognitive reserve in the rat hippocampus and parahippocampal regions

Age-related cognitive decline varies widely among individuals, with some showing resilience despite older age. This study examines synaptic markers of glutamatergic and GABAergic neurotransmission in the hippocampus and cortex of older rats with differing cognitive abilities, aiming to uncover mechanisms that contribute to cognitive resilience. We observed significant age-related reductions in vesicular glutamate transporter VGluT1, particularly in the stratum oriens (SO), radiatum (SR), and lacunosum-moleculare (SLM) of the dorsal CA3 and SLM of the dorsal CA1. Furthermore, loss of VGluT1 in the dorsal CA3-SLM correlated with severity of memory impairment. Higher levels of the vesicular GABA transporter (VGAT) were associated with better spatial learning in older rats, across several synaptic zones of the dorsal hippocampus, including the outer molecular layer of the dentate gyrus (DG), and the SO, SR, SLM, and pyramidal cell layers of both CA3 and CA1. This suggests that enhanced inhibitory neurotransmission specific to the dorsal aspect of the hippocampus may protect against age-related cognitive decline. While the dorsal hippocampus showed consistent age- and memory-related changes, markers in the ventral hippocampus remained largely intact. In the perirhinal cortex, VGluT1 declined with no changes in VGAT, while both markers remained unchanged in other cortical regions, including the lateral entorhinal, retrosplenial, and posterior parietal cortices. These findings highlight region-specific patterns of synaptic aging as potential markers of brain and cognitive reserve.

Luteolin ameliorates chronic stress-induced depressive-like behaviors in mice by promoting the Arginase-1+ microglial phenotype via a PPARγ-dependent mechanism

Accumulating evidence shows that neuroinflammation substantially contributes to the pathology of depression, a severe psychiatric disease with an increasing prevalence worldwide. Although modulating microglial phenotypes is recognized as a promising therapeutic strategy, effective treatments are still lacking. Previous studies have shown that luteolin (LUT) has anti-inflammatory effects and confers benefits on chronic stress-induced depression. In this study, we investigated the molecular mechanisms by which LUT regulates the functional phenotypes of microglia in mice with depressive-like behaviors. Mice were exposed to chronic restraint stress (CRS) for 7 weeks, and were administered LUT (10, 30, 40 mg· kg-1 ·day-1, i.g.) in the last 4 weeks. We showed that LUT administration significantly ameliorated depressive-like behaviors and decreased hippocampal inflammation. LUT administration induced pro-inflammatory microglia to undergo anti-inflammatory arginase (Arg)-1+ phenotypic polarization, which was associated with its antidepressant effects. Furthermore, we showed that LUT concentration-dependently increased the expression of PPARγ in LPS + ATP-treated microglia and the hippocampus of CRS-exposed mice, promoting the subsequent inhibition of the NLRP3 inflammasome. Molecular dynamics (MD) simulation and microscale thermophoresis (MST) analysis confirmed a direct interaction between LUT and peroxisome proliferator-activated receptor gamma (PPARγ). By using the PPARγ antagonist GW9662, we demonstrated that LUT-driven protection, both in vivo and in vitro, resulted from targeting PPARγ. First, LUT-induced Arg-1+ microglia were no longer detected when PPARγ was blocked. Next, LUT-mediated inhibition of the NLRP3 inflammasome and downregulation of pro-inflammatory cytokine production were reversed by the inhibition of PPARγ. Finally, the protective effects of LUT, which attenuated the microglial engulfment of synapses and prevented apparent synapse loss in the hippocampus of CRS-exposed mice, were eliminated by blocking PPARγ. In conclusion, this study showed that LUT ameliorates CRS-induced depressive-like behaviors by promoting the Arg-1+ microglial phenotype through a PPARγ-dependent mechanism, thereby alleviating microglial pro-inflammatory responses and reversing microglial phagocytosis-mediated synapse loss.

The Suprapyramidal and Infrapyramidal Blades of the Dentate Gyrus Exhibit Different GluN Subunit Content and Dissimilar Frequency-Dependent Synaptic Plasticity In Vivo

The entorhinal cortex sends afferent information to the hippocampus by means of the perforant path (PP). The PP input to the dentate gyrus (DG) terminates in the suprapyramidal (sDG) and infrapyramidal (iDG) blades. Different electrophysiological stimulation patterns of the PP can generate hippocampal synaptic plasticity. Whether frequency-dependent synaptic plasticity differs in the sDG and iDG is unclear. Here, we compared medial PP-DG responses in freely behaving adult rats and found that synaptic plasticity in the sDG is broadly frequency dependent, whereby long-term depression (LTD, > 24 h) is induced with stimulation at 1 Hz, short-term depression (< 2 h) is triggered by 5 or 10 Hz, and long-term potentiation (LTP) of increasing magnitudes is induced by 200 and 400 Hz stimulation, respectively. By contrast, although the iDG expresses STD following 5 or 10 Hz stimulation, LTD induced by 1 Hz is weaker, LTP is not induced by 200 Hz and LTP induced by 400 Hz stimulation is significantly smaller in magnitude than LTP induced in sDG. Furthermore, the stimulus-response relationship of iDG is suppressed compared to sDG. These differences may arise from differences in granule cell properties, or the complement of NMDA receptors. Patch clamp recordings, in vitro, revealed reduced firing frequencies in response to high currents, and different action potential thresholds in iDG compared to sDG. Assessment of the expression of GluN subunits revealed significantly lower expression levels of GluN1, GluN2A, and GluN2B in the middle molecular layer of iDG compared to sDG. Taken together, these data indicate that synaptic plasticity in the iDG is weaker, less persistent and less responsive to afferent frequencies than synaptic plasticity in sDG. Effects may be mediated by weaker NMDA receptor expression and differences in neuronal responses in iDG versus sDG. These characteristics may explain reported differences in experience-dependent information processing in the suprapyramidal and infrapyramidal blades of the DG.

Huntingtin plays an essential role in the adult hippocampus

The consequences of non-pathogenic huntingtin (HTT) reduction in the mature brain are of substantial importance as clinical trials for numerous HTT-lowering therapies are underway; many of which are non-selective in that they reduce both mutant and wild type protein variants. In this study, we injected CaMKII-promoted AAV-Cre directly into the hippocampus of adult HTT floxed mice to explore the role of wild-type huntingtin (wtHTT) in adult hippocampal pyramidal neurons and the broader implications of its loss. Our findings reveal that wtHTT depletion results in profound macroscopic morphological abnormalities in hippocampal structure, accompanied by significant reactive gliosis. At the synaptic level, we identified a marked reduction in presynaptic terminals 1-2 months following wtHTT loss; this was contrasted by an increased density of postsynaptic mushroom spines and larger amplitudes of spontaneous excitatory postsynaptic currents, indicative of disrupted synaptic homeostasis. Furthermore, intrinsic neuronal excitability was significantly diminished in CA1 pyramidal neurons lacking wtHTT, and we observed a complete loss of NMDA receptor-dependent long-term potentiation. Unexpectedly, synapse density returned to control levels 6-8 months following wtHTT loss, despite the ongoing presence of macroscopic morphological abnormalities, altered anxiety-related behaviors and clear impairments in spatial learning and memory. Overall, these findings uncover a previously unrecognized role of wtHTT as a critical regulator of hippocampal function in the mature brain, and highlight the hippocampus as a potentially vulnerable region to the adverse effects of non-selective HTT reduction.

Ventral Hippocampal CA1 GADD45B Regulates Susceptibility to Social Stress by Influencing NMDA Receptor-Mediated Synaptic Plasticity

Growth arrest DNA damage-inducible protein 45 β (GADD45B) has been reported to be a regulatory factor for active DNA demethylation and is implicated in the modulation of synaptic plasticity and chronic stress-related psychopathological processes. However, its precise role and mechanism of action in stress susceptibility remain elusive. In this study, we found a significant reduction in GADD45B expression specifically in the ventral, but not the dorsal hippocampal CA1 (dCA1) of stress-susceptible mice. Furthermore, we demonstrated that GADD45B negatively regulates susceptibility to social stress and NMDA receptor-dependent long-term potentiation (LTP) in the ventral hippocampal CA1 (vCA1). Importantly, through pharmacological inhibition using the NMDA receptor antagonist MK801, we provided further evidence supporting the hypothesis that GADD45B potentially modulates susceptibility to social stress by influencing NMDA receptor-mediated LTP. Collectively, these results suggested that modulation of NMDA receptor-mediated synaptic plasticity is a pivotal mechanism underlying the regulation of susceptibility to social stress by GADD45B.

Hippocampal subregion volumes and preadolescent depression risk in the ABCD sample

The hippocampus is central in the pathophysiology of depression. Subregions of the hippocampus (head, body, tail) have been implicated in adult depression, though research examining depression and hippocampal subregions in youth has been limited. This study aimed to examine associations between preadolescent hippocampal subregions and depression risk as well as their interactions with factors associated with depression risk, including biological sex and socioeconomic status (SES). Hippocampal subregions were extracted from the Adolescent Brain and Cognitive Development Study baseline sample (N = 10,469, ages 9-10 years). Depression risk factors included maternal lifetime depression, child depressive symptoms, and child internalizing and externalizing symptoms. Maternal depression was measured through the Family History Questionnaire, and child symptoms were measured through the Child Behavioral Checklist. Results identified associations between hippocampal volumes and future increases in internalizing symptoms (N = 9738). Further, associations between hippocampal subregions and depression risk were moderated by biological sex and SES: males, but not females, with maternal depression exhibited lower hippocampal tail volumes (N = 9826), and for preadolescents with low, but not high, SES, greater hippocampal head volumes predicted increased internalizing symptoms at baseline (N = 10,294) and at the 24-month follow up (N = 7069-7086). Together, this study demonstrates the importance of hippocampal subregions within preadolescent depression risk and identifies subgroups, including preadolescent males and those with low SES, that may be at particular risk.

Resting-state fMRI reveals altered functional connectivity associated with resilience and susceptibility to chronic social defeat stress in mouse brain

Chronic stress is a causal antecedent condition for major depressive disorder and associates with altered patterns of neural connectivity. There are nevertheless important individual differences in susceptibility to chronic stress. How functional connectivity (FC) amongst interconnected, depression-related brain regions associates with resilience and susceptibility to chronic stress is largely unknown. We used resting-state functional magnetic resonance imaging (rs-fMRI) to examine FC between established depression-related regions in susceptible (SUS) and resilient (RES) adult mice following chronic social defeat stress (CSDS). Seed-seed FC analysis revealed that the ventral dentate gyrus (vDG) exhibited the greatest number of FC group differences with other stress-related limbic brain regions. SUS mice showed greater FC between the vDG and subcortical regions compared to both control (CON) or RES groups. Whole brain vDG seed-voxel analysis supported seed-seed findings in SUS mice but also indicated significantly decreased FC between the vDG and anterior cingulate area compared to CON mice. Interestingly, RES mice exhibited enhanced FC between the vDG and anterior cingulate area compared to SUS mice. Moreover, RES mice showed greater FC between the infralimbic prefrontal cortex and the nucleus accumbens shell compared to CON mice. These findings indicate unique differences in FC patterns in phenotypically distinct SUS and RES mice that could represent a neurobiological basis for depression, anxiety, and negative-coping behaviors that are associated with exposure to chronic stress.

Hormonal contraceptives in adolescence impact the neuroimmune environment of the medial prefrontal cortex and hippocampus in female rats

Adolescence is a period of protracted neurodevelopment, during which the prefrontal cortex (PFC) undergoes significant remodeling. Microglia are integral to neurodevelopment and are sensitive to gonadal hormones, which increase during adolescence. Microglia and gonadal hormones can interact to influence adolescent development of the PFC (or medial prefrontal cortex [mPFC] in rodents). In females, gonadal hormones can be perturbed by using hormonal contraceptives (HCs). We predicted that HC administration over adolescence could affect microglia, other immunocompetent cells, and the neuroimmune environment of the developing mPFC. We also assessed HC effects on neuroimmune measures in the hippocampus, as the hippocampus also matures throughout adolescence and is sensitive to ovarian hormones. Intact post-pubertal female Sprague-Dawley rats received daily subcutaneous injections of vehicle or 10 ug ethinyl estradiol + 20 ug levonorgestrel (HCs) throughout adolescence from postnatal day (PND) 35-56. On PND 57 or 58, brains were collected for immunohistochemistry and qPCR. In the mPFC, HC-treated rats showed less Iba1 (microglia) immunolabeling and fewer Iba1+ cells. HC treatment also altered microglia morphology and reduced the spacing between microglia in the mPFC. In the hippocampus, HC-treated rats had reduced Iba1 immunolabeling in the dorsal CA1 and reductions in microglial cell complexity in dorsal CA1, ventral CA1, and ventral CA3. There were no effects of HCs on GFAP (astrocyte) immunolabeling in the mPFC or on astrocytes in any hippocampal subregion analyzed, except an increase in astrocyte number in the dorsal dentate gyrus. mPFC expression of genes related to phagocytosis (Cd68, Trem2) and neuroimmune signaling (Cx3cr1, Cx3cl1) were reduced in rats treated with HCs, but no gene expression changes were seen in the hippocampus. These data provide the first evidence that HCs given during the critical developmental period of adolescence can affect microglia properties in limbic brain regions.

Human mood disorder risk gene Synaptotagmin-14 contributes to mania-like behaviors in mice

Bipolar disorder (BD) and major depressive disorder (MDD) are the most prevalent mood disorders and cause considerable burden worldwide. Compelling evidence suggests a pronounced overlap between these two disorders in clinical symptoms, treatment strategies, and genetic etiology. Here we leverage a BD GWAS (1822 cases and 4650 controls) and a MDD GWAS (5303 cases and 5337 controls), followed by independent replications, to investigate their shared genetic basis among Han Chinese. We have herein identified a lead SNP rs126277 at the 1q32.2 locus, which also exhibited nominal associations with mood disorders and several relevant sub-clinical phenotypes (e.g., mania) in European populations. Bulk tissue and single-cell eQTL analyses suggest that the risk G-allele of rs126277 predicted lower SYT14 mRNA expression in human brains. We generated mice lacking Syt14 (Syt14-/-) and mice with insufficient expression of Syt14 in the hippocampus (Syt14-KD), and found that depletion of Syt14 resulted in mania-like behaviors including hyperactivity and anti-depressive behaviors, resembling aspects of mood disorders. We also confirmed that deficiency of this gene in the hippocampus was sufficient to induce hyperactivity in mice. RNA-sequencing analyses of the hippocampus of Syt14-/- mice revealed significant upregulation of Per1 as well as downregulation of Slc7a11 and Ptprb. Ultrastructural analyses showed significant alteration of the number of vesicles within 50 nm to the active zone and the width of synaptic cleft in the ventral hippocampus of Syt14-/- mice compared with the control mice. Overall, we have identified a novel mood disorder risk gene SYT14, and confirmed its impact on mania-like behaviors. While the current study identifies an essential mood disorder risk gene, further investigations elucidating the detailed mechanisms by which SYT14 contributes to the pathogenesis of the illnesses are needed.

Habituation to novel stimuli alters hippocampal plasticity associated with morphine tolerance in male Wistar rats

Chronic morphine exposure affects neuroplasticity in the hippocampus, a key area for learning and memory. Since, novelty exploration influence rodent hippocampal plasticity, the aim of this study was to investigate the effects of habituation to novel contexts and odors on hippocampal plasticity in morphine-tolerant rats. For this purpose, neurogenesis markers, dendritic spine density and mRNA levels for various genes encoding neurotrophic factors were evaluated in the hippocampus tissue (ventral, vH vs. dorsal, dH) of male rats. Habituation to the new environment was established using animal models of morphine tolerance. Following multiple exposures to a novel context (open field habituation, OFH) or a series of novel odors (odor habituation, OH), markers (Ki67 or DCX) associated with neurogenesis were found to be lower in the morphine-tolerant rats that underwent habituation than the non-habituated morphine-tolerant rats, with specific regions (dH, vH), being differently influenced by specific type of habituation (OFH, OH, respectively). Further results showed subregion and habituation specific effects on the number of dendritic spines per spine type or levels of neurotropic factors including BDNF and TrkB mRNA levels in the dH and vH in morphine-tolerant rats that underwent habituation as compared to the non-habituated morphine-tolerant rats. We provide new evidence that habituation to novel contexts and novel odors appears to affect hippocampal plasticity in morphine-tolerant rats and that pro-plasticity molecules appear to mediate habituation effects on morphine tolerance plasticity.

Amelioration of gap junction dysfunction in a depression model by loganin: Involvement of GSK-3β/β-catenin signaling

ETHNOPHARMACOLOGICAL RELEVANCE

Cornus officinalis Sieb. et Zucc has significant neuroprotective activity and has been widely studied for its potential to improve cognitive function. Our team's previous research has found that loganin isolated from Cornus officinalis has an antidepressant effect. Depression is a mental disorder accompanied by dysfunction of Connexin43 (Cx43)-formed astrocytic gap junctions. However, the precise mechanisms of loganin involved remain uncertain.

AIM OF THE STUDY

We aimed to examine the mechanism by which loganin produces its antidepressant properties.

MATERIALS AND METHODS

Using a chronic unpredictable stress (CUS) model of depression in rats, the study evaluated the behavioral responses to treatment with loganin, fluoxetine, and their combination. Biochemical analyses were conducted to measure the expression and phosphorylation status of Cx43, β-catenin, GSK-3β in the brain. In vitro experiments were also performed how loganin protects the gap junctions in astrocytes that have been exposed to corticosterone.

RESULTS

After four weeks of loganin treatment, rats exposed to CUS showed a decrease in depressive-like behaviors. When combined with fluoxetine, the antidepressant-like effects were observed faster than with either treatment alone. Loganin significantly increased Cx43 expression in the prefrontal cortex and ventral hippocampus, reversed Cx43 mimetic peptide Gap26-induced depressive-like behaviors, decreased Cx43 phosphorylation at Ser368, increased β-catenin levels, and promoted GSK-3β phosphorylation at Ser9. In vitro, loganin prevented corticosterone-induced damage to gap junctions between astrocytes, an effect that was negated by XAV-939 (β-catenin inhibitor).

CONCLUSION

These results implied that loganin could exert antidepressant-like effects by improving the gap junctions of astrocytes in the prefrontal cortex and hippocampus, acting through the GSK-3β/β-catenin signaling pathway. The combination of loganin with fluoxetine may provide a faster onset of antidepressant action compared to either treatment alone, highlighting the potential of loganin as a natural adjunct therapy for depression.

Cell-Type Specific Circuits in the Mammillary Body for Place and Object Recognition Memory

Mammillary body (MB) is traditionally viewed as a structural node of an anatomic circuit for emotion and memory. However, little is known about its molecular and cellular organizations. Here, a discovery that MB contains four subtypes of neurons that occupy different spatial subregions is reported. Of these, two subtypes of neurons are tagged by parvalbumin (PV) and dopamine receptor-D2 (Drd2) markers. PV neurons are spontaneously active, whereas Drd2 neurons are inactive at rest and generate rebound bursts. These two distinct electrophysiological properties are encoded by Kcnn4 and Cacna1h. PV and Drd2 neurons generate two distinct cell-type specific circuits by receiving inputs from two discrete subiculum neuronal classes. Gain- and loss-of-function studies on these cortical-subcortical circuits demonstrate their differential roles for place and object recognition memory. This finding provides a comprehensive molecular and structural atlas of MB neurons at single-cell resolution and reveals that MB contains molecularly, structurally, and functionally dissociable streams within its serial architecture.

Controlling the local extracellular electric field can suppress the generation and propagation of seizures and spikes in the hippocampus

OBJECTIVE

Neural activity such as theta waves, epileptic spikes and seizures can cross a physical transection using electric fields thus propagating by ephaptic coupling and independently of synaptic transmission. Recruitment of neurons in epilepsy occurs in part due to electric field coupling in addition to synaptic mechanisms. Hence, controlling the local electric field could suppress or cancel the generation of these epileptic events.

METHODS

4-aminopyridine (4-AP) was used to induce spontaneous epileptic spikes and seizures in longitudinal hippocampal slices in-vitro. Two extracellular recording electrodes were placed in the tissue, one at the edge of the slice on the temporal side at the focus of the epileptic activity and the other on the septal side to record the propagation. Two stimulating electrodes were placed outside the slice at the edge of the focal zone. An extracellular voltage clamp circuit maintained the voltage within the focus at 0V with respect to the bath ground.

RESULTS

Experiments showed that 100 % of the epileptic activity originated at the temporal region and propagated to the septal region of the slices thereby establishing the existence of a focus in the temporal end of the tissue. The clamp achieved 100 % suppression of all seizure activity in the tissue with current amplitudes between 70 and 250 nA. No spikes or seizures were observed in either the focus or the septal region when the clamp was "on". When the clamp was turned off, both the spikes and seizure events recovered immediately.

CONCLUSIONS

The experiments show that controlling the extracellular voltage within a focus can prevent the generation and the propagation of epileptiform activity from the focus with very low amplitudes currents.

Gut Microbiome, Diet and Depression: Literature Review of Microbiological, Nutritional and Neuroscientific Aspects

PURPOSE OF REVIEW

This review explores the intricate relationships among the gut microbiota, dietary patterns, and mental health, focusing specifically on depression. It synthesizes insights from microbiological, nutritional, and neuroscientific perspectives to understand how the gut-brain axis influences mood and cognitive function.

RECENT FINDINGS

Recent studies underscore the central role of gut microbiota in modulating neurological and psychological health via the gut-brain axis. Key findings highlight the importance of dietary components, including probiotics, prebiotics, and psychobiotics, in restoring microbial balance and enhancing mood regulation. Different dietary patterns exhibit a profound impact on gut microbiota composition, suggesting their potential as complementary strategies for mental health support. Furthermore, mechanisms like tryptophan metabolism, the HPA axis, and microbial metabolites such as SCFAs are implicated in linking diet and microbiota to depression. Clinical trials show promising effects of probiotics in alleviating depressive symptoms. This review illuminates the potential of diet-based interventions targeting the gut microbiota to mitigate depression and improve mental health. While the interplay between microbial diversity, diet, and brain function offers promising therapeutic avenues, further clinical research is needed to validate these findings and establish robust, individualized treatment strategies.

Dietary texture-driven masticatory activity and its impact on stress tolerance

OBJECTIVES

Although previous studies suggest that dietary texture-driven masticatory activity is correlated with stress tolerance, the underlying mechanisms, including neurotransmitter dynamics, remain unclear. This study investigated the effects of dietary texture-driven masticatory activity on stress tolerance in mice.

METHODS

Behavioral responses to stress were assessed using the repeated social defeat stress (R-SDS) and social interaction test (SIT) model. Neurotransmitter levels in stress-related brain regions were analyzed in mice fed a solid diet (promoting masticatory activity) or a powdered diet (decreasing masticatory activity).

RESULTS

Mice fed the powdered diet exhibited reduced stress tolerance compared with those fed the solid diet. Following the R-SDS, the powdered diet group displayed elevated gamma-aminobutyric acid (GABA) and norepinephrine levels in the prefrontal cortex. Before stress treatment, glutamic acid levels increased and those of choline decreased in the amygdala, whereas dopamine levels decreased in the powdered diet group after the R-SDS. In the locus coeruleus, mice on the powdered diet showed decreased glutamic acid and adenosine levels, alongside increased GABA levels. Serotonin levels decreased in the powdered diet group after the R-SDS, with no changes observed after the SIT. In the ventral hippocampus, GABA levels increased in the powdered diet group but decreased after the SIT.

CONCLUSIONS

This study demonstrates a correlation between masticatory activity and stress tolerance, evidenced by both behavioral and neurotransmitter changes. These findings suggest that reduced masticatory activity due to dietary texture contributes to decreased stress resilience.

Sex-specific influences of APOEε4 genotype on hippocampal neurogenesis and progenitor cells in middle-aged rats

BACKGROUND

Alzheimer's disease (AD) disproportionately and uniquely affects females, and these sex differences are further exacerbated by the presence of Apolipoprotein (APOE) ε4 alleles, the top genetic risk factor for late-onset AD. To expand our understanding about how late-onset AD risk might differentially influence males and females, this study explores how APOEε4 affects hippocampal neurogenesis and microglia, key neuroplastic markers involved in AD pathogenesis, differently by sex in middle-aged rats.

METHODS

A rat model expressing the humanized (h) APOEε4 allele was characterized to examine markers of adult neurogenesis (neural progenitor cells and new-born neurons) and immune cells (microglia) in the dentate gyrus of the hippocampus in 13 month-old male and female rats.

RESULTS

We observed basal sex differences in neurogenesis at middle age, as wildtype male rats had greater densities of neural progenitor cells and new-born neurons in the dentate gyrus than wildtype female rats. Male hAPOEε4 rats exhibited fewer neural progenitor cells, fewer new-born neurons, and more microglia than male wildtype rats. On the other hand, female hAPOEε4 rats exhibited more new-born neurons than female wildtype rats. Interestingly, females had more microglia than males regardless of genotype. Correlations were conducted to further elucidate any sex differences in the relationships between these biomarkers. Notably, there was a significant positive correlation between neural progenitor cells and new-born neurons, and a significant negative correlation between new-born neurons and microglia, but only in male rats.

CONCLUSION

In contrast to the clear pattern of effects of the hAPOEε4 risk factor on hippocampal neurogenesis in males, females had unaltered levels of neural progenitor cells and increased density of new-born neurons. Furthermore, relationships between neurogenesis and microglia were significantly correlated within males, and not females. This suggests that females may be presenting a compensatory response to the hAPOEε4 genotype at middle age. Collectively, these results exemplify the importance of thoroughly examining influences of sex on AD endophenotypes, as it may reveal sex-specific pathways and protective mechanisms relevant to AD.

Orofacial pain models induce impairment in spatial learning and working memory in rodents: A systematic review and meta-analysis

Orofacial pain is one of the most common causes of chronic pain leading to physical and cognitive disability. Several clinical and pre-clinical studies suggest that chronic pain results in cognitive impairment. However, there is a lack of meta-analyses examining the effects of orofacial pain models on behavioral learning and memory in rodents. Thus, this study aimed to evaluate whether orofacial pain models can impair learning and memory in rodents. The protocol was registered in PROSPERO (CRD42023355502). We used CAMARADES and SYRCLE to estimate the quality and the publication bias by using Egger's and Begg's test. Here, 21 studies were included in this systematic review and meta-analysis. We included 12 studies with trigeminal neuralgia models, 4 with migraine-like pain models, 4 with tooth nociception, and 1 with acute orofacial pain model. Spontaneous nociception and facial mechanical allodynia were observed in orofacial pain models. Regarding spatial learning we detected that latency to find the platform in the Morris water maze (MWM) was increased in orofacial pain models (related to facial mechanical allodynia or spontaneous nociception). Although the mean quality of the articles was high, we identify publication bias in the Begg's test for the time in the quadrant in the MWM. Our findings revealed that spontaneous nociception and facial mechanical allodynia in orofacial pain models contribute to the working memory and spatial learning dysfunction. Therefore, further studies are still needed to evaluate the influence of sex, age, social isolation, and environmental enrichment in orofacial pain-related learning and memory.

The expression of contextual fear conditioning involves the dorsal hippocampus TRPV1 receptor interacting with the NMDA/NO/cGMP signalling pathway

BACKGROUND AND PURPOSE

The dorsal hippocampus (dHIP) is pivotal for learning, memory, and defensive responses. Transient receptor potential vanilloid type 1 (TRPV1) receptors in the dHIP modulate contextual fear conditioning by triggering a cascade involving glutamate release, nitric oxide (NO) formation and cyclic guanosine monophosphate (cGMP) production. The present study investigated the involvement of dHIP TRPV1 receptors and their interaction with the glutamate/NO/cGMP signalling pathway in modulating the expression of contextual fear conditioning (CFC).

EXPERIMENTAL APPROACH

Male Wistar rats were submitted to an aversive contextual conditioning session and, 48 h later, were re-introduced to the same aversive environment where the freezing response and autonomic activity (evidenced by increased arterial pressure and heart rate and a decrease in tail temperature) were measured.

KEY RESULTS

The results demonstrated that the TRPV1 antagonist 6-I-CPS in dHIP reduced the expression of CFC, whereas the agonist capsaicin had the opposite effect. Furthermore, dHIP pre-treatment with an NMDA receptor antagonist (AP7), neuronal NO synthase inhibitor (N-propyl-L-arginine), NO scavenger (c-PTIO) or guanylate cyclase inhibitor (ODQ) attenuated capsaicin-induced increases in CFC. Finally, we observed that re-exposure to the aversive chamber increased dHIP NO levels in conditioned animals compared with a non-conditioned group, which was prevented by the administration of the TRPV1 antagonist, 6-I-CPS.

CONCLUSION AND IMPLICATIONS

Our study revealed that TRPV1 receptors in the dHIP play a crucial role in modulating contextual fear expression by acting through the NMDA receptor/NO/cGMP signalling pathway, providing important insights into the underlying mechanisms and potential therapeutic avenues associated with these pathways.

Memory-related neurophysiological mechanisms in the hippocampus underlying stress susceptibility

Stress-induced psychiatric symptoms, such as increased anxiety, decreased sociality, and depression, differ considerably across individuals. The cognitive model of depression proposes that biased negative memory is a crucial determinant in the development of mental stress-induced disorders. Accumulating evidence from both clinical and animal studies has demonstrated that such biased memory processing could be triggered by the hippocampus, a region well known to be involved in declarative memories. This review mainly describes how memory-related neurophysiological mechanisms in the hippocampus and their interactions with other related brain regions are involved in the regulation of stress susceptibility and discusses potential interventions to prevent and treat stress-related psychiatric symptoms. Further neurophysiological insights based on memory mechanisms are expected to devise personalized prevention and therapy to confer stress resilience.

Very-light-intensity exercise as minimal intensity threshold for activating dorsal hippocampal neurons: Evidence from rat physiological exercise model

Exercise benefits the brain, particularly the learning and memory center-the dorsal hippocampus (dHPC)-and holds promise for therapeutic applications addressing age-related cognitive deficits. While moderate-to-vigorous-intensity exercise is commonly recommended for health benefits, our translational research proposes the effectiveness of very-light-intensity exercise in enhancing cognitive functions. However, the intensity-dependent characteristics of HPC activation have yet to be fully delineated; therefore, there is no evidence of whether such easily accessible exercises for people of all ages and most fitness levels can activate HPC neurons. Here, we aimed to clarify this question using a physiologically sound rat exercise model. We used a previously established rat treadmill running model within a metabolic chamber and measured maximal oxygen uptake (V˙O2max) during an incremental running test. Referring to the American College of Sports Medicine's V˙O2max-based intensity classification, rats were assigned to one of five groups: resting control, very-light, light, moderate, and vigorous exercise intensity. We immunohistochemically assessed the effects of a single bout of exercise on dHPC neuronal activity and measured V˙O2 and blood lactate as exercise intensity indicators. dHPC neuronal activity increased with exercise intensity, even at light-intensity without blood lactate accumulation, and correlated positively with increasing V˙O2. The dorsal dentate gyrus and CA1 sub-regions were markedly activated even by very-light-intensity exercise. Our findings demonstrate the intensity-dependent activation of dHPC neurons, with very-light-intensity exercise as the minimal intensity threshold. These strongly support our hypothesis that very-light-intensity exercise serves as a viable memory-enhancing strategy, beneficial for various populations including low-fitness individuals and the elderly.

Female mice exhibit similar long-term plasticity and microglial properties between the dorsal and ventral hippocampal poles

The hippocampus is a heterogenous structure that exhibits functional segregation along its longitudinal axis. We recently showed that in male mice, microglia, the brain's resident immune cells, differ between the dorsal (DH) and ventral (VH) hippocampus, impacting long-term potentiation (LTP) mainly through the CX3CL1-CX3CR1 signaling. Here, we assessed the specific features of the hippocampal poles in female mice, demonstrating a similar LTP amplitude in VH and DH in both control and Cx3cr1 knock-out mice. In addition, the expression levels of Cx3cr1 and Cx3cl1 mRNA do not differ between the two poles in control mice. These data support the critical role of the CX3CL1-CX3CR1 signaling in setting the physiological amount of plasticity, equally between poles in females. Although BDNF is higher in DH compared to VH, the expression levels of inflammatory markers involved in plasticity and of phagocytosis markers in microglia are comparable between the two poles. In accordance, microglia soma and arborization area/perimeter, and microglial ultrastructure are similar across regions, with the exception of microglial density, cells arborization solidity and circularity that are higher in DH. Understanding the molecular processes underlying microglial sex differences and their potential implications for plasticity in specific brain regions is of major importance in physiological and pathological conditions.

Behavioral Animal Models and Neural-Circuit Framework of Depressive Disorder

Depressive disorder is a chronic, recurring, and potentially life-endangering neuropsychiatric disease. According to a report by the World Health Organization, the global population suffering from depression is experiencing a significant annual increase. Despite its prevalence and considerable impact on people, little is known about its pathogenesis. One major reason is the scarcity of reliable animal models due to the absence of consensus on the pathology and etiology of depression. Furthermore, the neural circuit mechanism of depression induced by various factors is particularly complex. Considering the variability in depressive behavior patterns and neurobiological mechanisms among different animal models of depression, a comparison between the neural circuits of depression induced by various factors is essential for its treatment. In this review, we mainly summarize the most widely used behavioral animal models and neural circuits under different triggers of depression, aiming to provide a theoretical basis for depression prevention.

Spatiotemporal Mapping of the Oxytocin Receptor at Single-Cell Resolution in the Postnatally Developing Mouse Brain

The oxytocin receptor (OXTR) has garnered increasing attention for its role in regulating both mature behaviors and brain development. It has been established that OXTR mediates a range of effects that are region-specific or period-specific. However, the current studies of OXTR expression patterns in mice only provide limited help due to limitations in resolution. Therefore, our objective was to generate a comprehensive, high-resolution spatiotemporal expression map of Oxtr mRNA across the entire developing mouse brain. We applied RNAscope in situ hybridization to investigate the spatiotemporal expression pattern of Oxtr in the brains of male mice at six distinct postnatal developmental stages (P7, P14, P21, P28, P42, P56). We provide detailed descriptions of Oxtr expression patterns in key brain regions, including the cortex, basal forebrain, hippocampus, and amygdaloid complex, with a focus on the precise localization of Oxtr+ cells and the variance of expression between different neurons. Furthermore, we identified some neuronal populations with high Oxtr expression levels that have been little studied, including glutamatergic neurons in the ventral dentate gyrus, Vgat+Oxtr+ cells in the basal forebrain, and GABAergic neurons in layers 4/5 of the cortex. Our study provides a novel perspective for understanding the distribution of Oxtr and encourages further investigations into its functions.

Transcriptomics study of hippocampus in mice exposed to heat stress

Heat stress (HS) triggers various pathophysiological responses in the brain, including neuroinflammation and cognitive impairments. The objective of this study was to examine the impact of HS by comparing the hippocampal transcriptomes of mice exposed to HS with those under control conditions. Our analysis revealed that HS exposure did not affect the number of SNP or InDel mutations in the mouse hippocampus, nor did it influence SNP functions, distribution, or types. However, HS did lead to differential gene expression in the hippocampus, with 210 differentially expressed genes (DEGs), including 72 upregulated and 138 downregulated genes. Gene Ontology (GO) analysis indicated that these DEGs are involved in hippocampal responses to various stimuli (chemical, oxygen-containing compounds, peptide hormones), metabolic processes (arachidonic acid, olefinic compound metabolism, lipid metabolism), and other functions. The regulation of these functions may be closely linked to specific DEGs, such as Card14, Ntrk1, Lcn2, Irs4, Cyp2c70, Hamp, Ambp, Gh, Mup19, and others, which exhibit the highest degree of differential variation. Furthermore, we observed that pre-treatment with taurine primarily modulated cognitive functions in the hippocampus following HS. Therefore, our study offers valuable insights for future research on heat stress-induced cognitive impairments and provides a theoretical foundation for developing taurine-based preventive strategies for high-risk populations.

Early-life malnutrition role in memory, emotional behavior and motor impairments in early brain lesions with potential for neurodevelopmental disorders: a systematic review with meta-analysis

OBJECTIVES

The present study aims to evaluate the impact of early exposure to brain injury and malnutrition on episodic memory and behavior.

METHODS

For this, a systematic review was carried out in the Medline/Pubmed, Web of Science, Scopus, and LILACS databases with no year or language restrictions.

RESULTS

Initially, 1759 studies were detected. After screening, 53 studies remained to be read in full. The meta-analysis demonstrated that exposure to double insults worsens episodic recognition memory but does not affect spatial memory. Early exposure to low-protein diets has been demonstrated to aggravate locomotor and masticatory sequelae. Furthermore, it reduces the weight of the soleus muscle and the muscle fibers of the masseter and digastric muscles. Early exposure to high-fat diets promotes an increase in oxidative stress and inflammation in the brain, increasing anxiety- and depression-like behavior and reducing locomotion.

DISCUSSION

Epigenetic modifications were noted in the hippocampus, hypothalamus, and prefrontal cortex depending on the type of dietetic exposure in early life. These findings demonstrate the impact of the double insult on regions involved in cognitive and behavioral processes. Additional studies are essential to understand the real impact of the double insults in the critical period.

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.

Permanent tactile sensory loss reduces neuronal activity in the amygdala and ventral hippocampus and alters anxiety-like behaviors

Tactile information from the whiskers (vibrissae) travels through the somatosensory cortex to the entorhinal cortex and the hippocampus, influencing development and psychological well-being. The lack of whiskers affects cognitive functions, spatial memory, neuronal firing, spatial mapping, and neurogenesis in the dorsal hippocampus. Recent studies underline the importance of tactile experiences in emotional health, noting that while tactile stimuli modulate the dorsal hippocampus, the effects of tactile deprivation on anxiety-like behaviors and neural activity in regions like the ventral hippocampus and amygdala are less understood. This study aims to investigate the impact of permanent tactile deprivation on modifying anxiety-like behaviors and c-Fos expression (a marker of neuronal activity) in the dorsolateral and central nucleus of the amygdala and the ventral hippocampus, two regions involved in emotional memory and anxiety. We sectioned the infraorbital nerve, responsible for transmitting whisker information, in CD1 mice to examine how tactile deprivation modifies the behavioral activity in the Elevated Plus Maze and Open-Field Test. Our data revealed a reduction in anxiety-related behaviors post-deprivation, which was linked to a significant decrease in c-Fos expression in the barrel cortex, as well as ventral hippocampus (CA1, dentate gyrus) and dorsolateral, central nucleus of the amygdala, suggesting impaired processing in emotional-regulator brain regions. In conclusion, tactile inputs reduce neuronal activity regulators in brain regions related to emotional regulation, which may trigger possible failures in risk perception or self-protective behaviors associated with the lack of appropriate anxiety responses.

Sex chromosomes and sex hormones differently shape microglial properties during normal physiological conditions in the adult mouse hippocampus

The brain presents various structural and functional sex differences, for which multiple factors are attributed: genetic, epigenetic, metabolic, and hormonal. While biological sex is determined by both sex chromosomes and sex hormones, little is known about how these two factors interact to establish this dimorphism. Sex differences in the brain also affect its resident immune cells, microglia, which actively survey the brain parenchyma and interact with sex hormones throughout life. However, microglial differences in density and distribution, morphology and ultrastructural patterns in physiological conditions during adulthood are largely unknown. Here, we investigated these aforementioned properties of microglia using the Four Core Genotypes (FCG) model, which allows for an independent assessment of gonadal hormones and sex chromosomal effects in four conditions: FCG XX and Tg XY- (both ovaries); Tg XXSry and Tg XYSry (both testes). We also compared the FCG results with XX and XY wild-type (WT) mice. In adult mice, we focused our investigation on the ventral hippocampus across different layers: CA1 stratum radiatum (Rad) and CA1 stratum lacunosum-moleculare (LMol), as well as the dentate gyrus polymorphic layer (PoDG). Double immunostaining for Iba1 and TMEM119 revealed that microglial density is influenced by both sex chromosomes and sex hormones. We show in the Rad and LMol that microglia are denser in FCG XX compared to Tg XYSry mice, however, microglia were densest in WT XX mice. In the PoDG, ovarian animals had increased microglial density compared to testes animals. Additionally, microglial morphology was modulated by a complex interaction between hormones and chromosomes, affecting both their cellular soma and arborization across the hippocampal layers. Moreover, ultrastructural analysis showed that microglia in WT animals make overall more contacts with pre- and post-synaptic elements than in FCG animals. Lastly, microglial markers of cellular stress, including mitochondrion elongation, and dilation of the endoplasmic reticulum and Golgi apparatus, were mostly chromosomally driven. Overall, we characterized different aspects of microglial properties during normal physiological conditions that were found to be shaped by sex chromosomes and sex hormones, shading more light onto how sex differences affect the brain immunity at steady-state.

Implications of prenatal exposure to hyperandrogen for hippocampal neurodevelopment and autism-like behavior in offspring

Autism spectrum disorder (ASD) is a highly heterogeneous neurodevelopmental disorder that significantly jeopardizes the physical and mental well-being of children. Autism spectrum disorder results from a combination of environmental and genetic factors. Hyperandrogenic exposure during pregnancy increases their risk of developing autism. Nevertheless, the prenatal exposure to androgens affects offspring neurodevelopment and the underlying mechanisms have not been fully elucidated. In the present study, administration of excessive dihydrotestosterone (DHT) to pregnant mice was found to impair neuronal development and dendritic spine formation in offspring, inducing autism-like behaviors. Furthermore, through mRNA transcriptome sequencing technology, the key molecule Nr4a2 was identified during this process of change. Overexpression of Nr4a2 and treatment with amodiaquine (AQ) significantly improved the abnormal phenotypes in offspring caused by prenatal exposure to androgens. Overall, Nr4a2 emerges as a crucial molecule involved in the impairment of offspring neurodevelopment due to prenatal androgen exposure, which provides a new perspective for the in-depth study of the influencing factors and underlying mechanisms.

Examining threat responses through a developmental lens

Adolescence has been characterized by risk taking and fearlessness. Yet, the emergence of anxiety disorders that are associated with fear peaks during this developmental period. Moreover, adolescents show heightened sensitivity to stress relative to children and adults. To address inconsistencies between the common characterization of adolescents as fearless and the evidence of heightened anxiety and stress during this time, we build upon foundational discoveries of threat-related circuitry and behavior in adult rodents by Joseph LeDoux and colleagues. Specifically, the conservation of this circuitry across species has provided opportunities for identifying mechanisms underlying threat responses that we have extended to developing humans and rodents. We elucidate situations in which adolescents show heightened threat responses and others where they appear fearless and link them to developmental changes of threat circuitry during this period. We discuss the potential adaptiveness of these threat responses for survival of the individual and species but also the potential risks for anxiety and stress. We end by offering potential new ways in which behavioral treatments for youth with anxiety and stress-related disorders may be optimized to target the developing vs developed brain.

Hippocampal Synaptic Plasticity: Integrating Memory and Anxiety Impairments in the Early Stages of Alzheimer's Disease

A decline in hippocampal function has long been associated with the progression of cognitive impairments in patients with Alzheimer's disease (AD). The disruption of hippocampal synaptic plasticity [primarily the reduction of long-term potentiation LTP] by excess production of soluble beta-amyloid (Aβ) has long been accepted as the mechanism by which AD pathology impairs memory, at least during the early stages of AD pathogenesis. However, the premise that hippocampal LTP underpins the formation of associative, long-term memories has been challenged. Here, we consider evidence that this canonical view of LTP needs to be refined. Similarly, the view that the hippocampus simply supports memory ignores the wealth of data showing that the hippocampus is functionally heterogeneous along its septo-temporal axis. The ventral (but not the dorsal) hippocampus plays a major role in modulating emotional reactions to conflict. Here, we suggest that hippocampal LTP is not involved in forming long-term associative memories, but instead contributes to the disambiguation of overlapping memories in situations of conflict and associative interference. This conceptualisation of hippocampal synaptic plasticity may help explain how early-stage AD pathology may impact both memory and anxiety.

Intriguing astrocyte responses in CA1 to reduced and rehabilitated masticatory function: Dorsal and ventral distinct perspectives in adult mice

OBJECTIVE

We sought to investigate the plasticity of diet-induced changes in astrocyte morphology of stratum lacunosum-moleculare (SLM) in CA1.

DESIGN

Three diet regimes were adopted in 15 mice, from the 21st postnatal day to 6 months. The first diet regimen was pellet feed, called Hard Diet (HD). The second, with reduced masticatory, received a pellet-diet followed by a powdered-diet, and it was identified as Hard Diet/Soft Diet (HD/SD). Finally, the group with rehabilitated masticatory was named Hard Diet/Soft Diet/Hard Diet (HD/SD/HD). In the end, euthanasia and brain histological processing were performed, in which astrocytic immunoreactivity to glial-fibrillary-acidic-protein (GFAP) was tested. In reconstructed astrocytes, morphometric analysis was performed.

RESULTS

Astrocyte morphometric revealed that changes in masticatory regimens impact astrocyte morphology. In the dorsal CA1, switching from a hard diet to a soft diet led to reductions in most variables, whereas in the ventral, fewer variables were affected, highlighting regional differences in astrocyte responses. Cluster analysis further showed that diet-induced changes in astrocyte morphology were reversible in the dorsal region, but not in the ventral region, indicating a persistent impact on astrocyte diversity and complexity in the ventral even after rehabilitation. Correlation tests between astrocyte morphology and behavioral performance demonstrated disrupted relationships under masticatory stress, with effects persisting after rehabilitation.

CONCLUSION

Changes in the diet result in significant alterations in astrocyte morphology, suggesting a direct link between dietary modulation and cellular structure. Morphometric analyses revealed distinct alterations in astrocyte morphology in response to changes in the masticatory regimen, with both dorsal/ventral regions displaying notable changes. Moreover, the regional differential effects on astrocytes underscore the complexity of mastication on neuroplasticity and cognitive function.

Social isolation during adolescence differentially affects spatial learning in adult male and female mice

Social isolation is a risk factor for cognitive impairment. Adolescents may be particularly vulnerable to these effects, because they are in a critical period of development marked by significant physical, hormonal, and social changes. However, it is unclear if the effects of social isolation on learning and memory are similar in both sexes or if they persist into adulthood after a period of recovery. We socially isolated male and female 129Sv/Ev mice throughout adolescence (postnatal days 29-56), provided a 2-week resocialization recovery period, and then tested spatial learning and cognitive flexibility in the active place avoidance task. After behavioral testing, mice were injected with 5'-bromo-2'-deoxyuridine (BrdU) so that lasting effects of social isolation on cell proliferation in the dentate gyrus could be examined. Tissue was also stained for doublecortin (DCX). We found that in males, isolation led to a modest impairment in the rate of initial spatial learning, whereas in females, initial learning was unaffected. However, when the location of the shock zone was switched during the conflict variant of the task, cognitive flexibility was impaired in females only. Similarly, social isolation reduced cell proliferation and the number of immature neurons in the ventral dentate gyrus only in females. Together, these findings indicate that social isolation during adolescence differentially impairs spatial processing in males and females, with effects that persist into adulthood.

Perspective: Hippocampal theta rhythm as a potential vestibuloacoustic biomarker of anxiety

Anxiety disorders are the most common mental illnesses - afflicting 19% of Americans every year and 31% within their lifetimes - yet diagnoses remain based on symptom checklists because existing technologies have yet to produce biomarkers sufficiently robust for clinical use. Some techniques provide superior spatial resolution of deep brain regions implicated in anxiety but have poor time resolution; while others measure signals in real time but lack spatial resolution. Often, the goal of probing deep brain regions in humans for anxiety research is to measure a putative analogue of a mammalian brain rhythm linked to behaviour that is suggestive of anxiety. This 4-12 Hz, 1-2 mV, behaviourally modulated, nearly sinusoidal "hippocampal theta rhythm" (hTheta) is one of the largest normal extracellular synchronous signals in mammals and although it has been linked to anxiety processes, its function remains unclear. This paper reviews the literature on hTheta as it relates to anxiety and sensory, in particular vestibuloacoustic, signals, concludes that hTheta can modulate sensory signals during anxiety and posits that such modulation of vestibular signals may be an anxiety biomarker that could be detected non-invasively in humans.

Chronic treatment with the antipsychotic lurasidone modulates the neuroinflammatory changes associated with the vulnerability to chronic mild stress exposure in female rats

Stress exposure is a key risk factor for the developmentof depressive-like conditions. However, despite the higher incidence of Major Depressive Disorder in the female population, classical stress-based experimental paradigms have primarily focused on males. In the present study, we used the well-established chronic mild stress (CMS) paradigm to investigate the development of anhedonia, a cardinal symptom of affective disorders, in the female animals and we also studied the potential effect of the antipsychotic drug lurasidone in normalizing the alterations brought about by stress exposure. We found that three weeks of CMS exposure produced a significant reduction of sucrose intake in 50% of the animals (vulnerable, CMS-V), whereas the others were resilient (CMS-R). The development of an anhedonic phenotype in CMS-V was associated with a significant elevation of different immune markers, such as Complement C3 and C4, and inflammatory cytokines, including INFß and Il1ß in dorsal and ventral hippocampus. Interestingly, sub-chronic treatment with the antipsychotic drug lurasidone was able to revert the anhedonic phenotype while normalizing most of the molecular alterations found in rats vulnerable to CMS exposure. This study extends the ability of lurasidone to normalize the anhedonic phenotype in CMS rats also to females. Moreover, we provide novel evidence on lurasidone's potential effectiveness in treating mental disorders characterized by immune-inflammatory dysfunction.

Depression-like behavior is associated with deficits in cognition and hippocampal neurogenesis in a subset of spinally contused male, but not female, rats

Depression and cognitive deficits present at higher rates among people with spinal cord injury (SCI) compared to the general population, yet these SCI comorbidities are poorly addressed. Sex and age appear to play roles in depression incidence, but consensus on the direction of their effects is limited. Systemic and cortical inflammation and disruptions in hippocampal neurogenesis have been identified as potential treatment targets, but a comprehensive understanding of these mechanisms remains elusive. We used a rodent SCI model to interrogate these gaps in knowledge. We examined post-injury depression-like behavior and cognitive deficits, as well as the association between affect, cognition, chronic hippocampal inflammation and hippocampal neurogenesis, in young and middle-aged male and female Sprague-Dawley rats. Depression-like behavior manifested in male and female subsets of SCI rats irrespective of age, at rates commensurate with the incidence of clinical depression. Changes in components of behavior were driven by sex and age, and affective outcomes were independent of common post-injury pathophysiological outcomes including locomotor functional deficits and spinal lesion severity. Interestingly, however, only male depression-like SCI rats exhibited deficits in hippocampal-associated spatial cognition. Neurogenesis was also disrupted in only SCI males in regions of the hippocampus responsible for affective outcomes. Decreased neurogenesis among middle-aged male subjects coincided with increases in numbers of the pro-inflammatory markers CD86 and iNOS, while middle-aged females had increased numbers of cells expressing Iba-1 and anti-inflammatory marker CD206. Overall, the present data suggest that post-SCI depression and cognition may be affected, in part, by sex- and age-dependent changes in hippocampal neurogenesis and inflammation. Hippocampal neurogenesis is a potential target to address psychological wellbeing after SCI, but therapeutic strategies must carefully consider sex and age as biological variables.

Linking new information to a short-lasting memory trace induces consolidation in the hippocampus

Novelty often influences the retention of nearby weak and transient memory traces, yet its precise role in shaping long-term memory storage remains elusive. Here, we demonstrate that a short-lasting memory is stabilized into a long-lasting one when new information is linked to the weak mnemonic trace in rats, resulting in the formation of long-term memories that are recalled together. An increased overlap between neuronal ensembles and de novo protein synthesis in the dorsal CA1 region of the hippocampus (dCA1) mediates this process. This intricate interconnectedness relies on both temporal and contextual relations between experiences, enhancing the adaptive value of memory consolidation. Finally, this phenomenon is negatively affected by aging, which is associated with reduced ensemble size after novelty exposure and diminished overlap between ensembles in aged dCA1. These findings provide valuable insights into the selectivity and malleability of memory consolidation and its decline during aging.

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.

Light-exercise-induced dopaminergic and noradrenergic stimulation in the dorsal hippocampus: Using a rat physiological exercise model

Exercise activates the dorsal hippocampus that triggers synaptic and cellar plasticity and ultimately promotes memory formation. For decades, these benefits have been explored using demanding and stress-response-inducing exercise at moderate-to-vigorous intensities. In contrast, our translational research with animals and humans has focused on light-intensity exercise (light exercise) below the lactate threshold (LT), which almost anyone can safely perform with minimal stress. We found that even light exercise can stimulate hippocampal activity and enhance memory performance. Although the circuit mechanism of this boost remains unclear, arousal promotion even with light exercise implies the involvement of the ascending monoaminergic system that is essential to modulate hippocampal activity and impact memory. To test this hypothesis, we employed our physiological exercise model based on the LT of rats and immunohistochemically assessed the neuronal activation of the dorsal hippocampal sub-regions and brainstem monoaminergic neurons. Also, we monitored the extracellular concentration of monoamines in the dorsal hippocampus using in vivo microdialysis. We found that even light exercise increased neuronal activity in the dorsal hippocampal sub-regions and elevated the extracellular concentrations of noradrenaline and dopamine. Furthermore, we found that tyrosine hydroxylase-positive neurons in the locus coeruleus (LC) and the ventral tegmental area (VTA) were activated even by light exercise and were both positively correlated with the dorsal hippocampal activation. In conclusion, our findings demonstrate that light exercise stimulates dorsal hippocampal neurons, which are associated with LC-noradrenergic and VTA-dopaminergic activation. This shed light on the circuit mechanisms responsible for hippocampal neural activation during exercise, consequently enhancing memory function.

Models of human hippocampal specialization: a look at the electrophysiological evidence

From an anatomical perspective, the concept that the anterior and posterior hippocampus fulfill distinct cognitive roles may seem unsurprising. When compared with the posterior hippocampus, the anterior region is proportionally larger, with visible expansion of the CA1 subfield and intimate continuity with adjacent medial temporal lobe (MTL) structures such as the uncus and amygdala. However, the functional relevance emerging from these anatomical differences remains to be established in humans. Drawing on both rodent and human data, several models of hippocampal longitudinal specialization have been proposed. For the brevity and clarity of this review, we focus on human electrophysiological evidence supporting and contravening these models with limited inclusion of noninvasive data. We then synthesize these data to propose a novel longitudinal model based on the amount of contextual information, drawing on previous conceptions described within the past decade.

Expansion of the neocortex and protection from neurodegeneration by in vivo transient reprogramming

Yamanaka factors (YFs) can reverse some aging features in mammalian tissues, but their effects on the brain remain largely unexplored. Here, we induced YFs in the mouse brain in a controlled spatiotemporal manner in two different scenarios: brain development and adult stages in the context of neurodegeneration. Embryonic induction of YFs perturbed cell identity of both progenitors and neurons, but transient and low-level expression is tolerated by these cells. Under these conditions, YF induction led to progenitor expansion, an increased number of upper cortical neurons and glia, and enhanced motor and social behavior in adult mice. Additionally, controlled YF induction is tolerated by principal neurons in the adult dorsal hippocampus and prevented the development of several hallmarks of Alzheimer's disease, including cognitive decline and altered molecular signatures, in the 5xFAD mouse model. These results highlight the powerful impact of YFs on neural proliferation and their potential use in brain disorders.

Defects of parvalbumin-positive interneurons are implicated in psychiatric disorders

Psychiatric disorders are a common cause of severe long-term disability and socioeconomic burden worldwide. Although our understanding of these disorders has advanced substantially over the last few years, little has changed the standards of care for these illnesses. Fast-spiking parvalbumin-positive interneurons (PVIs), a subpopulation of gamma-aminobutyric acid (GABA)ergic interneurons, are widely distributed in the hippocampus and have been reported to play an important role in various mental disorders. However, the mechanisms underlying the regulation of the molecular networks relevant to depression and schizophrenia (SCZ) are unknown. Here, we discuss the functions of PVIs in psychiatric disorders, including depression and SCZ. After reviewing several studies, we concluded that dysfunction in PVIs could cause depression-like behavior, as well as cognitive categories in SCZ, which might be mediated in large part by greater synaptic variability. In summary, this scientific review aims to discuss the current knowledge regarding the function of PVIs in depression and SCZ. Moreover, we highlight the importance of neurogenesis and synaptic plasticity in the pathogenesis of depression and SCZ, which seem to be mediated by PVIs activity. These findings provide a better understanding of the role of PVIs in psychiatric disorders.

Developmental 17-OHPC exposure disrupts behavior regulated by the mesocorticolimbic dopaminergic system in rats

The synthetic progestin, 17α-hydroxyprogesterone caproate (17-OHPC), is administered to pregnant individuals with the intention of reducing preterm birth. Although there is evidence that 17-OHPC is likely transferred from mother to fetus, there is little information regarding the potential effects of 17-OHPC administration on behavioral and neural development in offspring. Neonatal 17-OHPC exposure disrupts the development of the mesocorticolimbic dopaminergic pathway and associated behaviors in rats. 17-OHPC exposure altered dopaminergic innervation of prelimbic medial prefrontal cortex (mPFC) in neonates and adolescents and altered performance in measures of decision-making, set-shifting, and reversal-learning tasks. The present study tested the effects of developmental 17-OHPC exposure on numerous cognitive behaviors mediated by the mesocorticolimbic dopaminergic system, such as decision-making in a delay discounting task, latent inhibition following conditioned taste aversion (CTA), and spatial memory in the Morris Water Maze (MWM). The present work also aimed to further investigate response omissions in rats exposed to 17-OHPC during development and the potential role of dopamine D2 receptor in altering omissions in a delay discounting task. 17-OHPC exposure rendered rats less sensitive to an Eticlopride-induced increase in omissions in a delay discounting task when compared to controls. Quinpirole flattened the discount curve in both groups but did not significantly affect omissions in 17-OHPC-exposed or control rats. Following CTA, sucrose-pre-exposed 17-OHPC-exposed rats demonstrated decreased latent inhibition when compared to controls. In Morris Water Maze testing, 17-OHPC-exposed rats did not differ from controls after the first day of testing or during probe testing. These results suggest that exposure to 17-OHPC altered aspects of decision-making and latent inhibition in adult male rats, without affecting performance in a measure of spatial learning and memory. Further, the insensitivity of 17-OHPC-exposed males to an Eticlopride-induced increase in omissions suggests a dysfunction in the D2 receptor following exposure to this clinically used synthetic progestin.

The effects of moderate prenatal alcohol exposure on performance in hippocampal-sensitive spatial memory and anxiety tasks by adult male and female rat offspring

Moderate prenatal alcohol exposure (mPAE) results in structural alterations to the hippocampus. Previous studies have reported impairments in hippocampal-sensitive tasks, but have not compared performance between male and female animals. In the present study, performance in hippocampal-sensitive spatial memory and anxiety behavior tests were compared across adult male and female saccharin (SACC) control mPAE Long-Evans rat offspring. Two tests of spatial memory were conducted that were aimed at assessing memory for recently acquired spatial information: A delayed spatial alternation task using an M-shaped maze and a delayed match-to-place task in the Morris water task. In both tasks, rats in SACC and mPAE groups showed similar learning and retention of a spatial location even after a 2-h interval between encoding and retention. A separate group of adult male and female SACC and mPAE rat offspring were tested for anxiety-like behaviors in the elevated plus-maze paradigm. In this test, both male and female mPAE rats exhibited a significantly greater amount of time and a greater number of head dips in the open arms, while locomotion and open arm entries did not differ between groups. The results suggest that mPAE produces a reduction in anxiety-like behaviors in both male and female rats in the elevated plus-maze.

Distinct Ventral Hippocampus Network Properties in Dissociated Cultures

Extensive research has been focused in the past century on structural, physiological, and molecular attributes of the hippocampus. This interest was created by the unique involvement of the hippocampus in cognitive and affective functions of the brain. Functional analysis revealed that the hippocampus has divergent properties along its axial dimension to the extent that the dorsal sector (dorsal hippocampus, DH) has different connections with the rest of the brain than those of the ventral sector (VH). Still, longitudinal pathways connect the DH with the VH and dampen the functional differences between the sectors. To be able to identify the intrinsic functional difference between the DH and VH, we produced dissociated monolayer cultures from prenatal DH and VH and examined their properties at 10-20 days after plating by imaging the spontaneous activity of the network using Fluo-2 AM, a calcium indicator. Surprisingly, while DH and VH sectors produced dissociated cultures with similar morphological attributes, VH cultures were more active spontaneously than DH cultures. Furthermore, when stimulated to produce action potentials, VH neurons triggered network bursts in postsynaptic neurons more often than DH cultures. Finally, in both DH and VH cultures, electrical stimulation of single cells produced network bursts in response to a burst of action potentials rather than to single spikes. These experiments indicate that even in dissociated cultures, neurons of the VH are more excitable and sensitive to electrical stimulation than DH; hence, they are more likely to generate network bursts and epileptic seizures, as suggested for in vivo brains.

Postsynaptic dopamine D3 receptors selectively modulate μ-opioid receptor-expressing GABAergic inputs onto CA1 pyramidal cells in the rat ventral hippocampus

Although the actions of dopamine throughout the brain are clearly linked to motivation and cognition, the specific role(s) of dopamine in the CA1 subfield of the ventral hippocampus (vH) is unresolved. Prior preclinical studies suggest that dopamine D3 receptors (D3Rs) expressed on CA1 pyramidal cells exhibit a unique capacity to modulate mechanisms of long-term synaptic plasticity, but less is known about how interneuronal inputs modulate these cells. We hypothesized that inputs from μ-opioid receptor (MOR)-expressing inhibitory interneurons selectively modulate the activity of postsynaptic D3Rs expressed on CA1 principal cells to shape neurotransmission in the rat vH. We used the whole cell voltage-clamp technique to test this hypothesis by measuring evoked inhibitory postsynaptic currents (eIPSCs) from CA1 principal cells in vH slices or GABAA currents from acutely dissociated vH neurons. The eIPSC response recorded from CA1 neurons in vH slices was inhibited by either the MOR agonist DAMGO or the D3R agonist PD128907, but pretreatment with DAMGO occluded any further inhibition by PD128907. GABAA currents measured in acutely dissociated vH CA1 neurons were inhibited by D3R activation via PD128907, consistent with postsynaptic localization of D3 receptors. Kinetic alterations induced by the neuromodulatory agonists are consistent with selective targeting of postsynaptic D3Rs expressed on CA1 principal cells by MOR-expressing GABAergic inputs. Our findings suggest that postsynaptic D3R-mediated modulation of MOR-expressing GABAergic inputs is a site at which dopaminergic and opioidergic activity may contribute to disinhibition of vH excitatory neurotransmission and, thus, influence critical physiological processes such as synaptic plasticity and network oscillations.NEW & NOTEWORTHY We report that the activity of an inhibitory synapse on CA1 pyramidal cells in the rat ventral hippocampus is shaped by heterogeneous neuromodulators. Specifically, postsynaptic dopamine D3 receptors on ventral hippocampal CA1 pyramidal neurons are selectively targeted by an inhibitory input from µ-opioid receptor-expressing GABAergic terminals.

Flexible Behavioral Adjustment to Frustrative Nonreward in Anticipatory Behavior, but Not in Consummatory Behavior, Requires the Dorsal Hippocampus

The hippocampus (HC) is recognized for its pivotal role in memory-related plasticity and facilitating adaptive behavioral responses to reward shifts. However, the nature of its involvement in the response to reward downshifts remains to be determined. To bridge this knowledge gap, we explored the HC's function through a series of experiments in various tasks involving reward downshifts and using several neural manipulations in rats. In Experiment 1, complete excitotoxic lesions of the HC impaired choice performance in a modified T-maze after reducing the quantity of sugar pellet rewards. In Experiment 2, chemogenetic inhibition of the dorsal HC (dHC) disrupted anticipatory behavior following a food-pellet reward reduction. Experiments 3-5 impaired HC function by using peripheral lipopolysaccharide (LPS) administration. This treatment, which induces peripheral inflammation affecting HC function, significantly increased cytokine levels in the dHC (Experiment 3) and impaired anticipatory choice behavior (Experiment 4). None of these dorsal hippocampal manipulations affected consummatory responses in animals experiencing sucrose downshifts. Accordingly, we found no evidence of increased neural activation in either the dorsal or ventral HC, as measured by c-Fos expression, after a sucrose downshift task involving consummatory suppression (Experiment 6). The results highlight the HC's pivotal role in adaptively modulating anticipatory behavior in response to a variety of situations involving frustrative nonreward, while having no effect on adjustments on consummatory behavior. The data supporting this conclusion were obtained under heterogeneous experimental conditions derived from a multi-laboratory collaboration, ensuring the robustness and high reproducibility of our findings. Spatial orientation, memory update, choice of reward signals of different values, and anticipatory versus consummatory adjustments to reward downshift are discussed as potential mechanisms that could account for the specific effects observed from HC manipulations.

Acute stress induces different changes on the expression of CB1 receptors in the hippocampus of two lines of male rats differing in their response to stressors

The stress-induced alterations in cognitive processes and psychiatric disorders can be accelerated when acute stressors challenge the hippocampal functions. To address this issue, we used Western Blot (WB) and immunohistochemistry assays to investigate the impact of acute forced swimming (FS) on the expression of the CB1 cannabinoid receptors (CB1R) in the hippocampus (HC) of the male outbred Roman High- (RHA) and Low-Avoidance (RLA) rat lines, one of the most validated genetic models for the study of behavior related to fear/anxiety and stress-induced depression. The distinct responses to FS confirmed the different behavioral strategies displayed by the two phenotypes when exposed to stressors, with RLA and RHA rats displaying reactive vs. proactive coping, respectively. In control rats, the WB analysis showed lower hippocampal CB1R relative levels in RLA rats than in their RHA counterparts. After FS, RLA rats showed increased CB1R levels in the dorsal HC (dHC) vs. no change in the ventral HC (vHC), while RHA rats displayed no change in the dHC vs. a decrease in the vHC. In the tissue sections from dHC, FS elicited an increment over the control level of CB1R-like immunoreactivity (LI) in the CA1 and CA3 sectors of the Ammon's horn of RLA rats, while in RHA rats the density of CB1R-LI increased only in the CA1 sector. In tissue sections from the vHC, FS caused an increase over the control values of CB1R-LI only in the CA1 sector of RLA rats and a decrement of the CB1R-LI in the CA1 sector and dentate gyrus of control RHA rats. This study shows for the first time that, in baseline conditions, the CB1Rs are present in the dHC and the vHC of the Roman rat lines with a different distribution along the septo-temporal extension of the HC and that the FS induces rapid and distinct changes in the hippocampal expression of CB1R of RLA vs. RLA rats, in keeping with the view that endocannabinoid signaling may contribute to the molecular mechanisms that regulate the different responses of the dHC vs. the vHC to aversive situations in male Roman rats. Our results also provide evidence supporting the involvement of CB1R in the molecular underpinnings of the susceptibility of RLA rats and the resistance of RHA rats to stress-induced depression-like behavior.