Stress Regulation of Sustained Attention and the Cholinergic Attention System

Septohippocampal cholinergic system at the intersection of stress and cognition: Current trends and translational implications

Deficits in hippocampus-dependent memory processes are common across psychiatric and neurodegenerative disorders such as depression, anxiety and Alzheimer's disease. Moreover, stress is a major environmental risk factor for these pathologies and it exerts detrimental effects on hippocampal functioning via the activation of hypothalamic-pituitary-adrenal (HPA) axis. The medial septum cholinergic neurons extensively innervate the hippocampus. Although, the cholinergic septohippocampal pathway (SHP) has long been implicated in learning and memory, its involvement in mediating the adaptive and maladaptive impact of stress on mnemonic processes remains less clear. Here, we discuss current research highlighting the contributions of cholinergic SHP in modulating memory encoding, consolidation and retrieval. Then, we present evidence supporting the view that neurobiological interactions between HPA axis stress response and cholinergic signalling impact hippocampal computations. Finally, we critically discuss potential challenges and opportunities to target cholinergic SHP as a therapeutic strategy to improve cognitive impairments in stress-related disorders. We argue that such efforts should consider recent conceptualisations on the dynamic nature of cholinergic signalling in modulating distinct subcomponents of memory and its interactions with cellular substrates that regulate the adaptive stress response.

Distinct subpopulations of ventral pallidal cholinergic projection neurons encode valence of olfactory stimuli

To better understand the function of cholinergic projection neurons in the ventral pallidum (VP), we examined behavioral responses to appetitive (APP) and aversive (AV) odors that elicited approach or avoidance, respectively. Exposure to each odor increased cFos expression and calcium signaling in VP cholinergic neurons. Activity and Cre-dependent viral vectors selectively labeled VP cholinergic neurons that were activated and reactivated in response to either APP or AV odors, but not both, identifying two non-overlapping populations of VP cholinergic neurons differentially activated by the valence of olfactory stimuli. These two subpopulations showed differences in electrophysiological properties, morphology, and projections to the basolateral amygdala. Although VP neurons are engaged in both approach and avoidance behavioral responses, cholinergic signaling is only required for approach behavior. Thus, two distinct subpopulations of VP cholinergic neurons differentially encode valence of olfactory stimuli and play distinct roles in approach and avoidance behaviors.

Genetic loss of the dopamine transporter significantly impacts behavioral and molecular responses to sub-chronic stress in mice

Dopaminergic neurotransmission has emerged as a critical determinant of stress susceptibility and resilience. Although the dopamine transporter (DAT) is known to play a key role in maintaining dopamine (DA) homeostasis, its importance for the regulation of stress susceptibility remains largely unknown. Indeed, while numerous studies have examined the neurochemical and behavioral consequences of genetic loss of DAT, very few have compared responses to stress in wild-type and DAT-knockout (KO) animals. The current study compared the responses of male and female WT and DAT-KO mice to a model of sub-chronic stress. Our results reveal that DAT-KO mice are resistant to stress-induced increases in the latency to enter the light chamber of the light-dark emergence test and demonstrate that DAT-KO mice exhibit baseline reductions in forced swim test immobility and grooming time in the splash test of grooming behavior. In addition to these behavioral changes, our results highlight the importance of sex and dopaminergic neurotransmission on stress-induced changes in the expression and phosphorylation of several signal transduction molecules in the nucleus accumbens that have previously been implicated in the regulation of stress susceptibility, including ERK, GSK3β, and ΔFosB. Overall, these results provide further evidence of the importance of dopaminergic neurotransmission in regulating stress susceptibility and suggest that genetic loss of DAT prevents stress-induced increases in anxiety-like behavior.

Practical solutions for including sex as a biological variable (SABV) in preclinical neuropsychopharmacological research

Recently, many funding agencies have released guidelines on the importance of considering sex as a biological variable (SABV) as an experimental factor, aiming to address sex differences and avoid possible sex biases to enhance the reproducibility and translational relevance of preclinical research. In neuroscience and pharmacology, the female sex is often omitted from experimental designs, with researchers generalizing male-driven outcomes to both sexes, risking a biased or limited understanding of disease mechanisms and thus potentially ineffective therapeutics. Herein, we describe key methodological aspects that should be considered when sex is factored into in vitro and in vivo experiments and provide practical knowledge for researchers to incorporate SABV into preclinical research. Both age and sex significantly influence biological and behavioral processes due to critical changes at different timepoints of development for males and females and due to hormonal fluctuations across the rodent lifespan. We show that including both sexes does not require larger sample sizes, and even if sex is included as an independent variable in the study design, a moderate increase in sample size is sufficient. Moreover, the importance of tracking hormone levels in both sexes and the differentiation between sex differences and sex-related strategy in behaviors are explained. Finally, the lack of robust data on how biological sex influences the pharmacokinetic (PK), pharmacodynamic (PD), or toxicological effects of various preclinically administered drugs to animals due to the exclusion of female animals is discussed, and methodological strategies to enhance the rigor and translational relevance of preclinical research are proposed.

Distinct subpopulations of ventral pallidal cholinergic projection neurons encode valence of olfactory stimuli

The ventral pallidum (VP) mediates motivated behaviors largely via the action of VP GABA and glutamatergic neurons. In addition to these neuronal subtypes, there is a population of cholinergic projection neurons in the VP, whose functional significance remains unclear. To understand the functional role of VP cholinergic neurons, we first examined behavioral responses to an appetitive (APP) odor that elicited approach, and an aversive (AV) odor that led to avoidance. To examine how VP cholinergic neurons were engaged in APP vs. AV responses, we used an immediate early gene marker and in-vivo fiber photometry, examining the activation profile of VP cholinergic neurons in response to each odor. Exposure to each odor led to an increase in the number of cFos counts and increased calcium signaling of VP cholinergic neurons. Activity and cre-dependent viral vectors were designed to label engaged VP cholinergic neurons in two distinct contexts: (1) exposure to the APP odor, (2) followed by subsequent exposure to the AV odor, and vice versa. These studies revealed two distinct, non-overlapping subpopulations of VP cholinergic neurons: one activated in response to the APP odor, and a second distinct population activated in response to the AV odor. These two subpopulations of VP cholinergic neurons are spatially intermingled within the VP, but show differences in electrophysiological properties, neuronal morphology, and projections to the basolateral amygdala. Although VP cholinergic neurons are engaged in behavioral responses to each odor, VP cholinergic signaling is only required for approach behavior. Indeed, inhibition of VP cholinergic neurons not only blocks approach to the APP odor, but reverses the behavior, leading to active avoidance. Our results highlight the functional heterogeneity of cholinergic projection neurons within the VP. These two subpopulations of VP cholinergic neurons differentially encode valence of olfactory stimuli and play unique roles in approach and avoidance behaviors.

Priorities for research on neuromodulatory subcortical systems in Alzheimer's disease: Position paper from the NSS PIA of ISTAART

The neuromodulatory subcortical system (NSS) nuclei are critical hubs for survival, hedonic tone, and homeostasis. Tau-associated NSS degeneration occurs early in Alzheimer's disease (AD) pathogenesis, long before the emergence of pathognomonic memory dysfunction and cortical lesions. Accumulating evidence supports the role of NSS dysfunction and degeneration in the behavioral and neuropsychiatric manifestations featured early in AD. Experimental studies even suggest that AD-associated NSS degeneration drives brain neuroinflammatory status and contributes to disease progression, including the exacerbation of cortical lesions. Given the important pathophysiologic and etiologic roles that involve the NSS in early AD stages, there is an urgent need to expand our understanding of the mechanisms underlying NSS vulnerability and more precisely detail the clinical progression of NSS changes in AD. Here, the NSS Professional Interest Area of the International Society to Advance Alzheimer's Research and Treatment highlights knowledge gaps about NSS within AD and provides recommendations for priorities specific to clinical research, biomarker development, modeling, and intervention. HIGHLIGHTS: Neuromodulatory nuclei degenerate in early Alzheimer's disease pathological stages. Alzheimer's pathophysiology is exacerbated by neuromodulatory nuclei degeneration. Neuromodulatory nuclei degeneration drives neuropsychiatric symptoms in dementia. Biomarkers of neuromodulatory integrity would be value-creating for dementia care. Neuromodulatory nuclei present strategic prospects for disease-modifying therapies.

Sex-specific responses to juvenile stress on the dopaminergic system in an animal model of attention-deficit hyperactivity disorder

The etiology of attention-deficit hyperactivity disorder (ADHD) strongly suggests a genetic component as the main cause; however, environmental factors such as early adverse experiences in childhood may play an interactive role with the genetic susceptibility. Spontaneously hypertensive rats (SHRs), a genetic ADHD model, and control Wistar Kyoto rats (WKYs) were subjected to chronic unpredictable mild stress during the juvenile period. The behavioral characteristics were monitored, and dopamine-related factors in the core regions of dopaminergic pathways were measured. Higher ADHD symptom-related behaviors were observed in response to juvenile stress in male SHRs than control WKYs. For the SHRs subjected to juvenile stress, hyperactivity in males, recognition in females, and depressant potential in both sexes were markedly observed. In the expression of 17 dopamine-related genes and proteins, greater changes were detected in male SHRs subjected to juvenile stress, especially in dopamine metabolic factors. Dopamine clearance factors involved in dopamine degradation and transport, especially catechol-O-methyltransferase (COMT) and dopamine transporter (DAT), showed sex-specific differences induced by juvenile stress in dopamine metabolite assays. Moreover, stressed male SHRs treated with methylphenidate showed better improvement in behavior than the females, resulting in different levels of COMT and DAT amelioration. These results suggest that juvenile stress potentially increased the incidence of ADHD in a genetic rat model, which showed sex-specific differences based on the expression of COMT and DAT. Therefore, our results could help develop gender-specific diagnostics and healthcare options for juvenile stress in patients with ADHD.

The role of stress phenotypes in understanding childhood adversity as a transdiagnostic risk factor for psychopathology

Childhood adversity is a leading transdiagnostic risk factor for psychopathology, being associated with an estimated 31-62% of childhood-onset disorders and 23-42% of adult-onset disorders (Kessler et al., 2010). Major unresolved theoretical challenges stem from the nonspecific and probabilistic nature of the links between childhood adversity and psychopathology. The links are nonspecific because childhood adversity increases risk, through a range of mechanisms, for diverse forms of psychopathology and are probabilistic because not all individuals exposed to childhood adversity develop psychopathology. In this article, we propose a path forward by focusing on stress phenotypes, defined as biobehavioral patterns activated in response to stressors that can disrupt future functioning when persistent (e.g., reward seeking, social withdrawal, aggression). This review centers on the accumulating evidence that psychopathology appears to be more strongly predicted by behavior and biology during states of stress. Building on this observation, our theoretical framework proposes that we can model pathways from childhood adversity to psychopathology with greater specificity and certainty by understanding stress phenotypes, defined as patterns of behavior and their corresponding biological substrates that are elicited by stressors. This approach aims to advance our conceptualization of mediating pathways from childhood adversity to psychopathology. Understanding stress phenotypes will bring us closer to "precision mental health," a person-centered approach to identifying, preventing, and treating psychopathology. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

ChAT::Cre transgenic rats show sex-dependent altered fear behaviors, ultrasonic vocalizations and cholinergic marker expression

The cholinergic system is a critical regulator of Pavlovian fear learning and extinction. As such, we have begun investigating the cholinergic system's involvement in individual differences in cued fear extinction using a transgenic ChAT::Cre rat model. The current study extends behavioral phenotyping of a transgenic ChAT::Cre rat line by examining both freezing behavior and ultrasonic vocalizations (USVs) during a Pavlovian cued fear learning and extinction paradigm. Freezing, 22 kHz USVs, and 50 kHz USVs were compared between male and female transgenic ChAT::Cre+ rats and their wildtype (Cre-) littermates during fear learning, contextual and cue-conditioned fear recall, cued fear extinction, and generalization to a novel tone. During contextual and cued fear recall ChAT::Cre+ rats froze slightly more than their Cre- littermates, and displayed significant sex differences in contextual and cue-conditioned freezing, 22 kHz USVs, and 50 kHz USVs. Females showed more freezing than males in fear recall trials, but fewer 22 kHz distress calls during fear learning and recall. Females also produced more 50 kHz USVs during exposure to the testing chambers prior to tone (or shock) presentation compared with males, but this effect was blunted in ChAT::Cre+ females. Corroborating previous studies, ChAT::Cre+ transgenic rats overexpressed vesicular acetylcholine transporter immunolabeling in basal forebrain, striatum, basolateral amygdala, and hippocampus, but had similar levels of acetylcholinesterase and numbers of ChAT+ neurons as Cre- rats. This study suggests that variance in behavior between ChAT::Cre+ and wildtype rats is sex dependent and advances theories that distinct neural circuits and processes regulate sexually divergent fear responses.

Corticotropin releasing factor in the nucleus basalis of Meynert impairs attentional performance and reduces levels of glutamate and taurine in male and female rats

Psychiatric disorders that are characterized by impairments in sustained attention, including attention deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD), and major depression are also sensitive to exacerbation by stress. Sustained attention relies on cholinergic and non-cholinergic projections from the nucleus basalis of Meynert (NBM) in the basal forebrain to the medial prefrontal cortex (mPFC). We have previously shown that central administration of the stress neuropeptide corticotropin releasing factor (CRF) impairs performance on the sustained attention task (SAT) in adult male and female rats. The present study investigated whether this effect was mediated by CRF's action in the NBM. Rats were administered CRF in the NBM and subsequent SAT performance was measured. A high dose of CRF (100 ng) significantly impaired performance on non-signaled events across sex. Because performance on non-signaled events is believed to depend on non-cholinergic (i.e., GABA and glutamate) signaling, high performance liquid chromatography was used to quantify amino acid levels in the NBM and mPFC. We found females have higher levels of glutamate, glutamine, GABA glycine, and alanine in the NBM than males. Importantly, CRF in the NBM led to a local decrease of taurine and several amino acids involved in glutamate synthesis in males and females, changes which may mediate the CRF-induced SAT performance deficit. Together these studies suggest that CRF regulation of amino acids in the NMB contributes to stress-induced attention deficits.

Evaluation of changes in the cognitive function of adult cynomolgus monkeys under stress induced by audio-visual stimulation by applying modified finger maze test

Stress in life is ubiquitous and unavoidable. Prolonged exposure to severe stress can lead to physical intolerance and impair cognitive function. Non-human primates are considered to be the best animal model for studying cognitive function, especially memory and attention. The finger maze test, with the advantages of short training time and lower cost, is recommended to evaluate learning and memory in non-human primates. In this study, we modified the finger maze test method to evaluate the cognitive function of single-housed cynomolgus monkeys. The flexibility and attention of cynomolgus monkeys were assessed by performing the complex task test and the stranger intrusion interference test, respectively, which increased the difficulty of obtaining rewards, and the ability of long-term memory was also evaluated by the memory test. Furthermore, the changes in cognitive function of the cynomolgus monkeys were tested by using the finger maze test after audio-visual stimulation, and the changes in the cortisol levels during stimulation were also analyzed. We found that, after completing the learning test, there was no significant decrease in their success rate when monkeys processed multitasks at the same time. In the stranger intrusion interference test, all subjects were distracted, but the accuracy did not decrease. The monkeys completed the memory tests in the 1st and 2nd months after the learning tests, with a high success rate. However, the success rate decreased significantly at the end of the 4th month. During audio-visual stimulation, the plasma cortisol level significantly increased in the first 2 months and was maintained at a high level thereafter. One month after audio-visual stimulation, the accuracy of the memory test was significantly reduced, and the total time of distraction was significantly prolonged. In conclusion, chronic audio-visual stimulation can increase blood cortisol levels and impair cognitive function. The modified finger maze test can evaluate many aspects of cognitive function and assess the changes in the cognitive function of adult cynomolgus monkeys under stress.

Metric magnetic resonance imaging analysis reveals pronounced substantia-innominata atrophy in dementia with Lewy bodies with a psychiatric onset

Background

Dementia with Lewy bodies (DLB) is a type of dementia often diagnosed in older patients. Since its initial symptoms range from delirium to psychiatric and cognitive symptoms, the diagnosis is often delayed.

Objectives

In our study, we evaluated the magnetic resonance imaging (MRI) of patients suffering from DLB in correlation with their initial symptoms taking a new pragmatic approach entailing manual measurements in addition to an automated volumetric analysis of MRI.

Methods

A total of 63 patients with diagnosed DLB and valid 3D data sets were retrospectively and blinded evaluated. We assessed atrophy patterns (1) manually for the substantia innominata and (2) via FastSurfer for the most common supratentorial regions. Initial symptoms were categorized by (1) mild cognitive impairment (MCI), (2) psychiatric episodes, and (3) delirium.

Results

Manual metric MRI measurements revealed moderate, but significant substantia-innominata (SI) atrophy in patients with a psychiatric onset. FastSurfer analysis revealed no regional volumetric differences between groups.

Conclusion

The SI in patients with DLB and a psychiatric-onset is more atrophied than that in patients with initial MCI. Our results suggest potential differences in SI between DLB subtypes at the prodromal stage, which are useful when taking a differential-diagnostic approach. This finding should be confirmed in larger patient cohorts.

Chronic, but not sub-chronic, stress increases binge-like alcohol consumption in male and female c57BL6 mice

Stress is known to contribute to mental illness and alcohol use disorders, which are highly prevalent and lead to considerable disability. These stress-related disorders are characterized by significant sex differences, which remain poorly understood. Preclinical research comparing the effects of stress in males and females has the potential to provide new insights into the neurobiology of these conditions. The current study compared the effects of chronic and sub-chronic exposure to variable environmental stressors on binge-like alcohol consumption using the drinking-in-the-dark model in male and female c57BL6 mice. The results reveal that chronic, but not sub-chronic, exposure to variable stress increases alcohol intake in both sexes. Stress-induced alterations in gene expression were also compared in the nucleus accumbens, a brain region widely known to play a key role in stress susceptibility and reward processing. Real-time PCR data indicate that chronic, but not sub-chronic, environmental stress leads to downregulation of adenosine 2A (A2A) receptor mRNA. By contrast, sub-chronic stress increased CREB expression, while chronic stress did not. Several sex differences in the effects of stress on gene expression were also noted. Our results demonstrate that reductions in A2A receptor mRNA in the nucleus accumbens are associated with the increased binge drinking of chronically stressed animals, but future work will be required to determine the functional importance of this gene expression change. Continuing to define the molecular alterations associated with stress-induced increases in alcohol intake has the potential to provide insights into the development and progression of stress-related disorders.

Early life maternal separation induces sex-specific antidepressant-like responses but has minimal effects on adult stress susceptibility in mice

Early life stress is known to increase the risk of depression and anxiety disorders, which are highly prevalent conditions that disproportionately affect women. However, the results of preclinical studies have been mixed, with some work suggesting that early life stress promotes anxiety-like behavior and/or increases susceptibility to subsequent stressors, and other research suggesting that early life stress reduces anxiety-like behavior and/or confers resilience to subsequent stress exposure. It is likely that factors such as sex and the timing and severity of early life and adult stress exposure dictate whether a particular early life experience promotes adaptive vs. maladaptive behavior later in life. Most work in this area has focused exclusively on males, but several sex differences in the effects of early life stress on subsequent stress susceptibility have been reported. The current study examined the impact of early life maternal separation on susceptibility to behavioral alterations induced by 3 days of variable stress in adulthood in male and female c57BL6 mice. Our results indicate that 3 days of adult stress is sufficient to increase anxiety-like behavior in several paradigms and to increase immobility in the forced swim test. In contrast, a history of maternal separation reduces anxiety-like behavior in several tests, particularly in males. These findings could contribute to our understanding of sex differences in mental illness by demonstrating that males are more likely than females to display adaptive responses to mild early life stressors.

α7 nicotinic acetylcholine receptors in the hippocampal circuit: taming complexity

Cholinergic innervation of the hippocampus uses the neurotransmitter acetylcholine (ACh) to coordinate neuronal circuit activity while simultaneously influencing the function of non-neuronal cell types. The α7 nicotinic ACh receptor (nAChR) subtype is highly expressed throughout the hippocampus, has the highest calcium permeability compared with other subtypes of nAChRs, and is of high therapeutic interest due to its association with a variety of neurological disorders and neurodegenerative diseases. In this review, we synthesize research describing α7 nAChR properties, function, and relationship to cognitive dysfunction within the hippocampal circuit and highlight approaches to help improve therapeutic development.

Sex differences in anxiety and depression: circuits and mechanisms

Epidemiological sex differences in anxiety disorders and major depression are well characterized. Yet the circuits and mechanisms that contribute to these differences are understudied, because preclinical studies have historically excluded female rodents. This oversight is beginning to be addressed, and recent studies that include male and female rodents are identifying sex differences in neurobiological processes that underlie features of these disorders, including conflict anxiety, fear processing, arousal, social avoidance, learned helplessness and anhedonia. These findings allow us to conceptualize various types of sex differences in the brain, which in turn have broader implications for considering sex as a biological variable. Importantly, comparing the sexes could aid in the discovery of novel therapeutics.

Repurposing Cholinesterase Inhibitors as Antidepressants? Dose and Stress-Sensitivity May Be Critical to Opening Possibilities

When stress becomes chronic it can trigger lasting brain and behavioral changes including Major Depressive Disorder (MDD). There is conflicting evidence regarding whether acetylcholinesterase inhibitors (AChEIs) may have antidepressant properties. In a recent publication, we demonstrated a strong dose-dependency of the effect of AChEIs on antidepressant-related behavior in the mouse forced swim test: whereas the AChEI donepezil indeed promotes depression-like behavior at a high dose, it has antidepressant-like properties at lower doses in the same experiment. Our data therefore suggest a Janus-faced dose-response curve for donepezil in depression-related behavior. In this review, we investigate the mood-related properties of AChEIs in greater detail, focusing on both human and rodent studies. In fact, while there have been many studies showing pro-depressant activity by AChEIs and this is a major concept in the field, a variety of other studies in both humans and rodents show antidepressant effects. Our study was one of the first to systematically vary dose to include very low concentrations while measuring behavioral effects, potentially explaining the apparent disparate findings in the field. The possibility of antidepressant roles for AChEIs in rodents may provide hope for new depression treatments. Importantly, MDD is a psychosocial stress-linked disorder, and in rodents, stress is a major experimental manipulation for studying depression mechanisms, so an important future direction will be to determine the extent to which these depression-related effects are stress-sensitive. In sum, gaining a greater understanding of the potentially therapeutic mood-related effects of low dose AChEIs, both in rodent models and in human subjects, should be a prioritized topic in ongoing translational research.