Prenatal stress exposure increases the excitation of dopamine neurons in the ventral tegmental area and alters their reponses to psychostimulants

Preventive putative effect of agmatine on cognitive and molecular outcomes in ventral tegmental area of male offspring following physical and psychological prenatal stress

Prenatal stress (PS) results from a maternal experience of stressful events during pregnancy, which has been associated with an increased risk of behavioral disorders including substance abuse and anxiety in the offspring. PS is known to result in heightened dopamine release in the ventral tegmental area (VTA), in part through the effects of corticotropin-releasing hormone, which directly excites dopaminergic cells. It has recently been suggested that agmatine plays a role in modulating anxiety-like behaviors. In this study, we investigated whether agmatine could reduce negative cognitive outcomes in male mice prenatally exposed to psychological/physical stress, and whether this could be associated with molecular changes in VTA. Agmatine (37.5 mg/kg) was administrated 30 min prior to PS induction in pregnant Swiss mice. Male offspring were evaluated in a series of behavioral and molecular assays. Findings demonstrated that agmatine reduced the impairment in locomotor activity induced by both psychological and physical PS. Agmatine also decreased heightened conditioned place preference to morphine seen in PS offspring. Moreover, agmatine ameliorated the anxiety-like behavior and drug-seeking behavior induced by PS in the male offspring. Molecular effects were seen in VTA as the enhanced brain-derived neurotrophic factor (BDNF) induced by PS in the VTA was reduced by agmatine. Behavioral tests indicate that agmatine exerts a protective effect on PS-induced impairments in male offspring, which could be due in part to agmatine-associated molecular alterations in the VTA. Taken together, our data suggest that prenatal treatment with agmatine exerts protective effect against negative consequences of PS on the development of affective circuits in the offspring.

Cross-talk between the HPA axis and addiction-related regions in stressful situations

Addiction is a worldwide problem that has a negative impact on society by imposing significant costs on health care, public security, and the deactivation of the community economic cycle. Stress is an important risk factor in the development of addiction and relapse vulnerability. Here we review studies that have demonstrated the diverse roles of stress in addiction. Term searches were conducted manually in important reference journals as well as in the Google Scholar and PubMed databases, between 2010 and 2022. In each section of this narrative review, an effort has been made to use pertinent sources. First, we will provide an overview of changes in the Hypothalamus-Pituitary-Adrenal (HPA) axis component following stress, which impact reward-related regions including the ventral tegmental area (VTA) and nucleus accumbens (NAc). Then we will focus on internal factors altered by stress and their effects on drug addiction vulnerability. We conclude that alterations in neuro-inflammatory, neurotrophic, and neurotransmitter factors following stress pathways can impact related mechanisms on craving and relapse susceptibility.

Adult stress exposure blunts dopamine system hyperresponsivity in a neurodevelopmental rodent model of schizophrenia

Stress is a major risk factor for the development of both schizophrenia and depression, and comorbidity between the two is common in schizoaffective disorders. However, the effects of stress exposure (i.e. chronic mild stress-CMS) on depression-related phenotypes in a neurodevelopmental model relevant to schizophrenia (i.e. methylazoxymethanol acetate—MAM) have yet to be explored and could provide insight into shared mechanisms of disease. To this end, we combined the prenatal MAM model with adult CMS exposure and explored the resultant pathophysiology using the social approach test (SAT), immobility in the forced swim test (FST) and amphetamine-induced hyperlocomotion (AIH) as depression- and schizophrenia-related endophenotypes and performed extracellular recordings of ventral tegmental area (VTA) DA neurons. MAM rats exhibited a reduction in social approach and increased VTA DA neuron activity compared to SAL rats or CMS groups. Separate cohorts of MAM animals were subjected to FST and AIH testing (counterbalanced order) or FST only. CMS groups exhibited increased FST immobility. Post-FST, both MAM groups (MAM-CON, MAM-CMS) exhibited blunted locomotor response to amphetamine compared with their SAL counterparts exposed to the same tests. Post-FST, MAM rats exhibited comparable VTA population activity to SAL rats, and CMS groups exhibited attenuated VTA population activity. Apomorphine administration results were consistent with the model suggesting that reductions in VTA DA neuron activity in MAM rats following FST exposure resulted from over-excitation, or depolarization block. These data suggest stress-induced DA downregulation in MAM rats, as FST exposure was sufficient to block the DA hyperresponsivity phenotype.

Adolescent chronic unpredictable stress leads to increased anxiety and attention deficit/hyperactivity-like symptoms in adulthood

BACKGROUND

Early-life adversities during development (e.g., child abuse and neglect) are linked to multiple behavioral and cognitive dysfunctions, such as attention deficit/hyperactivity disorder (ADHD) and anxiety disorders, which have high comorbidity. However, the impact of adversities during adolescence, a crucial period in early life for these disorders, is understudied. Using a chronic unpredictable stress (CUS) model in rats, we investigated whether adversities in adolescence could lead to increased anxiety and ADHD-like symptoms in adulthood.

METHODS

Mid- to late-adolescent (5-7-week-old) male and female Sprague-Dawley rats underwent a mild CUS procedure for 2 weeks. Various stressors were applied in an unpredictable way. Rats of both sexes were then trained with a 2-choice reaction time (2-CRT) task during adulthood, which are designed to detect ADHD-like symptoms, including increased impulsivity and lapse of attention. In addition, an open field test was conducted to examine if CUS resulted in a persistent increase in anxiety-like behavior during adulthood.

RESULTS

Both male and female rats with CUS exposure travelled shorter distances in the open field and spent less time in the center zone, indicating increased anxiety. In the 2-CRT task, rats of both sexes with CUS exposure showed increased impulsivity. Augmented lapses of attention were observed in female but not male rats.

CONCLUSION

Chronic unpredictable stress during adolescence increases anxiety and leads to ADHD-like symptoms in both male and female rats in adulthood. The deficits are more severe in females than in males. These observations support that adversities during adolescence persistently increase anxiety, which is comorbid with attention deficits.

Effects of early life stress during stress hyporesponsive period (SHRP) on anxiety and curiosity in adolescent rats

Repeated exposure to adverse experiences in early life, termed Early Life Stress (ELS), can increase anxiety disorders later in life. Anxiety is directly associated with curiosity, a form of intrinsic drive state associated with increased novelty-seeking behaviour and risk taking for challenging opportunities and could probably modulate learning and memory. In humans, elevated curiosity during adolescence tends to elicit increased exploration, novelty seeking, high risk-taking behaviour and heightened emotionality. Such behaviours are beneficial in maintaining social skills and cognitive functions later in life. We investigated whether ELS-induced anxiety impacts curiosity-like behaviour at adolescence in an animal model. ELS was induced by subjecting Sprague Dawley rat pups to maternal separation and isolation (MS) stress during the stress hyporesponsive period (SHRP) from post-natal days (PND) 4-PND 14. This rat model was tested for anxiety, spontaneous exploratory behaviour and curiosity-like behaviour in a custom-designed arena during adolescence (PND 30-45). ELS-induced changes in the stress were confirmed by corticosterone, while, basal dopamine level was estimated to understand the neurochemical basis of MS stress-induced changes in curiosity. We observed an increase in the levels of anxiety and intrinsic drive state such as curiosity-like behaviour, which was associated with elevated plasma corticosterone and dopamine in MS animals during adolescence suggesting the impact of ELS during SHRP on adolescent behaviour.

Early environmental enrichment prevents cognitive impairments and developing addictive behaviours in a mouse model of prenatal psychological and physical stress

Environmental enrichment (EE) has shown remarkable effects in improving cognition and addictive behaviour. We tested whether EE could help recover from prenatal stress exposure. Mature Swiss Webster male and virgin female mice were placed together until vaginal plugs were detectable. Next, pregnant rodents were randomized into the control, physically and psychologically stressed groups. The application of stress was initiated on the 10th day of pregnancy and persisted for a week to induce stress in the mice. Open field and elevated plus-maze (EPM) tests were utilized as explorative and anxiety assays, respectively. A passive avoidance shuttle-box test was carried out to check anxiety-modulated behaviour. Morris water maze (MWM) test was undertaken to evaluate spatial learning and memory. Conditioned place preference (CPP) test was selected for evaluation of tendency to morphine consumption. Our results showed that prenatal stress elevated anxiety-like behaviour in the offspring which EE could significantly alleviate after weaning. We also found a higher preference for morphine use in the physical stress and psychological stress offspring group. However, no difference was observed among the genders. Application of EE for the stress group improved several parameters of the cognitive behaviour significantly. Although prenatal stress can lead to detrimental behavioural and cognitive outcomes, it can in part be relieved by early exposure to EE. However, some outcomes linked to prenatal stress exposure may not be diminished by EE therapy. In light of such irreversible effects, large-scale preventive actions promoting avoidance from stress during pregnancy should be advised.

Moderate prenatal ethanol exposure leads to attention deficits in both male and female rats

BACKGROUND

Attention deficits caused by prenatal ethanol (EtOH) exposure (PE) are a prevalent condition in fetal alcohol spectrum disorders (FASDs). Importantly, the deficits are observed in individuals with FASD who have normal IQs and show no dysmorphic facial features caused by heavy PE. These observations suggest that even moderate PE could lead to attention deficits. This possibility was investigated in the present study using a rat model.

METHODS

Pregnant Sprague Dawley rats were administered EtOH (3 g/kg/day) or vehicle via intragastric gavage on gestational days 8 to 20. The blood EtOH concentration (BEC) in EtOH-treated rats was 87.7 ± 1.2 mg/dl (1 h after the gavage), similar to the BECs reported in other moderate PE studies in rodents. Moderate PE did not produce teratogenic effects on birthweight or litter size. The adult offspring underwent a 2-choice reaction time task.

RESULTS

Moderate PE led to augmented action impulsivity in both sexes, indicated by more rapid response initiation and more premature responses. Deficits were more marked in males than in females. No greater lapses of attention, assessed by incorrect or relatively slow responses, were observed in rats of either sex with moderate PE. In addition, no deficits in learning or motor function were detected after moderate PE. Interestingly, rats with moderate PE completed more trials than controls.

CONCLUSIONS

Our results confirm that moderate PE leads to attention deficits in both sexes, which is demonstrated by greater action impulsivity, but not more lapses of attention. This effect differs from that of heavy PE, as shown in our previous study, which is manifested as impaired action impulsivity and lapses of attention in both sexes.

Hyperexcitability of VTA dopaminergic neurons in male offspring exposed to physical or psychological prenatal stress

Prenatal stress (PS) exposure leads to cognitive and behavioral alterations in offspring including an increased risk of substance abuse and anxiety disorders. Signalling from dopamine (DA) neurons of the ventral tegmental area (VTA) in the mesoaccumbal and mesocortical pathways plays a vital role in drug dependency and anxiety behavior. To provide further knowledge about the changes in drug seeking behavior and anxiety behaviors in prenatally stressed mice, we conducted ex vivo investigations in VTA brain slices of adult male PS offspring to evaluate the effects of two types of PS (physical vs. psychological) on activity of DA neurons. Elevated plus maze (EPM) was used to assess anxiety-like behaviors and conditioned place preference (CPP) was used to evaluate drug reinforcing effects in mice. An increased anxiety-like behavior and preference to morphine was observed in prenatally stressed mice. PS VTA DA cells exhibited greater I_h current and a higher frequency and amplitude of sEPSCs, which were consistent with a greater degree of pre- or postsynaptic excitability of the VTA. This was confirmed by lower rheobase and lower firing thresholds in PS VTA neurons, as well as increases in spontaneous firing frequency. When taken together, these data suggest that alterations in VTA DA neurons in this mouse model of prenatal stress might be associated with later life alterations in drug seeking and anxiety-like behaviors through their role in mesocortical and mesoaccumbal pathways.

Pre-clinical models of reward deficiency syndrome: A behavioral octopus

Individuals with mood disorders or with addiction, impulsivity and some personality disorders can share in common a dysfunction in how the brain perceives reward, where processing of natural endorphins or the response to exogenous dopamine stimulants is impaired. Reward Deficiency Syndrome (RDS) is a polygenic trait with implications that suggest cross-talk between different neurological systems that include the known reward pathway, neuroendocrine systems, and motivational systems. In this review we evaluate well-characterized animal models for their construct validity and as potential models for RDS. Animal models used to study substance use disorder, major depressive disorder (MDD), early life stress, immune dysregulation, attention deficit hyperactivity disorder (ADHD), post traumatic stress disorder (PTSD), compulsive gambling and compulsive eating disorders are discussed. These disorders recruit underlying reward deficiency mechanisms in multiple brain centers. Because of the widespread and remarkable array of associated/overlapping behavioral manifestations with a common root of hypodopaminergia, the basic endophenotype recognized as RDS is indeed likened to a behavioral octopus. We conclude this review with a look ahead on how these models can be used to investigate potential therapeutics that target the underlying common deficiency.

Wheel-running exercise before and during gestation against acute and sensitized cocaine psychomotor-activation in offspring

While animal research has consistently reported preventive effects of exercise against drug abuse vulnerability, little is known about the influence of the developmental stage during which exercise is displayed on addictive drugs responsiveness. This study aimed to determine whether prenatal exercise could attenuate acute cocaine reactivity and psychomotor sensitization in youth offspring. We used a split-plot factorial design where C57BL/6 J females were randomly assigned into sedentary or exercised (wheel-running) conditions before and during gestation, the wheels being removed on gestational day 18. Offspring were weaned, gendered and individually housed on 24-28 days old. At 38-42 days old, they were tested for their acute psychomotor responsiveness to 8 mg/kg cocaine and their initiation of sensitization over 8 additional once-daily administrations, the long-term expression of sensitization occurring 30 days later. Adolescent females born from exercised mothers were much less responsive to the acute psychomotor-stimulating effect of cocaine than those born from sedentary mothers (d = 0.75, p = 0.02), whereas there was no evidence for such a difference in males (d = 0.34, p = 0.17). However, we did not find sizeable attenuating effects of prenatal exercise on the initiation and the long-term expression of the psychomotor-activating effect of cocaine, in either sex (Cohen's ds varying from -0.13 to 0.39). These results suggest that prenatal exercise may induce initial protection against cocaine responsiveness in youth females, a finding that warrants further research.

Prenatal ethanol exposure increases risk of psychostimulant addiction

Prenatal ethanol exposure (PE) causes many cognitive and behavioral deficits including increased drug addiction risk, demonstrated by enhanced ethanol intake and behavioral phenotypes associated with addiction risk. Additionally, preclinical studies show that PE persistently changes the function of dopaminergic neurons in the ventral tegmental area, a major neural substrate for addiction, and alters these neurons' responses to psychostimulants. Accordingly, PE could also lead to increased risk of addiction to drugs of abuse, other than ethanol. In the present study, addiction risk was examined utilizing paradigms of amphetamine conditioned place preference (CPP) and intravenous self-administration. Ethanol was administered to pregnant dams via intragastric gavage (6  g/kg, during gestational days 8-20). Behavioral tests were conducted in adult male offspring. Amphetamine at a low dose (0.3  mg/kg, i.p.) induced CPP in PE but not control rats, whereas at a higher dose (0.6  mg/kg, i.p.) both groups acquired CPP. There was no group difference in amphetamine-induced CPP reinstatement. Furthermore, PE rats self-administered more amphetamine at a low dose (0.02  mg/kg/infusion) than controls, while no group differences were observed at a higher dose (0.1  mg/kg/infusion). Rats with PE also exhibited greater reactivity to contextual drug cues after extended abstinence and amphetamine-induced reinstatement of drug seeking. These results support that PE persistently leads to increased psychostimulant addiction risk later in life, manifested in many elements of addictive behavior following limited psychostimulant exposure. The observations provide insights into prevention strategies for drug addiction in individuals with fetal alcohol spectrum disorders.

Impaired contextual fear-conditioning in MAM rodent model of schizophrenia

The methylazoxymethanol acetate (MAM) rodent neurodevelopmental model of schizophrenia exhibits aberrant dopamine system activation attributed to hippocampal dysfunction. Context discrimination is a component of numerous behavioral and cognitive functions and relies on intact hippocampal processing. The present study explored context processing behaviors, along with dopamine system activation, during fear learning in the MAM model. Male offspring of dams treated with MAM (20mg/kg, i.p.) or saline on gestational day 17 were used for electrophysiological and behavioral experiments. Animals were tested on the immediate shock fear conditioning paradigm, with either different pre-conditioning contexts or varying amounts of context pre-exposure (0-10 sessions). Amphetamine-induced locomotor activity and dopamine neural activity was measured 1-week after fear conditioning. Saline, but not MAM animals, demonstrated enhanced fear responses following a single context pre-exposure in the conditioning context. One week following fear learning, saline rats with 2 or 7min of context pre-exposure prior to fear conditioning also demonstrated enhanced amphetamine-induced locomotor response relative to MAM animals. Dopamine neuron recordings showed fear learning-induced reductions in spontaneous dopamine neural activity in MAM rats that was further reduced by amphetamine. Apomorphine administration confirmed that reductions in dopamine neuron activity in MAM animals resulted from over excitation, or depolarization block. These data show a behavioral insensitivity to contextual stimuli in MAM rats that coincide with a less dynamic dopamine response after fear learning.

Plasticity of the Maternal Brain Across the Lifespan

Maternal behavior is dynamic and highly sensitive to experiential and contextual factors. In this review, this plasticity will be explored, with a focus on how experiences of females occurring from the time of fetal development through to adulthood impact maternal behavior and the maternal brain. Variation in postpartum maternal behavior is dependent on estrogen sensitivity within the medial preoptic area of the hypothalamus and activation within mesolimbic dopamine neurons. This review will discuss how experiences across the lifespan alter the function of these systems and the multigenerational consequences of these neuroendocrine and behavioral changes. These studies, based primarily on the examination of maternal behavior in laboratory rodents and nonhuman primates, provide mechanistic insights relevant to our understanding of human maternal behavior and to the mechanisms of lifelong plasticity.

Altered monoamine system activities after prenatal and adult stress: A role for stress resilience?

INTRODUCTION

Prenatal stress (PNS) and stress in adulthood are risk factors for development of major depressive disorder. The present study aimed to 1) confirm previous neuroendocrine and behavioral changes induced by PNS, and 2) to characterize the effect of early- and late life stress on the in vivo activity of monoamine systems.

METHODS

Gestational dams were restrained thrice daily under bright illumination from gestational day (GD)11-20. Behavior and neuroendocrine responses to the forced swim test (FST) were determined in adult (50-80 days) offspring, and electrophysiological single unit recordings of dorsal raphe nucleus serotonin (5-HT), ventral tegmental area dopamine (DA) and locus coeruleus norepinephrine (NE) neurons were obtained at baseline and 24h after the FST.

RESULTS

Gestational dams did not habituate to chronic restraint stress, and PNS reduced the birth weight of offspring. In adulthood, swim stress elevated CORT levels longer in PNS animals, while it had no effect on swim behaviors. Baseline firing activity of 5-HT neurons was decreased in PNS animals, while the firing activity of NE and DA neurons was increased. Swim stress had no effect on the firing on 5-HT neurons, but normalized the firing activity of catecholamine neurons in PNS animals.

CONCLUSION

The present data confirm previously established effects on neuroendocrine and physiological measures, and demonstrate an altering effect of PNS and stress on monoamine system activities in adulthood. Since PNS did not result in a depressive-like phenotype, these central changes following PNS might play reflect adaptive changes contributing to stress resilience in adulthood.

Stress psychobiology in the context of addiction medicine: from drugs of abuse to behavioral addictions

In this chapter, we briefly review the basic biology of psychological stress and the stress response. We propose that psychological stress and the neurobiology of the stress response play in substance use initiation, maintenance, and relapse. The proposed mechanisms for this include, on the one hand, the complex interactions between biological mediators of the stress response and the dopaminergic reward system and, on the other hand, mediators of the stress response and other systems crucial in moderating key addiction-related behaviors such as endogenous opioids, the sympathetic-adrenal-medullary system, and endocannabinoids. Exciting new avenues of study including genomics, sex as a moderator of the stress response, and behavioral addictions (gambling, hypersexuality, dysfunctional internet use, and food as an addictive substance) are also briefly presented within the context of stress as a moderator of the addictive process.

Prenatal Stress Produces Persistence of Remote Memory and Disrupts Functional Connectivity in the Hippocampal-Prefrontal Cortex Axis

Prenatal stress is a risk factor for the development of neuropsychiatric disorders, many of which are commonly characterized by an increased persistence of aversive remote memory. Here, we addressed the effect of prenatal stress on both memory consolidation and functional connectivity in the hippocampal-prefrontal cortex axis, a dynamical interplay that is critical for mnemonic processing. Pregnant mice of the C57BL6 strain were subjected to restraint stressed during the last week of pregnancy, and male offspring were behaviorally tested at adulthood for recent and remote spatial memory performance in the Barnes Maze test under an aversive context. Prenatal stress did not affect the acquisition or recall of recent memory. In contrast, it produced the persistence of remote spatial memory. Memory persistence was not associated with alterations in major network rhythms, such as hippocampal sharp-wave ripples (SWRs) or neocortical spindles. Instead, it was associated with a large decrease in the basal discharge activity of identified principal neurons in the medial prefrontal cortex (mPFC) as measured in urethane anesthetized mice. Furthermore, functional connectivity was disrupted, as the temporal coupling between neuronal discharge in the mPFC and hippocampal SWRs was decreased by prenatal stress. These results could be relevant to understand the biological basis of the persistence of aversive remote memories in stress-related disorders.

Early life stress causes refractoriness to haloperidol-induced catalepsy

The use of classic antipsychotic drugs is limited by the occurrence of extrapyramidal motor symptoms, which are caused by dopamine (DA) receptor blockade in the neostriatum. We examined the impact of early-life stress on haloperidol-induced catalepsy using the rat model of prenatal restraint stress (PRS). Adult "PRS rats," i.e., the offspring of mothers exposed to restraint stress during pregnancy, were resistant to catalepsy induced by haloperidol (0.5-5 mg/kg i.p.) or raclopride (2 mg/kg s.c.). Resistance to catalepsy in PRS rats did not depend on reductions in blood or striatal levels, as compared with unstressed control rats. PRS rats also showed a greater behavioral response to the DA receptor agonist, apomorphine, suggesting that PRS causes enduring neuroplastic changes in the basal ganglia motor circuit. To examine the activity of this circuit, we performed a stereological counting of c-Fos(+) neurons in the external and internal globus pallidus, subthalamic nucleus, and ventral motor thalamic nuclei. Remarkably, the number of c-Fos(+) neurons in ventral motor thalamic nuclei was higher in PRS rats than in unstressed controls, both under basal conditions and in response to single or repeated injections with haloperidol. Ventral motor thalamic nuclei contain exclusively excitatory projection neurons that convey the basal ganglia motor programming to the cerebral cortex. Hence, an increased activity of ventral motor thalamic nuclei nicely explains the refractoriness of PRS rats to haloperidol-induced catalepsy. Our data raise the interesting possibility that early-life stress is protective against extrapyramidal motor effects of antipsychotic drugs in the adult life.