Alteration in anxiety-related behaviors and reduction of serotonergic neurons in raphe nuclei in adult rats prenatally exposed to ethanol

Prenatal ethanol exposure leads to persistent anxiety-like behavior during adulthood indicated by reduced horizontal and vertical exploratory behaviors

Background

Fetal alcohol spectrum disorders (FASD) caused by prenatal ethanol exposure (PE) consist of many cognitive/behavioral deficits. Studies have reported that PE leads to impairments of learning and memory, attention, executive function, and anxiety. Open field (OF) is a common behavioral model which offers comprehensive ethological information. Here, we analyzed multiple parameters of OF to examine anxiety behavior and habituation after PE.

Material and Methods

Pregnant Sprague Dawley rats were gavaged twice/day with 0 or 3 g/kg/treatment ethanol (15% w/v) during gestational day (GD) 8-20, mimicking second-trimester heavy PE in humans. The control and PE adult offspring were subjected to OF task in different ambient light levels with or without acute stress.

Results

Prenatal ethanol exposure did not influence the overall locomotor activities or habituation in the OF. In lower ambient light, no PE effects could be detected. In higher ambient light, female PE rats showed less activities in the center zone, indicative of increased anxiety. Males show lower activities in the center zone only after acute stress. Rats spent <2% of the time in the center zone compared to >75% of the time in the corner zone where they engaged in frequent rearing activities (vertical exploration; exploratory rearing). Prenatal ethanol exposure led to lower rearing activities in the corner in both males and females. Acute stress masks the PE effects in males but not in females.

Discussion

The results support that heavy PE leads to persistent anxiety-like behavior during adulthood in both sexes. This conclusion is supported by using multiple parameters of exploratory behavior in the OF, including the rearing activities in the corner to reach reliable quantification of anxiety-like behavior.

Prenatal ethanol exposure causes anxiety-like phenotype and alters synaptic nitric oxide and endocannabinoid signaling in dorsal raphe nucleus of adult male rats

Mood disorders, including anxiety and depression caused by prenatal ethanol exposure (PE) are prevalent conditions in fetal alcohol spectrum disorders (FASDs). Prenatal ethanol exposure is associated with persistent dysfunctions of several neurotransmitter systems, including the serotonin (5-HT) system, which plays a major role in mood regulation and stress homeostasis. While PE is known to disrupt the development of the 5-HT system, the cellular mechanisms by which it alters the function of dorsal raphe nucleus (DRn) 5-HT neurons and their synaptic inputs remain unknown. Here, we used a second-trimester binge-drinking pattern PE (two daily gavages of 15% w/v ethanol at 3 g/kg, 5-6 h apart) during gestational days 8 - 20 and measured anxiety-like behaviors of adult male rats using the elevated plus (EPM) and zero (ZM) mazes. We also employed ex-vivo electrophysiological and pharmacological approaches to unravel the mechanisms by which PE alters the excitability and synaptic transmission onto DRn 5-HT neurons. We found that PE enhanced anxiety-like behaviors in adult male rats and induced a persistent activation of DRn 5-HT neurons. The PE-induced activation of DRn 5-HT neurons was largely mediated by potentiation of DRn glutamate synapses, which was caused by activation of the nitrergic system and impaired endocannabinoid signaling. As such, the present study reveals "push-pull" effects of PE on nitrergic and eCB signaling, respectively, which mediate the enhanced activity of DRn 5-HT neurons and could contribute to anxiety-like behaviors observed in animal model of FASD.

Gestational Factors throughout Fetal Neurodevelopment: The Serotonin Link

Serotonin (5-HT) is a critical player in brain development and neuropsychiatric disorders. Fetal 5-HT levels can be influenced by several gestational factors, such as maternal genotype, diet, stress, medication, and immune activation. In this review, addressing both human and animal studies, we discuss how these gestational factors affect placental and fetal brain 5-HT levels, leading to changes in brain structure and function and behavior. We conclude that gestational factors are able to interact and thereby amplify or counteract each other's impact on the fetal 5-HT-ergic system. We, therefore, argue that beyond the understanding of how single gestational factors affect 5-HT-ergic brain development and behavior in offspring, it is critical to elucidate the consequences of interacting factors. Moreover, we describe how each gestational factor is able to alter the 5-HT-ergic influence on the thalamocortical- and prefrontal-limbic circuitry and the hypothalamo-pituitary-adrenocortical-axis. These alterations have been associated with risks to develop attention deficit hyperactivity disorder, autism spectrum disorders, depression, and/or anxiety. Consequently, the manipulation of gestational factors may be used to combat pregnancy-related risks for neuropsychiatric disorders.

Anxious Behavior of Adult CD1 Mice Perinatally Exposed to Low Concentrations of Ethanol Correlates With Morphological Changes in Cingulate Cortex and Amygdala

Perinatal ethanol (EtOH) exposure is associated with high incidence of behavioral disorders such as depression and anxiety. The cerebral areas related with these consequences involve the corticolimbic system, in particular the prefrontal cortex, hippocampus, amygdala, and cingulate cortex, although the latter has not been thoroughly studied yet. Different animal models of prenatal or perinatal EtOH exposure have reported morphofunctional alterations in the central nervous system, which could explain behavioral disorders along life; these results focus on youth and adolescents and are still controversial. In the light of these inconclusive results, the aim of this work was to analyze adult behavior in CD1 mice perinatally exposed to low concentrations of EtOH (PEE) during gestation and lactation, and describe the morphology of the cingulate cortex and amygdala with a view to establishing structure/function/behavior correlations. Primiparous CD1 female mice were exposed to EtOH 6% v/v for 20 days prior to mating and continued drinking EtOH 6% v/v during pregnancy and lactation. After weaning, male pups were fed food and water ad libitum until 77 days of age, when behavioral and morphological studies were performed. Mouse behavior was analyzed through light–dark box and open field tests. Parameters related to anxious behavior and locomotor activity revealed anxiogenic behavior in PEE mice. After behavioral studies, mice were perfused and neurons, axons, serotonin transporter, 5HT, CB1 receptor (CB1R) and 5HT1A receptor (5HT1AR) were studied by immunofluorescence and immunohistochemistry in brain sections containing cingulate cortex and amygdala. Cingulate cortex and amygdala cytoarchitecture were preserved in adult PEE mice, although a smaller number of neurons was detected in the amygdala. Cingulate cortex axons demonstrated disorganized radial distribution and reduced area. Serotonergic and endocannabinoid systems, both involved in anxious behavior, showed differential expression. Serotonergic afferents were lower in both brain areas of PEE animals, while 5HT1AR expression was lower in the cingulate cortex and higher in the amygdala. The expression of CB1R was lower only in the amygdala. In sum, EtOH exposure during early brain development induces morphological changes in structures of the limbic system and its neuromodulation, which persist into adulthood and may be responsible for anxious behavior.

Altered brain functional connectivity and behaviour in a mouse model of maternal alcohol binge-drinking

Prenatal and perinatal alcohol exposure caused by maternal alcohol intake during gestation and lactation periods can have long-lasting detrimental effects on the brain development and behaviour of offspring. Children diagnosed with Foetal Alcohol Spectrum Disorders (FASD) display a wide range of cognitive, emotional and motor deficits, together with characteristic morphological abnormalities. Maternal alcohol binge drinking is particularly harmful for foetal and early postnatal brain development, as it involves exposure to high levels of alcohol over short periods of time. However, little is known about the long-term effects of maternal alcohol binge drinking on brain function and behaviour. To address this issue, we used pregnant C57BL/6 female mice with time-limited access to a 20% v/v alcohol solution as a procedure to model alcohol binge drinking during gestation and lactational periods. Male offspring were behaviourally tested during adolescence (30 days) and adulthood (60 days), and baseline neural metabolic capacity of brain regions sensitive to alcohol effects were also evaluated in adult animals from both groups. Our results show that prenatal and postnatal alcohol exposure caused age-dependent changes in spontaneous locomotor activity, increased anxiety-like behaviour and attenuated alcohol-induced conditioned place preference in adults. Also, significant changes in neural metabolic capacity using cytochrome c oxidase (CCO) quantitative histochemistry were found in the hippocampal dentate gyrus, the mammillary bodies, the ventral tegmental area, the lateral habenula and the central lobules of the cerebellum in adult mice with prenatal and postnatal alcohol exposure. In addition, the analysis of interregional CCO activity correlations in alcohol-exposed adult mice showed disrupted functional brain connectivity involving the limbic, brainstem, and cerebellar regions. Finally, increased neurogenesis was found in the dentate gyrus of the hippocampus of alcohol-exposed offspring, suggesting neuroadaptive effects due to early alcohol exposure. Our results demonstrate that maternal binge-like alcohol drinking causes long-lasting effects on motor and emotional-related behaviours associated with impaired neuronal metabolic capacity and altered functional brain connectivity.

A 5-HT2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, mitigates developmental neurotoxicity of ethanol to serotonergic neurons

Prenatal ethanol exposure causes the reduction of serotonergic (5-HTergic) neurons in the midbrain raphe nuclei. In the present study, we examined whether an activation of signaling via 5-HT2A and 5-HT2C receptors during the fetal period is able to prevent the reduction of 5-HTergic neurons induced by prenatal ethanol exposure. Pregnant Sprague-Dawley rats were given a liquid diet containing 2.5 to 5.0% (w/v) ethanol on gestational days (GDs) 10 to 20 (Et). As a pair-fed control, other pregnant rats were fed the same liquid diet except that the ethanol was replaced by isocaloric sucrose (Pf). Each Et and Pf group was subdivided into two groups; one of the groups was treated with 1 mg/kg (i.p.) of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), an agonist for 5-HT2A/2C receptors, during GDs 13 to 19 (Et-DOI or Pf-DOI), and another was injected with saline vehicle only (Et-Sal or Pf-Sal). Their fetuses were removed by cesarean section on GD 19 or 20, and fetal brains were collected. An immunohistological examination of 5-HTergic neurons in the fetuses on embryonic day 20 using an antibody against tryptophan hydroxylase revealed that the number of 5-HTergic neurons in the midbrain raphe nuclei was significantly reduced in the Et-Sal fetuses compared to that of the Pf-Sal and Pf-DOI fetuses, whereas there were no significant differences between Et-DOI and each Pf control. Thus, we concluded that the reduction of 5-HTergic neurons that resulted in prenatal ethanol exposure could be alleviated by the enhancement of signaling via 5-HT2A/2C receptors during the fetal period.

Docosahexaenoic acid partially ameliorates deficits in social behavior and ultrasonic vocalizations caused by prenatal ethanol exposure

Prenatal ethanol exposure disrupts social behavior in humans and rodents. One system particularly important for social behavior is the somatosensory system. Prenatal ethanol exposure alters the structure and function of this area. Docosahexaenoic acid (DHA), an omega 3 polyunsaturated fatty acid, is necessary for normal brain development and brains from ethanol-exposed animals are DHA deficient. Thus, we determined whether postnatal DHA supplementation ameliorated behavioral deficits induced by prenatal ethanol exposure. Timed pregnant Long-Evans rats were assigned to one of three groups: ad libitum access to an ethanol-containing liquid diet, pair fed an isocaloric isonutritive non-alcohol liquid diet, or ad libitum access to chow and water. Pups were assigned to one of two postnatal treatment groups; gavaged intragastrically once per day between postnatal day (P)11 and P20 with DHA (10 mg/kg in artificial rat milk) or artificial rat milk. A third group was left untreated. Isolation-induced ultrasonic vocalizations (iUSVs) were recorded on P14. Social behavior and play-induced USVs were tested on P28 or P42. Somatosensory performance was tested with a gap crossing test around P33 or on P42. Anxiety was tested on elevated plus maze around P35. Animals exposed to ethanol prenatally vocalized less, play fought less, and crossed a significantly shorter gap than control-treated animals. Administration of DHA ameliorated these ethanol-induced deficits such that the ethanol-exposed animals given DHA were no longer significantly different to control-treated animals. Thus, DHA administration may have therapeutic value to reverse some of ethanol's damaging effects.

Prenatal alcohol exposure results in long-term serotonin neuron deficits in female rats: modulatory role of ovarian steroids

BACKGROUND

Previous studies on male rodents found that prenatal alcohol exposure (PAE) decreases the number of serotonin immunoreactive (5-HT-ir) neurons in the brainstem. However, data on the effects of PAE in females are lacking. In light of known sex differences in responsiveness of the 5-HT system and known effects of estrogen (E2 ) and progesterone (P4 ) in the brain, we hypothesized that sex steroids will modulate the adverse effects of PAE on 5-HT neurons in adult females.

METHODS

Adult females from 3 prenatal groups (Prenatal alcohol-exposed [PAE], Pair-fed [PF], and ad libitum-fed Controls [C]) were ovariectomized (OVX), with or without hormone replacement, or underwent Sham OVX. 5-HT-ir cells were examined in key brainstem areas.

RESULTS

Our data support the hypothesis that PAE has long-term effects on the 5-HT system of females and that ovarian steroids have a modulatory role in these effects. Intact (Sham OVX) PAE females had marginally lower numbers of 5-HT-ir neurons in the dorsal raphe nucleus of the brainstem compared with PF and C females. This marginal difference became significant following removal of hormones by OVX. Replacement with E2 restored the number of 5-HT-ir neurons in PAE females to control levels, while P4 reversed the effects of E2 . Importantly, despite these differential responses of the 5-HT system to ovarian steroids, there were no differences in E2 and P4 levels among prenatal treatment groups.

CONCLUSIONS

These data demonstrate long-term, adverse effects of PAE on the 5-HT system of females, as well as differential sensitivity of PAE compared with control females to the modulatory effects of ovarian steroids on 5-HT neurons. Our findings have important implications for understanding sex differences in 5-HT dysfunction in depression/anxiety disorders and the higher rates of these mental health problems in individuals with fetal alcohol spectrum disorder.

Prenatal ethanol exposure impairs passive avoidance acquisition and enhances unconditioned freezing in rat offspring

Previous studies have suggested that ethanol exposure during brain development affects responses to fear and anxiety after maturity. To clarify in detail the impaired behavior related to fear and anxiety seen in rat offspring prenatally exposed to ethanol, their behaviors were observed using an elevated T-maze (ETM) test, which allows assessment of passive avoidance acquisition and one-way escape separately, and an elevated open platform (EOP) test for the assessment of unconditioned freezing against innate fear. The ETM test revealed that acquisition of passive avoidance was significantly inhibited in prenatally ethanol-exposed rats, while their escape behavior was not altered. In the EOP test, the duration of the freezing behavior was significantly elongated in prenatally ethanol-exposed offspring. Thus, we concluded that prenatal ethanol exposure could impair acquisition of passive avoidance, while it could facilitate a response related to unconditioned fears in rat offspring.