Growth and behavioral differences in a C57BL/6J mouse model of prenatal alcohol exposure

BACKGROUND

Prenatal alcohol exposure (PAE) can produce behavioral deficits in the presence or absence of growth and morphological deficits. Here, we describe a murine PAE model having parallels to the clinical diagnosis of alcohol-related neurodevelopmental deficit (ARND).

METHODS

Pregnant C57BL/6J mice were gavaged with alcohol (ALC, 3 g/kg) or maltodextrin daily on embryonic days (E) E8.5 through E17.5. Blood alcohol levels were 211 ± 14 mg/dL at 30 min post-gavage. Offspring behavior was tested at adolescence.

RESULTS

ALC dams gained less weight during the alcohol exposure period (p = 0.035). ALC male and female pups weighed more than controls at P15 (p ≤ 0.001) and P22 (p ≤ 0.001), but not at P37, perhaps because their dams were pair-housed. During the training session for accelerating rotarod, control offspring trended to stay longer on the rotarod than did ALC offspring [F_(1,54) = 2.892, p = 0.095]. In the Y-maze, ALC offspring had a higher percent alternation than did controls [F_(1,54) = 16.577, p < 0.001], but activity level did not appear to differ. In the fear-conditioning test, there was no ALC effect in the training trial. In the contextual test, there was a group × minute effect for males [F_(4,120) = 2.94, p = 0.023], and ALC trended to freeze less than controls in minute 1 (p = 0.076) and froze less in minute 2 (p = 0.02). In the cue test, there was a trend for a group-sex interaction [F_(1,53) = 3.008, p = 0.089] on overall freezing, such that ALC males (p < 0.05) again froze less than control males, whereas ALC females (p < 0.05) froze more than control females.

CONCLUSIONS

This mouse model of PAE, using a repeated intermediate exposure, produces modest behavioral impairments that are consistent along the continuum of PAE models, including deficits in associative memory and hyper-responsivity. The lack of growth or morphological deficits suggests these mice may model aspects of ARND.

Neonatal ethanol exposure impairs long-term context memory formation and prefrontal immediate early gene expression in adolescent rats

Fetal alcohol exposure leads to severe disruptions in learning and memory involving the hippocampus and prefrontal cortex in humans. Animal model research on FASD has documented impairment of hippocampal neuroanatomy and function but animal studies of cognition involving the prefrontal cortex are sparse. We have found that a variant of contextual fear conditioning in which both the hippocampus and prefrontal cortex is required, the Context Preexposure Facilitation Effect (CPFE), is particularly sensitive to neurobehavioral disruption caused by neonatal ethanol exposure during the third trimester equivalent of human pregnancy in the rat (i.e., PD4-9). In the CPFE, learning about the context, acquiring a context-shock association, and retrieving contextual fear are temporally separated across three days. The current study asked whether neonatal alcohol exposure impairs context learning, consolidation, or retrieval and examined prefrontal and hippocampal molecular signaling as correlates of this impairment. Long-Evans rats that received oral intubation of ethanol (AE; 5.25 g/kg/day, split into two doses) or underwent sham-intubation (SI) from PND4-9 were tested on the CPFE on PD31-33. Extending our previous reports, ethanol abolished both post-shock and retention test freezing in the CPFE. Assays (qPCR) of immediate early gene expression revealed that ethanol disrupted prefrontal but not hippocampal expression of c-Fos, Arc, Egr-1, and Npas4 during context learning. Finally, ethanol-exposed animals were unimpaired in a standard contextual fear conditioning procedure in which learning about the context and acquiring a context-shock association occurs concurrently. These findings implicate impaired prefrontal function in cognitive deficits arising from 3rd-trimester equivalent alcohol exposure in the rat.

Long-Lasting Effects of Prenatal Ethanol Exposure on Fear Learning and Development of the Amygdala

Prenatal ethanol exposure (PrEE) produces developmental abnormalities in brain and behavior that often persist into adulthood. We have previously reported abnormal cortical gene expression, disorganized neural circuitry along with deficits in sensorimotor function and anxiety in our CD-1 murine model of fetal alcohol spectrum disorders, or FASD (El Shawa et al., 2013; Abbott et al., 2016). We have proposed that these phenotypes may underlie learning, memory, and behavioral deficits in humans with FASD. Here, we evaluate the impact of PrEE on fear memory learning, recall and amygdala development at two adult timepoints. PrEE alters learning and memory of aversive stimuli; specifically, PrEE mice, fear conditioned at postnatal day (P) 50, showed deficits in fear acquisition and memory retrieval when tested at P52 and later at P70–P72. Interestingly, this deficit in fear acquisition observed during young adulthood was not present when PrEE mice were conditioned later, at P80. These mice displayed similar levels of fear expression as controls when tested on fear memory recall. To test whether PrEE alters development of brain circuitry associated with fear conditioning and fear memory recall, we histologically examined subdivisions of the amygdala in PrEE and control mice and found long-term effects of PrEE on fear memory circuitry. Thus, results from this study will provide insight on the neurobiological and behavioral effects of PrEE and provide new information on developmental trajectories of brain dysfunction in people prenatally exposed to ethanol.

Differential expression of the immediate early genes c-Fos, Arc, Egr-1, and Npas4 during long-term memory formation in the context preexposure facilitation effect (CPFE)

The context preexposure facilitation effect (CPFE) is a contextual fear conditioning paradigm in which learning about the context, acquiring the context-shock association, and retrieving/expressing contextual fear are temporally dissociated into three distinct phases (context preexposure, immediate-shock training, and retention). The current study examined changes in the expression of plasticity-associated immediate early genes (IEGs) during context and contextual fear memory formation on the preexposure and training days of the CPFE, respectively. Using adolescent Long-Evans rats, preexposure and training day expression of the IEGs c-Fos, Arc, Egr-1, and Npas4 in the medial prefrontal cortex (mPFC), dorsal hippocampus (dHPC), and basolateral amygdala (BLA) was analyzed using qPCR as an extension of previous studies from our lab examining Egr-1 via in situ hybridization (Asok, Schreiber, Jablonski, Rosen, & Stanton, 2013; Schreiber, Asok, Jablonski, Rosen, & Stanton, 2014). In Expt. 1, context preexposure induced expression of c-Fos, Arc, Egr-1 and Npas4 significantly above that of home-cage (HC) controls in all three regions. In Expt. 2, immediate-shock was followed by a post-shock freezing test, resulting in increased mPFC c-Fos expression in a group preexposed to the training context but not a control group preexposed to an alternate context, indicating expression related to associative learning. This was not seen with other IEGs in mPFC or with any IEG in dHPC or BLA. Finally, when the post-shock freezing test was omitted in Expt. 3, training-related increases were observed in prefrontal c-Fos, Arc, Egr-1, and Npas4, hippocampal c-Fos, and amygdalar Egr-1 expression. These results indicate that context exposure in a post-shock freezing test re-engages IEG expression that may obscure associatively-induced expression during contextual fear conditioning. Additionally, these studies suggest a key role for long-term synaptic plasticity in the mPFC in supporting the CPFE.

Sex-specific effects of developmental alcohol exposure on cocaine-induced place preference in adulthood

Fetal Alcohol Syndrome (FAS) is associated with high rates of drug addiction in adulthood. One possible basis for increased drug use in this population is altered sensitivity to drug-associated contexts. This experiment utilized a rat model of FASD to examine behavioral and neural changes in the processing of drug cues in adulthood. Alcohol was given by intragastric intubation to pregnant rats throughout gestation and to rat pups during the early postnatal period (ET group). Controls consisted of a non-treated group (NC) and a pair-fed group given the intubation procedure without alcohol (IC). On postnatal day (PD) 90, rats from all treatment groups were given saline, 0.3mg/kg, 3.0mg/kg, or 10.0mg/kg cocaine pairings with a specific context in the conditioned place preference (CPP) paradigm. While control animals of both sexes showed cocaine CPP at the 3.0 and 10.0mg/kg doses, ET females also showed cocaine CPP at 0.3mg/kg. This was accompanied by a decrease in c-Fos/GAD₆₇ cells in the nucleus accumbens (NAc) shell and GAD₆₇-only cells in the NAc shell and PFC at this 0.3mg/kg dose. ET males failed to show cocaine CPP at the 3.0mg/kg dose. This was associated with an increase in c-Fos only-labeled cells in the NAc core and PFC at this 3.0mg/kg dose. These results suggest that developmental alcohol exposure has a sexually-dimorphic effect on cocaine's conditioning effects in adulthood and the NAc.

Adolescent transitions in reflexive and non-reflexive behavior: Review of fear conditioning and impulse control in rodent models

Adolescence is a time of critical brain changes that pave the way for adult learning processes. However, the extent to which learning in adolescence is best characterized as a transitional linear progression from childhood to adulthood, or represents a period that differs from earlier and later developmental stages, remains unclear. Here we examine behavioral literature on associative fear conditioning and complex choice behavior with rodent models. Many aspects of fear conditioning are intact by adolescence and do not differ from adult patterns. Sufficient evidence, however, suggests that adolescent learning cannot be characterized simply as an immature precursor to adulthood. Across different paradigms assessing choice behavior, literature suggests that adolescent animals typically display more impulsive patterns of responding compared to adults. The extent to which the development of basic conditioning processes serves as a scaffold for later adult decision making is an additional research area that is important for theory, but also has widespread applications for numerous psychological conditions.

An animal model of fetal alcohol spectrum disorder: Trace conditioning as a window to inform memory deficits and intervention tactics

Animal models of Fetal Alcohol Spectrum Disorders (FASD) afford the unique capacity to precisely control timing of alcohol exposure and alcohol exposure amounts in the developing animal. These models have powerfully informed neurophysiological alterations associated with fetal and perinatal alcohol. In two experiments presented here we expand use of the Pavlovian Trace Conditioning procedure to examine cognitive deficits and intervention strategies in a rat model of FASD. Rat pups were exposed to 5g/kg/day ethanol on postnatal days (PD) 4-9, simulating alcohol exposure in the third trimester in humans. During early adolescence, approximately PD 30, the rats were trained in the trace conditioning task in which a light conditioned stimulus (CS) and shock unconditioned stimulus (US) were paired but separated by a 10-s stimulus free trace interval. Learning was assessed in freezing behavior during shock-free tests. Experiment 1 revealed that neonatal ethanol exposure significantly impaired hippocampus-dependent trace conditioning relative to controls. In Experiment 2 a serial compound conditioning procedure known as 'gap filling' completely reversed the ethanol-induced deficit in trace conditioning. We also discuss prior data regarding the beneficial effects of supplemental choline and novel preliminary data regarding the pharmacological cognitive enhancer physostigmine, both of which mitigate the alcohol-induced cognitive deficit otherwise seen in trace conditioning controls. We suggest trace conditioning as a useful tool for characterizing some of the core cognitive deficits seen in FASD, and as a model for developing effective environmental as well as nutritional and pharmacological interventions.

Selective cognitive deficits in adult rats after prenatal exposure to inhaled ethanol

Increased use of ethanol blends in gasoline suggests a need to assess the potential public health risks of exposure to these fuels. Ethanol consumed during pregnancy is a teratogen. However, little is known about the potential developmental neurotoxicity of ethanol delivered by inhalation, the most likely route of exposure from gasoline-ethanol fuel blends. We evaluated the potential cognitive consequences of ethanol inhalation by exposing pregnant Long Evans rats to clean air or ethanol vapor from gestational days 9-20, a critical period of neuronal development. Concentrations of inhaled ethanol (5000, 10,000, or 21,000 ppm for 6.5h/day) produced modeled peak blood ethanol concentrations (BECs) in exposed dams of 2.3, 6.8, and 192 mg/dL, respectively. In offspring, no dose-related impairments were observed on spatial learning or working memory in the Morris water maze or in operant delayed match-to-position tests. Two measures showed significant effects in female offspring at all ethanol doses: 1) impaired cue learning after trace fear conditioning, and 2) an absence of bias for the correct quadrant after place training during a reference memory probe in the Morris water maze. In choice reaction time tests, male offspring (females were not tested) from the 5000 and 10,000 ppm groups showed a transient increase in decision times. Also, male offspring from the 21,000 ppm group made more anticipatory responses during a preparatory hold period, suggesting a deficit in response inhibition. The increase in anticipatory responding during the choice reaction time test shows that inhaled ethanol yielding a peak BEC of ~200mg/dL can produce lasting effects in the offspring. The lack of a dose-related decrement in the effects observed in females on cue learning and a reference memory probe may reflect confounding influences in the exposed offspring possibly related to maternal care or altered anxiety levels in females. The surprising lack of more pervasive cognitive deficits, as reported by others at BECs in the 200mg/dL range, may reflect route-dependent differences in the kinetics of ethanol. These data show that response inhibition was impaired in the offspring of pregnant rats that inhaled ethanol at concentrations at least 5 orders of magnitude higher than concentrations observed during normal automotive transport and fueling operations, which rarely exceed 100 ppb.

Significant long-term, but not short-term, hippocampal-dependent memory impairment in adult rats exposed to alcohol in early postnatal life

In rodents, ethanol exposure in early postnatal life is known to induce structural and functional impairments throughout the brain, including the hippocampus. Herein, rat pups were administered one of three ethanol doses over postnatal days (PD) 4-9, a period of brain development comparable to the third trimester of human pregnancy. As adults, control and ethanol rats were trained and tested in a variant of hippocampal-dependent one-trial context fear conditioning. In Experiment 1, subjects were placed into a novel context and presented with an immediate footshock (i.e., within ∼8 sec). When re-exposed to the same context 24 hr later low levels of conditioned freezing were observed. Context pre-exposure 24 hr prior to the immediate shock reversed the deficit in sham-intubated and unintubated control rats, enhancing freezing behavior during the context retention test. Even with context pre-exposure, however, significant dose-dependent reductions in contextual freezing were seen in ethanol rats. In Experiment 2, the interval between context pre-exposure and the immediate shock was shortened to 2 hr, in addition to the standard 24 hr. Ethanol rats trained with the 2 hr, but not 24 hr, interval displayed retention test freezing levels roughly equal to controls. Results suggest the ethanol rats can encode a short-term context memory and associate it with the aversive footshock 2 hr later. In the 24 hr ethanol rats the short-term context memory is poorly transferred or consolidated into long-term memory, we propose, impeding the memory's subsequent retrieval and association with shock.

Anxiety vulnerability in women: a two-hit hypothesis

Females are twice as likely to develop an anxiety disorder compared to males, and thus, are believed to possess an innate vulnerability that increases their susceptibility to develop an anxiety disorder. However, studies using aversive learning paradigms to model anxiety disorders in humans and animals have revealed contradictory results. While females exhibit the ability to rapidly acquire stimulus-response associations, which may result from a greater attentional bias towards threat, females are also capable to readily extinguish these associations. Thus, there is little evidence to suggest that the female sex represents a vulnerability factor of anxiety, per se. However, if females are to possess a second vulnerability factor that increases the inflexibility of stimulus-response associations, then an anxiety disorder may be more likely to develop. Behavioral inhibition (BI) is a vulnerability factor associated with the formation of inflexible stimulus-response associations. In this "two hit" model of anxiety vulnerability, females possessing a BI temperament will rapidly acquire stimulus-response associations that are resistant to extinction, resulting in the development of an anxiety disorder. In this review we explore evidence for a "two-hit" hypothesis underlying anxiety vulnerability in females. We explore the literature for evidence of a sex difference in attentional bias towards threat that may lead to the facilitated acquisition of stimulus-response associations in females. We also provide evidence that BI is associated with inflexible stimulus-response association formation. We conclude with data generated from our laboratory that highlights the additive effect of the female sex and behavioral inhibition vulnerabilities using a model behavior for anxiety disorder-susceptibility, active avoidance.

Cyr S, Jablonski S, Hunt P, Klintsova A, Stanton M. Effects of exercise and environmental complexity on deficits in trace and contextual fear conditioning produced by neonatal alcohol exposure in rats

In rodents, voluntary exercise and environmental complexity increases hippocampal neurogenesis and reverses spatial learning and long-term potentiation deficits in animals prenatally exposed to alcohol. The present experiment extended these findings to neonatal alcohol exposure and to delay, trace, and contextual fear conditioning. Rats were administered either 5.25 g/kg/day alcohol via gastric intubation or received sham-intubations (SI) between Postnatal Day (PD) 4 and 9 followed by either free access to a running wheel on PD 30-41 and housing in a complex environment on PD 42-72 (wheel-running plus environmental complexity; WREC) or conventional social housing (SHSH) from PD 30 to 72. Adult rats (PD 80 ± 5) received 5 trials/day of a 10-s flashing-light conditioned stimulus (CS) paired with .8 mA footshock either immediately (delay conditioning) or after a 10-s trace interval (trace conditioning) for 2 days. Neonatal alcohol exposure impaired context and trace conditioning, but not short-delay conditioning. The WREC intervention did not reverse these deficits, despite increasing context-related freezing in ethanol-exposed and SI animals.

Long-term consequences of developmental alcohol exposure on brain structure and function: therapeutic benefits of physical activity

Developmental alcohol exposure both early in life and during adolescence can have a devastating impact on normal brain structure and functioning, leading to behavioral and cognitive impairments that persist throughout the lifespan. This review discusses human work as well as animal models used to investigate the effect of alcohol exposure at various time points during development, as well as specific behavioral and neuroanatomical deficits caused by alcohol exposure. Further, cellular and molecular mediators contributing to these alcohol-induced changes are examined, such as neurotrophic factors and apoptotic markers. Next, this review seeks to support the use of aerobic exercise as a potential therapeutic intervention for alcohol-related impairments. To date, few interventions, behavioral or pharmacological, have been proven effective in mitigating some alcohol-related deficits. Exercise is a simple therapy that can be used across species and also across socioeconomic status. It has a profoundly positive influence on many measures of learning and neuroplasticity; in particular, those measures damaged by alcohol exposure. This review discusses current evidence that exercise may mitigate damage caused by developmental alcohol exposure and is a promising therapeutic target for future research and intervention strategies.