Unraveling the complex relationship between prenatal alcohol exposure, hippocampal LTP, and learning and memory

Prenatal alcohol exposure (PAE) has been extensively studied for its profound impact on neurodevelopment, synaptic plasticity, and cognitive outcomes. While PAE, particularly at moderate levels, has long-lasting cognitive implications for the exposed individuals, there remains a substantial gap in our understanding of the precise mechanisms underlying these deficits. This review provides a framework for comprehending the neurobiological basis of learning and memory processes that are negatively impacted by PAE. Sex differences, diverse PAE protocols, and the timing of exposure are explored as potential variables influencing the diverse outcomes of PAE on long-term potentiation (LTP). Additionally, potential interventions, both pharmacological and non-pharmacological, are reviewed, offering promising avenues for mitigating the detrimental effects of PAE on cognitive processes. While significant progress has been made, further research is required to enhance our understanding of how prenatal alcohol exposure affects neural plasticity and cognitive functions and to develop effective therapeutic interventions for those impacted. Ultimately, this work aims to advance the comprehension of the consequences of PAE on the brain and cognitive functions.

Impact of two different rodent diets on maternal ethanol consumption, serum ethanol concentration and pregnancy outcome measures

Recent studies report varying levels of ethanol consumption by rodents maintained on different commercially available laboratory diets. As varied ethanol consumption by dams may impact offspring outcome measures in prenatal ethanol exposure paradigms, we compared ethanol consumption by rats maintained on the Envigo 2920 diet, used in our vivarium, with an isocalorically equivalent PicoLab 5L0D diet used in some alcohol consumption studies. Compared to 5L0D diet, female rats maintained on 2920 diet consumed 14% less ethanol during daily 4-h drinking sessions prior to pregnancy and 28% less ethanol during gestation. Rat dams consuming 5L0D diet gained significantly less weight during pregnancy. However, their pup birth weights were significantly higher. A subsequent study revealed that hourly ethanol consumption was not different between diets during the first 2 h, but was significantly lower on 2920 diet at the end of the third and fourth hours. The mean serum ethanol concentration in 5L0D dams after the first 2 h of drinking was 46 mg/dL compared to 25 mg/dL in 2920 dams. Further, ethanol consumption at the 2-h blood sampling time point was more variable in 2920 dams compared to 5L0D dams. An in vitro analysis mixing each powdered diet with 5% ethanol in acidified saline revealed that a 2920 diet suspension adsorbed more aqueous medium than the 5L0D diet suspension. The total ethanol remaining in aqueous supernatant of 5L0D mixtures was nearly twice the amount of ethanol in supernatants of the 2920 mixtures. These results suggest that the 2920 diet expands to a greater extent in aqueous medium than the 5L0D diet. We speculate that increasing adsorption of water and ethanol by the 2920 diet may reduce or delay the amount of ethanol absorbed and may decrease serum ethanol concentration to a greater extent than would be predicted from the amount of ethanol consumed.

Effect of moderate prenatal ethanol exposure on the differential expression of two histamine H3 receptor isoforms in different brain regions of adult rat offspring

We have reported that prenatal alcohol exposure (PAE) elevates histamine H₃ receptor (H3R) agonist-mediated inhibition of glutamatergic neurotransmission in the dentate gyrus. Here, we hypothesized that PAE alters the expression of two prominent H3R isoforms namely, the rH_3A and rH_3C isoforms, which have differing intrinsic activities for H3R agonists, in a manner that may contribute to heightened H3R function in PAE rats. In contrast to our predictions, we found different effects of sex and PAE in various brain regions with significant interactions between sex and PAE in dentate gyrus and entorhinal cortex for both isoforms. Subsequently, to confirm the PAE-and sex-induced differences on H3R isoform mRNA expression, we developed a polyclonal antibody selective for the rH_3A inform. Western blots of rH_3A mRNA-transfected HEK-293 cells identified a  ~ 48 kDa band of binding consistent with the molecular weight of rH_3A, thus confirming antibody sensitivity for rH_3A protein. In parallel, we also established a pan-H3R knockout mice line to confirm antibody specificity in rodent brain membranes. Both qRT-PCR and H3R agonist-stimulated [³⁵S]-GTPγS binding confirmed the absence of mH_3A mRNA and H3 receptor-effector coupling in H3R knockout (KO) mice. Subsequent western blotting studies in both rat and mouse brain membranes were unable to detect rH_3A antibody binding at ~48 kDa. Rather, the H3RA antibody bound to a  ~ 55 kDa band in both rat and mouse membranes, including H3R KO mice, suggesting H3RA binding was not specific for H3Rs in rodent membranes. Subsequent LC/MS analysis of the ~55 kDa band in frontal cortical membranes identified the highly abundant beta subunit of ATPase in both WT and KO mice. Finally, LC/MS analysis of the ~48 kDa band from rH_3A mRNA-transfected HEK-293 cell membranes was able to detect rH_3A protein, but its presence was below the limits of quantitative reliability. We conclude that PAE alters rH_3A and rH_3C mRNA expression in some of the same brain regions where we have previously reported PAE-induced alterations in H3R-effector coupling. However, interpreting the functional consequences of altered H3R isoform expression was limited given the technical challenges of measuring the relatively low abundance of rH_3A protein in native membrane preparations.

Prenatal Alcohol Exposure Increases Histamine H3 Receptor-Mediated Inhibition of Glutamatergic Neurotransmission in Rat Dentate Gyrus

BACKGROUND

We have reported that prenatal alcohol exposure (PAE)-induced deficits in dentate gyrus, long-term potentiation (LTP), and memory are ameliorated by the histamine H3 receptor inverse agonist ABT-239. Curiously, ABT-239 did not enhance LTP or memory in control offspring. Here, we initiated an investigation of how PAE alters histaminergic neurotransmission in the dentate gyrus and other brain regions employing combined radiohistochemical and electrophysiological approaches in vitro to examine histamine H3 receptor number and function.

METHODS

Long-Evans rat dams voluntarily consumed either a 0% or 5% ethanol solution 4 hours each day throughout gestation. This pattern of drinking, which produces a mean peak maternal serum ethanol concentration of 60.8 ± 5.8 mg/dl, did not affect maternal weight gain, litter size, or offspring birthweight.

RESULTS

Radiohistochemical studies in adult offspring revealed that specific [3 H]-A349821 binding to histamine H3 receptors was not different in PAE rats compared to controls. However, H3 receptor-mediated Gi /Go protein-effector coupling, as measured by methimepip-stimulated [35 S]-GTPγS binding, was significantly increased in cerebral cortex, cerebellum, and dentate gyrus of PAE rats compared to control. A LIGAND analysis of detailed methimepip concentration-response curves in dentate gyrus indicated that PAE significantly elevates receptor-effector coupling by a lower affinity H3 receptor population without significantly altering the affinities of H3 receptor subpopulations. In agreement with the [35 S]-GTPγS studies, a similar range of methimepip concentrations also inhibited electrically evoked field excitatory postsynaptic potential responses and increased paired-pulse ratio, a measure of decreased glutamate release, to a significantly greater extent in dentate gyrus slices from PAE rats than in controls.

CONCLUSIONS

These results suggest that a PAE-induced elevation in H3 receptor-mediated inhibition of glutamate release from perforant path terminals as 1 mechanism contributing the LTP deficits previously observed in the dentate gyrus of PAE rats, as well as providing a mechanistic basis for the efficacy of H3 receptor inverse agonists for ameliorating these deficits.

Long Term Depression in Rat Hippocampus and the Effect of Ethanol during Fetal Life

Alcohol (ethanol) disturbs cognitive functions including learning and memory in humans, non-human primates, and laboratory animals such as rodents. As studied in animals, cellular mechanisms for learning and memory include bidirectional synaptic plasticity, long-term potentiation (LTP), and long-term depression (LTD), primarily in the hippocampus. Most of the research in the field of alcohol has analyzed the effects of ethanol on LTP; however, with recent advances in the understanding of the physiological role of LTD in learning and memory, some authors have examined the effects of ethanol exposure on this particular signal. In the present review, I will focus on hippocampal LTD recorded in rodents and the effects of fetal alcohol exposure on this signal. A synthesis of the findings indicates that prenatal ethanol exposure disturbs LTD concurrently with LTP in offspring and that both glutamatergic and γ-aminobutyric acid (GABA) neurotransmissions are altered and contribute to LTD disturbances. Although the ultimate mode of action of ethanol on these two transmitter systems is not yet clear, novel suggestions have recently appeared in the literature.

Increase of KCC2 in hippocampal synaptic plasticity disturbances after perinatal ethanol exposure

Low to moderate perinatal ethanol exposure (PEE) may have disastrous consequences for the central nervous system resulting notably in permanent cognitive deficits. Learning and memory are mediated in the hippocampus by long-term potentiation (LTP) and long term depression (LTD), two forms of synaptic plasticity. PEE decreases LTP but also abnormally facilitates LTD (Kervern et al. ) through a presently unknown mechanism. We studied in rat hippocampus slice, the involvement of the chloride co-transporters NKCC1 and KCC2, in the role of GABAA inhibitions in facilitated LTD after moderate PEE. After PEE and in contrast to control slices, facilitated LTD in CA1 field was reduced by the GABAA receptor antagonist bicuculline with no changes in sensitivity to bicuculline and in GABA and benzodiazepine binding sites. Also, sensitivity to diazepam was unaltered, whereas aberrant LTD was blocked. Immunohistochemistry and protein analysis demonstrated an increase in KCC2 protein level at cell membrane in CA1 after PEE with no change in NKCC1 expression. Specifically, both monomeric and dimeric forms of KCC2 were increased in CA1. Bumetanide (10-100 μM), a dose-dependent blocker of NKCC1 and KCC2, or VU0240551 (10 μM) a specific antagonist of KCC2, corrected the enhanced LTD and interestingly bumetanide also restored the lower LTP after PEE. These results demonstrate for the first time an upregulation of the KCC2 co-transporter expression after moderate PEE associated with disturbances in GABAergic neurotransmission modulating bidirectional synaptic plasticity in the hippocampus. Importantly, bumetanide compensated deficits in both LTP and LTD, revealing its potential therapeutic properties.

Moderate Prenatal Alcohol Exposure Alters Functional Connectivity in the Adult Rat Brain

BACKGROUND

Past studies of moderate prenatal alcohol exposure (PAE) have focused on specific brain regions, neurotransmitter systems, and behaviors. However, the effects of PAE on brain function and behavior are complex and not limited to discrete brain regions. Thus, there is a critical need to understand the global effects of moderate PAE on neural function. A primary aim of this research was to explore the functional relationships in neural activity of spatially distinct areas by applying a widely used computational algorithm-group-independent component analysis (gICA)-to resting-state functional magnetic resonance imaging data from rats exposed to either an alcohol or saccharin control solution via maternal consumption during pregnancy.

METHODS

Long-Evans rat dams consumed either 5% (v/v) alcohol or a saccharin control solution throughout gestation. Adult offspring from each prenatal treatment group were anesthetized for functional, structural, and perfusion magnetic resonance-based image acquisition sequences. gICA was applied to the functional data to extract components. To determine connectivity, component time-course correlations were computed and compared. Additionally, spectral power analyses were utilized as an additional measure of functional connectivity. Finally, blood perfusion-assessed by arterial spin labeling-and whole-brain volumetric analyses were evaluated.

RESULTS

Analyses revealed 17 components in several brain regions such as the cortex, hippocampus, and thalamus. PAE was associated with reductions in coordinated activity between components, especially in males. PAE was also associated with reductions in low-frequency spectral power, an effect that was more robust in females. Brain volumetric analyses revealed sex-dependent reductions in females while blood flow analyses revealed sex-dependent reductions in males.

CONCLUSIONS

Moderate PAE leads to persistent changes in functional connectivity in the absence of whole-brain volume or blood flow measures. Future studies will investigate the relationships between alterations in functional network connectivity and behavior.

Aberrant NMDA-dependent LTD after perinatal ethanol exposure in young adult rat hippocampus

Irreversible cognitive deficits induced by ethanol exposure during fetal life have been ascribed to a lower NMDA-dependent synaptic long-term potentiation (LTP) in the hippocampus. Whether NMDA-dependent long-term depression (LTD) may also play a critical role in those deficits remains unknown. Here, we show that in vitro LTD induced with paired-pulse low frequency stimulation is enhanced in CA1 hippocampus field of young adult rats exposed to ethanol during brain development. Furthermore, single pulse low frequency stimulation, ineffective at this age (LFS600), induced LTD after ethanol exposure accompanied with a stronger response than controls during LFS600, thus revealing an aberrant form of activity-dependent plasticity at this age. Blocking NMDA receptor or GluN2B containing NMDA receptor prevented both the stronger response during LFS600 and LTD whereas Zinc, an antagonist of GluN2A containing NMDA receptor, was ineffective on both responses. In addition, LFS600-induced LTD was revealed in controls only with a reduced-Mg(2+) medium. In whole dissected hippocampus CA1 field, perinatal ethanol exposure increased GluN2B subunit expression in the synaptic compartment whereas GluN2A was unaltered. Using pharmacological tools, we suggest that LFS600 LTD was of synaptic origin. Altogether, we describe a new mechanism by which ethanol exposure during fetal life induces a long-term alteration of synaptic plasticity involving NMDA receptors, leading to an aberrant LTD. We suggest this effect of ethanol may reflect a delayed maturation of the synapse and that aberrant LTD may also participates to long-lasting cognitive deficits in fetal alcohol spectrum disorder.

Differential effects of the histamine H(3) receptor agonist methimepip on dentate granule cell excitability, paired-pulse plasticity and long-term potentiation in prenatal alcohol-exposed rats

BACKGROUND

We previously reported that prenatal alcohol-induced deficits in dentate gyrus (DG) long-term potentiation (LTP) are ameliorated by the histamine H3 receptor inverse agonist ABT-239. ABT-239 did not enhance LTP in control rats, suggesting that the possibility of a heightened H3 receptor-mediated inhibition of LTP in prenatal alcohol-exposed (PAE) offspring.

METHODS

To further investigate this mechanism, we examined the effect of methimepip, a selective histamine H3 receptor agonist, on DG granule cell responses and LTP in saccharin control and PAE rats. Long-Evans rat dams voluntarily consumed either a 0 or 5% ethanol solution 4 hours each day throughout gestation. Adult male offspring from these dams were anesthetized with urethane and electrodes implanted into the entorhinal cortical perforant path and the DG.

RESULTS

In control offspring, methimepip reduced the coupling of fast excitatory postsynaptic field potentials to population spikes (E-S coupling), the probability of glutamate release, as measured by paired-pulse ratio (PPR) and diminished DG LTP. Similar reductions in E-S coupling and LTP were observed in saline-treated PAE offspring. In contrast to the control group, methimepip did not exacerbate PAE-induced reductions in E-S coupling or LTP.

CONCLUSIONS

While the effects of methimepip in control offspring were consistent with speculation of a PAE-induced enhancement of H3 receptor-mediated inhibition of E-S coupling and LTP, the absence of an added effect of methimepip in PAE offspring could indicate either an inability to further inhibit these responses with methimepip in PAE rats or the presence of more complex regulatory neural interactions with in vivo recordings in PAE rats. Follow-up studies of H3 receptor-mediated responses in DG slice preparations are under way to provide clearer insights into the role of the H3 receptor regulation of excitatory transmission in PAE rats.

Effects of exposure to moderate levels of ethanol during prenatal brain development on dendritic length, branching, and spine density in the nucleus accumbens and dorsal striatum of adult rats

Reductions in measures of dendritic morphology in the agranular insular cortex have been identified as consequences of prenatal exposure to moderate levels of ethanol in the rat. Motivated by the strong connectivity between this region of frontal cortex and the striatum and a growing body of data linking specific components of the mesocortical/limbic system to effects of ethanol and ethanol self-administration, the current study investigated the effects of moderate fetal ethanol exposure on the dendritic morphology of medium spiny neurons (MSNs) in several regions of the striatum. Throughout gestation, pregnant rat dams either consumed a saccharin solution (control) or achieved average daily blood ethanol concentrations of 84 mg% via voluntary consumption of a 5% ethanol solution. The brains of adult male offspring were extracted and processed for Golgi-Cox staining. MSNs from the dorsomedial striatum, dorsolateral striatum and the nucleus accumbens core and shell were sampled for analysis. Relative to saccharin controls, robust reductions in dendritic length and branching, but not spine density, were observed in the shell of the nucleus accumbens in fetal-ethanol-exposed rats. No significant prenatal ethanol effects were found in the other regions of the striatum. These findings suggest that exposure to moderate levels of ethanol in utero can have profound effects on brain regions related to reward processing and provide possible clues relevant to understanding increased self-administration of drugs of abuse in animals exposed to ethanol during brain development.

Effects of a novel cognition-enhancing agent on fetal ethanol-induced learning deficits

BACKGROUND

Drinking during pregnancy has been associated with learning disabilities in affected offspring. At present, there are no clinically effective pharmacotherapeutic interventions for these learning deficits. Here, we examined the effects of ABT-239, a histamine H₃ receptor antagonist, on fetal ethanol-induced fear conditioning and spatial memory deficits.

METHODS AND RESULTS

Long-Evans rat dams stably consumed a mean of 2.82 g ethanol/kg during a 4-hour period each day during pregnancy. This voluntary drinking pattern produced a mean peak serum ethanol level of 84 mg/dl. Maternal weight gain, litter size and birth weights were not different between the ethanol-consuming and control groups. Female adult offspring from the control and fetal alcohol-exposed (FAE) groups received saline or 1 mg ABT-239/kg 30 minutes prior to fear conditioning training. Three days later, freezing time to the context was significantly reduced in saline-treated FAE rats compared to control. Freezing time in ABT-239-treated FAE rats was not different than that in controls. In the spatial navigation study, adult male offspring received a single injection of saline or ABT-239 30 minutes prior to 12 training trials on a fixed platform version of the Morris Water Task. All rats reached the same performance asymptote on Trials 9 to 12 on Day 1. However, 4 days later, first-trial retention of platform location was significantly worse in the saline-treated FAE rats compared control offspring. Retention by ABT-239-treated FAE rats was similar to that by controls. ABT-239's effect on spatial memory retention in FAE rats was dose dependent.

CONCLUSIONS

These results suggest that ABT-239 administered prior to training can improve retention of acquired information by FAE offspring on more challenging versions of hippocampal-sensitive learning tasks. Further, the differential effects of ABT-239 in FAE offspring compared to controls raises questions about the impact of fetal ethanol exposure on histaminergic neurotransmission in affected offspring.

Effects of the cognition-enhancing agent ABT-239 on fetal ethanol-induced deficits in dentate gyrus synaptic plasticity

Prenatal ethanol exposure causes deficits in hippocampal synaptic plasticity and learning. At present, there are no clinically effective pharmacotherapeutic interventions for these deficits. In this study, we examined whether the cognition-enhancing agent 4-(2-{2-[(2R)-2-methylpyrrolidinyl]ethyl}-benzofuran-5-yl) benzonitrile (ABT-239), a histamine H(3) receptor antagonist, could ameliorate fetal ethanol-induced long-term potentiation (LTP) deficits. Long-Evans rat dams consumed a mean of 2.82 g/kg ethanol during a 4-h period each day. This voluntary drinking pattern produced a mean peak serum ethanol level of 84 mg/dl. Maternal weight gain, offspring litter size, and birth weights were not different between ethanol-consuming and control groups. A stimulating electrode was implanted in the entorhinal cortical perforant path, and a recording electrode was implanted in the dorsal dentate gyrus of urethane-anesthetized adult male offspring. Baseline input/output responses were not affected either by prenatal ethanol exposure or by 1 mg/kg ABT-239 administered 2 h before data collection. No differences were observed between prenatal treatment groups when a 10-tetanus train protocol was used to elicit LTP. However, LTP elicited by 3 tetanizing trains was markedly impaired by prenatal ethanol exposure compared with control. This fetal ethanol-induced LTP deficit was reversed by ABT-239. In contrast, ABT-239 did not enhance LTP in control offspring using the 3-tetanus train protocol. These results suggest that histamine H(3) receptor antagonists may have utility for treating fetal ethanol-associated synaptic plasticity and learning deficits. Furthermore, the differential effect of ABT-239 in fetal alcohol offspring compared with controls raises questions about the impact of fetal ethanol exposure on histaminergic modulation of excitatory neurotransmission in affected offspring.

Prenatal ethanol exposure in rats decreases levels of complexin proteins in the frontal cortex

BACKGROUND

Rodents that are prenatally exposed to ethanol have been shown to exhibit a wide range of cognitive deficits, including impairments in memory, attention and executive function. To determine a potential molecular substrate for cognitive dysfunction in adulthood, we measured regional levels of the presynaptic proteins complexin I and II in a rat model of prenatal ethanol exposure, as levels of these proteins are altered in cognitive-related synaptic plasticity.

METHODS

Pregnant female rats received either a liquid ethanol diet (36% ethanol-derived calories) or a liquid control diet (maltose-dextrin isocalorically substituted for ethanol, matched in amount [g/kg body wt/day of gestation] to an ethanol-consuming partner), or were given ad libitum-fed access to standard laboratory chow and water. Levels of complexin I, II and the ubiquitous presynaptic marker synaptophysin were measured in the frontal cortex and hippocampus of adult male offspring, using ELISA.

RESULTS

Prenatal exposure to ethanol did not alter levels of presynaptic proteins in the hippocampus or levels of synaptophysin in the prefrontal cortex. However, rats prenatally exposed to ethanol displayed significantly lower levels of both complexin I and II in the prefrontal cortex compared to control animals.

CONCLUSIONS

These data indicate that prenatal exposure to ethanol is associated with a selective loss of complexin proteins in the frontal cortex. These proteins are known to be important for activity-dependent neurotransmission, and have previously been shown to mediate synaptic plasticity and cognition. These combined findings suggest that further study of complexin proteins as a substrate for cognitive impairment related to prenatal exposure to ethanol is warranted.

Hippocampal cell proliferation is reduced following prenatal ethanol exposure but can be rescued with voluntary exercise

The ingestion of ethanol during pregnancy has a number of deleterious consequences for the unborn offspring, producing structural and functional deficits that affect the brain and many other organs into adulthood. The hippocampus is a brain area that is particularly sensitive to ethanol's adverse effects. In a previous study we showed that voluntary exercise can ameliorate deficits in long-term potentiation and behavior that occur following prenatal ethanol exposure (Eur J Neurosci, 2005, 21, 1719-1726). In the present study, we investigated the effects of prenatal ethanol exposure on neurogenesis in adulthood, and tested the hypothesis that voluntary exercise would ameliorate any deficits observed. Sprague-Dawley females were administered one of three diets throughout gestation: (i) ethanol (E), a liquid diet containing 36.5% ethanol-derived calories; (ii) pair-fed (PF), a liquid control diet, with maltose-dextrin isocalorically substituted for ethanol, in the amount consumed by an E partner (g/kg body wt/day of gestation); and (iii) ad-libitum-fed control (C), normal laboratory chow and water, ad libitum. The offspring were housed individually at postnatal day (PND) 35, and at PND 50 were randomly assigned to cages either with or without an exercise wheel. BrdU (200 mg/kg, I.P.) was injected on PND 57, and animals terminated either 24 h (proliferation) or 4 weeks (neurogenesis) later. Our results demonstrate that prenatal ethanol exposure significantly decreases both cell proliferation and neurogenesis in the adult dentate gyrus. Animals in the PF condition also showed reduced neurogenesis. In contrast, all animals that engaged in voluntary exercise showed a significant increase in cell proliferation and neurogenesis. These results indicate that prenatal ethanol exposure can suppress both cell proliferation and neurogenesis, and that these effects may be, at least in part, nutritionally mediated. Importantly, voluntary exercise appears to have beneficial effects for these long-lasting deficits in hippocampal volume and cell number that have been observed in animals exposed to ethanol in utero.

Moderate Fetal Alcohol Exposure Impairs the Neurogenic Response to an Enriched Environment in Adult Mice

BACKGROUND

Prenatal ethanol exposure results in a spectrum of cognitive and behavioral deficits and affects an estimated thirteen percent of children born in the United States. The basis of prenatal ethanol-induced impairment of brain function has been widely studied in animal models, where significant changes in the physiological and structural plasticity of hippocampal function have been documented. Here, we explored the possibility that exposure to moderate levels of alcohol in utero might also result in long-lasting impairment of adult hippocampal neurogenesis, a novel form of plasticity that occurs throughout adulthood.

METHODS

Female mice were trained to voluntarily consume 10% EtOH throughout pregnancy using the two-bottle choice paradigm, which results in moderate blood alcohol levels of approximately 121 mg/dl, as previously described (Allan et al., 2003). Offspring were exposed to standard or enriched living conditions for 8-12 weeks post-weaning. BrdU was administered at 50 mg/kg for 12 consecutive days. Mice in each housing condition were sacrificed at either 24 hrs or four weeks following the final BrdU injection, and BrdU cells within the dentate gyrus were evaluated using immuno-histochemical methods.

RESULTS

Neither fetal alcohol exposure (FAE) nor enriched environment affected the number of proliferating progenitors within the subgranular zone (SGZ) of the dentate gyrus. However, FAE severely impaired the neurogenic response to enriched environment. Control mice housed in enriched environment displayed a two-fold increase in hippocampal neurogenesis, whereas FAE mice responded to enriched environment with neither enhanced progenitor survival nor enhanced neuronal differentiation.

CONCLUSIONS

These observations indicate that moderate FAE results in a long-term, persistent defect in neurogenic responses to behavioral challenge.

Prenatal ethanol exposure reduces mGluR5 receptor number and function in the dentate gyrus of adult offspring

BACKGROUND

Previous studies in our laboratory indicated that metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis is markedly reduced in the hippocampal formation of adult rat offspring whose mothers drank moderate amounts of ethanol during pregnancy. In the present study, we extended these observations by measuring the impact of prenatal ethanol exposure on proteins associated with the mGluR5 receptor-effector system along with two mGluR5 agonist-mediated responses in dentate gyrus of adult offspring.

METHODS

Sprague-Dawley rat dams consumed one of three diets throughout gestation: (1) a BioServ liquid diet that contained 5% ethanol (v/v), (2) pair-fed an isocalorically equivalent amount of 0% ethanol liquid diet, or (3) lab chow ad libitum. Microdissected slices of dentate gyrus were prepared from adult female offspring from each diet group and used for (1) Western blot analyses of mGluR5, the G-proteins Galphaq and Galpha11, and phospholipase C-beta1; (2) 2-chloro-5-hydroxyphenylglycine (CHPG)-stimulated growth associated protein 43 (GAP-43) phosphorylation; or (3) CHPG potentiation of electrically evoked [H]-D-aspartate (D-ASP) release from dentate gyrus slices.

RESULTS

In tissue prepared from untreated control rats, CHPG produced a dose-dependent increase in phosphate incorporation into GAP-43, with maximal agonist stimulation occurring at 20 microM of CHPG. CHPG produced a quantitatively similar dose-dependent increase in the potentiation of electrically evoked D-ASP release from dentate gyrus slices from untreated controls. Fetal ethanol exposure reduced the amount of dentate gyrus mGluR5 receptor protein by 36% compared with the diet control groups. There were no significant differences between diet groups in the two G-proteins or phospholipase C-beta1 protein. Fetal ethanol exposure reduced CHPG-stimulated GAP-43 phosphorylation to approximately one half the amount of CHPG stimulation observed in the control diet groups. Prenatal ethanol exposure also reduced CHPG potentiation of D-ASP release to a similar degree compared with control.

CONCLUSIONS

These results indicate that prenatal exposure to moderate quantities of ethanol reduces mGluR5 expression in the dentate gyrus of adult offspring. Although the subcellular site(s) for reduced mGluR5 expression cannot be discerned from Western blot data, the quantitatively similar effects of prenatal ethanol exposure on mGluR5 agonist stimulation of presynaptically localized GAP-43 phosphorylation and CHPG potentiation of evoked D-ASP release suggest that the presynaptic nerve terminal is one site where prenatal ethanol exposure has reduced mGluR5 receptor number and function. Furthermore, these data implicate these neurochemical alterations as one factor contributing to the hippocampal synaptic plasticity and behavioral deficits that we have observed previously in prenatal ethanol-exposed offspring.

Fetal Alcohol Exposure Alters GAP-43 Phosphorylation and Protein Kinase C Responses to Contextual Fear Conditioning in the Hippocampus of Adult Rat Offspring

BACKGROUND

The growth- and plasticity-associated protein GAP-43 plays a significant role in the establishment and remodeling of neuronal connections. We have previously shown that GAP-43 levels, protein kinase C (PKC) activity, and GAP-43 phosphorylation increase during contextual fear conditioning and that fetal alcohol exposure (FAE) decreases PKC activity and GAP-43 phosphorylation in the hippocampus of adult offspring. Drawing on these observations, we hypothesized that FAE manifests its cognitive impairment by disrupting PKC activation and membrane translocation, thereby decreasing GAP-43 phosphorylation and function.

METHODS

Three groups of pregnant rat dams (FAE and two control diet groups) were placed on different diet regimens. Offspring from each of these groups were placed into each of four test groups, a contextual fear conditioned (CFC) group, a naïve unhandled group, and two nonlearning stress control groups. Hippocampi were dissected, homogenized, and used to prepare a cytosolic and a membrane fraction. These fractions were probed for total GAP-43, PKC-phosphorylated GAP-43, and several PKC subtypes. PKC activity also was measured in total homogenates.

RESULTS

Compared with both control diet groups, FAE animals showed a deficit in the activation of PKC in the hippocampus at 24 hr but not at 1.5 hr after CFC. Likewise, we found that the amount of GAP-43 and its phosphorylation were decreased 24 hr after CFC in FAE rats but not at early times after training. Analysis of the translocation of various PKC isoforms revealed that FAE animals had decreased levels of membrane-bound PKC beta2 and PKC epsilon 24 hr after CFC.

CONCLUSIONS

Considering the role of PKC activation and GAP-43 phosphorylation in synaptic plasticity, our results suggest that deficient translocation of PKC beta2 and PKC epsilon in the hippocampus may mediate the electrophysiological and behavioral deficits observed in fetal alcohol exposed animals.

Chronic prenatal ethanol exposure impairs conditioned responding and enhances GABA release in the hippocampus of the adult guinea pig

In this study, we assessed the effects of chronic prenatal ethanol exposure (CPEE) on spatial navigation in the water maze, conditioned responding using food-reinforced lever pressing, and amino acid neurotransmitter release from the hippocampus of the adult guinea pig. Pregnant guinea pigs were treated with ethanol (3 g/kg of maternal body weight/day), isocaloric-sucrose/pair-feeding, or water throughout gestation. Adult offspring were trained in two-lever operant chambers to respond for sucrose pellets, with one lever designated as the reward lever. There were no group differences in response acquisition or lever discrimination on a fixed-ratio 1 (FR-1) schedule. During extinction sessions, CPEE offspring maintained higher levels of responding on the previously reinforced lever, suggesting that CPEE increases perseveration and/or impairs response inhibition but does not affect operant responding for an appetitive reinforcer or the ability to discriminate rewarding from nonrewarding stimuli. In contrast, there was no effect of CPEE on performance in the water maze in the maternal ethanol regimen used in this study. CPEE did not alter electrically evoked glutamate or GABA release from hippocampal brain slices. However, when slices were tested after delivery of a tetanizing stimulation (five 5-s trains at 100 Hz), post-tetanic potentiation of electrically stimulated GABA release was greater in hippocampal slices obtained from CPEE offspring, whereas post-tetanic potentiation of electrically stimulated glutamate release was unaffected. These data suggest that conditioned learning is a sensitive behavioral measure of CPEE-induced brain injury. Increased activity-dependent potentiation of GABA release in the hippocampus may contribute to alterations in synaptic plasticity observed in CPEE offspring.

Neurosteroid modulation of glutamate release in hippocampal neurons: lack of an effect of a chronic prenatal ethanol exposure paradigm

BACKGROUND

Pregnenolone sulfate (PREGS) is a promnesic neurosteroid that is abundantly expressed in the hippocampus of rodents. Studies have shown that the modulation of postsynaptic ligand-gated ion channels by this neurosteroid is impaired in preparations from the brains of fetal ethanol-exposed animals. In this study, we examined whether the presynaptic actions of PREGS also are affected by exposure to ethanol in utero.

METHODS

Rat dams were exposed to one of the following diets during pregnancy: (1) 5% ethanol liquid diet, (2) 0% ethanol liquid diet with pair-feeding, and (3) ad libitum controls. We then studied the presynaptic actions of PREGS on (1) alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-mediated miniature excitatory postsynaptic currents (mEPSCs) recorded from cultured hippocampal neurons in the whole-cell patch-clamp configuration and (2) paired-pulse facilitation of NMDA receptor-dependent excitatory postsynaptic potentials that were intracellularly recorded from CA1 pyramidal neurons in hippocampal slices from adult rats.

RESULTS

Chronic prenatal ethanol exposure affected neither basal mEPSC frequency nor its potentiation by PREGS. Basal paired-pulse facilitation (i.e., in the absence of PREGS) was unaffected by fetal ethanol exposure. Chronic prenatal ethanol exposure did not affect the PREGS-induced potentiation of paired-pulse facilitation.

CONCLUSIONS

Chronic prenatal ethanol exposure does not affect the basal probability of glutamate release in immature or mature hippocampal neurons. Moreover, the presynaptic actions of the neurosteroid PREGS also are unaffected by this exposure. Given that modulation of glutamate release could have a role in the mechanism of the promnesic actions of this neurosteroid, future studies are warranted to determine whether PREGS can ameliorate learning and memory deficits in fetal ethanol-exposed animals.

Changes in protein kinase C (PKC) activity, isozyme translocation, and GAP-43 phosphorylation in the rat hippocampal formation after a single-trial contextual fear conditioning paradigm

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

Effects of chronic prenatal ethanol exposure on cGMP content and glutamate release in the hippocampus of the neonatal guinea pig

The glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS)-cGMP signal transduction system plays key neurotrophic and intercellular communication roles in the hippocampus. In the guinea pig, chronic prenatal ethanol exposure (CPEE), via maternal ethanol administration, suppresses the hippocampal glutamate-NMDA receptor-NOS pathway in the near-term fetus and decreases stimulated glutamate release in the hippocampus of young postnatal offspring, with no effect on NMDA receptor number or NOS activity. At present, the effect of CPEE on cGMP, a key second messenger of the glutamate signal transduction system, in the hippocampus is not known. The objective of this study was to test the hypothesis that CPEE suppresses the hippocampal glutamate signal transduction system in the neonatal guinea pig at the levels of cGMP content and glutamate release. Timed pregnant guinea pigs received chronic oral administration of 4 g ethanol/kg maternal body weight/day, isocaloric-sucrose/pair-feeding, or water treatment throughout gestation. CPEE decreased brain and hippocampal weights at postnatal day (PD) 1 and PD 5 (P<.05). CPEE did not affect basal, NMDA (1, 10, or 100 microM)-stimulated, or K(+) (15 or 30 mM)-stimulated cGMP content in transverse hippocampal slices at PD 1 or 5. At 60 mM K(+), however, CPEE decreased stimulated hippocampal cGMP content at PD 1 (P<.05) and increased stimulated cGMP content at PD 5 (P<.05). In transverse hippocampal slices, CPEE did not affect basal or K(+) (40 or 45 mM)-stimulated glutamate release at PD 1 or 5, or NMDA (50 microM)-stimulated glutamate release at PD 1, but did decrease NMDA (50 microM)-stimulated glutamate release at PD 5 (P<.05). The data demonstrate that the effects of CPEE on stimulated cGMP content and glutamate release in the hippocampus of the neonatal guinea pig are stimulating agent- and age-dependent.

Impaired acquisition in the water maze and hippocampal long-term potentiation after chronic prenatal ethanol exposure in the guinea-pig

In the hippocampus, the CA1 region is selectively vulnerable to the effects of chronic prenatal ethanol exposure. In the guinea-pig, the number of CA1 pyramidal cells is decreased after chronic prenatal ethanol exposure. We tested the hypotheses that chronic prenatal ethanol exposure (through maternal ethanol ingestion) results in impairments in spatial learning and short- and long-term plasticity in the CA1 region of the postnatal guinea-pig hippocampus. Timed, pregnant guinea-pigs were treated with ethanol (4 g/kg maternal body weight/day), isocaloric sucrose/pair-feeding, or water throughout gestation. Offspring were studied between postnatal days 40 and 80. In the Morris water maze, animals exposed to ethanol prenatally showed slower acquisition of an escape response to a hidden platform over 5 days of training. The amplitude of the field excitatory postsynaptic potential in the CA1 region in response to contralateral CA3 stimulation was decreased in offspring exposed to ethanol prenatally. Two forms of short-term plasticity (paired-pulse and frequency facilitation) were unaffected by chronic prenatal ethanol exposure. Long-term potentiation (LTP) in response to high-frequency CA3 stimulation was induced reliably and maintained over 60 min in isocaloric-sucrose and water control animals. However, LTP failed to be induced in the CA1 area of the hippocampus in prenatal ethanol-exposed offspring. These data show that chronic prenatal ethanol exposure, through maternal ethanol administration, impairs spatial performance and LTP in CA1 neurons. Hippocampal dysfunction could contribute importantly to the cognitive and behavioural deficits resulting from chronic prenatal ethanol exposure.

In vitro ethanol exposure decreases potassium-stimulated, but not veratridine-stimulated, glutamate release in the guinea pig hippocampus

In this study we determined the effect of in vitro ethanol exposure on stimulated glutamate release in transverse hippocampal slices (400-microm thickness) of the young postnatal guinea pig (PD 12) by using two chemical stimuli with different mechanisms of action. Ethanol (50 mM) decreased K+ (45 mM)-, but not veratridine (10 microM)-, stimulated glutamate release. The study findings demonstrate that in vitro ethanol exposure produces differential inhibition of stimulated glutamate release in the hippocampus, dependent on the stimulating agent.

Ethanol neurobehavioral teratogenesis and the role of the hippocampal glutamate-N-methyl-D-aspartate receptor-nitric oxide synthase system

The purpose of this review is to evaluate a proposed mechanism for ethanol neurobehavioral teratogenesis in the hippocampus, involving suppression of the glutamate-N-methyl-D-aspartate (NMDA) receptor-nitric oxide synthase (NOS) system. It is postulated that suppression of this signal transduction system in the fetus by chronic maternal consumption of ethanol plays a key role in hippocampal dysmorphology and dysfunction in postnatal life. This mechanism is evaluated critically based on the current literature and our research findings. In view of the apparent time course for loss of CA1 pyramidal cells in the hippocampus produced by chronic prenatal ethanol exposure that manifests in early postnatal life, it is proposed that therapeutic intervention, which targets the glutamate-NMDA receptor-NOS system, may prevent or lessen the magnitude of postnatal hippocampal dysfunction.

Effects of chronic prenatal ethanol exposure on hippocampal glutamate release in the postnatal guinea pig

This study was designed to test the hypothesis that chronic prenatal ethanol exposure decreases basal and stimulated L-glutamate release in the hippocampus of young, postnatal guinea pigs. Timed, pregnant guinea pigs were randomly assigned to one of the following three chronic treatment groups: 4 g ethanol/kg maternal body weight/day, isocaloric-sucrose and pair-feeding to the ethanol group, and water. Each oral treatment was given daily throughout gestation. Spontaneous locomotor activity was increased on postnatal day (PD) 10, and brain and hippocampal weights were decreased on PD 12 in the offspring of the ethanol group compared with the isocaloric-sucrose/pair-fed and water groups. On PD 12, the 45 mM K(+)- and 10 microM veratridine-stimulated release of glutamate in transverse hippocampal slices was decreased in the ethanol group compared with the two control groups. This alteration in glutamate release produced by chronic prenatal ethanol exposure may decrease the efficiency of excitatory synaptic transmission in the hippocampus during postnatal life.

Effects of prenatal alcohol exposure on the hippocampus: spatial behavior, electrophysiology, and neuroanatomy

Prenatal exposure to alcohol can result in fetal alcohol syndrome (FAS), characterized by growth retardation, facial dysmorphologies, and a host of neurobehavioral impairments. Neurobehavioral effects in FAS, and in alcohol-related neurodevelopmental disorder, include poor learning and memory, attentional deficits, and motor dysfunction. Many of these behavioral deficits can be modeled in rodents. This paper reviews the literature suggesting that many fetal alcohol effects result, at least in part, from teratogenic effects of alcohol on the hippocampus. Neurobehavioral studies show that animals exposed prenatally to alcohol are impaired in many of the same spatial learning and memory tasks sensitive to hippocampal damage, including T-mazes, the Morris water maze, and the radial arm maze. Direct evidence for hippocampal involvement is provided by neuroanatomical studies of the hippocampus documenting reduced numbers of neurons, lower dendritic spine density on pyramidal neurons, and decreased morphological plasticity after environmental enrichment in rats exposed prenatally to alcohol. Electrophysiological studies also demonstrate changes in synaptic activity in in vitro hippocampal brain slices isolated from prenatal alcohol-exposed animals. Considered together, these observations demonstrate that prenatal exposure to alcohol can result in abnormal hippocampal development and function. Such studies provide a better understanding of neurological deficits associated with FAS in humans, and may also contribute to the development of strategies to ameliorate the effects of prenatal alcohol exposure on behavior.

Two prenatal alcohol use screening instruments compared with a physiologic measure

OBJECTIVE

To compare two prenatal alcohol use screening instruments with a physiologic measure of prenatal alcohol use.

DESIGN

Retrospective comparison of the Prenatal Alcohol Use Interview (PAUI) and the ACOG Antepartum Record with CDTect.

SETTING

An inner-city, high-volume, prenatal clinic.

PARTICIPANTS

Fifty-six women selected and enrolled at their first prenatal visits.

MAIN OUTCOME MEASURE

An assessment of relative sensitivity and specificity of two prenatal alcohol use screening instruments.

RESULTS

Women identified as Drinkers by the CDTect were more likely to be identified as Drinkers by the PAUI (59%) than by the ACOG Antepartum Record (19%). Also, the PAUI had a lower false negative rate (41%) than the ACOG record (80%). That is, the PAUI was less likely to identify as Quitters women the CDTect identified as Drinkers than was the ACOG record.

CONCLUSION

The PAUI is a more sensitive screen than the ACOG record and should be the instrument preferred for screening prenatal alcohol use.