Exposure to ethanol and nicotine during the brain growth spurt: spatial DMP performance in male rats

Ablation of hippocampal neurogenesis impairs contextual fear conditioning and synaptic plasticity in the dentate gyrus

Although hippocampal neurogenesis has been described in many adult mammals, the functional impact of this process on physiology and behavior remains unclear. In the present study, we used two independent methods to ablate hippocampal neurogenesis and found that each procedure caused a limited behavioral deficit and a loss of synaptic plasticity within the dentate gyrus. Specifically, focal X irradiation of the hippocampus or genetic ablation of glial fibrillary acidic protein-positive neural progenitor cells impaired contextual fear conditioning but not cued conditioning. Hippocampal-dependent spatial learning tasks such as the Morris water maze and Y maze were unaffected. These findings show that adult-born neurons make a distinct contribution to some but not all hippocampal functions. In a parallel set of experiments, we show that long-term potentiation elicited in the dentate gyrus in the absence of GABA blockers requires the presence of new neurons, as it is eliminated by each of our ablation procedures. These data show that new hippocampal neurons can be preferentially recruited over mature granule cells in vitro and may provide a framework for how this small cell population can influence behavior.

Chronic nicotine induces growth retardation in neonatal rat pups

In the United State, 20% of pregnant women smoke. One of the most consistent adverse outcomes is reduced birth weight in the off-spring. Animal studies using chronic nicotine, the major psychoactive tobacco ingredient, have shown conflicting results, questioning the role of nicotine in growth retardation. To evaluate the direct effects of nicotine during a period equivalent to the human third trimester, we developed an oral gastric intubation model using neonatal rat pups. Nicotine (6 mg/kg/day) was dissolve in milk-formula and delivered during three feedings daily from postnatal day (P)1 to P7. Nicotine immediately and significantly [P<0.05] decreased weight gain per day (WGD) by 13.5% (+/-) 1 day after onset of treatment in both genders and throughout the treatment period. This resulted in significantly lower body weight at P4 and P5 in male and female pups, respectively. After nicotine withdrawal, WGD returned to control level within 1 day, whereas total body weight recovered by P18. There were no long-term consequences on body weight or growth pattern in either gender. The nicotinic acetylcholine receptor (nAChR) antagonist dihydro-beta-erythroidine (DHbetaE) reversed nicotine's effects on WGD suggesting an involvement of heteromeric alpha4beta2, whereas methyllycaconitine (MLA) an antagonist for the homomeric alpha7-type receptor was ineffective. The immediate decrease of growth in neonatal pups suggests that nicotine's effect on birth weight results from direct anorexic rather then indirect effects due to placental dysfunction or increased fetal hypoxia. The postnatal oral gastric intubation model seems to accurately reflect the direct effects of nicotine in neonates.

Artificial rearing alters the response of rats to natural and drug-mediated rewards

Artificial rearing (AR) of infant rats permits precise control over key features of the early environment without maternal influence. The present study examined the behavioral response of AR rats towards natural and drug-mediated rewards, as well as their exploratory and affective behaviors. Adolescent AR rats showed increased preference for sucrose consumption relative to chow and demonstrated greater activity in the open field and in the elevated plus-maze compared to maternally reared (MR) rats. With respect to measures of emotionality, AR rats showed enhanced avoidance of the open arms of the plus-maze, indicating increased anxiety, but they did not differ from MR rats in exploring the center of the open field. Adult AR rats displayed a stronger conditioned response to morphine in a place preference test. These findings support the potential of the AR model to contribute to understanding the role of early experience in the development of behavioral motivation.

Short-Term Memory Impairment and Reduced Hippocampal c-Fos Expression in an Animal Model of Fetal Alcohol Syndrome

BACKGROUND

Previous work in our laboratory has shown that exposure to ethanol during the brain growth spurt impairs spatial short-term memory in rats on the delayed matching-to-place (DMP) version of the Morris water maze. The objectives of this study were to ascertain whether this impairment could: 1) be prevented by increasing the length of encoding time and 2) be related to hippocampal c-Fos expression.

METHODS

Using an artificial rearing model, male Long-Evans rats were fed 6.5 g/Kg/day of ethanol from postnatal days 6-9, with controls fed an isocaloric amount of maltose dextrin. As adults, rats in each treatment condition were trained and subsequently tested on either the DMP version of the Morris water maze, or on a random platform version (RAN) that incorporated the same performance requirements, but disallowed spatial learning. Brains were processed for c-Fos expression.

RESULTS

Ethanol-exposed rats showed longer search trials during training and took longer to learn the DMP task. When the delay between search and recall trials was increased from 60 sec to 120 min, the performance of ethanol-exposed rats was impaired compared with that of controls after a 10 sec, but not after a 45 sec, encoding time. Brain c-Fos expression was increased in hippocampus, prefrontal cortex and visual cortex in rats trained on the DMP compared to the RAN task. Furthermore, in the DMP-trained rats, hippocampal c-Fos expression was lower in ethanol-exposed rats.

CONCLUSIONS

These results suggest that the short-term memory impairment of ethanol-exposed rats 1) can be improved slightly by an increase in encoding time and 2) is related to a decrease in c-Fos expression in the hippocampus.

Early postnatal ethanol administration does not affect prepulse inhibition in rats

Human prenatal ethanol exposure is associated with relatively widespread cognitive deficits but it is unclear whether general deficits in responsivity to sensory stimuli contribute to or underlie the deficits in later or more complex stages of information processing. The present experiment assessed the effects of early postnatal ethanol administration in rats on prepulse inhibition, with animals tested in adolescence (postnatal day (PD) 35) and early adulthood (PD 70). Animals were assigned to receive ethanol (5.25 g/kg) via intragastric intubation on PD 4-9, sham-intubation, or to a naive control group. Pre-exposure to ethanol did not differentially affect the magnitude of the response to the startle stimulus alone nor did it affect the percent inhibition of the startle response on trials with a prepulse stimulus. Male rats exhibited a greater percent inhibition than female rats on PD 35 at all interstimulus intervals (ISIs) except the shortest, 4 ms. Female rats exhibited a greater percent inhibition than male rats at all ISIs on PD 70. Collectively, these data demonstrate that cognitive deficits associated with early exposure to ethanol may not be attributable to deficits in sensorimotor gating, at least to the extent this construct is measured by prepulse inhibition.

Spontaneously hypertensive and Wistar Kyoto rats differ in delayed matching-to-place performance and response to dietary long-chain polyunsaturated fatty acids

Spontaneously hypertensive rats (SHR) were used as an animal model of attention deficit hyperactivity disorder (ADHD). This study investigated whether, in comparison with its progenitor strain, Wistar-Kyoto rats (WKY), SHR would show deficits in spatial short-term memory in the delayed-matching-to-place (DMP) version of the Morris water maze and be more distracted by exposure to a novel stimulus during recall trials. It also addressed whether dietary supplementation with long-chain polyunsaturated fatty acids (LCPUFA) during development would increase brain docosahexaenoic acid (DHA) and improve SHR behavioral performance. Beginning at weaning (21 days), male SHR and WKY were fed either a control or LCPUFA supplemented diet [0.5% arachidonic acid (AA) and 0.9% DHA], and behavioral testing began at 8 weeks. The first three tasks comprised a series of problems, each consisting of an initial search trial and subsequent recall trials. The intertrial interval (ITI) between the search and recall trial was either 60 s or 60 min. Surprisingly, in contrast to SHR, WKY did not appear to use a spatial short-term memory strategy to solve the problem. Notwithstanding, the performance of both strains was affected by the delay, such that they showed longer path lengths at the long compared with the short ITI. There was no effect of dietary supplementation on DMP performance. SHR fed the control diet were less responsive to a novel stimulus introduced on the first recall trial than WKY, and this tended to increase with supplementation. Analysis of brain fatty acid composition indicated that supplementation did increase DHA in the phosphatidylethanolamine fraction in WKY; however, in SHR, there was either no change (phosphatidylethanolamine) or paradoxical decreases (phosphatidylcholine and phosphatidyserine/phosphatidylinositol). Further research is needed to determine whether SHR are an appropriate model for studying a possible relationship between dietary LCPUFA and the behavioral symptoms of ADHD.

Testing the spatial- versus object-learning distinction: water-maze performance of male rats exposed to ethanol during the brain growth spurt

This study investigated the effects of exposure to ethanol during the brain growth spurt on a visual-discrimination (VD) and a place-learning task (PL) using intra-maze cues in the water maze. Artificially reared male Long-Evans rats were exposed to ethanol (ET) in a binge pattern from postnatal days 6-9 (6.5 g kg(-1) x day(-1); BAC approximately 330 mg/dl) or an isocaloric maltose-dextrin solution (gastrostomy control). A third suckled control group was reared by lactating dams. In experiment 1, rats were trained to discriminate horizontal- (H) versus vertical-striped (V) cues, with the positive cue providing escape from water. Groups did not differ with V+, but ET rats made more errors with H+. In experiment 2, the ET group was impaired in learning the spatial location of a submerged platform relative to intra-maze cues. In both tasks, acquisition deficits among ET rats were characterized by impairment emerging at trial 2, with intact reference memory on trial 1, and the ET group reached a comparable level of performance to controls by the end of training. In summary, because impairment was related to task characteristics, a clear distinction between impaired spatial- versus cue-based learning was not supported. However, these findings do support an effect of exposure to ethanol during the brain growth spurt on recent event, but not reference, memory.