Alterations in the development of the main olfactory bulb of the mouse after ethanol exposure

Effects of prenatal alcohol exposure on the olfactory system development

Fetal Alcohol Spectrum Disorders (FASD), resulting from maternal alcohol consumption during pregnancy, are a prominent non-genetic cause of physical disabilities and brain damage in children. Alongside common symptoms like distinct facial features and neurocognitive deficits, sensory anomalies, including olfactory dysfunction, are frequently noted in FASD-afflicted children. However, the precise mechanisms underpinning the olfactory abnormalities induced by prenatal alcohol exposure (PAE) remain elusive. Utilizing rodents as a model organism with varying timing, duration, dosage, and administration routes of alcohol exposure, prior studies have documented impairments in olfactory system development caused by PAE. Many reported a reduction in the olfactory bulb (OB) volume accompanied by reduced OB neuron counts, suggesting the OB is a brain region vulnerable to PAE. In contrast, no significant olfactory system defects were observed in some studies, though subtle alterations might exist. These findings suggest that the timing, duration, and extent of fetal alcohol exposure can yield diverse effects on olfactory system development. To enhance comprehension of PAE-induced olfactory dysfunctions, this review summarizes key findings from previous research on the olfactory systems of offspring prenatally exposed to alcohol.

Fetal alcohol exposure leads to abnormal olfactory bulb development and impaired odor discrimination in adult mice

Background

Children whose mothers consumed alcohol during pregnancy exhibit widespread brain abnormalities and a complex array of behavioral disturbances. Here, we used a mouse model of fetal alcohol exposure to investigate relationships between brain abnormalities and specific behavioral alterations during adulthood.

Results

Mice drank a 10% ethanol solution throughout pregnancy. When fetal alcohol-exposed offspring reached adulthood, we used high resolution MRI to conduct a brain-wide screen for structural changes and found that the largest reduction in volume occurred in the olfactory bulbs. Next, we tested adult mice in an associative olfactory task and found that fetal alcohol exposure impaired discrimination between similar odors but left odor memory intact. Finally, we investigated olfactory bulb neurogenesis as a potential mechanism by performing an in vitro neurosphere assay, in vivo labeling of new cells using BrdU, and in vivo labeling of new cells using a transgenic reporter system. We found that fetal alcohol exposure decreased the number of neural precursor cells in the subependymal zone and the number of new cells in the olfactory bulbs during the first few postnatal weeks.

Conclusions

Using a combination of techniques, including structural brain imaging, in vitro and in vivo cell detection methods, and behavioral testing, we found that fetal alcohol exposure results in smaller olfactory bulbs and impairments in odor discrimination that persist into adulthood. Furthermore, we found that these abnormalities in olfactory bulb structure and function may arise from deficits in the generation of new olfactory bulb neurons during early postnatal development.

Persistent deficits in heart rate response habituation following neonatal binge ethanol exposure

BACKGROUND

We have previously shown that the rate of habituation of the heart rate orienting response to a novel odor in rats is negatively affected by neonatal ethanol exposure. Thus far, however, only young rats (16 days of age) have been tested. Given the persistence of attention and memory problems evident in humans exposed to ethanol in utero, the purpose of this experiment was to examine the longer-term consequences of ethanol exposure on response habituation.

METHODS

Ethanol (5.25 g/kg/d) was administered intragastrically to male and female Sprague-Dawley rats on postnatal days (PD) 4 to 9, and controls were given sham intubations. Animals were tested for heart rate orienting and response habituation to a novel olfactory stimulus (amyl acetate) on PD 16, 23, or 30.

RESULTS

Animals tested on PD 16 or 23 showed normal heart rate deceleration to the novel odor, a measure of the orienting response. However, ethanol-treated subjects showed impaired response habituation compared with sham controls. While controls exhibited complete habituation within 4 to 5 trials, ethanol-treated animals continued to respond throughout the testing session, with little decrement in heart rate response magnitude across 10 stimulus presentations. A different pattern of responding was observed in animals tested during adolescence (PD 30). Control animals failed to show the typical heart rate decrease indicative of orienting, and instead showed a tendency toward tachycardia. In contrast, ethanol-treated animals tested on PD 30 showed orienting bradycardia that persisted for several trials.

CONCLUSIONS

These data suggest that there are relatively long-term consequences of neonatal ethanol exposure on nonassociative memory. This impairment in habituation may be relevant to the distractibility and poor focused attention that is pervasive among humans diagnosed with fetal alcohol spectrum disorders.

Gestational ethanol exposure alters the behavioral response to ethanol odor and the expression of neurotransmission genes in the olfactory bulb of adolescent rats

Fetal exposure to ethanol is highly predictive of the propensity to ingest ethanol during adolescence and in utero chemosensory plasticity has been implicated as a contributing factor in this process. Recent rodent studies have shown that fetal ethanol exposure results in a tuned unconditioned sniffing and neurophysiological olfactory response to ethanol odor in infant animals. Importantly, a significant proportion of increased ethanol avidity at this age can be attributed to the tuned behavioral response to ethanol odor. These effects are absent in adults. Using behavioral methods and comprehensive gene expression profiling to screen for robust transcriptional differences induced in the olfactory bulb, we examined whether ethanol exposure via maternal diet results in an altered responsiveness to ethanol odor that persists into late adolescence and, if so, the molecular mechanisms that may be associated with such effects. Compared to controls, fetal exposure altered: the adolescent sniffing response to ethanol odor consistent with the previously observed changes in infant animals; and the expression of genes involved in synaptic transmission and plasticity as well as neuronal development (both cell fate and axon/neurite outgrowth). These data provide evidence for a persistence of olfactory-mediated responsiveness to ethanol into the period of adolescence. Further, they provide insight into an important relationship between fetal exposure to ethanol, adolescent odor responsiveness to the drug and potential underlying molecular mechanisms for the odor-guided behavioral response.

Alcohol exposure during the first two trimesters equivalent alters granule cell number and neurotrophin expression in the developing rat olfactory bulb

Although alcohol has been shown to affect brain development adversely, the underlying mechanism of alcohol's actions are poorly understood. The present study addressed the hypothesis that alcohol affects growth factor availability during critical periods of neural growth by measuring the mRNA expression of brain-derived neurotrophic factor (BDNF), a potent developmental growth factor. Multiple offspring of timed-pregnant rat dams given alcohol (6.0 g/kg per day) or control treatments during gestation were sacrificed at either embryonic (E) day 21 or E33 (usually postnatal day 10) when their olfactory bulbs were processed for molecular analyses or neuron counting. BDNF mRNA levels were measured by reverse-transcription-polymerase chain reaction, and DNA methylation of the BDNF gene was quantified by Southern blot analyses following digestion with methylation-sensitive enzymes. Estimates of total granule cell number were obtained by counting those cells using unbiased stereological techniques. There was a significant decrease in BDNF mRNA levels in the alcohol-exposed offspring of both ages compared with controls. In addition, the number of olfactory bulb granule cells significantly decreased in the E33 but not the E21 rat pups exposed to alcohol compared with their appropriate aged controls. Finally, BDNF DNA of alcohol-exposed animals was less susceptible to digestion with the methylation-sensitive enzyme HpaII compared with controls, suggesting that the DNA of the alcohol exposed pups was hypermethylated. Our results indicate that exposure to alcohol during early brain development in the rat, a period equivalent to the first two trimesters in humans, can have a detrimental effect on normal development of the olfactory bulb by reducing the number of BDNF-synthesizing neurons. Although the exact mechanism for the alcohol-induced neuronal loss is unknown, the inappropriate transcription of the BDNF gene is one mechanism that may account for the complexity of effects observed in offspring exposed to heavy alcohol exposure in utero.

Fetal alcohol syndrome: early olfactory learning as a model system to study neurobehavioral deficits

The goal of basic research examining the deficits underlying fetal alcohol syndrome is to develop an animal model which allows investigation and assessment of the neural and cognitive impairments resulting from prenatal alcohol exposure. The following review focuses on animal models and their relationship to human deficits following prenatal alcohol exposure. In addition, this review examines a unique, well-established model system which may permit an increased understanding of the role of alcohol on the developing brain and cognitive behavior. Specifically, large metabolic, neurochemical, neuropharmacological, morphological and neurophysiological changes in young rats have been reported as a consequence of early olfactory preference conditioning, a form of learning that normally occurs during both human and rat development. This olfactory odor preference training paradigm can be used to assess changes in learning as well as the neural substrates underlying this learning. Olfactory preference training has been used to examine: 1) learning, as demonstrated by a behavioral preference for an odor previously paired with stimulation which mimics maternal care; 2) metabolism, by measuring 2-deoxyglucose uptake and distribution in response to the trained odor; 3) neurotransmitter levels, by using in vivo microdialysis, to examine changes in neurotransmitter levels in the olfactory bulb in response to a trained odor. Using in vivo microdialysis enables measurement of both baseline responsiveness of alcohol-exposed pups as well as learned responses at several different developmental ages. The established neural features of this olfactory model include an increase in behavioral preference for a trained odor, increases in 2-DG uptake in specific foci within the olfactory bulb in response to the odor, and increases in dopamine in response to olfactory preference training stimuli, as well as conditioned increases in norepinephrine following olfactory preference training. Using these known behavioral, metabolic and neurochemical indices in control pups allows identification of some of the neurotransmitter systems involved in deficits and the neurobiological basis for impairments induced by prenatal alcohol exposure.

Early postnatal alcohol exposure acutely and permanently reduces the number of granule cells and mitral cells in the rat olfactory bulb: a stereological study

This study demonstrates that exposure to alcohol during a period of rapid brain growth can lead to severe and permanent deficits in the number of granule cells and mitral cells in the main olfactory bulb. Sprague-Dawley rat pups were reared artificially and were administered alcohol over postnatal days (PD) 4 through 9, a period of brain development comparable to part of the human third trimester. The daily alcohol dose of 6.6 g/kg was concentrated into two of the twelve daily feedings, producing high peak blood alcohol concentrations followed by near total clearance. Pups were either sacrificed on PD10 or were allowed to grow to adulthood and sacrificed on PD115. The total number of granule cells and mitral cells in the main olfactory bulb were estimated with the aid of unbiased stereological principles and systematic sampling techniques. Exposure to alcohol resulted in significant reductions in the number of both granule cells and mitral cells on PD10. Significant deficits in both neuronal populations remained on PD115. The results support the hypothesis that alcohol exposure can kill developing neurons and lead to permanent neuronal deficits. Substantial developmental changes also occurred in the total number of mitral cells and granule cells between PD10 and PD115 in the control groups. In untreated rats, the number of granule cells increased from 2.20 x 10(6) on PD10 to 5.06 x 10(6) on PD115, while the number of mitral cells decreased from 5.30 x 10(4) to 4.33 x 10(4) over the same time period. These results demonstrate that there is a natural loss of mitral cells during postnatal development at the same time that granule cell number is increasing.

The effects of prenatal alcohol exposure on behavioral and neuroanatomical components of olfaction

Prenatal alcohol exposure is associated with deficits in odor-associative learning in very young rat pups. One alternative explanation for these findings is that rather than a learning deficit per se, alcohol-exposed pups may display a sensory deficit. The present study was designed to examine the effects of prenatal alcohol exposure on behavioral and neuroanatomical components involved in olfaction. The subjects in this study were pups exposed to 35% ethanol-derived calorie (EDC) liquid diet from gestation days (GD) 6-20. Two control groups were included, a 0% EDC pair-fed and an ad lib lab chow group. In Experiment 1, respiratory response to a novel odor was examined in pups tested at either 3, 4, or 10 days of age. The 35% EDC offspring clearly detected the odor. Furthermore, there was an apparent alcohol-related development delay in respiratory rate as shown by a lower baseline respiratory rate at PN 3 relative to controls which was no longer apparent by PN 4. Experiment 2 examined the volume of two neuroanatomical structures involved in olfaction, the main olfactory bulb (MOB) and the vomeronasal organ (VNO) in 3-day-old pups. Prenatal alcohol exposure was associated with a decreased volume of the MOB although the VNO was unaffected.

Acute and long-term neuronal deficits in the rat olfactory bulb following alcohol exposure during the brain growth spurt

This study demonstrates that there is a relative recovery in the number of olfactory bulb granule cells following an initial alcohol-induced deficit, while the number of mitral cells remains permanently and severely depressed. The importance of pattern of exposure in influencing the severity of alcohol-induced neuronal loss in the olfactory bulb is also demonstrated. Sprague-Dawley rat pups were reared artificially and were administered alcohol over postnatal days (PD) 4 through 9, a period of rapid brain growth comparable to part of the human third trimester. Two groups received a daily alcohol dose of 4.5 g/kg, administered either as a 5.1% or 10.2% solution. A third group received a higher daily alcohol dose of 6.6 g/kg administered continuously as a 2.5% solution. Pups were either sacrificed on PD 10 or were allowed to grow to adulthood and sacrificed on PD 90. The number of mitral cells and granule cells and the area of the subependymal zone were determined from single sections. On PD 10, immediately following the alcohol exposure, both the mitral cells and the granule cells were significantly reduced in number, relative to controls, in both of the groups receiving the concentrated (5.1% and 10.2%) alcohol treatments. On PD 90, however, only the mitral cell number remained significantly reduced in the groups receiving the concentrated solutions, while the number of granule cells no longer differed significantly from that of controls. The group receiving the higher daily dose (6.6 g/kg) in continuous fractions had no significant cell loss at 10 or 90 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Nipple attachment behavior in rat pups exposed to alcohol in utero

Two experiments were conducted to further characterize impaired nipple attachment behavior in 5-day-old rat pups exposed to alcohol prenatally. In Experiment 1, animals were given two different nipple attachment tests. In the pups-on-mother test, which required animals to first locate a nipple, alcohol-exposed pups displayed longer attachment latencies than controls. Furthermore, more than 50% of these pups failed to attach during testing. In the pups-on-nipple test, in which no search for the nipple was necessary, prenatal alcohol did not reliably affect attachment. To assess the extent to which alcohol-exposed pups utilize an olfactory cue for attachment, in Experiment 2, nipple attachment was observed using the pups-on-nipple test before and after olfactory cue removal. Similar to controls, the alcohol-exposed animals only attached to the normally scented nipples. Therefore, prenatal alcohol exposure impaired nipple search performance in young rats, but not attachment once at the nipple. Moreover, these pups performed like controls in response to olfactory cue manipulation. Possible mechanisms for the poor search behavior are discussed.

Synaptogenesis in the hippocampal dentate gyrus: effects of in utero ethanol exposure

In animal models of Fetal Alcohol Syndrome, ethanol causes a number of changes in brain development, with many of these changes being very transient. This is especially true for the process of synaptogenesis in different brain areas. Our quantitative electron microscopic study of synaptogenesis in the molecular layer of the rat dentate gyrus supports the above statement, by demonstrating that ethanol has no effect on the appearance of synapses in the dentate gyrus during early postnatal life (10-30 days old). However, prenatal ethanol exposure does appear to affect the process of synapse turnover, which is indicated by the significantly delayed appearance of complex (curved) synapses and multiple synaptic contacts on single axonal terminals. Efficient synapse turnover is thought to be required for the normal maintenance of neuronal plasticity, which in turn ensures an animal's ability to respond to novel environments, tasks and injuries. It would seem that the prenatal neurotoxicology of ethanol may manifest itself by more subtle mechanisms at sites of structural and functional importance.

The effects of prenatal alcohol exposure on odor associative learning in rats

Alcohol was administered to pregnant females via a liquid diet that contained either 35% ethanol-derived calories (35% EDC) or 0% EDC on gestation days 6-20. An ad lib lab chow group (LC) was also included. In Experiment 1, odor-aversion learning was examined in 10-day-old offspring. While both the 0% EDC and LC groups displayed odor aversions, the 35% EDC offspring did not. In Experiment 2, learning was assessed in an appetitive paradigm in three-day-old offspring. Once again, the 35% EDC offspring showed no evidence of learning. Experiment 3 examined odor-aversion learning in adults. Both alcohol-exposed offspring and controls learned the odor association equally well. These findings suggest that odor associative learning is a sensitive indicator for alcohol-related learning deficits in rat pups although these deficits may dissipate as the offspring matures. Since odor associations play a critical role in neonatal behaviors, these deficits may help explain other behavioral anomalies noted following prenatal alcohol exposure.

Morphometric study of the glomerular population in the mouse olfactory bulb: numerical density and size distribution along the rostrocaudal axis

A morphometric study of the glomerular population in the olfactory bulb of the mouse has been carried out by using stereological methods. On the basis of the assumption that the glomerular population is a polydispersed system of spheres, glomerular profile distributions obtained from profile measurements were subjected to a mathematical unfolding procedure to obtain the actual glomerular size distribution. We used a distribution-free method to account for the combined effects of overprojection due to section thickness and truncation (two missing profile mechanisms). Results proved better than those obtained directly from profile measurements without stereological analysis. Several new findings were obtained. First, significant variations of the glomerulus sizes were found along the rostrocaudal axis. The glomeruli are larger in the middle region of the olfactory bulb, whereas their numerical density decreases in the same region. Moreover, the profile density is homogeneous along the rostrocaudal axis. In other words, the relative surface occupied by the periglomerular cells in the glomerular layer is invariant. As a consequence, it may be concluded that the variations in size and numerical density are inversely correlated. Thus, since the glomeruli are larger in the middle region, their number per unit volume is logically smaller in this same area. Finally, the computerization of all these data has led us to estimate the number of glomeruli (1,810 +/- 41) in the olfactory bulb of the mouse. In order to get a comparative idea of their advantages and disadvantages, other standard stereological methods were used in the present study to determine this number. Functional interpretations of the variations of the size and numerical density along the rostrocaudal axis of the olfactory bulb are discussed with respect to ontogenetic and morphofunctional data obtained elsewhere.

Effects of prenatal alcohol exposure on the sexually dimorphic nucleus of the preoptic area of the hypothalamus in male and female rats

Prenatal alcohol exposure can produce a variety of behavioral and physiological alterations, including changes in sexually dimorphic behaviors. It has been proposed that alcohol alters these behaviors by altering prenatal androgen and/or other steroid levels. This study was designed to examine the effects of prenatal alcohol exposure on a sexually dimorphic neuroanatomical structure, namely, the sexually dimorphic nucleus of the preoptic area of the hypothalamus (SDN-POA). This nucleus is larger in males than females and is sensitive to perinatal sex steroid exposure. The SDN-POA was examined in 70-80-day-old male and female rats whose mothers received on days 6-20 of pregnancy either a liquid diet containing 35% ethanol-derived calories (EDC) or a 0% EDC isocaloric pair-fed liquid diet. An ad libitum lab chow control group was also included (LC). Both volume and average cell size of the SDN-POA were markedly smaller in alcohol-exposed males relative to 0% EDC and LC controls. In contrast, prenatal alcohol exposure did not appear to affect SDN-POA volume or cell size in females. Prenatal alcohol exposure did not significantly alter the volume of a nearby nucleus, the nucleus of the anterior commissure, in either sex. These findings support the hypothesis that prenatal alcohol exposure alters sexual differentiation in males, perhaps by altering some aspect of the prenatal androgen environment. The absence of any effect in 35% EDC females suggests that males and females may be differentially sensitive to alcohol's effects on this nucleus.

Heredity and alcohol-induced brain anomalies: effects of alcohol on anomalous prenatal development of the corpus callosum and anterior commissure in BALB/c and C57BL/6 mice

Using two inbred strains of mice which have similar rates of alcohol metabolism, we asked whether prenatal alcohol exposure would cause greater incidence and severity of defects in the development of two forebrain fiber tracts, the corpus callosum and the anterior commissure, in mice prone to these defects (BALB/c) than in mice not prone to these defects (C57BL/6). Pregnant animals were fed 0.6 kcal/g body weight of a Sustacal-based liquid diet containing 0, 15, 17.5, 20, or 25% ethanol-derived calories from day 7 to fetal assessment on day 18 of gestation. Most of alcohol's greatest effects and the greatest strain differences in alcohol's effects on fetal variables were produced by the 17.5% diet. This dose had inhibitory effects on fetal body, brain, and midsagittal corpus callosum and anterior commissure growth. All these effects, except that on brain weight, were significantly greater in C57s than in BALBs. When the results were compared with prenatal growth curves for normal untreated mice, the effect of alcohol on corpus callosum but not anterior commissure growth was largely explained by its effects on overall development. The 17.5% diet had a greater specific effect on size of the anterior commissure in C57s than BALBs but increased the incidence and severity of its permanent dysmorphology in BALBs more than in C57s. Anterior commissure size and morphology may be sensitive indicators of alcohol's effects on prenatal brain development. Hereditary differences in rate of maternal alcohol metabolism no doubt have important consequences for risks arising from prenatal alcohol exposure. However, this study clearly indicates that inherited factors, other than those that influence rate of alcohol metabolism, are important influences on the overall fetal response and the specific responses of the anterior commissure to prenatal alcohol exposure.