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.

Purkinje cell deficits in nonhuman primates following weekly exposure to ethanol during gestation

The most serious features of fetal alcohol syndrome (FAS) are mental retardation and other behavioral problems resulting from alcohol-induced damage to the developing central nervous system (CNS). The mechanism by which alcohol induces its neuroteratogenic effects is unknown. One hypothesis is that gestational alcohol exposure results in a reduction in neuronal number. This study demonstrates that gestational exposure to ethanol in a non-human primate species induces permanent dose-related deficits in the number of cerebellar Purkinje cells. Ethanol was administered via nasogastric tube once per week to 15 gravid pigtailed macaques (Macaca nemistrina) in one of the following doses: 0.0 (intubated controls), 1.2, 1.8, 2.5, 3.3, and 4.1 g/kg/dose. Offspring were reared with parental surrogates and were sacrificed at 6 months of age; 8-microns-thick, parasagittal sections were cut through the paraffin-embedded cerebellar vermis. Purkinje cells were quantified, the length of the Purkinje cell line was determined stereologically, and Purkinje cell linear frequency was calculated. The number of Purkinje cells and their linear frequencies were significantly reduced in the alcohol-treated subjects, and the deficits were dose-dependent. The groups receiving 2.5 g/kg/dose and above were most severely affected and had an average deficit in Purkinje cell number of 11.8%, relative to controls. Alcohol had no effect on the length of the Purkinje cell line. The findings suggest that alcohol-induced reduction in neuronal number may be an important factor underlying the CNS dysfunction in FAS.

The protective effect of neuronal nitric oxide synthase (nNOS) against alcohol toxicity depends upon the NO-cGMP-PKG pathway and NF-kappaB

Fetal alcohol syndrome (FAS) stems from maternal alcohol abuse during pregnancy and is an important cause of mental retardation and hyperactivity in children. In the developing brain, alcohol can kill neurons, leading to microencephaly. However, due to their genetic makeup, some individuals are less vulnerable than others to alcohol's neurotoxic effects. Animal studies have demonstrated that one particular gene, neuronal nitric oxide synthase (nNOS), protects developing neurons in vivo against alcohol-induced death. We utilized pharmacologic techniques to demonstrate that nNOS protects neurons against alcohol toxicity by activating the NO-cGMP-PKG signaling pathway. Cerebellar granule cell cultures derived from mice carrying a null mutation for nNOS (nNOS-/- mice) were substantially more vulnerable than cultures from wild-type mice to alcohol-induced cell death. However, activation of the pathway at sites downstream of nNOS protected the cultures against alcohol toxicity. Conversely, blockade of the pathway rendered wild-type cultures vulnerable to alcohol-induced death. We further identified NF-kappaB as the downstream effector through which nNOS and the NO-cGMP-PKG pathway signal their neuroprotective effects. Tumor necrosis factor-alpha (TNF-alpha), which activates NF-kappaB, ameliorated alcohol-induced cell death in nNOS-/- and wild-type cultures, while an NF-kappaB inhibitor (NFi) blocked the protective effects of TNF-alpha and worsened alcohol-induced cell death. Furthermore, NFi blocked the protective effects of NO-cGMP-PKG pathway activators, demonstrating that NF-kappaB is downstream of the NO-cGMP-PKG pathway. As wild-type neurons matured in culture, they became resistant to alcohol toxicity. However, this maturation-dependent alcohol resistance did not occur in nNOS-/- mice and could be reversed in wild-type mice with NFi, demonstrating that nitric oxide and NF-kappaB are crucial for the development of alcohol resistance with age. Thus, nNOS protects developing neurons against alcohol toxicity by activating the NO-cGMP-PKG-NF-kappaB pathway and is crucial for the acquisition of maturation-dependent alcohol resistance.

Genetic absence of nNOS worsens fetal alcohol effects in mice. I: behavioral deficits

BACKGROUND

Alcohol abuse during pregnancy often induces neuropsychological problems in the offspring, including learning disorders, attention deficits, and behavior problems, all of which are prominent components of fetal alcohol spectrum disorders (FASD). However, not all children who were exposed to alcohol in utero are equally affected by it. While some children have major deficits, others are spared. This unequal vulnerability is likely due largely to differences in fetal genetics. Some fetuses appear to have certain genotypes that make them much more prone to FASD. However, to date, no gene has been identified that worsens alcohol-induced brain dysfunction. Nitric oxide (NO) is a gaseous molecule that can protect developing neurons against alcohol-induced death. In the brain, NO is produced by neuronal nitric oxide synthase (nNOS). In this study, we examined whether homozygous mutation of the nNOS gene in mice worsens the behavioral deficits of developmental alcohol exposure.

METHODS

Wild-type and nNOS(-/-) mice received alcohol (0.0, 2.2, or 4.4 mg/g) daily over postnatal days (PDs) 4 to 9. Beginning on PD 85, the mice underwent a series of behavioral tests, including open field activity, the Morris water maze, and paired pulse inhibition.

RESULTS

For the wild-type mice, alcohol impaired performance only in the water maze. In contrast, for the nNOS(-/-) mice, alcohol impaired performance on all 3 tasks. Furthermore, the nNOS(-/-) mice were substantially more impaired than wild-type mice in their performance on all 3 of the behavioral tests and at both the low (2.2) and high (4.4) doses of alcohol.

CONCLUSIONS

Targeted disruption of the nNOS gene worsens the behavioral impact of developmental alcohol exposure and allows alcohol-induced learning problems to emerge that are not seen in wild type. This is the first demonstration that a specific genotype can interact with alcohol to worsen functional brain deficits in an animal model of FASD.

Alcohol exposure during the brain growth spurt promotes hippocampal seizures, rapid kindling, and spreading depression

BACKGROUND

Epilepsy is a prominent sign of brain dysfunction and a cause of substantial disability in some children with fetal alcohol syndrome. The hippocampal formation is vulnerable to alcohol-induced pathologic changes and is the source of seizure activity in a variety of epileptic conditions. This study tests the hypothesis that developmental alcohol exposure facilitates epileptic activity and promotes kindling within hippocampal circuitry.

METHODS

Rat pups received either a moderate dose (2.0 g/kg) or a high dose (3.75 g/kg) of alcohol via intragastric intubation over postnatal days 4 to 10. Intubated control and suckle control groups were also included. Upon reaching adulthood (postnatal days 85-100), the rats underwent electrophysiologic testing. A double-barrel potassium-sensitive microelectrode was placed into the right dentate gyrus stratum granulosum for the recording of extracellular field potential and extracellular potassium concentration. Stimuli were delivered via an electrode positioned in the CA3 subregion of the left hippocampus. To assess whether alcohol promotes hippocampal seizures and rapid kindling, the parameters of maximal dentate activation (MDA) were measured before, during, and after a series of stimulation-induced seizures.

RESULTS

Developmental exposure to the high dose of alcohol permanently altered several parameters of MDA. Time to onset of MDA and stimulus threshold for afterdischarge production were both decreased, whereas the duration of the afterdischarge was increased. Although the moderate alcohol dose reduced time to onset of MDA, it did not affect any other MDA parameters. Over the course of the repeated induced seizures, spreading depression occurred more often and with fewer stimuli in the high-dose alcohol group than in any other group. The series of repeated electrographic seizures induced rapid kindling in all of the treatment groups. However, the kindling effect was enhanced in a dose-response manner by the previous alcohol exposures.

CONCLUSIONS

These findings demonstrate that exposure to alcohol during brain development can permanently alter the physiology of the hippocampal formation, thus promoting epileptic activity, enhancing kindling, and facilitating spreading depression. The relative roles of alcohol intoxication and withdrawal in these abnormal physiologic responses remain unknown.

Reduced seizure threshold and hippocampal cell loss in rats exposed to alcohol during the brain growth spurt

BACKGROUND

Epilepsy is a prominent sign of neurologic dysfunction in some children with fetal alcohol syndrome (FAS). However, it is unknown whether the epileptic disorders in these children are directly due to the neuroteratogenic effects of alcohol or to some other factor accompanying maternal alcoholism. The hippocampus is vulnerable to alcohol-induced pathologic changes, and dysfunction of the hippocampus often manifests as epilepsy. We examined the effect of alcohol exposure during development on the seizure threshold and examined the relationship between alteration of seizure threshold and alcohol-induced neuronal loss from the hippocampus.

METHODS

Rat pups received 0.85, 2.5, or 3.75 g/kg of alcohol via intragastric intubation daily over postnatal days (PD) 4-9. An intubated control and a suckle control group were also included. To assess the effect of a single day of alcohol exposure, an additional group received 3.75 g/kg of alcohol on PD 4 alone. Behavioral seizure thresholds were determined by intravenous infusion of the proconvulsant, pentylenetetrazol (PTZ), on PD 31 or on PD 90. In addition, electrographic seizure thresholds were determined by recording extracellular field potentials from the dentate gyrus. The number of hippocampal CA1 pyramidal cells, CA3 pyramidal cells, and granule cells of the dentate gyrus were determined by stereology.

RESULTS

Daily exposure to alcohol resulted in a dose-dependent decrease in the seizure threshold and in the selective loss of CA1 pyramidal cells. Reduction in the seizure threshold was significantly correlated with loss of CA1 pyramidal cells. Recordings of extracellular field potentials confirmed the alcohol-induced reduction in seizure threshold, demonstrated that PTZ-induced seizures involve hippocampal-parahippocampal circuitry, and provided evidence that the hippocampal formation is the generator of the PTZ-induced seizures in alcohol-exposed animals.

CONCLUSIONS

These findings demonstrate that exposure of the developing brain to alcohol can permanently reduce the threshold for both behavioral and electrographic seizures and can selectively kill hippocampal CA1 pyramidal cells. Both the pathologic findings and the physiologic recordings support the concept that the reduced seizure threshold in alcohol-exposed animals is due to hippocampal pathology.