Alcohol exposure during the first two trimesters equivalent alters granulecell 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 presen t study addressed the hypothesis that alcohol affects growth factor availab ility during critical periods of neural growth by measuring the mRNA expres sion of brain-derived neurotrophic factor (BDNF), a potent developmental gr owth 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 ol factory bulbs were processed for molecular analyses or neuron counting. BDN F mRNA levels were measured by reverse-transcription-polymerase chain react ion, and DNA methylation of the BDNF gene was quantified by Southern blot a nalyses following digestion with methylation-sensitive enzymes. Estimates o f total granule cell number were obtained by counting those cells using unb iased stereological techniques. There was a significant decrease in BDNF mR NA levels in the alcohol-exposed offspring of both ages compared with contr ols. In addition, the number of olfactory bulb granule cells significantly decreased in the E33 but not the E21 rat pups exposed to alcohol compared w ith their appropriate aged controls. Finally, BDNF DNA of alcohol-exposed a nimals was less susceptible to digestion with the methylation-sensitive enz yme Hpall compared with controls, suggesting that the DNA of the alcohol ex posed pups was hypermethylated. Our results indicate that exposure to alcoh ol during early brain development in the rat, a period equivalent to the fi rst two trimesters in humans, can have a detrimental effect on normal devel opment of the olfactory bulb by reducing the number of BDNF-synthesizing ne urons. Although the erect mechanism for the alcohol-induced neuronal loss i s unknown, the inappropriate transcription of the BDNF gene is one mechanis m that may account for the complexity of effects observed in offspring expo sed to heavy alcohol exposure in utero. (C) 1999 John Wiley & Sons. Inc.