Neurons in the hilus region of the rat hippocampus are depleted in number by exposure to alcohol during early postnatal life

We have previously shown that exposing rats to a relatively high dose of et hanol during early postnatal life resulted in a deficit in spatial learning ability. This ability is controlled, at least in part, by the hippocampal formation. The purpose of the present study was to determine whether exposu re of rats to ethanol during early postnatal life affected the number of sp ecific neurons in the hippocampus. Wistar rats were exposed to a relatively high daily dose of ethanol between postnatal days 10 and 15 by placing the m for 3 h each day in a chamber containing ethanol vapor. The blood ethanol concentration was about 430 mg/dl at the end of the exposure period. Group s of ethanol-treated (ET) rats, separation controls (SC), and mother-reared controls (MRC) were anesthetized and killed at 16 days of age by perfusion with phosphate-buffered glutaraldehyde (2.5%). The Cavalieri principle was used to determine the volume of various subdivisions of the hippocampal fo rmation (CA1, CA2+CA3, hilus, and granule cell layer), and the physical dis ector method was used to estimate the numerical densities of neurons within each subdivision. The total number of neurons was calculated by multiplyin g estimates of the numerical density with the volume. There were, on averag e, about 441,000 granule cells in the granule cell layer and 153,000 to 177 ,000 pyramidal cells in both the CA1 and CA2+CA3 regions in all three treat ment groups. In the hilus region, ET rats had about 27,000 neuronal cells. This was significantly fewer than the average of 38,000 such neurons estima ted to be present in both MRC and SC animals. Thus, neurons in the hilus re gion may be particularly vulnerable to the effects of a high dose of ethano l exposure during early postnatal life. (C) 2000 Wiley-Liss, Inc.