Neuronal Loss in the Developing Cerebral Cortex of Normal and Bax-Deficient Mice: Effects of Ethanol Exposure

Fetal alcohol spectrum disorder is associated with defects in neuronal generation, migration, and differentiation. The present study tested the hypothesis that ethanol exposure during the period of naturally occurring neuronal death causes a time- and Bax-dependent neuronal loss. Wild-type and Bax knockout mice were given a pair of injections (two hours apart) of ethanol (2.5 g/kg) or saline on postnatal day (P) 4, P7, P10, or P13. Mean blood ethanol concentration was 435 mg/dl one hour after the second injection. The total numbers of neurons in individual layers of somatosensory cortex were stereologically determined in 30-day-old mice and the expressions of active caspase 3 immunopositivity and terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) were determined 2-36 h after the first injection. On P30, ethanol caused up to 36% neuronal loss in cortical laminae of wild-type mice. In contrast, no ethanol-induced loss was detected in Bax knockout mice. Ethanol also caused an acute bilaminar (layers II/III and V) increase in caspase 3 immunoexpression and TUNEL in wild-type mice. The changes in the expression of these markers were age- and lamina-dependent. No ethanol-induced expression of caspase 3 or TUNEL was detected in Bax knockout animals. Thus, ethanol-induced death of cortical neurons is Bax-dependent, occurs concurrently in all layers, but does not correspond to lamina- and age-dependent expression of DNA fragmentation. Both ethanol-induced and naturally occurring death of cortical neurons rely on caspase 3-dependent and independent mechanisms.