Drug-Induced Apoptosis: Mechanism by which Alcohol and Many Other Drugs Can Disrupt Brain Development

Abstract

Maternal ingestion of alcohol during pregnancy can cause a disability syndrome termed Fetal Alcohol Spectrum Disorder (FASD), which may include craniofacial malformations, structural pathology in the brain, and a variety of long-term neuropsychiatric disturbances. There is compelling evidence that exposure to alcohol during early embryogenesis (4th week of gestation) can cause excessive death of cell populations that are essential for normal development of the face and brain. While this can explain craniofacial malformations and certain structural brain anomalies that sometimes accompany FASD, in many cases these features are absent, and the FASD syndrome manifests primarily as neurobehavioral disorders. It is not clear from the literature how alcohol causes these latter manifestations. In this review we will describe a growing body of evidence documenting that alcohol triggers widespread apoptotic death of neurons and oligodendroglia (OLs) in the developing brain when administered to animals, including non-human primates, during a period equivalent to the human third trimester of gestation. This cell death reaction is associated with brain changes, including overall or regional reductions in brain mass, and long-term neurobehavioral disturbances. We will also review evidence that many drugs used in pediatric and obstetric medicine, including general anesthetics (GAs) and anti-epileptics (AEDs), mimic alcohol in triggering widespread apoptotic death of neurons and OLs in the third trimester-equivalent animal brain, and that human children exposed to GAs during early infancy, or to AEDs during the third trimester of gestation, have a significantly increased incidence of FASD-like neurobehavioral disturbances. These findings provide evidence that exposure of the developing human brain to GAs in early infancy, or to alcohol or AEDs in late gestation, can cause FASD-like neurodevelopmental disability syndromes. We propose that the mechanism by which alcohol, GAs and AEDs produce neurobehavioral deficit syndromes is by triggering apoptotic death and deletion of neurons and OLs (or their precursors) from the developing brain. Therefore, there is a need for research aimed at deciphering mechanisms by which these agents trip the apoptosis trigger, the ultimate goal being to learn how to prevent these agents from causing neurodevelopmental disabilities.

Keywords: alcohol; anesthetics; anti-epileptics; apoptosis; fetal alcohol spectrum disorder.

Figure 1

Computer plots showing laminar degeneration of layer II and V neurons in the primary visual cortex of neonatal rhesus monkeys following exposure to GABA agonists, and absence of a similar pattern in control or ketamine-exposed neonatal monkey brain.

Figure 2

Cellular degeneration induced by alcohol in the caudate/putamen of fetal rhesus monkey brain as detected by activated caspase 3 (AC3) immunohistochemistry. There is a marked increase in AC3-positive neuronal profiles in the alcohol-exposed caudate (A) and putamen (C), compared to the sparse display of apoptotic profiles in control caudate (B) or control putamen (not shown).

Figure 3

These panels are computer plots of sections cut through the basal ganglia [caudate (Ca) and putamen (Pu)] of the fetal rhesus monkey brain following exposure to an NMDA antagonist anesthetic (ketamine), or to GABA agonist anesthetics (propofol or isoflurane). Apoptotic neurons (red dots) are sparsely distributed in the basal ganglia region of the control brain, and are much more heavily concentrated in the basal ganglia region following exposure to any of the three anesthetic agents. In the anesthesia-exposed brains, apoptotic OLs (white dots) are densely and diffusely distributed throughout all white matter areas, including the corpus callosum (CC), Corona Radiata (CR), Centrum Semi-Ovale (CSO) and Internal capsule (IC).

Figure 4

Coronal sections of neonatal mouse brain 24 h after exposure to saline (A) or alcohol (B–D). All sections stained by DeOlmos cupric silver method (marker of dead or dying cells). Sections B–D are cut at 3 rostro-caudal levels to show the massive extent and remarkable bilateral symmetry of alcohol-induced neurodegeneration.

Figure 5

Sections from the internal capsule (A); corpus callosum (B); and corona radiata (C) of a G120 fetal macaque 8 h after exposure to alcohol, showing that in each of these white matter regions there are abundant oligodendroglia (OLs) that stain positive for activated caspase 3 (AC3), signifying that they are dying by apoptosis. Within 5 h after alcohol is administered, large numbers of OLs in diffusely scattered distribution throughout the white matter become AC3 positive, then rapidly progress to an advanced stage of degeneration in which they become fragmented and reduced to particulate debris that is rapidly phagocytized by macrophages and removed from the scene.

Figure 6

These panels depict the cingulate cortex and corpus callosum of a 10-day-old mouse brain, 72 h after treatment with saline (left) or a single high dose (5 g/kg) of alcohol (right). Both brain sections are shown at the same magnification and are cut from the same rostrocaudal level. Note the decreased cortical mass and also the decreased size of the corpus callosum of the alcohol brain. The number of neuronal profiles within the demarcated area of the saline vs. ethanol brain is 881 vs. 488, which represents a 45% cell loss within the cingulate region. Adapted from Olney et al. [7].

Figure 7

These histological sections depict the parietal cortex (PC), cingulate cortex (Cing), rostral hippocampus (HC) and corpus callosum (CC) of a 7-day-old C57BL/6 mouse 8 h following subcutaneous treatment with saline (left) or ethanol (right). Both sections have been stained immunocytochemically with antibodies to activated caspase-3. The saline control brain shows a pattern of caspase-3 activation that occurs normally in the 7-day-old mouse brain, and is attributable to physiological cell death. The pattern of caspase-3 activation closely resembles the pattern of silver staining shown in Figure 4C, but the density of caspase staining is not as great as the density of silver staining, because the silver stain marks all neurons and fragments thereof that have degenerated over a 24 h period, and caspase-3 marks only those neurons that are undergoing caspase-3 activation at a given survival interval, in this case the 8 h interval. Adapted from Olney et al. [7].