Behavioral consequences of abnormal cortical development: insights into developmental disabilities.

Cerebral cortical development occurs in precisely-timed stages that can be divided into neurogenesis, neuronal migration and neuronal differentiation. These events occur during discrete time windows that span the late prenatal and early postnatal periods in both rodents and primates, including humans. Insults at particular developmental stages can lead to distinctive cortical abnormalities including cortical hypoplasia (reduced cell number), cortical ectopias (abnormalities in migration) and cortical dysplasias (abnormalities in the shapes or numbers of dendrites). In this review, we examine some of the most extensively-studied animal models of disrupted stages of cortical development and we compare long-term anatomical, neurochemical, and behavior abnormalities in these models. The behavioral abnormalities in these models range from alterations in simple motor behaviors to food hoarding and maternal behaviors as well as cognitive behaviors. Although we examine concisely animal models of cortical hypoplasia and cortical ectopias, we focus here on developmental manipulations that affect cortical differentiation, particularly, those that interrupt the normal ontogeny of the neurotransmitter-defined cortical afferent systems: norepinephrine, serotonin, dopamine and acetylcholine. All of these afferents presumably play a critical role in the maturation of their cortical targets; the timing of the afferents' entry into the cortex and their effects on their cortical targets, however, are different. We, therefore, compare the specific anatomical, neurochemical and behavioral effects of manipulations of the different cortical afferents. Because of the considerable evidence that cortical development proceeds differently in the two sexes, when data are available, we address whether perinatal insults differentially affect the sexes. Finally, we discuss how these developmental studies provide insights into cellular and neurochemical correlates of behavioral functional abnormalities and the relevance of these data to understanding developmental disabilities in humans.