SYNCHRONOUS DEVELOPMENT OF PYRAMIDAL NEURON DENDRITIC SPINES AND PARVALBUMIN-IMMUNOREACTIVE CHANDELIER NEURON AXON TERMINALS IN LAYER-III OF MONKEY PREFRONTAL CORTEX

Postnatal development of the primate cerebral cortex involves an initi al proliferation and the subsequent attrition of cortical synapses. Al though these maturational changes in synaptic density have been observ ed across the cortical mantle, little is known about the precise time course of developmental refinements in synaptic inputs to specific pop ulations of cortical neurons. We examined the postnatal development of two markers of excitatory and inhibitory inputs to a subpopulation of layer III pyramidal neurons in areas 9 and 46 of rhesus monkey prefro ntal cortex. These neurons are of particular interest because they pla y a major role in the flow ofinformation both within and between corti cal regions. Quantitative reconstructions of Golgi-impregnated mid-lay er III pyramidal neurons revealed substantial developmental changes in the relative density of dendritic spines, the major site of excitator y inputs to these neurons. Relative spine density;on both the apical a nd basilar dendritic trees increased by 50% during the first two postn atal months, remained at a plateau through 1.5 years of age, and then decreased over the peripubertal age range until stable adult levels we re achieved. As a measure of the postnatal changes in inhibitory input to the axon initial segment of these pyramidal neurons, we determined the density of parvalbumin-immunoreactive axon terminals belonging to the chandelier class of local circuit neurons. The density of these d istinctive axon terminals (cartridges) exhibited a temporal pattern of change that exactly paralleled the changes in dendritic spine density . These results suggest that subpopulations of cortical neurons may be regulated by dynamic interactions between excitatory and inhibitory i nputs during development and, in concert with other data, they emphasi ze the cellular specificity of postnatal refinements in cortical circu itry.