Based on the previous discussion, there are several developmental characteristics that are similar across neurotransmitter systems, and there is no reason to believe that the situation is different for the prefrontal cortex. For example, the progenitor cells responsible for the elaboration of both intrinsic and extrinsic transmitter systems in the cortex become postmitotic long before they show appreciable differentiation of their respective neurotransmitter phenotypes. In addition, virtually all neurotransmitter systems are present in the cortex at birth, but require further maturation. Finally, the postnatal development of most transmitter systems continues beyond the weanling period (similar to early adolescence in humans), during which important changes in gonadal maturity are occurring, and overlaps with the postweanling period (similar to late adolescence when changes in emotional maturity are occurring). Although there is a dearth of information
Figure 6.5 Low-power darkfield photomicrographs of dopamine-immunoreactive varicose fibers in medial prefrontal cortex of rats at postnatal days Pll, P20, and P45 and during early adulthood. At Pll, there are very few fibers in evidence, but the density increases steadily until adulthood, particularly in the deeper layers V and VI. Bar = 100 |um.
(Reprinted with permission from Benes, F. M., S. L. Vincent, R. Molloy, and Y. Khan, 1996. Increased interaction of dopamine-immunoreactive varicosities with GABA neurons of rat medial prefrontal cortex occurs during the postweanling period. Synapse 23:237-245.)
regarding human brain, available evidence suggests that postnatal maturation of both the cholinergic and serotonergic systems may continue throughout childhood and adolescence, and may, in some cases, persist throughout the lifespan of the normal individual. Based on observations in rats, however, it seems likely that changes in the dopamine system are also occurring until the early adult period.
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