Abstract
Immunohistochemical methods are used to characterize the distribution of noradrenergic and serotonergic fibers in primary visual cortex of cynomolgus monkeys (Macaca fascicularis) at various postnatal ages. Previous studies in adult squirrel monkeys have shown that serotonergic fibers are generally restricted to the upper four cortical laminae and are especially dense in layer IV, whereas noradrenergic fibers are especially dense in layers V and VI, moderate in layers I, II, and III, and virtually absent in layer IV (Morrison, J. H., S. L. Foote, M. E. Molliver, F. E. Bloom, and H. G. W. Lidov (1982) Proc. Natl. Acad. Sci. U.S.A. 2401–2405; Morrison, J. H., S. L. Foote, D. O'Connor, and F. E. Bloom (1982) Brain Res. Bull. 9: 309–319). Since these monoamines, especially norepinephrine, have been hypothesized to play an essential role in the developmental plasticity of visual cortex organization (e.g., Kasamatsu, T., and J. D. Pettigrew (1976) Science 194: 206–209; Pettigrew, J. D., and T. Kasamatsu (1978) Nature 271: 761–763), the present study examined the postnatal development of these innervation patterns, especially just before and just after the reported “critical period” for visual plasticity. A dense serotonergic innervation of layer IV is present at birth along with sparse innervation of other laminae. The adult pattern of serotonergic innervation, which is similar to that in the squirrel monkey but even more specifically laminated, becomes evident by 6 weeks of age. In the adult pattern, the most dense innervation remains in layers IVb and IVc. A much lower density of noradrenergic than of serotonergic fibers is evident at all ages examined. As with serotonin, the lowest density of fibers is observed at birth. By about 2 months of age these noradrenergic fibers have become more dense, and their laminar distribution is similar to that of adult cynomolgus which is similar to adult squirrel monkey. These studies indicate that: (1) both types of innervation display a continuum of development, with no abrupt changes, (2) serotonergic innervation is more dense than noradrenergic innervation at every age examined, (3) these two transmitter systems exhibit very different laminar innervation patterns as early as birth, and (4) the greater laminar specialization of area 17 in cynomolgus versus squirrel monkeys is accompanied by corresponding enhanced laminar specialization of these monoaminergic afferents.