PATTERNING OF THE NEOCORTICAL PROJECTIONS FROM THE RAPHE NUCLEI IN PERINATAL RATS - INVESTIGATION OF POTENTIAL ORGANIZATIONAL MECHANISMS

Serotoninergic (5-HT) fibers in the cerebral cortex of perinatal rats have a pattern that coincides with the boundaries of primary sensory a reas and within the primary somatosensory cortex form the rattunculus. This patterned immunoreactivity (IR) appears about 60 hours after bir th and disappears between postnatal days (P-) 12 and 15. Three experim ents were carried out to evaluate mechanisms that might underlie the p recise patterning of the 5-HT-IR. Retrograde labelling with fluorescen t tracers in perinatal rats revealed only a coarse rostrocaudal topogr aphy in the raphe-cortical projection and the existence of raphe cells projecting to multiple cortical locations. Thus, a precise point-to-p oint, raphe-cortical projection does not underlie the patterned cortic al 5-HT-IR. Ablation of the thalamus prior to the age at which pattern ed 5-HT-IR could be seen in the developing cortex caused a complete lo ss of patterned immunoreactivity. This suggests that 5-HT fibers may r equire the presence of thalamocortical axons to achieve the pattern ob served in normal animals. Serotoninergic raphe neurons transplanted to the cortices of newborn rats exhibited extensive axonal outgrowth, bu t did not form a somatotopic pattern. This result also suggests that s pecific spatiotemporal interactions between growing 5-HT and thalamoco rtical axons may be necessary for the somatotopic patterning of the fo rmer fibers. (C) 1994 Wiley-Liss, Inc.