CONTRIBUTIONS OF RAPHE-CORTICAL AND THALAMOCORTICAL AXONS TO THE TRANSIENT SOMATOTOPIC PATTERN OF SEROTONIN IMMUNOREACTIVITY IN RAT CORTEX

Two experiments were carried out to evaluate the relative contribution s of thalamocortical and raphe-cortical fibers to the transient somato topically organized pattern of serotonin (5-HT) immunoreactivity that appears in the primary somatosensory cortex (SI) of rats during the fi rst 2 weeks of life. In the first experiment, the specific 5-HT uptake inhibitors, fluoxetine and paroxetine, were administered systemically , animals were killed 3, 6, or 12 h later, and cortices evaluated for 5-HT immunoreactivity. Fluoxetine treatment had no appreciable effect on the density of 5-HT immunoreactivity in the cortex. Paroxetine trea tment caused a reduction in 5-HT immunoreactivity which was maximal 6 h after administration. Examination of the cortices of these animals r evealed a loss of very fine dust-like 5-HT immunoreactivity, but a vib rissae-related pattern remained visible in thicker fibers. In a second experiment, raphe-cortical fibers were destroyed by systemic administ ration of 5,7-dihydroxytryptamine on the day of birth. Six days after this manipulation, 5-HT was applied directly to the cortex in vivo and the animals were then killed and cortices processed to demonstrate 5- HT immunoreactivity. The cortices of these rats revealed a fine dust-l ike immunoreactivity organized in a somatotopic pattern, but only very few 5-HT-positive axons. The results of these experiments suggest tha t both raphe-cortical axons and thalamocortical fibers contribute to t he patterned 5-HT immunoreactivity observed in SI of perinatal rats.