The topographical distribution of serotoninergic terminals in the spinal cord of the cat: quantitative radioautographic studies.

In vitro uptake of labeled serotonin (5-HT) has allowed the radioautographic detection of serotoninergic fibers in the cat spinal cord at thoracic and lumbar levels. The specificity of labeling was ensured by the conditions of incubation ([3H]5-HT was 10(-7) M) and by test control experiments with concomitant 10(-6) M NA or 2 X 10(-6) M fluoxetine which led respectively to an unmodified or a negative radioautographic pattern. Macroscopic radioautography with [14C]5-HT gave the framework for analysis of 5-HT innervation at this level which appeared dense in the grey matter and more scattered in white matter except in the periphery. In the grey matter labeled structures appeared at histological level as randomly oriented fibers. They displayed enlargements which were identified by electron microscopy as varicosities (mean diameter 0.7 micrometer) containing clear and large granular vesicles. To estimate the density of 5-HT varicosities in grey matter, semi-automatic counting was performed on histological radioautographs, using an image analyzer. Maximal concentrations of boutons were found in the periependymal region, the sympathetic lateral column, the whole dorsal horn and the motor neuron area of ventral horn. In a densely innervated region, the number of boutons was estimated to be at least 2.65 X 10(6)/cu.mm. In white matter, labeled fibers were longitudinally oriented in the periphery and obliquely in other regions, except in the dorsal funiculus, where they were nearly absent. Previous histofluorescence and biochemical data are confirmed and completed by the results of this approach which displays several advantages in resolution, sensitivity and possibilities of automated quantification. Moreover owing to the dynamic abilities of the radioautographic method, the spinal cord provides a good model for the study of the properties (namely the axonal flow) and the functions of the serotoninergic neurons.