[3H]-serotonin retrograde labelling in serotonergic fibers

Autoradiography, following retrograde axonal transport with [3H]-serotonin (10(-4) M) was used to identify, selectively, the serotonergic neurons of medullary raphe that innervate the spinal cord. In combination with this technique, immunocytochemical detection of endogenous serotonin showed some raphe magnus cell bodies immunostained by 5-HT antibodies and [3H]5-HT radiolabelled. Serotonergic fibers of lateral and dorsolateral funiculus of ligated spinal cord were also characterized following retrograde axonal transport with 10(-4) M [3H]-serotonin. At the ultrastructural level, mainly myelinated, although unmyelinated labelled fibers were also identified as swollen axons. Clusters of large membranous organelles, predominantly mitochondria, some dense lamellar bodies, multivesicular bodies and lysosomes accumulated on the distal segment of the ligation. Autoradiographic reaction was always observed to be very intense and silver grains overlapped mainly mitochondria (41%) and clusters of membranous organelles (37%). However, smooth endoplasmic reticulum seemed to be devoid of silver grains. From our results, serotonergic myelinated spinal fibers were labelled after uptake-retrograde axonal transport with [3H]5-HT. Mitochondria and membrane organelles could convey tritiated compounds derived from [3H]-serotonin uptake towards the serotonergic cell bodies.