Direct catecholaminergic innervation of spinal dorsal horn neurons with axons ascending the dorsal columns in cat

Previous ultrastructural studies have shown that catecholamine-containing nerve terminals in the spinal dorsal horn form synaptic junctions with dendrites and somata, but the identity of the neurons giving rise to these structures is largely unknown. In this study we have investigated the possibility that spinomedullary neurons, which project through the dorsal columns to the dorsal column nuclei, are synaptic targets for descending catecholaminergic axons. Neurons with axons ascending the dorsal columns were retrogradely labelled after uptake of horseradish peroxidase by their severed axons in the thoracic (T10-T12) or cervical (C2-C3) dorsal columns. After the retrogradely labelled neurons were visualized, the tissue was immunocytochemically stained with antisera raised against tyrosine hydroxylase or dopamine-beta-hydroxylase. Three hundred forty-three retrogradely labelled neurons within laminae III-V of the lumbosacral dorsal horn were examined under high power with the light microscope. In Triton X-100 treated material, over 60% of cells were found to have dopamine-beta-hydroxylase-immunoreactive varicosities closely apposed to their somata and proximal dendrites. The number of contacts per cell varied from 1 to 22, with a mean number of 4.5. Fewer cells (34%) received contacts from axons immunoreactive for tyrosine hydroxylase as a consequence of the weaker immunoreaction produced by this antiserum. Correlated light and electron microscopic analysis confirmed that many of these contacts were regions of synaptic specialization and that immunostained boutons contained pleomorphic (round to oval) agranular vesicles together with several dense core vesicles. These observations suggest that catecholamines regulate sensory transmission through this spinomedullary pathway by a direct postsynaptic action upon its cells of origin. Such an action would be predicted to suppress transmission generally through this pathway.