Spinal dopaminergic system of the rat: light and electron microscopic study using an antiserum against dopamine, with particular emphasis on synaptic incidence.

The mapping of the spinal dopaminergic innervation has been performed in the adult rat using an anti-dopamine antiserum. Immunoreactive fibers were detected with the light microscope in the dorsal horn (mainly in laminae III-IV), in the intermediolateral cell column (IML), in the peri-ependymal region and in the ventral horn. The ultrastructural analysis of dopaminergic innervation showed mainly axodendritic contacts and fewer axosomatic ones. In the ventral horn and the IML, the pattern of dopaminergic innervation exhibited a majority of classical synapses. In the dorsal horn, dopaminergic innervation was partly non-synaptic (at cervical level), whereas numerous axodendritic synapses were observed at thoraco-lumbar level. Previous studies described the non-synaptic organization of serotonergic and noradrenergic projections in the dorsal horn. It is thus hypothesized that the monoaminergic systems, involved in pain modulation within the dorsal horn, act partly through volume transmission. In contrast, these systems would modulate the motor and autonomic functions through classical synapses.