Neuropeptide Y-immunoreactive synapses in the intermediolateral cell column of rat and rabbit thoracic spinal cord

Neuropeptide Y (NPY)-immunoreactive nerve fibers in the intermediolateral cell column of rat spinal cord segments T 2–T 3 and T 8–T 10 and rabbit segments T 3–T 6 were studied with light and electron microscopic immunocytochemistry. Plexuses of NPY-immunoreactive nerve fibers were found by light microscopy. NPY-positive synapses were present electron microscopically but non-immunoreactive synapses greatly outnumbered NPY-immunoreactive ones. In the lateral horn of rat T 9, 30% of the vesicle-containing NPY-positive axon profiles formed synapses, 95% of which were axodendritic. These synaptic connections may mediate the effects of brainstem NPY neurons on the activity of sympathetic preganglionic neurons.