Possible target neurons of 5-hydroxytryptamine fibers in the lamprey spinal cord: immunohistochemistry combined with intracellular staining with Lucifer yellow.

Intracellular recordings were made from 76 neurons belonging to various cell types in the lamprey spinal cord, and these neurons were subsequently stained with Lucifer yellow. Sections were made of spinal cords containing Lucifer-yellow-filled neurons, and in the same sections 5-hydroxytryptamine (5-HT)-containing neurons and fibers were made visible with immunohistochemical methods. Motoneurons and lateral cells appeared to send part of their dendrites into a dense ventromedial 5-HT plexus, and these dendrites were adjacent to 5-HT varicosities. No or few 5-HT varicosities have been found adjacent to cell bodies or dendrites of sensory dorsal cells, giant interneurons, and edge cells. The combined application of intracellular staining and immunohistochemistry appeared to be suited to screen for possible transmitter-identified contacts on morphologically identified neurons.