A confocal and electron microscopic study of contacts between 5‐HT fibres and feline dorsal horn interneurons in pathways from muscle afferents

Morphological substrates of actions of serotonin upon dorsal horn interneurons with input from group II muscle afferents were investigated by using two experimental approaches. Twelve interneurons were intracellularly labelled with rhodamine‐dextran, and serotoninergic fibres were identified by immunofluorescence. Appositions between the serotoninergic axons and these interneurons were examined with a dual‐channel confocal microscope. A further four interneurons were intracellularly labelled with horseradish peroxidase, and serotoninergic axons were identified by immunocytochemistry; these neurons were prepared for combined light and electron microscopy. Confocal microscopy revealed serotoninergic varicosities in apposition to both cell bodies and dendrites. Similar total numbers of appositions were found on the soma, and on dendrites within 100 μm from the soma, on the most completely labelled neurons. The number of appositions on 100‐μm segments of dendrites decreased with increasing distances from the soma (from 14.6 within 100 μm, to 3.8 and 2.4 at 100–300 μm, and more than 300 μm distances, respectively). Electron microscopic analysis of two neurons revealed that few of the apparent contacts on cell bodies were synaptic, but, in contrast, many varicosities apposed to proximal dendrites formed synapses. The evidence suggests that serotonin may have more powerful synaptic effects upon the dendrites of this class of dorsal horn interneurons than on their cell bodies. J. Comp. Neurol. 387:430–438, 1997. © 1997 Wiley‐Liss, Inc.