A CONFOCAL AND ELECTRON-MICROSCOPIC STUDY OF CONTACTS BETWEEN 5-HT FIBERS AND FELINE DORSAL HORN INTERNEURONS IN PATHWAYS FROM MUSCLE AFFERENTS

Morphological substrates of actions of serotonin upon dorsal horn inte rneurons with input from group II muscle afferents were investigated b y using two experimental approaches. Twelve interneurons were intracel lularly labelled with rhodamine-dextran, and serotoninergic fibres wer e identified by immunofluorescence. Appositions between the serotonine rgic axons and these interneurons were examined with a dual-channel co nfocal microscope. A further four interneurons were intracellularly la belled with horseradish peroxidase, and serotoninergic axons were iden tified by immunocytochemistry; these neurons were prepared for combine d Light and electron microscopy. Confocal microscopy revealed serotoni nergic varicosities in apposition to both cell bodies and dendrites. S imilar total numbers of appositions were found on the soma,and on dend rites within 100 mu m from the soma, on the most completely labelled n eurons. The number of appositions an 100-mu m segments of dendrites de creased with increasing distances from the soma (from 14.6 within 100 mu m, to 3.8 and 2.4 at 100-300 mu m, and more than 300 mu m 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 sy napses. The evidence suggests that serotonin may have more powerful sy naptic effects upon the dendrites of this class of dorsal horn interne urons than on their cell bodies. (C) 1997 Wiley-Liss, Inc.