Ultrastructure of chemically defined neuron systems in the dorsal horn of the monkey. III. Serotonin immunoreactivity.

The ultrastructural organization of serotoninergic axons and terminals in the superficial dorsal horn of the monkey was examined by the PAP immunohistochemical method. Terminals with serotonin-like immunoreactivity (SLI) were identified in lamina I (marginal zone) and lamina IIo (outer substantia gelatinosa). Labelled profiles contained many small, round, clear vesicles and usually a few granular vesicles (70 nm diameter). Most synaptic junctions were symmetrical with sparse pre- and post-synaptic densities. Most frequently, terminals formed axodendritic synapses on large and small dendrites; axosomatic and axospinous contacts were infrequent. In addition SLI terminals were found apposed to unlabelled LGV-type terminals (containing several large granular vesicles of 75-90 nm). The appositions commonly met some criteria of axo-axonic synapses and the SLI terminal was suspected to be presynaptic. The unlabelled LGV terminal was often presynaptic to a dendrite, and it had characteristics similar to those observed for some primary afferents, particularly those which may contain substance P, a proposed transmitter for nociceptive C-fibers. Most of these 'triplet' complexes (SLI terminal apposing and LGV terminal synapsing onto dendrite) were found in the apical region of lamina I. The axodendritic and axosomatic serotoninergic contacts onto dorsal horn neurons may be a basis for some of the reported post-synaptic effects on dorsal horn cells of either local serotonin iontophoresis or of stimulation of the brainstem raphe, the probable origin of the serotoninergic terminals. These effects include both depression and excitation of the responses of the dorsal horn cells to electrical or natural stimulation of primary afferents, particularly C-fibers and nociceptors. Likewise, the contacts of SLI terminals with LGV terminals may provide a morphological substrate for the presynaptic effects also observed for serotonin iontophoresis or raphe stimulation, including changes in the excitability of primary afferent C-fibers.