Choline acetyltransferase-immunoreactive profiles are presynaptic to primary sensory fibers in the rat superficial dorsal horn.

The specific aim of this study was to search for morphological counterparts to the known antinociceptive effects of cholinomimetic drugs at the spinal cord level. For this, the light microscopic and ultrastructural distribution of choline acetyltransferase immunoreactivity was studied in laminae I-III of the rat cervical spinal cord. Immunoreactivity was present in cell bodies in lamina III, and in dendrites and axons of all three laminae. Immunoreactive axonal varicosities were often presynaptic to the central varicosities of type II synaptic glomeruli in lamina II and lamina III, less often presynaptic to the central elements of type I glomeruli in lamina II, and often presynaptic to dendrites in both type I and type II glomeruli. In addition, immunoreactive dendrites were often postsynaptic to the central varicosities of glomeruli of all morphological types. These results indicate that 1) primary sensory fibers excite cholinergic interneurons; 2) the acetylcholine released by the axon terminals of these interneurons modulates both nociceptive and non-nociceptive sensory information at the spinal cord level through both pre- and postsynaptic mechanisms. Furthermore, our results reinforce current ideas on reciprocal sensory interaction between thick and fine afferent fibers.