Myelinated and unmyelinated primary afferent axons form contacts with cholinergic interneurons in the spinal dorsal horn.

Cholinergic interneurons in laminae III/IV of the dorsal horn contain co-localised gamma-aminobutyric acid (GABA) and frequently form axoaxonic synapses with terminals of primary afferents. They are therefore probably last-order interneurons involved in presynaptic inhibition. The purpose of the present investigation was to determine if these cells receive direct input from primary afferents. Relationships between primary afferents and interneurons were investigated in adult rats. Myelinated primary afferents were labelled with the B-subunit of cholera toxin (CTb). Unmyelinated afferents were labelled with IB4 lectin and an antibody to identify calcitonin-gene-related peptide (CGRP). Cholinergic neurons were labelled with an antibody raised against choline acetyltransferase and examined with a confocal microscope. Cells were reconstructed with NeuroLucida for Confocal and afferent contacts plotted. Interneurons (N=30) received an average of 20.2+/-11.9 (SD) contacts from CTb-labelled primary afferents, which were preferentially distributed on proximal and intermediate dendrites. Interneurons with dendrites which extended into lamina II (N=20) received an average of 27.4+/-19.0 IB4 contacts (on intermediate and distal dendrites) and 9.2+/-6.8 CGRP contacts. It is concluded that cholinergic interneurons receive contacts from both myelinated and unmyelinated primary afferents and different classes of afferent target particular dendritic domains. Cholinergic interneurons are likely to be components of an inhibitory feedback pathway that is monosynaptically activated by primary afferents.