Transganglionic labelling of primary sensory afferents in the rat lumbar spinal cord: comparison between wheatgerm agglutinin and the I-B4 isolectin from Bandeiraea simplicifolia.

We recently reported that the I-B4 isolectin from Bandeiraea simplicifolia could be used as a transganglionic neuronal tracer which appears to be selective for unmyelinated cutaneous afferents (C fibres) and their terminals in the superficial dorsal horn. As terminals in the superficial dorsal horn are also labelled by wheatgerm agglutinin, we sought to compare these two neuronal tracers. Three days after the injection of 1% wheatgerm agglutinin-HRP or 1% BSI-B4-HRP into the sciatic nerve of adult rats the lumbar spinal cord was processed for HRP reactivity. The majority of labelled structures was found in the superficial dorsal horn, with fewer labelled structures seen in the overlying white matter (including Lissauer's tract). In wheatgerm agglutinin-HRP experiments most labelled structures were synaptic terminals (63%) and unmyelinated axons (32%). About 3% of wheatgerm agglutinin-HRP-labelled structures were fine myelinated fibres (which were found only in lamina I and outer lamina II) and about 2% of label was located in neuronal somata. In contrast, label from BSI-B4-HRP experiments was found only in synaptic terminals (37%) and unmyelinated axons (63%). Previous studies have shown that small diameter dorsal root ganglion neurons and their terminals in the superficial dorsal horn express a range of structurally related carbohydrates that contain binding sites for BSI-B4 or wheatgerm agglutinin or both. Comparison of the labelling patterns produced by the two transganglionic tracers in the present study suggests that unmyelinated sciatic afferents express wheatgerm agglutinin and BSI-B4 binding sites, but some thin myelinated afferents, and a distinct form of synaptic terminal in lamina I/II outer, express the wheatgerm agglutinin binding site and not the BSI-B4 binding site.