Structural and neurochemical comparison of vagal and spinal afferent neurons projecting to the rat lung

Afferent information from the lung is conveyed both to the brainstem and to the spinal cord by primary afferent fibres originating from vagal sensory (jugular-nodose ganglion complex = JNC) and dorsal root ganglion (DRG) neurons, respectively. Most interest, so far, has been paid to the vagal pathway while much less is known about spinal afferents. Here we provide the first direct comparison of rat pulmonary spinal and vagal pulmonary afferent neurons with respect to structural (soma size) and two neurochemical characteristics (binding of lectin IB4, immunoreactivity to calcitonin gene-related peptide = CGRP). After retrograde labelling from the lung, all possible combinations of CGRP-immunoreactivity and IB4-binding were observed, and the neurochemically defined subpopulations occurred in the same order of frequency in DRG and JNC: (1) IB4−/CGRP+ (DRG: 48%, JNC: 47%); (2) IB4−/CGRP− (DRG: 35%, JNC: 29%); (3) IB4+/CGRP+ (DRG: 12%, JNC: 21%) and (4) IB4+/CGRP− (DRG: 5%, JNC: 3%). In the IB4−/CGRP− population, pulmonary DRG neurons were slightly, but significantly larger than those in JNC (mean diameter: 33 μm versus 30 μm). This group is likely to contain slowly and rapidly adapting mechanoreceptors, which may be differently distributed among rat vagal and spinal afferent pathways. In rat DRG, labelling patterns IB4−/CGRP+, IB4+/CGRP+ and IB4+/CGRP− are generally characteristic for different nociceptor subtypes. With respect to these features and soma size, no further distinction between spinal and vagal afferents became obvious, although this does not exclude elicitation of entirely different responses when these pathways are stimulated.