Relationship of substance P to afferent characteristics of dorsal root ganglion neurones in guinea-pig

Differences in expression of oligosaccharides, neuropeptides, carbonic anhydrase and neurofilament in rat primary afferent neurons retrogradely labelled via skin, muscle or visceral nerves

Dorsal root ganglion neurons innervating skin via the saphenous nerve, muscle via the gastrocnemius nerve and viscera via the splanchnic nerve, were identified by retrograde tracing with Fast Blue applied to the cut nerve. Only neuronal profiles with nuclei were counted. At the survival times used no changes in immunohistochemical labelling patterns were detectable in the axotomized neurons. Percentages of Fast Blue-labelled neuronal profiles that were immunolabelled were calculated. The values for markers of carbohydrate groups were for skin, muscle and viscera, respectively: the lectin peanut agglutinin 55%, 24%, and 50%; the lectin soybean agglutinin 72%, 56%, 61%; the antibody 2C5 (against lactoseries groups) 43%, 20%, 6%; the antibodies SSEA-4 (against globoseries groups) 6%, 12%, 0% and SSEA-3 (against globoseries groups) 6%, 5%, 0%. The values for neurofilament rich profiles were for skin, muscle and viscera, respectively: 34%, 43%, 19%, and for carbonic anhydrase were 10%, 33%, 2%. Values for neuropeptides were, for calcitonin gene-related peptide 51%, 70%, 99%, for substance P 21%, 51%, 82%, and for somatostatin 10%, 2% and 0%. The population of skin afferents therefore contained the highest proportion of profiles expressing galactose containing carbohydrate groups labelled by 2C5 and the lectins and the highest proportion of cells with somatostatin. In contrast they had the lowest proportions of cells with calcitonin gene-related peptide and substance P, compared with the other tissues. Muscle afferents had the highest proportions compared with the other tissues of the neurofilament-rich, carbonic anhydrase-positive and SSEA-4-labelled profiles, but the lowest proportions of profiles with lectin binding. The splanchnic visceral afferents had the highest proportions, compared with the other tissues, of neuronal profiles labelled for calcitonin gene-related peptide and substance P, but the lowest proportions of neurofilament rich profiles and of profiles with carbonic anhydrase or 2C5 labelling and they totally lacked any labelling for globoseries carbohydrates and somatostatin. Both the muscle and skin afferent populations had clear small cell and large cell peaks in their size distributions, with the small cell peak being larger for skin than muscle afferents and the large cell peak being more marked for muscle afferents. The visceral afferent profiles had a unimodal size distribution with the peak size being between the small and large cell peaks of the somatic afferent units. This study therefore shows that the patterns of immunohistochemical labelling and cell size of primary afferent neurons differ according to their peripheral target tissue.

Primary afferent tachykinins are required to experience moderate to intense pain

The excitatory neurotransmitter glutamate coexists with the peptide known as substance P in primary afferents that respond to painful stimulation. Because blockers of glutamate receptors reliably reduce pain behaviour, it is assumed that 'pain' messages are mediated by glutamate action on dorsal horn neurons. The contribution of substance P, however, is still unclear. We have now disrupted the mouse preprotachykinin A gene (PPT-A), which encodes substance P and a related tachykinin, neurokinin A. We find that although the behavioural response to mildly painful stimuli is intact in these mice, the response to moderate to intense pain is significantly reduced. Neurogenic inflammation, which results from peripheral release of substance P and neurokinin A, is almost absent in the mutant mice. We conclude that the release of tachykinins from primary afferent pain-sensing receptors (nociceptors) is required to produce moderate to intense pain.

Substance P and nociceptive afferent neurones

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