The effects of neonatal capsaicin treatment on the sensory innervation of the nipple and on the milk ejection reflex in the rat

Summary

Much of the sensory innervation of the nipple is provided by fibers of small calibre (Aδ and C). In order to determine the contribution of unmyelinated C-fibers to this innervation and to the physiology of lactation, mammary afferents were studied in rats that had succeeded in lactating after neonatal treatment with capsaicin, a neurotoxin which selectively destroys C-fibers. After subcutaneous injection of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) into the nipple of capsaicintreated lactating rats, cell counts and surface area estimates of peroxidase-labelled and unlabelled cells were made in the corresponding dorsal root ganglia (DRG) and compared to values obtained in untreated lactating females that had received similar tracer injections. The segmental distribution of HRP-labelled primary sensory neurons in the capsaicintreated rats was similar to that in untreated controls, but the number of labelled cells was significantly reduced at each segmental level. This reduction reflected a marked decrease in C-fibers, since there was a striking reduction in the number of small HRP-labelled and unlabelled cell bodies in the DRG and unmyelinated fibers in the dorsal roots. Peroxidase labelling within the dorsal horn of capsaicin-treated rats was also substantially diminished. About 40% of the females that had been treated neonatally with capsaicin gave birth and lactated. Although the average weight gain of their litters was retarded with respect to that of litters of untreated controls, the milk ejection reflex appeared to function normally. Our observations thus indicate that although C-fibers are an important component of the sensory innervation of the nipple, their destruction early in postnatal development is not sufficient to impair reflex milk ejection.