Cutaneous lesions in capsaicin-pretreated rats. A trophic role of capsaicin-sensitive afferents?

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Abstract
1. The time course and regional distribution of ‘spontaneous’ cutaneous lesions in rats desensitized to capsaicin as newborns was correlated to behavioural observations and regional distribution of substance P-like immunoreactivity (SP-LI) and tachykinin-like immunoreactivity (TK-LI) in various skin areas. 2. ‘Spontaneous’ skin lesions in the form of wounds, scabs and areas of alopecia were observed in 80–90% of rats desensitized to capsaicin. No major sex-related differences were observed with regard to incidence and distribution of the lesions with the possible exception of a lesser tendency to bilateral lesions in female rats. 3. ‘Spontaneous’ skin lesions were almost restricted to the head: the areas most frequently affected were snouts, periocular and retroauricular regions and ventral area of the neck. 4. No major differences were observed between capsaicin- or vehicle-treated animals in spontaneous or novelty-induced grooming as well as in open-field gross behaviour. Likewise, no differences were observed in the mouse-killing behaviour. 5. Both SP-LI and TK-LI in various skin areas were significantly reduced by systemic capsaicin pretreatment. The rank order of various skin areas for SP-LI or TK-LI levels was: snouts > thigh > neck > abdomen ≃ retroauricular region. 6. Intradermal injection of Arg-neurokinin B, a potent and water soluble derivative of neurokinin B, produced a similar plasma extravasation (Evans blue leakage technique) in the skin of vehicle- or capsaicin-pretreated rats. 7. In capsaicin-desensitized rats fur regrowth (measured at abdominal level, 28 days after shaving) was significantly less than in vehicle-treated animals. 8. The s. c. injection of 1 N HCl in the dorsal thoracic region (an area devoid of ‘spontaneous’ lesions in capsaicin-desensitized animals) produced cutaneous ulcers whose area and depth were greater in capsaicin- than vehicle-treated rats. 9. These findings are consistent with the hypothesis that capsaicin-sensitive nerves play a trophic role in the rat skin and contribute to its ability to react and repair injuries. The most consistent explanation for the restricted localization of ‘spontaneous’ skin lesions to the head seems to be that ‘normal’ injurious factors (such as grooming) operate on a distrophic skin to induce lesions by repeated microtrauma.