Kathleen B. English

Substance P axons and sensory threshold increase in burn-graft human skin

BACKGROUND Our knowledge of afferent nerve fiber reinnervation of grafted skin following third-degree burn is limited by a lack of quantitative histological and psychophysical assessment from the same cutaneous area. The current study compares fiber profile and functional recovery measurements in injured and control skin from the same subject. MATERIALS AND METHODS Nerve regeneration and modality-specific sensory thresholds were compared using immunocytochemical labeling with protein gene product 9.5 antibody to stain all axons and anti-substance P to label substance P axons (which are predominantly unmyelinated), as well as computerized instrumentation to obtain psychophysical estimates. RESULTS Compared to control skin, threshold measures of pinprick (P < 0.001), warming (P < 0.001), touch (P < 0.001), and vibration (P < 0.01) were significantly elevated in burn-graft skin and correlated with histological analysis of skin biopsies obtained from the same site. Immunohistochemical staining of all axons innervating the dermis and epidermis revealed a significant reduction in burn-graft relative to control skin (54% decrease, P < 0.0001). In contrast, the incidence of substance P nerve fibers was significantly elevated in burn-graft (177% increase, P < 0.05) and appeared to correlate with patient reports of pruritus and pain. CONCLUSIONS Observations support the hypothesis that sensory regeneration is fiber-size-dependent in burn-graft skin. The findings that substance P fiber growth increased while total fiber count decreased and that thermal threshold showed the greatest degree of functional recovery suggest that unmyelinated neurons have the greater ability to transverse scar tissue and reinnervate grafted skin following third-degree burn injury.

5-HT2 and 3 receptor antagonists suppress the response of rat type I slowly adapting mechanoreceptor: an in vitro study.

Previous experiments have shown an increase in rat type I mechanoreceptor responsiveness during arterial serotonin (5-hydroxytryptamine) infusion and the presence of serotonin immunostaining in Merkel cells. The current findings demonstrate that the 5-HT(2) antagonists ritanserin and ketanserin, as well as the 5-HT(3) antagonist MDL 72222, reduce type I response to a standardized mechanical stimulus in an in vitro skin preparation. In addition, ritanserin blocked the enhancement of type I response produced by 5-HT. These experiments suggest that serotonin is released during mechanical distortion of the Merkel cell membrane and alters action potential generation by the type I ending. In addition, it is possible that serotonin, released from outside the type I complex, influences mechanoreceptor responsiveness. For example, serotonin generated during inflammatory events could enhance type I response to mechanical stimulation and thereby increase symptoms of mechanical allodynia.

Primary culture of cells arising from a neuroendocrine skin carcinoma

A carcinoma arising in the skin of the lip metastasized to the lymph nodes in the neck of a 53-year-old white man. Electron microscopy of the initial excisional biopsy specimen revealed that the tumor cells contained dense-cored vesicles (100 nm in diameter) in their cytoplasm and were joined by simple junctions. Cells from the nodal metastases were found to be immunoreactive for neuronspecific enolase, keratin intermediate filaments, and chromogranin A, but not for neurofilaments. The tumor was thus classified as a neuroendocrine skin carcinoma. In addition, its metastatic cells shared immunoreactive and ultrastructural characteristics of Merkel cells, which are situated in the basal epidermis of normal skin. Primary cultures from a nodal metastasis were established and characterized. The cells attached and proliferated on culture flask surfaces. The population-doubling time was 2 days. This is the first report where cells from a neuroendocrine skin carcinoma have been demonstrated to retain their characteristic ultrastructure in an in vitro environment (10 days). Studies of cells cultured from neuroendocrine skin carcinomas may prove useful in understanding the pathobiology of this disease and help define the in vitro growth requirements of nontransformed Merkel cells as well.

Chemosensitivity of the Rat Type I Slowly Adapting Mechanoreceptor

An in vitro lateral thoracic skin preparation of the adult rat was used to test the effect of serotonin (5, 50, 500 μM) and control solutions on the response of the type I slowly adapting mechanoreceptor to a standard mechanical stimulus. Serotonin (5-HT) significantly increased the magnitude of the type I response to mechanical indentation: 50 μM 5-HT infusion enhanced responsiveness more effectively than 5 μM 5-HT. In the absence of mechanical stimulation, little or no change in spontaneous discharge relative to control was observed, and recovery to baseline levels occurred within three stimulus trials. In vitro and in vivo control experiments showed no statistically significant change in responsiveness over a similar number of stimulus cycles. It was concluded that 5-HT modulates, but does not activate the rat type I receptor or alter its ability to encode the depth and/or velocity of mechanical displacement.