Neurogenic inflammation, vascular permeability, and mast cells.

Electrical stimulation (ES) of sensory nerves causes increased vascular permeability and vasodilatation, a process known as neurogenic inflammation. The purpose of this study was to assess the role of mast cells in neurogenic inflammation induced by ES of sensory nerves. ES of the rat saphenous nerve for 1, 3, 5, 15, or 30 min induced a 166 to 436% increase in the amount of 125I-albumin deposited in the skin. Through the initial 15 min of ES, the histamine content of the skin remained unchanged. However, 30 min of ES caused a 22.1% decrease in skin histamine (p less than 0.05). ES for 5 min followed by measurement of vascular permeability from 0 to 30 min thereafter resulted in maximal increases in 125I-albumin in the skin immediately after cessation of the pulse of ES. When skin histamine was measured at various intervals after a 5-min pulse of ES, no change in the histamine content was observed through the subsequent 30 min. When mast cell degranulation was assessed histologically, 5 min of ES failed to stimulate mast cell degranulation. However, 30 min of ES caused a significant increase in the proportion of degranulating mast cells. When draining venous plasma histamine was monitored before, during and after ES, no change in plasma histamine was observed. In contrast, the intradermal injection of 5 micrograms of compound 48/80 produced a significant increase in plasma histamine. In order to examine the possibility that histamine might be released but remain in the skin after ES, skin "blisters" were developed by intradermal injections of saline. There was a significant increase in the amount of 125I-albumin extravasated into blister fluid measured after 3, 5, and 10 min of ES and a significant increase in histamine after 5 or 10 min. Therefore, prolonged ES of sensory nerves can cause mast cell degranulation. However, ES causes increased vascular permeability at times when no mast cell activation can be observed. These data suggest that the initial phases of neurogenic inflammation are independent of mast cell activation.