Endocrine and vagal controls of sympathetically dependent neurogenic inflammation

Recently the very significant role of the postganglionic sympathetic neuron (PGSN) terminal in the production of neurogenic inflammation has been appreciated. An important model of this sympathetically dependent inflammation is venular plasma extravasation (PE) and neutrophil attraction produced by local intra-articular injection of the potent inflammatory mediator bradykinin (BK). Sympathetic-dependent PE in the synovium has been proposed as a protective mechanism in arthritis. In a recent series of studies, a novel mechanism has been discovered by which activation of primary afferent nociceptors exerts a potent feedback inhibition of PGSN-dependent PE. Activation of nociceptive afferents was shown to be involved in this feedback system. Such a negative feedback control of the acute inflammatory response would have survival value; the inflammatory response, as initiated by a high degree of positive feedback, and the inflammatory process itself when persisting can result in significant tissue injury. If indeed HPA axis activity plays a significant physiological role in the modulation of neurogenic inflammation, then physiological processes that modulate the HPA axis would be expected to influence neurogenic inflammation. A dramatic effect of this kind has been demonstrated, in the rat, for vagal afferent activity. In the presence of subdiaphragmatic (or celiac branch) vagotomy, the potency of nociceptive afferent activity to inhibit sympathetically dependent, BK-induced PE was increased by four orders of magnitude compared to vagus-intact animal. Hypoactivity or hyperactivity of these vagally mediated mechanisms could contribute to diseases characterized by either an inadequate or an exaggerated inflammatory response.