The sympathetic nervous response in inflammation

Over the past decades evidence has accumulated clearly demonstrating a pivotal role for the sympathetic nervous system (SNS) and its neurotransmitters in regulating inflammation. The first part of this review provides the reader with an overview showing that the interaction of the SNS with the immune system to control inflammation is strongly context-dependent (for example, depending on the activation state of the immune cell or neuro-transmitter concentration). In the second part we focus on autoimmune arthritis as a well investigated example for sympathetically controlled inflammation to show that the SNS and catecholamines play a differential role depending on the time point of ongoing disease. A model will be developed to explain the proinflammatory effects of the SNS in the early phase and the anti-inflammatory effects of catecholamines in the later phase of autoimmune arthritis. In the final part, a conceptual framework is discussed that shows that a major purpose of increased SNS activity is nourishment of a continuously activated immune system at a systemic level using energy-rich fuels (glucose, amino acids, lipids), while uncoupling from central nervous regulation occurs at sites of inflammation by repulsion of sympathetic fibers and local adrenoceptor regulation. This creates zones of ‘permitted local inflammation’. However, if this ‘inflammatory configuration’ persists and is strong, as in autoimmunity, the effects are detrimental because of the resultant chronic catabolic state, leading to cachexia, high blood pressure, insulin resistance, and increased cardiovascular mortality, and so on. Today, the challenge is to translate this conceptual knowledge into clinical benefit.