Nonsynaptic Noradrenaline Release in Neuro-Immune Responses

Evidence has recently been obtained that the branches of the autonomic nervous system, mainly, the sympathetic [25], regulate cytokine production. Not only the primary (thymus, bone marrow) and secondary (spleen, tonsils, and lymph nodes) lymphoid organs, but also many other tissues are involved in immune responses and are heavily influenced by noradrenaline (NA) derived from varicose axon terminals of the sympathetic nervous system [25, 100]. Besides NA released from nonsynaptic varicosities of noradrenergic terminals [92], circulating catecholamines (adrenaline, dopamine, NA) are also able to influence immune responses, the production of pro- and anti-inflammatory cytokines by different immune cells. The sympathetic nervous system (catecholamines) and the hypothalamic-pituitary-adrenal (HPA) axis (cortisol) are the major integrative and regulatory components of different immune responses. In our laboratory convincing evidence has been obtained that NA released non-synaptically [90, 92] from sympathetic axon terminals and enhanced in concentration in the close proximity of immune cells is able to inhibit production of proinflammatory (TNF-α, IFN-γ, IL-12, IL-1) and increase antiinflammatory cytokines (IL-10) in response to LPS [25, 91], indicating a fine-tuning control of the production of TNF-α and other cytokines by sympathetic innervation under stressful conditions. This effects are mediated via β2-adrenoceptors expressed on immune cells and coupled to cAMP levels.