[Behavior-immunity relationship: the role of cytokines]

There are several phenomena in which the immune and the central nervous systems regulate each other. However, their mechanisms are poorly understood. Since cytokines have a central role in the regulation of the immune response, this review describes their participation in two forms of neuro-immune communication, immunomodulation by psychological stress and behavioral conditioning of immune response. The role of cytokines in the endocrine and behavioral effects of acute phase, where cytokines have an effect in functions of the central nervous system, is also reviewed. The effects of psychological stress are described as both immunosuppressing and immunoenhancing. Among them, a relevant immunosuppressing one is the reduction of IL-1, IL-2, and IFN-gamma levels. In contrast, some of the pro-inflammatory effects of stress are mediated by an increase in the levels of IL-6, IL-1, and TNF mediated by the neurotransmitter Substance P. A possible role for IL-1 and IFN-beta as possible messengers in immune regulation by behavioral conditioning is proposed. Pro-inflammatory cytokines in turn can activate the hypothalamus-pituitary-adrenal axis and induce sickness behavior during the acute phase response, during which the parasympathetic nervous system serves as pathway for their detection by the central nervous system. An account is given about recent findings on the regulation of cytokine expression by neurotransmitters from the sympathetic nervous system (epinephrine and norepinephrine), a key piece in all these mechanisms of brain-immune communication. Possible mechanisms and pathways of communication between the brain and the immune system, as well as the possible participation of other cytokines are discussed.