The Mechanism of Action of Cytokines to Control the Release of Hypothalamic and Pituitary Hormones in Infection

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Abstract
Abstract: During infection, bacterial and viral products, such as bacterial lipopolysaccharide (LPS), cause the release of cytokines from immune cells. These cytokines can reach the brain by several routes. Furthermore, cytokines, such as interleukin‐1 (IL‐1), are induced in neurons within the brain by systemic injection of LPS. These cytokines determine the pattern of hypothalamic‐pituitary secretion that characterizes infection. IL‐2, by stimulation of cholinergic neurons, activates neural nitric oxide synthase (nNOS). The nitric oxide (NO) released diffuses into corticotropin‐releasing hormone (CRH)‐secreting neurons and releases CRH. IL‐2 also acts in the pituitary to stimulate adrenocorticotropic hormone (ACTH) secretion. On the other hand, IL‐1α blocks the NO‐induced release of luteinizing hormone‐releasing hormone (LHRH) from LHRH neurons, thereby blocking pulsatile LH but not follicle‐stimulating hormone (FSH) release and also inhibiting sex behavior that is induced by LHRH. IL‐1α and granulocyte macrophage colony‐stimulating factor (GMCSF) block the response of the LHRH terminals to NO. The mechanism of action of GMCSF to inhibit LHRH release is as follows. It acts on its receptors on γ‐aminobutyric acid (GABA)ergic neurons to stimulate GABA release. GABA acts on GABAa receptors on the LHRH neuronal terminal to block NOergic stimulation of LHRH release. IL‐1α inhibits growth hormone (GH) release by inhibiting GH‐releasing hormone (GHRH) release, which is mediated by NO, and stimulating somatostatin release, also mediated by NO. IL‐1α‐induced stimulation of PRL release is also mediated by intra‐hypothlamic action of NO, which inhibits release of the PRL‐inhibiting hormone dopamine. The actions of NO are brought about by its combined activation of guanylate cyclase‐liberating cyclic guanosine monophosphate (cGMP) and activation of cyclooxygenase (COX) and lipoxygenase (LOX) with liberation of prostaglandin E2 and leukotrienes, respectively. Thus, NO plays a key role in inducing the changes in release of hypothalamic peptides induced in infection by cytokines. Cytokines, such as IL‐1β, also act in the anterior pituitary gland, at least in part via induction of inducible NOS. The NO produced inhibits release of ACTH. The adipocyte hormone leptin, a member of the cytokine family, has largely opposite actions to those of the proinflammatory cytokines, stimulating the release of FSHRF and LHRH from the hypothalamus and FSH and LH from the pituitary directly by NO.