Contribution of the adrenal glands and splenic nerve to LPS-induced splenic cytokine production in the rat.

Both the hypothalamic pituitary adrenal axis (HPAA) and the sympathetic nervous system (SNS) can inhibit immune function and are regarded as the primary efferent pathways for neural-immune interactions. To determine if this relationship is maintained in vivo in response to an inflammatory stimulus, rats were injected intravenously (iv) with various doses of lipopolysaccharide (LPS) and splenic cytokine mRNA and protein levels were measured at several dose and time intervals post-injection. The spleen was chosen as the target organ because both the neural and hormonal inputs to the spleen can be selectively removed by splenic nerve cut (SNC) and adrenalectomy (ADX), respectively. Data from our dose response studies established that maximum levels of splenic cytokines were induced in response to relatively low doses of LPS. Minimal changes in LPS-induced splenic cytokine levels were observed in response to ADX, SNC, or a combination of the two procedures across several doses of LPS. These results suggest that there are aspects of immune regulation that are functionally removed from these central modulatory systems and that the counter-regulatory responses induced by LPS have minimal impact on the concurrent induction of cytokines by this inflammatory stimulus. The conceptual model of neural-immune regulation as an inhibitory feedback system, at least with regards to the early activational effects induced by an inflammatory stimulus, was not supported by these studies.