Background: 

The OVLT is an ideal location at which circulating pyrogens may act to communicate with the brain during a systemic immune challenge. Its dense vasculature and fenestrated capillaries allow direct access of these pyrogens to brain tissue located in close vicinity to the thermoregulatory structures of the preoptic area without impediment of the blood-brain barrier. We thus aimed to detect direct cellular responses within the OVLT to exposure with the exogenous pyrogens lipopolysaccharide (LPS) and fibroblast-stimulating lipopeptide-1 (FSL-1), or the endogenous pyrogens tumor necrosis factor (TNF), interleukin-1 (IL-1) and IL-6. For all these substances the respective receptors are present in this specialized brain structure.

Methods: 

To study specific pyrogen-mediated cellular activation, a primary micro-culture of the OVLT was established from topographically excised brain tissue of 5–6 days old rat pups. The Fura-2 ratio imaging technique was used to quantify pyrogen-induced calcium signaling in OVLT micro-culture chambers superfused with the respective substance. Subsequent immunolabeling with antisera directed against cell-specific marker proteins allowed identification of the phenotype of a given responsive cell. Additionally, concentrations of TNF and IL-6 were determined in the supernatants of LPS-stimulated OVLT-cultures.

Results: 

LPS caused fast, transient rises in intracellular calcium concentrations in >16% of neurons, 7% of astrocytes and >3% of microglial cells investigated. There were no cellular responses to stimulation with FSL-1. TNF evoked calcium signals in 11% of neurons, 22% of astrocytes and 5% of microglial cells tested. A considerable population of neurons (11%) and less than 3% of astrocytes and microglial cells responded to IL-6. About 10% of microglial cells and 4–5% of astrocytes and neurons were activated by IL-1. Incubation with LPS caused a time-dependent release of substantial amounts of TNF and IL-6 into the supernatants of the OVLT micro-cultures.

Conclusions: 

The demonstration of direct cellular responses of OVLT-intrinsic cells to stimulations with LPS or cytokines reinforces the suggested role of this brain structure as true sensor for circulating endogenous and even exogenous pyrogens. Compared to other brain sites with an incomplete blood-brain barrier the considerable populations of neurons responsive to LPS and IL-6 makes the OVLT unique among these specialized central nervous structures. Due to its sensitivity to LPS, the OVLT might thus be involved in a rapid induction of brain-controlled illness responses including fever prior to formation of endogenous pyrogens (cytokines). The responsiveness to IL-6, which is circulating in increased amounts and for prolonged periods under inflammatory conditions, may participate in the manifestation of delayed sickness symptoms in vivo. Supported by the DFG (GE 649/6–1).