Substance P enhances the secretion of tumor necrosis factor-alpha from neuroglial cells stimulated with lipopolysaccharide.

The neuropeptide, substance P (SP), can stimulate secretion of TNF-alpha from macrophages. Neuroglia have SP receptors and subserve various macrophage-like functions in the central nervous system. We investigated whether SP stimulates secretion of TNF-alpha from primary cultures of neuroglial cells containing both astrocytes (approximately 90%) and microglia (approximately 10%). SP alone had no effect; however in the presence of LPS (10 ng/ml), SP (1 to 10 nM) caused a dose-dependent increase in TNF-alpha secretion above the level measured in response to LPS alone. The effective doses of SP correlated with 125I-labeled Bolton Hunter-conjugated SP binding (Kd 0.2 nM) to these cultures. Incubation with LPS did not change the number or affinity of SP-binding sites. In cultures enriched for microglia (> 99% pure), LPS stimulated the secretion of TNF-alpha but SP caused no enhancement. Microglia have no detectable 125I-labeled Bolton-Hunter-conjugated SP binding sites in the presence or absence of LPS. These results indicate that the action of SP is mediated through astrocytes. We investigated whether IL-1 mediates the SP enhancement of TNF-alpha secretion. Addition of IL-1-neutralizing antisera to mixed cultures stimulated with both LPS and 10 nM SP decreased TNF-alpha secretion to the level observed with LPS alone. LPS alone stimulated the secretion of IL-1 in a dose-dependent manner in the primary cultures, and this LPS-mediated IL-1 secretion was enhanced by SP. This enhancement was not observed in microglial cultures. SP may therefore play a role in neuropathologies in which these cytokines have been implicated.