Substance P-induced nerve growth factor release is down-regulated by serotonin in serum-free cultured osteoarthritis macrophage-like synovial cells

Abstract Purpose Pain-induced neurogenic inflammation modulates osteoarthritis activity by neuron-derived molecules such as neurokinine A, substance P (SP), and serotonin (5-hydroxytryptamine, 5-HT). SP is regulated by nerve growth factor (NGF), however, the combinatory effect of SP, 5-HT and IL-1s on NGF expression is unknown in osteoarthritis. The objective of this study was to examine the involvement of these transmitters on NGF overexpression as a major inductor of neurogenic inflammation. Methods Serum-free cultured macrophage-like synovial cells from 5 OA patients were stimulated with SP, 5-HT and interleukin lβ (IL-1β), and supernatants were analysed for NGF. Results SP at a concentration of 200 ng/ml stimulated the NGF release (mean ± SEM) by 3.1-fold ± 0.90 (P = 0.04) in the supernatant but not at 50 ng/ml. 10 μM 5-ΗΤ or 200 IU/ml IL-1β alone did not stimulate NGF significantly (1.6-fold ± 0.20, P = 0.2 and 1.5-fold ± 0.22, P = 1.2). However, 50 ng/ml SP + 200 IU/ml IL-1β increased NGF concentrations as combinatory effect by 2.5-fold ± 0.37 (P = 0.02). In contrast, NGF dropped to 0.3-fold ± 0.03 (P = 0.0009) in response 10 μM 5-ΗΤ + 50 ng/ml SP. The triple combination of 10 μM 5-HT + 200 IU IL-1β/ml + 50 ng SP/ml abolished 5-ΗΤ-induced NGF suppression (1.8-fold ± 0.37, P = 0.01)⋅ Conclusion SP might trigger synovitis via a self-perpetuating feedback loop in OA pain. The SP-mediated response is down regulated by a combination of 5-HT. The synergistic or antagonistic effect of SP, 5-HT and IL-1β should be further investigated.