Functional NK-1 (substance P) receptors have been demonstrated previously on astrocytes
from primary newborn rat brain cultures and human astrocytoma cells lines by specific
[125I]-Bolton Hunter substance P (SP) binding and by SP-induced phosphoinositol turnover.
In addition, these cells have been shown to release cytokines upon stimulation with
interleukin-1 (IL-1) and lipopolysaccharide (LPS). Since SP has also been shown to
induce cytokine release from rat glial cells, this neuropeptide may contribute to
the pathophysiology of neuronal inflammation in humans by stimulating cytokine production
in the brain. We, therefore, explored whether SP could induce U-373 MG human astrocytoma
cells, via specific NK-1 receptor activation, to secrete interleukin-6 (IL-6), a cytokine
implicated as a key mediator of immune and inflammatory responses. SP stimulated IL-6
production in a concentration-dependent manner with an MC50 (concentration inducing
50% of the maximum response) of 45 nM. IL-6 was detected in the cell culture supernatant
fluids 2 h post stimulation and secretion peaked at 12 h. SP induced IL-6 secretion
was not mediated by IL-1 since neutralizing anti-IL-1 (alpha and beta) antibody treatment
had no effect on the SP response. The selective NK-1 receptor agonist, [Sar9, Met(O2)11]-SP,
was comparably effective to SP in stimulating IL-6 secretion; however, selective NK-2
and NK-3 receptor agonists were 250-500-fold less effective. In addition, the non-peptide
NK-1 receptor antagonist, (+/-)CP-96,345, inhibited SP (Ki = 4 nM), but not IL-1-induced
IL-6 release. These selectivity and specificity studies confirmed the presence of
functional NK-1 type receptors linked to IL-6 release. The results of this study support
a role for SP as a modulator of immune and/or inflammatory processes in the human
CNS.