SEROTONIN 5HT 2A RECEPTOR ACTIVATION INHIBITS INDUCIBLE NITRIC OXIDE SYNTHASE ACTIVITY IN C6 GLIOMA CELLS

C6-glioma cells endogenously express both 5HT 2A receptors and inducible nitric oxide synthase (iNOS). iNOS can be induced by transcriptional activation to produce nitric oxide (NO) in response to a challenge with lipopolysaccharide (LPS). Experiments were conducted to determine whether 5HT 2A receptor activation could modify the production of NO in response to LPS. Incubation of 10μg/ml LPS with C6-glioma cells for a period of 24 hours resulted in a 2.6 fold increase in nitrite levels, as a measure of NO levels, over vehicle treated controls. Co-incubation with the selective 5HT 2A receptor partial agonist (±) -2,5-dimethoxy-4-iodoamphetamine (DOI) produced a dose-dependent inhibition of the LPS-induced nitrite levels of 22% with an IC 50 of 16nM. The full agonists serotonin (5HT) and α-methyl-5HT produced an inhibition of approximately 30% at a concentration of 1μM. The inhibitory effect of 1μM DOI was blocked by the 5HT 2A receptor antagonists spiperone and ritanserin (10nM). Inhibition of protein kinase C (PKC) using 100nM chelerythrine prevented the DOI-mediated decrease in LPS-induced nitrite levels. Addition of DOI to the cells after 1hr following the LPS addition did not produce a decrease in nitrite levels indicating iNOS was not modified post-translationally. The data demonstrate that iNOS activity can be modulated by serotonin 5HT 2A receptor activation, most likely at the initiation of the induction process, via PKC. We therefore suggest that there may be a link between the serotonergic system and NO-mediated immune responses in the brain.