SEROTONIN 5HT(2A) RECEPTOR ACTIVATION INHIBITS INDUCIBLE NITRIC-OXIDESYNTHASE ACTIVITY IN C6 GLIOMA-CELLS

C6-glioma cells endogenously express both 5HT(2A) receptors and induci ble nitric oxide synthase (iNOS) iNOS can be induced by transcriptiona l activation to produce nitric oxide (NO) in response to a challenge w ith lipopolysaccharide (LPS) Experiments were conducted to determine w hether 5HT(2A) receptor activation could modify the production of NO i n response to LPS. Incubation of 10 mu g/ml LPS with C6-glioma cells f or a period of 24 hours resulted in a 2.6 fold increase in nitrite lev els, as a measure of NO levels, over vehicle treated controls. Co-incu bation with the selective 5HT(2A) receptor partial agonist (+/-)-2,5-d imethoxy-4-iodoamphetamine (DOI) produced a dose-dependent inhibition of the LPS-induced nitrite levels of 22% with an IC50 of 16nM. The ful l agonists serotonin (5HT) and alpha-methyl-5HT produced an inhibition of approximately 30% at a concentration of 1 mu M. The inhibitory eff ect of 1 mu M DOI was blocked by the 5HT(2A) receptor antagonists spip erone and ritanserin (10nM) Inhibition of protein kinase C (PKC) using 100nM chelerythrine prevented the DOI-mediated decrease in LPS-induce d 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 activatio n, most likely at the initiation of the induction process, via PKC. We therefore suggest that there may be a link between the serotonergic s ystem and NO-mediated immune responses in the brain.