Methylmercury (MeHg) is an environmental toxicant that is known to induce lymphocyte apoptosis; however, little is known about the molecular mechanism involved. Data showed that MOLT-3 cells were more sensitive to MeHg-induced cytotoxic effects than Jurkat clone E6-1 cells, suggesting that the lymphocytic muscarinic cholinergic system may be involved since the expressions of five subtypes (M1-M5) of muscarinic acetylcholine receptor (mAChR) in MOLT-3 cells are higher than in Jurkat cells. The role of mAChR-linked pathways in MeHg-induced apoptosis in human leukemic T cells was examined in this study. Treatment of the MOLT-3 cells with 1 microM MeHg produced induction of c-Fos expression, apoptotic cell death, and downregulation of mAChR. MeHg-induced c-Fos expression was significantly reduced by pretreatment with atropine (a nonselective mAChR antagonist), or 4-DAMP (a selective M1/M3 mAChR antagonist), whereas pirenzipine (a selective M1 mAChR antagonist) or himbazine (a selective M2/M4 mAChR antagonist) did not reduce this induction, suggesting that MeHg-induced c-Fos expression through the activation of the mAChR, at least M3 subtype, is involved. Pretreatment with 4-DAMP or SB 203580 (a specific p38 inhibitor) resulted in decreases in the level of phosphorylated p38, c-Fos expression, and apoptotic cell death induced by MeHg. Taken together, these data suggest that the mAChR-p38-dependent pathway participates in the increase of c-Fos expression, which is involved in MeHg-induced lymphocyte apoptosis. In addition, a noncytotoxic concentration of MeHg (0.1 microM) inhibited PHA/PMA-stimulated interleukin (IL)-2 production, and this inhibition was reversed by pretreatment with atropine or 4-DAMP. Overall, this study provides initial evidence that MeHg may alter the immune system by targeting the lymphocytic mAChR.