We investigated the effects of neomycin on nicotinic acetylcholine receptor-induced responses in bovine adrenal chromaffin cells. Neomycin inhibited the nicotinic agonist dimethylphenylpiperazinium iodide (DMPP)-induced norepinephrine secretion in a concentration-dependent manner. Neomycin had also an inhibitory effect on the DMPP-induced increase in cytosolic Ca++ concentration ([Ca++]i). This effect was further confirmed by inhibition of the DMPP-induced fluorescence quenching of fura-2 upon Mn++ entry. Under the same conditions, however, neomycin did not change the bradykinin-induced [Ca++]i increase, which follows the downstream signal of phospholipase C phospholipase C activation in this cell. The inhibitory effect of neomycin on the DMPP-induced [Ca++]i increase was apparent when the neomycin treatment was performed simultaneously with DMPP, suggesting a direct action on the nicotinic receptor. The direct inhibitory action of neomycin on the nicotinic receptor was also evident when neomycin inhibited the DMPP-induced cytosolic Ca++ increase, which is not affected by nifedipine nor omega-conotoxin MVIIC, and the cytosolic Na+ increase, which is not affected by tetrodotoxin. In addition, we observed that neomycin inhibited the binding of nicotine to the acetylcholine receptor in a noncompetitive manner. The data suggest that neomycin inhibits the nicotinic acetylcholine receptor directly, which results in blockage of the nicotinic receptor-mediated signaling without involvement of phospholipase C.