Inhibitory effects of tramadol on nicotinic acetylcholine receptors in adrenal chromaffin cells and in Xenopus oocytes expressing alpha 7 receptors.

1. Tramadol has been used clinically as an analgesic; however, the mechanism of its analgesic effects is still unknown. 2. We used bovine adrenal chromaffin cells to investigate effects of tramadol on catecholamine secretion, nicotine-induced cytosolic Ca(2+) concentration ([Ca(2+)](i)) increases and membrane current changes. We also investigated effects of tramadol on alpha7 nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. 3. Tramadol concentration-dependently suppressed carbachol-induced catecholamine secretion to 60% and 27% of the control at the concentration of 10 and 100 microM, respectively, whereas it had little effect on veratridine- or high K(+)-induced catecholamine secretion. 4. Tramadol also suppressed nicotine-induced ([Ca(2+)](i)) increases in a concentration-dependent manner. Tramadol inhibited nicotine-induced inward currents, and the inhibition was unaffected by the opioid receptor antagonist naloxone. 5. Tramadol inhibited nicotinic currents carried by alpha7 receptors expressed in Xenopus oocytes. 6. Tramadol inhibited both alpha-bungarotoxin-sensitive and -insensitive nicotinic currents in bovine adrenal chromaffin cells. 7. In conclusion, tramadol inhibits catecholamine secretion partly by inhibiting nicotinic AChR functions in a naloxone-insensitive manner and alpha7 receptors are one of those inhibited by tramadol.