Amitriptyline does not block the action of ATP at human P2X4 receptor

Background and purpose: Amitriptyline is a tricyclic antidepressant that is also widely used to treat neuropathic pain in humans, but the mechanism of this anti-hyperalgesic effect is unknown. Microglia in the mouse spinal cord become activated in neuropathic pain, and expression of P2X4 receptors by these microglia is increased. Antisense RNA targeting P2X4 receptors suppresses the development of tactile allodynia in rats. This suggests that blockade of P2X4 receptors might be the mechanism by which amitriptyline relieves neuropathic pain. Experimental approach: We expressed human, rat and mouse P2X receptors (P2X2, P2X4, P2X7) in human embryonic kidney cells and evoked inward currents by applying ATP. We compared the action of ATP on control cells and cells treated with amitriptyline. Key results: Amitriptyline (10 μM), either applied acutely or by pre-incubation for 2-6 h, had no effect on inward currents evoked by ATP (0.3-100 μM) at human P2X4 receptors. At rat and mouse receptors, amitriptyline (10 μM) caused a modest reduction in the maximum responses to ATP, without changes in EC 50 values, but it had no effect at 1 μM. Amitriptyline also had no effects on currents evoked by ATP at rat P2X2 receptors, or at rat or human P2X7 receptors. Conclusion and implications: The results do not support the view that amitriptyline owes its pain-relieving actions in man to the direct blockade of P2X4 receptors. © 2010 The British Pharmacological Society.