P2X receptors are neurotransmitter-gated ion channels that open in response to extracellular ATP. The divalent cation zinc is a modulator of the ATP response through P2X₂ homomeric receptors. Both zinc and ATP are released from synaptic vesicles in mammals, and concentrations of zinc that are likely to occur in some neuronal synapses elicit potentiation of ATP-evoked current through rat P2X₂. Previous work has shown that two histidines in the ectodomain of rat P2X₂ are required for the potentiating effect of zinc. This dissertation work improves understanding of the mechanism of zinc action at rat P2X₂, and also addresses the question of whether or not zinc modulation of P2X₂ is likely to be relevant in humans.

I hypothesized that histidines 120 and 213 in the extracellular domain of rat P2X₂ bind directly to zinc. In this dissertation, I provide evidence that these two histidines directly participate in a zinc binding site, and that the zinc is coordinated across the P2X₂ subunit interface. Additionally, I show that this binding site is relatively inflexible to changing positions of the histidines. Because human P2X₂ contains only one of the critical extracellular histidines, I also hypothesized that zinc would not cause potentiation of current through human P2X₂ receptors. I show in this work that in human P2X₂, the effect of physiologically relevant concentrations of zinc is inhibition of ATP-elicited current rather than the potentiation seen in rat P2X₂. The in vivo actions of zinc on human P2X₂ are likely to be profoundly different from the zinc effects which experiments on mice and rats would suggest, and should therefore be reexamined.