Allosteric modulation of 5-HT(1A) receptors by zinc: Binding studies

5-HT_1A receptors were studied via [³H]WAY-100635 and [³H]8-OH-DPAT binding to rat brain cortical membranes. We characterized the effect of zinc (Zn²⁺) on the binding properties of the 5-HT_1A receptor. The allosteric ternary complex model was applied to determine the dissociation constant (K_A) of Zn²⁺ and their cooperativity factors (α) affecting the dissociation constants (K_D, K_i) of [³H]WAY-100635, [³H]8-OH-DPAT, and serotonin (5-HT), the endogenous neurotransmitter. Zn²⁺ (5 μM-1 mM) inhibited the binding of agonist/antagonist to 5-HT1A receptors, mostly by decreasing both the ligands' affinity and the maximal number of sites. In [³⁵S]GTPγS binding assays Zn²⁺ behaved as insourmountable antagonist of 5-HT1A receptors, in agreement with radioligand binding assays. The residues involved in the formation of the inhibitory binding site on the 5-HT1A receptor were assessed by using N-ethyl-maleimide (NEM) or diethylpyrocarbonate (DEPC) which modify preferentially cysteine and histidine residues, respectively. Exposure to both agents did not block the negative allosteric effects of Zn²⁺ on agonist and antagonist binding. Our findings represent the first quantitative analysis of allosteric binding interactions for 5-HT_1A receptors. The physiological significance of Zn²⁺ modulation of 5-HT_1A receptors is unclear, but the colocalization of 5-HT_1A receptors and Zn²⁺ in the nervous system (e.g. in the hippocampus and cerebral cortex) suggests that Zn²⁺ released at nerve terminals may modulate signals generated by the 5-HT_1A receptors in vivo. Finally, these findings suggest that synaptic Zn²⁺ may be a factor influencing the effectiveness of therapies that rely on 5-HT_1A receptor activity.