5-HT(1A) receptors were studied via [(3)H]WAY-100635 and [(3)H]8-OH-DPAT binding to rat brain cortical membranes. We characterized the effect of zinc (Zn(2+)) 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 Zn(2+) and their cooperativity factors (alpha) affecting the dissociation constants (K(D), K(i)) of [(3)H]WAY-100635, [(3)H]8-OH-DPAT, and serotonin (5-HT), the endogenous neurotransmitter. Zn(2+) (5microM-1mM) inhibited the binding of agonist/antagonist to 5-HT1A receptors, mostly by decreasing both the ligands' affinity and the maximal number of sites. In [(35)S]GTPgammaS binding assays Zn(2+) 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 Zn(2+) 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 Zn(2+) modulation of 5-HT(1A) receptors is unclear, but the colocalization of 5-HT(1A) receptors and Zn(2+) in the nervous system (e.g. in the hippocampus and cerebral cortex) suggests that Zn(2+) released at nerve terminals may modulate signals generated by the 5-HT(1A) receptors in vivo. Finally, these findings suggest that synaptic Zn(2+) may be a factor influencing the effectiveness of therapies that rely on 5-HT(1A) receptor activity.