Concentration-Dependent Dual Mode of Zn Action at Serotonin 5-HT1A Receptors: In Vitro and In Vivo Studies

Recent data has indicated that Zn can modulate serotonergic function through the 5-HT_1A receptor (5-HT_1AR); however, the exact mechanisms are unknown. In the present studies, radioligand binding assays and behavioural approaches were used to characterize the pharmacological profile of Zn at 5-HT_1ARs in more detail. The influence of Zn on agonist binding to 5-HT_1ARs stably expressed in HEK293 cells was investigated by in vitro radioligand binding methods using the agonist [³H]-8-OH-DPAT. The in vivo effects of Zn were compared with those of 8-OH-DPAT in hypothermia, lower lip retraction (LLR), 5-HT behavioural syndrome and the forced swim (FST) tests. In the in vitro studies, biphasic effects, which involved allosteric potentiation of agonist binding at sub-micromolar Zn concentrations and inhibition at sub-millimolar Zn concentrations, were found. The in vivo studies showed that Zn did not induce LLR or elements of 5-HT behavioural syndrome but blocked such effects induced by 8-OH-DPAT. Zn decreased body temperature in rats and mice; however, Zn failed to induce hypothermia in the 5-HT_1A autoreceptor knockout mice. In the FST, Zn potentiated the effect of 8-OH-DPAT. However, in the FST performed with the 5-HT_1A autoreceptor knockout mice, the anti-immobility effect of Zn was partially blocked. Both the binding and behavioural studies suggest a concentration-dependent dual mechanism of Zn action at 5-HT_1ARs, with potentiation at low dose and inhibition at high dose. Moreover, the in vivo studies indicate that Zn can modulate both presynaptic and postsynaptic 5-HT_1ARs; however, Zn’s effects at presynaptic receptors seem to be more potent.