This work demonstrates that chronic treatment with the new antidepressant agomelatine, a MT1 and MT2 receptor agonist and 5-HT2C receptor antagonist, as well as with venlafaxine, a selective serotonin and noradrenaline reuptake inhibitor, markedly reduces depolarization-evoked endogenous release of glutamate from hippocampal synaptosomes. Interestingly, the depolarization-evoked release of GABA was not modified, suggesting that both drugs selectively inhibit glutamatergic release without affecting GABA transmission. These results are in line with previous studies, both in vivo and ex vivo, showing similar effects on glutamatergic transmission of a number of traditional antidepressants and of agomelatine ([23, 24]; discussed and reviewed in Ref.  and ).
The reduction of glutamate release with no changes in GABA release suggests an alteration in the balance between excitatory and inhibitory neurotransmission that could improve the signal to noise ratio in glutamate transmission, when it becomes compromised by excessive release. In this regard, we have demonstrated that chronic treatment with different classes of antidepressants, includ-ing agomelatine, was able to completely prevent the marked increase of depolarization-evoked glutamate release from prefrontal and frontal cortex synaptosomes induced by acute stress [14, 19].
Another intriguing result of this study is that, despite chronic agomelatine inhibited glutamate release evoked by 15 mM KCl depolarization, it had no effect on release induced by 0.5 mM ionomycin. Indeed, if it is generally agreed that electrical/chemical depolarization mainly induces the fusion of vesicles of the readily releasable pool (RRP) , ionomycin also promotes calcium-dependent fusion of vesicles, but mainly from the reserve pool. Therefore, our results suggest that agomelatine may selectively affect the RRP of vesicles, thereby altering a physiologically relevant pool for neurotransmitter release. In line with this hypothesis, we found that the reduction of glutamate release induced by chronic agomelatine and venlafaxine was accompanied by reduced accumulation of SNARE complexes in synaptic membranes (the fraction of synaptosomes containing the RRP). These data are in line with our previous results showing that traditional antidepressant-induced reduction of glutamate release is accounted for by changes in protein-protein interactions regulating the assembly of the SNARE complex , and by reduction of complex accumulation in presynaptic membranes . However, contrary to previous results with traditional antidepressants (2 weeks of treatment) , here we did not find major changes in the expression of syntaxin-1 after 3 weeks of treatment with agomelatine or venlafaxine. Because the same result was obtained with both drugs, we suggest that the difference from other antidepressants is probably due to the different timing of drug-treatment (3 vs 2 weeks). However, the main finding is that, after treatment for 3 weeks, agomelatine and venlafaxine still reduced both the accumulation of SNARE complexes at the level of the RRP and the physiological release of glutamate.
Indeed, the SNARE complex and associated proteins play a critical role in vesicle docking, priming, fusion and synchronization of neurotransmitter release at presynaptic membranes and it was established that the SNARE complex corresponds to the minimal machinery for membrane fusion in eukaryotic cells, forming a stable complex that make the vesicles competent for fusion [21, 22, 26]. Therefore, a reduction of SNARE complex accumulation in synaptic membranes is consistent with reduced neurotransmitter release.
In conclusion, the present study demonstrated that chronic agomelatine dampens hippocampal glutamate neurotransmission, a likely component of the therapeutic action of antidepressants. The intriguing finding of reduced accumulation of SNARE complexes in presynaptic membranes suggests selected mechanisms in the exocytotic machinery as possible molecular targets of these drugs.