The novel antidepressant agomelatine reduces release of glutamate and related presynaptic mechanisms in rat hippocampus

Introduction: Agomelatine is a potent melatonergic receptor (MT1/MT2) agonist and 5HT2C receptor antagonist with proven antidepressant efficacy in both animal models of depression and in depressed patients. In previous studies we had found that following a 2 week-chronic treatment with different antidepressants [1] depolarization-evoked release of glutamate in the hippocampus but not GABA was markedly reduced, suggesting that antidepressants may change the balance between excitatory and inhibitory neurotransmission. The functional changes in glutamatergic neurotransmission were accounted for by alterations of neurotransmitter release regulating protein-protein interactions: in particular, we found a reduction of syntaxin-1/aCalcium-CaM dependent Kinase II (aCaMKII) interaction, suggested to promote the formation of exocytotic SNARE complex, and an increase of syntaxin-1/Munc-18 interaction, that reduces formation of SNARE complex and neurotransmitter release [1,2]. Purpose: In order to determine whether also a chronic treatment with agomelatine would induce similar synaptic functional and molecular changes, we measured depolarization-evoked release of glutamate and selected presynaptic molecular mechanisms involved in the regulation of glutamatergic transmission in the hippocampus of chronically treated rats. Methods: Rats were intraperitoneally treated with agomelatine (40 mg/kg), the reference antidepressant venlafaxine (10 mg/kg) or vehicle at 5 pm (2 h before the start of the 12 h-dark-phase), for 21 days. 17 hrs after the final administrations, rat hippocampi were dissected and synaptosomes and synaptic membranes were prepared by centrifugation on Percoll gradients followed by differential centrifugation and ultracentrifugation. Glutamate release was measured in freshly purified synaptosomes by using superfusion technique [1]. aCaMKII and Munc-18 were independently immunoprecipitated with specific antibodies and coimmunoprecipitated syntaxin-1 was measured by SDS-PAGE and Western blot. SDS-resistant SNARE complex was measured by SDS-PAGE and Western blot. Results: Chronic treatment with agomelatine significantly (p<0.05) reduced depolarization-evoked release of glutamate in hippocampus (by 30%), comparable to venlafaxine. However, treatment with either venlafaxine or agomelatine did not affect glutamate release evoked by ionomycin or GABA release. Moreover, in an analysis of the protein-protein interactions regulating SNARE complex no difference in syntaxin-1/aCaMKII interaction and a slight increase of syntaxin-1/Munc-18 interaction (+19% and +27% with agomelatine and venlafaxine, p<0.05 and p<0.001 respectively) was found. Since we found reduction of glutamate release and no major alterations of protein-protein interactions regulating glutamate release, we speculated that the reduction of glutamate release might be due to downstream molecular mechanisms involved in neurotransmitter release. We therefore measured the level of SNARE complex, the protein complex mediating docking and release of synaptic vesicles. Two versions of SDS-resistant SNARE complex, ~80 kDa and ~110 kDa, were visualized using antibodies for syntaxin-1 and SNAP25, respectively. Both complexes were decreased by agomelatine and venlafaxine (-20%, p<0.05). Conclusion: Chronic treatment with agomelatine, comparable to what had previously been seen with other antidepressants, reduced depolarization-evoked release of glutamate but not GABA release in the rat hippocampus, suggesting a dampening of excitatory neurotransmission also by this novel antidepressant. After a three-week treatment the presynaptic molecular changes were not observed in the priming interactions for SNARE complex, but in the complex itself