In the present study, the effect of MPEP was blocked by pretreatment with a tryptophan hydroxylase inhibitor, PCPA, suggesting that serotonergic transmission plays a role in
the effect of the mGlu5 receptor antagonist in the NSF test. It should be noted that this GDC973 is the first report to demonstrate the involvement of serotonergic transmission in the effect of an mGlu5 receptor antagonist in the NSF test. Previously, we demonstrated that treatment with PCPA (300 mg/kg twice daily for 3 days) caused a 74.8% reduction in the 5-HT content in the frontal cortex in mice, compared with a vehicle-treated group, and abolished the head-twitch response induced by a 5-HT release-promoting agent, PCA (11). Therefore, the treatment condition with PCPA used in this study is sufficient for the pharmacological depletion of 5-HT in mouse brain. This finding is consistent with previous reports that the antidepressant-like effect of MTEP
was attenuated by PCPA treatment in the TST (20), indicating Navitoclax mouse that serotonergic transmission may play a key role in the actions of mGlu5 receptor antagonists across animal models. Next, we investigated the involvement of the 5-HT receptor subtype in the effect of MPEP in the NSF test. 5-HT1A and 5-HT2A/2C receptors were investigated in the present study because these receptors play important roles in the antidepressant and anxiolytic-like effects of agents (24) and (25). We found that the effect of MPEP was blocked by a 5-HT2A/2C receptor antagonist, ritanserin, but not by a 5-HT1A receptor antagonist, WAY100635, in the NSF test. These results suggest that the stimulation of the 5-HT2A/2C receptor, however but not the 5-HT1A receptor, mediates the effect of MPEP in the NSF test. These findings are consistent with previous reports
that the antidepressant and anxiolytic effects of MTEP were attenuated by ritanserin but not WAY100635 in the TST and Vogel conflict drinking test (20) and (21). Given that both MPEP and MTEP do not have activities at 5-HT receptors and mGlu5 receptor antagonists have been reported to increase 5-HT release in the prefrontal cortex and hippocampus (21), (26) and (27), the blockade of mGlu5 receptors may indirectly stimulate the 5-HT2A/2C receptor through an increase in 5-HT release, leading to the antidepressant/anxiolytic effects in animal models, including the NSF test. Although the effects of both an mGlu5 receptor antagonist and ketamine in the NSF test are mediated through serotonergic transmission, the mechanism of the mGlu5 receptor antagonist differs from that of ketamine, since we previously reported that the 5-HT1A receptor, but not the 5-HT2A/2C receptor, is involved in the effect of ketamine (11). Ketamine reportedly increases 5-HT release via the stimulation of the AMPA receptor (10) in the prefrontal cortex, which may lead to the stimulation of the postsynaptic 5-HT1A receptor and its subsequent effects.