mGlu1 and mGlu5 have reciprocal distributions in the brain and are located predominantly postsynaptically where they are thought to augment neurotransmission. Antagonists of both mGlu1 and mGlu5 have shown anxiolytic activity in animal models.124 For mGlu1 antagonists, the glutamate analog AIDA (Figure 11c) and the allosteric antagonist JNJ16259685 are both effective after systemic administration in the Vogel test, and positive results were also reported for AIDA in the EPM and open field, but not the four-plate test.131,132 The mGlu5 allosteric antagonist MPEP was efficacious in fear-potentiated startle, EPM, conflict tests, and stress-induced hypothermia with systemic administration,124,133 as was the analog MTEP (Figure 11c).134 In general, anxiolytic effects were observed without sedation; however, group I mGlu receptors are involved in learning and memory processes, consequently blockade of mGlu1 and mGlu5 may be problematic in this respect. Fenobam (Figure 11c), a drug efficacious in patients with anxiety, is a potent negative allosteric modulator of mGlu5135 representing the first validation of the potential anxiolytic efficacy of a group I mGlu antagonist in humans.
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