Immunotherapy of Alzheimer’s disease (AD) is a promising approach to reduce the accumulation of amyloid-beta (Aβ) peptides, a critical event in the onset of the disease. Emerging evidence indicate that metabotropic glutamate receptors (mGluRs) are involved in the progression of AD, representing potential targets for new treatments. Studies have highlighted the potential role of group II mGluRs, mGluR2 and mGluR3, in the modulation of Aβ production, although their respective contribution remains unclear, due to the lack of specific tools to study them. Here, we showed in vitro that enhancing mGluR2 activity increases Aβ1-42 peptide production whereas activation of mGluR3 has no effect. We show that this difference likely results from the direct interaction of APP with mGluR3, but not with mGluR2, that prevents APP amyloidogenic cleavage and Aβ1-42 peptides production. To selectively study the mGluR2 contribution to the amyloidosis in vivo, we used a nanobody specifically targeting mGluR2 and exhibiting positive allosteric modulation effects. This nanobody crosses the blood-brain barrier and can be detected in brains up to 7 days, after a single peripheral injection. A chronic treatment of a mouse model of AD (5xFAD mice) with this brain-penetrating nanobody accelerated amyloid aggregation and exacerbated memory deficits. Interestingly, this chronic treatment had no effect in wild-type mice. Our results confirm that a selective mGluR2 activation exacerbates AD development, suggesting that therapeutic benefices could be obtained with blockers of this receptor.