Transient mGlu5R inhibition enhances the survival of granule cell precursors in the neonatal cerebellum

The generation of the most abundant neurons of the cerebellum, the granule cells, relies on a balance between clonal expansion and apoptosis during the first 10 days after birth in the external germinal layer (EGL). The amino acid glutamate controls such critical phases of cell development in other systems through specific receptors such as metabotropic glutamate receptor 5 (mGlu5R). However, the function of mGlu5Rs on the proliferation and survival of granule cell precursors (GCPs) remains elusive. We found mGlu5R mRNA transcripts in EGL using RT-PCR and observed mGlu5R-mediated Ca2+ responses in GCPs in acute slices as early as postnatal day (P) 2-3. Using in vivo injections of the selective non-competitive mGlu5R antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP) in P7-P9 mice, we found a 20% increase in the number of proliferative GCPs labeled at P7 with the S-phase marker bromodeoxyuridine (BrdU), but no increase in cell proliferation examined 2 h following a BrdU injection. Furthermore, similar treatments led to a significant 70% decrease in the number of apoptotic GCPs in the EGL as determined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. In contrast, in vivo treatment with the mGlu5R agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) resulted in a ∼60% increase in the number of TUNEL-labeled GCPs compared to control. These findings identify a unique role for glutamate acting at mGlu5Rs as a functional switch regulating GCP survival in the EGL, thus controlling the total number of cerebellar granule cells produced.

► Functional expression of mGlu5R in the neonatal cerebellar neurogenic zone. ► mGlu5R inhibition enhances the survival of granule cell precursors (GCP). ► Gene array analysis supports mGlu5R function on GCP apoptosis.