Cultured cerebellar granule cells grown in medium containing 10 mM K+ undergo apoptosis after 4-5 days in vitro (DIV), and, at that time, the activity of metabotropic glutamate (mGlu) receptors coupled to polyphosphoinositide (PI) hydrolysis begins to decline. In granule cells at 4 DIV, the mGlu receptor subtype mGlu5 was expressed at high levels. The expression of another PI-coupled mGlu receptor, the mGlu1a, was low at 4 DIV but increased during the following days. In cultures at 4-5 DIV, the few cells that already showed an apoptotic phenotype were devoid of mGlu5 receptors, but they all expressed mGlu1a receptors. The development of apoptosis was accelerated after treating the cultures with: (i) mGlu5 antisense oligonucleotides; (ii) the mixed mGlu receptor antagonist, (+)-alpha-methyl-4-carboxyphenylglycine; or (iii) the glutamate depleting enzyme, alanine aminotransferase. In contrast, an induced overexpression of mGlu5 receptors protected cultured granule cells against apoptotic death. We suggest that the activity of mGlu5 receptors supports cell survival, and a decline in the expression of mGlu5 receptors gives access to programmed cell death in cerebellar granule cells developing in primary cultures.