An extensive literature shows that astrocytes exhibit metabotropic glutamate receptor 5 (mGluR5)–dependent increases in cytosolic calcium ions (Ca²⁺) in response to glutamatergic transmission and, in turn, modulate neuronal activity by their Ca²⁺-dependent release of gliotransmitters. These findings, based on studies of young rodents, have led to the concept of the tripartite synapse, in which astrocytes actively participate in neurotransmission. Using genomic analysis, immunoelectron microscopy, and two-photon microscopy of astrocytic Ca²⁺ signaling in vivo, we found that astrocytic expression of mGluR5 is developmentally regulated and is undetectable after postnatal week 3. In contrast, mGluR3, whose activation inhibits adenylate cyclase but not calcium signaling, was expressed in astrocytes at all developmental stages. Neuroglial signaling in the adult brain may therefore occur in a manner fundamentally distinct from that exhibited during development.