FUNCTIONAL CA²⁺-PERMEABLE AMPA RECEPTORS DRIVE CALCIUM SIGNALING AND NEUROGLIAL COMMUNICATION IN HIPPOCAMPAL ASTROCYTES

Glutamate is the main excitatory neurotransmitter, acting through ionotropic receptors such as AMPA receptors (AMPARs). While their neuronal role is well established, AMPAR function in astrocytes remains poorly understood, particularly in the hippocampus. Here, we investigated the expression and functional properties of astrocytic AMPARs, focusing on their contribution to calcium signaling and glia communication. Using western blotting and immunohistochemistry, we confirmed the expression of GluA1 and GluA2 subunits in primary hippocampal astrocytes, together with detectable levels of GluA4. Functional expression was further demonstrated by whole-cell patch-clamp recordings, which revealed glutamate-evoked AMPAR currents defining two distinct astrocytic populations with differential electrophysiological properties. Kinetic analysis of current desensitization showed profiles consistent with modulation by transmembrane AMPAR regulatory proteins (TARPs), a finding supported by additional western blot detection of the auxiliary subunits. Calcium imaging experiments demonstrated that AMPAR activation elicited intracellular Ca²⁺ transients in a subset of astrocytes, responses that were strongly potentiated by cyclothiazide. Notably, part of these calcium signals persisted in the absence of extracellular Na⁺, indicating the presence of calcium-permeable AMPARs (CP-AMPARs). However, sodium influx was required for full response amplification, suggesting a synergistic interaction between Na⁺ and Ca²⁺ entry. Pharmacological blockade of CP-AMPARs significantly reduced calcium responses, supporting the coexistence of calcium-permeable and calcium-impermeable AMPAR subtypes in astrocytes. Sniffer-calcium assays revealed that astrocytic AMPAR activation could trigger ATP-mediated gliotransmission, facilitating the propagation of intercellular calcium waves. These findings suggest that AMPARs in astrocytes may contribute to calcium signaling and glia communication in the hippocampus.