Glutamatergic and purinergic receptor-mediated calcium transients in bergmann glial cells

Astrocytes respond to neuronal activity with [Ca²⁺]_i increases after activation of specific receptors. Bergmann glial cells (BGs), astrocytes of the cerebellar molecular layer (ML), express various receptors that can mobilize internal Ca²⁺. BGs also express Ca²⁺ permeable AMPA receptors that may be important for maintaining the extensive coverage of Purkinje cell (PC) excitatory synapses by BG processes. Here, we examined Ca²⁺ signals in single BGs evoked by synaptic activity in cerebellar slices. Short bursts of high-frequency stimulation of the ML elicited Ca²⁺ transients composed of a small-amplitude fast rising phase, followed by a larger and slower rising phase. The first phase resulted from Ca²⁺ influx through AMPA receptors, whereas the second phase required release of Ca²⁺ from internal stores initiated by P2 purinergic receptor activation. We found that such Ca²⁺ responses could be evoked by direct activation of neurons releasing ATP onto BGs or after activation of metabotropic glutamate receptor 1 on these neurons. Moreover, examination of BG and PC responses to various synaptic stimulation protocols suggested that ML interneurons are likely the cellular source of ATP.