Calcium-permeable AMPA receptors trigger vesicular glutamate release from Bergmann gliosomes

Abstract

The Bergmann glia is equipped with Ca2+-permeable AMPA receptors for glutamate, indispensable for structural and functional relations between the Bergmann glia and parallel/climbing fibers-Purkinje cell synapses. To better understand roles for the Bergmann AMPA receptors, herein we investigate on gliotransmitter release and Ca2+ signals in isolated Bergmann glia processes obtained from adult rat cerebellum. We found that: 1) the rat cerebellar purified astrocyte processes (gliosomes) expressed astrocytic and Bergmann markers and exhibited negligible contamination by nerve terminals, microglia, or oligodendrocytes; 2) activation of Ca2+-permeable AMPA receptors caused Ca2+ signals in the processes, and the release of glutamate from the processes; 3) effectiveness of rose bengal, trypan blue or bafilomycin A1, indicated that activation of the AMPA receptors evoked vesicular glutamate release. Cerebellar purified nerve terminals appeared devoid of glutamate-releasing Ca2+-permeable AMPA receptors, indicating that neuronal contamination may not be the source of the signals detected. Ultrastructural analysis indicated the presence of vesicles in the cytoplasm of the processes; confocal imaging confirmed the presence of vesicular glutamate transporters in Bergmann glia processes. We conclude that: a vesicular mechanism for release of the gliotransmitter glutamate is present in mature Bergmann processes; entry of Ca2+ through the AMPA receptors located on Bergmann processes is coupled with vesicular glutamate release. The findings would add a new role for a well-known Bergmann target for glutamate (the Ca2+-permeable AMPA receptors) and a new actor (the gliotransmitter glutamate) at the cerebellar excitatory synapses onto Purkinje cells.

Keywords: (S)-AMPA (PubChem CID: 158397); (S)-CPW 399 (PubChem CID: 657004); 1-Naphthylacetyl spermine trihydrochloride (PubChem CID: 16219727); Cerebellum; Fura 2am (PubChem CID: 3086531); GYKI 52466 (PubChem CID: 3538); Gliosomes; Gliotransmitter release; GluA2-lacking AMPA receptor; Neuroglia; Rose bengal (PubChem CID: 32343); Trypan blue (PubChem CID 9562061).