Quantal transmitter release by glioma cells: quantification of intramembrane particle changes

Glial cells in situ are able to release neurotransmitters such as glutamate or acetylcholine (ACh). Glioma C6BU-1 cells were used to determine whether the mechanisms of ACh release by a glial cell line are similar or not to quantal release from neurones. Individual C6BU-1 cells, pre-filled with ACh, were moved into contact with a Xenopus myocyte that was used as a real-time ACh detector. Upon electrical stimulation, C6BU-1 cells generated evoked ACh impulses which were Ca(2+)-dependent and quantal (quantal steps of ca. 100 pA). Changes in plasma membrane ultrastructure were investigated by using a freeze-fracture technique designed for obtaining large and flat replicas from monolayer cell cultures. A transient increase in the density of medium and large size intramembrane particles--and a corresponding decrease of small particles--occurred in the plasma membrane of C6BU-1 cells stimulated for ACh release. Changes in interaction forces between adjacent medium and large particles were investigated by computing the radial distribution function and the interaction potential. In resting cells, the radial distribution function revealed a significant increase in the probability to find two particles separated by an interval of 24 nm; the interaction potential suggested repulsive forces for intervals shorter than 24 nm and attractive forces between 24 and 26 nm. In stimulated cells, this interaction was displaced to 21 nm and made weaker, despite of the fact that the overall particle density increased. The nature of this transient change in intramembrane particles is discussed, particularly with regard to the mediatophore proteolipid which is abundant in the membranes C6-BU-1 like in those of cholinergic neurones. In conclusion, evoked ACh release from pre-filled C6-BU-1 glioma cells is quantal and Ca(2+)-dependent. It is accompanied by a transient changes in the size distribution and the organisation of intramembrane particles in the plasma membrane. Thus, for the release characteristics, glioma cells do not differ fundamentally from neurones.