SPATIAL HETEROGENEITY OF INTRACELLULAR CA2+ SIGNALS IN AXONS OF BASKET CELLS FROM RAT CEREBELLAR SLICES

1. Using tight-seal whole-cell recording and digital fluorescence imag ing, we studied intracellular calcium (ca(i)(2+)) dynamics in cerebell ar basket cells, whose dendrites, axon and presynaptic terminals are c oplanar, an optimal configuration for simultaneous optical measurement s of all functional domains. 2. In Cs+-loaded neurones, depolarizing p ulses induced large Ca-i(2+) transients in single axonal varicosities and synaptic terminals, contrasting with much weaker signals between v aricosities or in the somato-dendritic domain. 3. Axonal branch points consistently displayed [Ca2+](i) rises of similar magnitude and time course to those in axonal terminals and varicosities. 4. In biocytin-f illed basket cells, varicosity-like swellings were present along the a xon including its branch points. Thus, axonal enlargements are not due to fluorescence-induced cell damage. 5. The spatial heterogeneity of Ca-i(2+) signals was also observed in Kf-loaded cells upon depolarizin g trains, suggesting that this behaviour is an intrinsic property of C a-i(2+) homeostasis in basket cells. 6. We conclude that depolarizatio n of basket cell axons evokes high local Ca-i(2+) signals in synaptic terminals, en passant varicosities and branch points. While high [Ca2](i) in presynaptic structures presumably triggers transmitter release , Ca-i(2+) transients at branch points may control signal transmission in the axonal arborization.