Determinants of the Time Course of Facilitation at the Granule Cell to Purkinje Cell Synapse

Short-term facilitation is a widely observed form of synaptic enhancement that is not well understood. Although presynaptic calcium has long been implicated in this process, its role is unclear, particularly at synapses in the mammalian brain. We tested the role of presynaptic residual free calcium (Cares) in facilitation of synapses between granule cells and Purkinje cells in rat cerebellar slices. Paired-pulse facilitation of synaptic currents resulted in an approximately 2.5-fold enhancement that decayed with a time constant of approximately 200 msec, as assessed by voltage-clamp recordings. Measurements of Cares using fluorescent calcium-sensitive indicators revealed that Cares decayed more rapidly than did facilitation. Manipulation of Cares dynamics by introducing EGTA into presynaptic terminals sped the decays of Cares and facilitation in a dose-dependent manner. When Cares was reduced to a brief impulse lasting several milliseconds, facilitation was still present, although reduced in amplitude and duration. Facilitation decayed with an intrinsic time constant of approximately 40 msec. These results suggest that facilitation at this synapse is produced by a calcium-driven process with a high affinity and a slow effective off-rate. A combination of Cares dynamics and the properties of a calcium-driven reaction determine the time course and amplitude of facilitation.