Long-lasting potentiation of GABAergic inhibitory synaptic transmission in cerebellar Purkinje cells: Its properties and possible mechanisms

The cellular basis of motor learning in the cerebellum has been attributed mostly to long-term depression (LTD) at excitatory parallel fiber (PF)-Purkinje cell (PC) synapses. LTD is induced when PFs are activated in conjunction with a climbing fiber (CF), the other excitatory input to PCs. Recently, by using whole-cell patch-clamp recording from PCs in cerebellar slices, a new form of synaptic plasticity was discovered. Stimulation of excitatory CFs induced a long-lasting (usually longer than 30 min) 'rebound potentiation (RP)' of γ-amino-butyric acid A (GABA(A))-receptor mediated inhibitory postsynaptic currents (IPSCs). As in LTD, induction of RP requires transient elevation of intracellular calcium concentration ([Ca²⁺](i)) due to activation of voltage-gated Ca²⁺ channels. Activity of inhibitory synapses seems also to be necessary for RP to occur. RP is mainly due to up-regulation of postsynaptic GABA(A) receptor function, since PC response to bath-applied exogenous GABA is also potentiated with a time course similar to RP. The difference in the time scale between the Ca²⁺ transients (10-30 sec) and the durations of RP (> 30 min) strongly suggests that some intracellular biochemical machinery is involved. Pharmacological evidence suggests that protein kinases are involved in RP of inhibitory synapses and LTD of excitatory PF synapses. Besides the well-described LTD, RP could be a cellular mechanism that plays an important role in motor learning.