Interplay between facilitation, depression, and residual calcium at three presynaptic terminals.

Synapses display remarkable alterations in strength during repetitive use. Different types of synapses exhibit distinctive synaptic plasticity, but the factors giving rise to such diversity are not fully understood. To provide the experimental basis for a general model of short-term plasticity, we studied three synapses in rat brain slices at 34 degrees C: the climbing fiber to Purkinje cell synapse, the parallel fiber to Purkinje cell synapse, and the Schaffer collateral to CA1 pyramidal cell synapse. These synapses exhibited a broad range of responses to regular and Poisson stimulus trains. Depression dominated at the climbing fiber synapse, facilitation was prominent at the parallel fiber synapse, and both depression and facilitation were apparent in the Schaffer collateral synapse. These synapses were modeled by incorporating mechanisms of short-term plasticity that are known to be driven by residual presynaptic calcium (Ca(res)). In our model, release is the product of two factors: facilitation and refractory depression. Facilitation is caused by a calcium-dependent increase in the probability of release. Refractory depression is a consequence of release sites becoming transiently ineffective after release. These sites recover with a time course that is accelerated by elevations of Ca(res). Facilitation and refractory depression are coupled by their common dependence on Ca(res) and because increased transmitter release leads to greater synaptic depression. This model captures the behavior of three different synapses for various stimulus conditions. The interplay of facilitation and depression dictates synaptic strength and variability during repetitive activation. The resulting synaptic plasticity transforms the timing of presynaptic spikes into varying postsynaptic response amplitudes.

Calcium dependence and recovery kinetics of presynaptic depression at the climbing fiber to Purkinje cell synapse.

Short-term depression is a widespread form of use-dependent plasticity found in the peripheral and central nervous systems of invertebrates and vertebrates. The mechanism behind this transient decrease in synaptic strength is thought to be primarily the result of presynaptic "depletion" of a readily releasable neurotransmitter pool, which typically recovers with a time constant of a few seconds. We studied the mechanism and dynamics of recovery from depression at the climbing fiber to Purkinje cell synapse, where marked presynaptic depression has been described previously. Climbing fibers are well suited to studies of recovery from depression because they display little, if any, facilitation (even under conditions of low-release probability), which can obscure rapid recovery from depression for hundreds of milliseconds after release. We found that recovery from depression occurred in three kinetic phases. The fast and intermediate components could be approximated by exponentials with time constants of 100 msec and 3 sec at 24 degrees C. A much slower recovery phase was also present, but it was only prominent during prolonged stimulus trains. The fast component was enhanced by raising extracellular calcium and was eliminated by lowering presynaptic calcium, suggesting that, on short time scales, recovery from depression is driven by residual calcium. During regular and Poisson stimulus trains, recovery from depression was dramatically accelerated by accumulation of presynaptic residual calcium, maintaining synaptic efficacy under conditions that would otherwise deplete the available transmitter pool. This represents a novel form of presynaptic plasticity in that high levels of activity modulate the rate of recovery as well as the magnitude of depression.

Mechanism and kinetics of heterosynaptic depression at a cerebellar synapse.

High levels of activity at a synapse can lead to spillover of neurotransmitter from the synaptic cleft. This extrasynaptic neurotransmitter can diffuse to neighboring synapses and modulate transmission via presynaptic receptors. We studied such modulation at the synapse between granule cells and Purkinje cells in rat cerebellar slices. Brief tetanic stimulation of granule cell parallel fibers activated inhibitory neurons, leading to a transient elevation of extracellular GABA, which in turn caused a short-lived heterosynaptic depression of the parallel fiber to Purkinje cell EPSC. Fluorometric calcium measurements revealed that this synaptic inhibition was associated with a decrease in presynaptic calcium influx. Heterosynaptic inhibition of synaptic currents and calcium influx was eliminated by antagonists of the GABAB receptor. The magnitude and time course of the depression of calcium influx were mimicked by the rapid release of an estimated 10 microM GABA using the technique of flash photolysis. We found that inhibition of presynaptic calcium influx peaked within 300 msec and decayed in <3 sec at 32 degrees C. These results indicate that presynaptic GABAB receptors can sense extrasynaptic GABA increases of several micromolar and that they rapidly regulate the release of neurotransmitter primarily by modulating voltage-gated calcium channels.

Contributions of calcium-dependent and calcium-independent mechanisms to presynaptic inhibition at a cerebellar synapse.

Activation of either adenosine A1 receptors or GABAB receptors inhibits many excitatory synapses in the mammalian brain. However, the extent to which different mechanisms contribute to such synaptic modulation is unclear. We examined the manner in which activation of adenosine A1 receptors and GABAB receptors modulates synaptic strength at the granule cell to Purkinje cell synapse in rat cerebellar slices. Optical determination of presynaptic calcium influx revealed that presynaptic calcium channels were modulated by 2-chloroadenosine (2CA) and baclofen, agonists of the adenosine A1 receptor and the GABAB receptor, respectively. 2CA and baclofen differentially affected three classes of calcium channels without altering the shape of the presynaptic volley, suggesting that changes in presynaptic waveform do not contribute significantly to synaptic modulation. 2CA affected neither the amplitude nor the frequency of spontaneous miniature postsynaptic currents, whereas baclofen reduced the frequency by approximately 40% without affecting the amplitude. In addition, 2CA and baclofen do not change either fiber excitability or presynaptic residual calcium. Taken together, our data indicate that activation of the adenosine A1 receptor reduces synaptic strength by modulating presynaptic calcium channels. Baclofen modulates presynaptic calcium channels as well but also affects release processes downstream from calcium entry.