Abstract
Persistent, bidirectional changes in synaptic signaling (that is, potentiation and depression of the synapse) can be induced by the precise timing of individual pre- and postsynaptic action potentials. However, far less attention has been paid to the ability of paired trains of action potentials to elicit persistent potentiation or depression. We examined plasticity following the pairing of spike trains in the touch mechanosensory neuron (T cell) and S interneuron (S cell) in the medicinal leech. Long-term potentiation (LTP) of T to S signaling was elicited when the T-cell spike train preceded the S-cell train. An interval 0 to +1 sec between the T- and S-cell spike trains was required to elicit long-term potentiation (LTP), and this potentiation was NMDA receptor (NMDAR)-dependent. Long-term depression (LTD) was elicited when S-cell activity preceded T-cell activity and the interval between the two spike trains was -0.2 sec to -10 sec. This surprisingly broad temporal window involved two distinct cellular mechanisms; an NMDAR-mediated LTD (NMDAR-LTD) when the pairing interval was relatively brief (<-1 sec) and an endocannabinoid-mediated LTD (eCB-LTD) when longer pairing intervals were used (-1 to -10 sec). This eCB-LTD also required activation of a presynaptic transient receptor potential vanilloid (TRPV)-like receptor, presynaptic Ca(2+) release from intracellular stores and activation of voltage-gated Ca(2+) channels (VGCCs). These findings demonstrate that the pairing of spike trains elicits timing-dependent forms of LTP and LTD that are supported by a complex set of cellular mechanisms involving NMDARs and endocannabinoid activation of TRPV-like receptors.