Abstract
The adenylyl cyclase activator forskolin or membrane permeable cAMP analogues enhance synaptic transmission presynaptically at many synapses, but exact mechanism underlying these effects is unknown. We have studied the intracellular mechanism of the cAMP action at the calyx of Held, where it is possible to make simultaneous pre- and postsynaptic recordings. Forskolin (0.5-50 μM) presynaptically potentiated EPSCs in 8-22 days old rats with the magnitude of potentiation being reduced with postnatal development. Loading cAMP (0.5 mM) into calyceal terminal in simultaneous pre- and postsynaptic whole-cell recordings also markedly potentiated EPSCs. The forskolin-induced synaptic potentiation was associated with an increase in release probability (p) and size of readily releasable pool (RRP) of synaptic vesicles deduced from experiments using tetanic stimulation or MK-801. Forskolin had no effect on the peak amplitude of presynaptic Ca2+ currents or K+ currents recorded from calyceal terminals, suggesting that it directly stimulates exocytotic machinery downstream of Ca2+ influx. Phorbol ester applied in addition to forskolin further potentiated EPSCs with no apparent occlusion, suggesting that the mechanism underlying synaptic potentiations by cAMP and PKC are distinct. Whereas neither a PKA inhibitor nor Ih channel blocker affected the forskolin-induced EPSC potentiation. Direct loading of the cAMP-GEF activator 8CPT-2Me-cAMP into calyceal terminal potentiated EPSCs. We conclude that cAMP-induced synaptic potentiation is caused by increasing both p and RRP size, possibly through cAMP-GEF pathway at the calyx of Held. [Jpn J Physiol 54 Suppl:S153 (2004)]
