Dynamic control of presynaptic ca2+inflow by fast-inactivating K+ channelsin hippocampal mossy fiber boutons

Analysis of presynaptic determinants of synaptic strength has been difficul t at cortical synapses, mainly due to the lack of direct access to presynap tic elements. Here we report patch-clamp recordings from mossy fiber bouton s (MFBs) in rat hippocampal slices. The presynaptic action potential is ver y short during low-frequency stimulation but is prolonged up to 3-fold duri ng high-frequency stimulation. Voltage-gated K+ channels in MFBs inactivate rapidly but recover from inactivation very slowly, suggesting that cumulat ive K+ channel inactivation mediates activity-dependent spike broadening. P rolongation of the presynaptic voltage waveform leads to an increase in the number of Ca2+ ions entering the terminal per action potential and to a co nsecutive potentiation of evoked excitatory postsynaptic currents at MFB-CA 3 pyramidal cell synapses. Thus, inactivation of presynaptic K+ channels co ntributes to the control of efficacy of a glutamatergic synapse in the cort ex.