TARGET REGION-SPECIFICITY OF THE Excitability of large HIPPOCAMPAL mossy fiber terminals
The exceptionally large boutons (LMFBs) formed by the axons of dentate gyrus granule cells excite mossy cells in the hilus and pyramidal cells in the CA3 region. Here we tested whether different axonal excitability contributes to the different functions of these two populations of synapses using direct axonal patch-clamp recordings and subsequent morphological identification. First, we compared the passive membrane properties of the two LMFB populations. Membrane capacitance was smaller and input resistance was larger in the hilar-LMFBs due to their smaller size and membrane surface compared to CA3-LMFBs. These differences depended on the region but not on their distance from the parent soma (if LMFBs of same regions were considered). However, in spite of the differences in their passive properties, action potential (AP) shape and kinetics were uniform in LMFBs. The activity dependent plasticity (broadening) of the APs was also similar in hilar- and CA3-LMFBs. High-voltage-activated calcium currents that are responsible for AP-release coupling were also compared. Albeit the peak calcium current was smaller in hilar-LMFBs than in the CA3, the density was similar, because hilar-LMFBs have smaller membrane surface. Pharmacological isolation of the main presynaptic calcium channels (N- and P/Q-types) showed that their contributions to the total current were also similar in the two regions, which also explains why the voltage-dependence and kinetics of the total calcium currents were indistinguishable in hilar- and CA3-LMFBs. These results suggest that despite the distinct function and postsynaptic targets, the digital axonal code is uniform among the large mossy fiber boutons.