Compartmentalized Ca2+ Channel Regulation at Divergent Mossy-Fiber Release Sites Underlies Target Cell-Dependent Plasticity

Hippocampal mossy fibers (MFs) innervate CA3 targets via anatomically distinct presynaptic elements: MF boutons (MFBs) innervate pyramidal cells (PYRs), whereas filopodial extensions (Fils) of MFBs innervate st. lucidum interneurons (SLINs). Surprisingly, the same high-frequency stimulation (HFS) protocol induces presynaptically expressed LTP and LTD at PYR and SLIN inputs, respectively. This differential distribution of plasticity indicates that neighboring, functionally divergent presynaptic elements along the same axon serve as autonomous computational elements capable of modifying release independently. Indeed we report that HFS persistently depresses voltage-gated calcium channel (VGCC) function in Fil terminals, leaving MFB VGCCs unchanged despite similar contributions of N- and P/Q-type VGCCs to transmission at each terminal. Selective Fil VGCC depression results from HFS-induced mGluR7 activation leading to persistent P/Q-type VGCC inhibition. Thus, mGluR7 localization to MF-SLIN terminals and not MFBs allows for MF-SLIN LTD expression via depressed presynaptic VGCC function, whereas MF-PYR plasticity proceeds independently of VGCC alterations.