Neurotransmitter release depends critically on close spatial coupling of Ca(2+) entry to synaptic vesicles at the nerve terminal; however, the molecular substrates determining their physical proximity are unknown. Using the calyx of Held synapse, where "microdomain" coupling predominates at immature stages and developmentally switches to "nanodomain" coupling, we demonstrate that deletion of the filamentous protein Septin 5 imparts immature synapses with striking morphological and functional features reminiscent of mature synapses. This includes synaptic vesicles tightly localized to active zones, resistance to the slow Ca(2+) buffer EGTA and a reduced number of Ca(2+) channels required to trigger single fusion events. Disrupting Septin 5 organization acutely transforms microdomain to nanodomain coupling and potentiates quantal output in immature wild-type terminals. These observations suggest that Septin 5 is a core molecular substrate that differentiates distinct release modalities at the central synapse.

Reliable neuronal spiking is critical for a myriad of computations performed by neural circuits. This is particularly evident for sound localization cues in the auditory brainstem circuits that detect timing and intensity differences of sounds arriving at two ears. The calyx of Held-principal neuron synapse in the medial nucleus of the trapezoid body (MNTB) in this circuit is traditionally viewed as a reliable relay, which converts contralateral excitatory inputs to inhibitory outputs to ipsilateral superior olive neurons that code interaural timing and intensity differences. However, recent studies demonstrated large variability in the incidence of postsynaptic spike failures at this synapse, challenging the view that this synapse is a fail-safe relay. Using combined imaging and paired recordings in mature (P16 -P19) mouse brainstem slices, we show that spike failure rates of MNTB neurons are strongly correlated with differences in gross morphology of the calyx terminal and quantal properties under standard in vitro-and in vivo-like conditions. MNTB neurons innervated by calyces with simple morphologies (mainly digits) express strong short-term synaptic depression and a high incidence of spike failures after high-frequency stimulation. Conversely, MNTB neurons innervated by structurally complex calyces (digits and numerous bouton-like swellings) exhibit initial facilitation followed by slow depression and very few spike failures. Our results indicate that the calyx of Held-MNTB synapse is likely organized as a structural and functional continuum, in that correlated heterogeneities in calyx morphology and short-term plasticity serve as a filter for regulating the inhibition delivered to superior olive neurons during sound localization.

In the presence of monoamines, L-type Ca 2+ channels on the dendrites of motoneurons contribute to persistent inward currents (PICs) that can amplify synaptic inputs two-to sixfold. However, the exact location of the L-type Ca 2+ channels is controversial, and the importance of the location as a means of regulating the input-output properties of motoneurons is unknown. In this study, we used a computational strategy developed previously to estimate the dendritic location of the L-type Ca 2+ channels and test the hypothesis that the location of L-type Ca 2+ channels varies as a function of motoneuron size. Compartmental models were constructed based on dendritic trees of five motoneurons that ranged in size from small to large. These models were constrained by known differences in PIC activation reported for low-and high-conductance motoneurons and the relationship between somatic PIC threshold and the presence or absence of tonic excitatory or inhibitory synaptic activity. Our simulations suggest that L-type Ca 2+ channels are concentrated in hotspots whose distance from the soma increases with the size of the dendritic tree. Moving the hotspots away from these sites (e.g., using the hotspot locations from large motoneurons on intermediate-sized motoneurons) fails to replicate the shifts in PIC threshold that occur experimentally during tonic excitatory or inhibitory synaptic activity. In models equipped with a size-dependent distribution of L-type Ca 2+ channels, the amplification of synaptic current by PICs depends on motoneuron size and the location of the synaptic input on the dendritic tree.