Decoding glutamate receptor activation by the Ca2+ sensor protein hippocalcin in rat hippocampal neurons.

Hippocalcin is a Ca(2+)-binding protein that belongs to a family of neuronal Ca(2+)sensors and is a key mediator of many cellular functions including synaptic plasticity and learning. However, the molecular mechanisms involved in hippocalcin signalling remain illusive. Here we studied whether glutamate receptor activation induced by locally applied or synaptically released glutamate can be decoded by hippocalcin translocation. Local AMPA receptor activation resulted in fast hippocalcin-YFP translocation to specific sites within a dendritic tree mainly due to AMPA receptor-dependent depolarization and following Ca(2+)influx via voltage-operated calcium channels. Short local NMDA receptor activation induced fast hippocalcin-YFP translocation in a dendritic shaft at the application site due to direct Ca(2+)influx via NMDA receptor channels. Intrinsic network bursting produced hippocalcin-YFP translocation to a set of dendritic spines when they were subjected to several successive synaptic vesicle releases during a given burst whereas no translocation to spines was observed in response to a single synaptic vesicle release and to back-propagating action potentials. The translocation to spines required Ca(2+)influx via synaptic NMDA receptors in which Mg(2+) block is relieved by postsynaptic depolarization. This synaptic translocation was restricted to spine heads and even closely (within 1-2 microm) located spines on the same dendritic branch signalled independently. Thus, we conclude that hippocalcin may differentially decode various spatiotemporal patterns of glutamate receptor activation into site- and time-specific translocation to its targets. Hippocalcin also possesses an ability to produce local signalling at the single synaptic level providing a molecular mechanism for homosynaptic plasticity.

Estimating transmitter release rates and quantal amplitudes in central synapses from postsynaptic current fluctuations.

Several approaches recently introduced to analyze release rates in central synapses advanced our understanding of synaptic neurotransmission, however, leaving many questions still unresolved. In this work we present evidence that a new method recently developed by Sakaba and Neher to study neurotransmission in calyx of Held, a giant glutamatergic synapse, could be also applied for estimating release rate functions and averaged quantal sizes in small central synapses. By means of different simulation approaches applied to reproduce GABAergic neurotransmission in the hippocampus we have shown that possible problems with a spatial voltage clamp which can occur in synaptic connections distributed over a large area of dendritic tree are not crucial for applicability of the method when synapses are compactly distributed or located proximally and when release rates are below 1 ms(-1). In another set of simulations we have also shown that at above mentioned release rates desensitization and/or saturation of postsynaptic GABAA receptors does not prevent accurate estimates of release rate and averaged quantal size. Thus, we conclude that the new approach based on analysis of fluctuations of postsynaptic currents under conditions of stationary release or moderately nonstationary conditions might be applicable to studies of small central synapses.