Multivesicular release underlies short term synaptic potentiation independent of release probability change in the supraoptic nucleus

The ubiquitous nature of multivesicular release

'Simplicity is prerequisite for reliability' (E.W. Dijkstra [1]) Presynaptic action potentials trigger the fusion of vesicles to release neurotransmitter onto postsynaptic neurons. Each release site was originally thought to liberate at most one vesicle per action potential in a probabilistic fashion, rendering synaptic transmission unreliable. However, the simultaneous release of several vesicles, or multivesicular release (MVR), represents a simple mechanism to overcome the intrinsic unreliability of synaptic transmission. MVR was initially identified at specialized synapses but is now known to be common throughout the brain. MVR determines the temporal and spatial dispersion of transmitter, controls the extent of receptor activation, and contributes to adapting synaptic strength during plasticity and neuromodulation. MVR consequently represents a widespread mechanism that extends the dynamic range of synaptic processing.

Interplay between synchronization of multivesicular release and recruitment of additional release sites support short-term facilitation at hippocampal mossy fiber to CA3 pyramidal cells synapses

Synaptic short-term plasticity is a key regulator of neuronal communication and is controlled via various mechanisms. A well established property of mossy fiber to CA3 pyramidal cell synapses is the extensive short-term facilitation during high-frequency bursts. We investigated the mechanisms governing facilitation using a combination of whole-cell electrophysiological recordings, electrical minimal stimulation, and random-access two-photon microscopy in acute mouse hippocampal slices. Two distinct presynaptic mechanisms were involved in short-term facilitation, with their relative contribution dependent on extracellular calcium concentration. The synchronization of multivesicular release was observed during trains of facilitating EPSCs recorded in 1.2 mM external Ca(2+) ([Ca(2+)]e). Indeed, covariance analysis revealed a gradual augmentation in quantal size during trains of EPSCs, and application of the low-affinity glutamate receptor antagonist γ-D-glutamylglycine showed an increase in cleft glutamate concentration during paired-pulse stimulation. Whereas synchronization of multivesicular release contributed to the facilitation in 1.2 mM [Ca(2+)]e, variance-mean analysis showed that recruitment of more release sites (N) was likely to account for the larger facilitation observed in 2.5 mM [Ca(2+)]e. Furthermore, this increase in N could be promoted by calcium microdomains of heterogeneous amplitudes observed in single mossy fiber boutons. Our findings suggest that the combination of multivesicular release and the recruitment of additional release sites act together to increase glutamate release during burst activity. This is supported by the compartmentalized spatial profile of calcium elevations in boutons and helps to expand the dynamic range of mossy fibers information transfer.

Short-term potentiation of GABAergic synaptic inputs to vasopressin and oxytocin neurones

The magnocellular vasopressin (VP) and oxytocin (OT) neurones undergo long-term synaptic plasticity to accommodate prolonged hormone demand. By contrast, rapidly induced,transient synaptic plasticity in response to brief stimuli could enable the activation of magnocellular neurones in response to acute challenges. Here, we report a robust short-term potentiation of asynchronous GABAergic synaptic inputs (STP(GABA)) to VP and OT neurones of the hypothalamic supraoptic nucleus elicited by repetitive extracellular electrical stimulation.The STP(GABA) required extracellular Ca2+, but did not require activation of glutamate, VP or OT receptors or nitric oxide synthesis. Presynaptic action potential generation was necessary for the induction, but not the maintenance, of STP(GABA). The STP(GABA) led to a minutes-long GABA(A)receptor-dependent increase in spike frequency in VP neurones, but not in OT neurones,consistent with an excitatory function of GABA in only VP neurones and with the generation of prolonged bursts of action potentials in VP neurones. Therefore, this short-term plasticity of GABAergic synaptic inputs is likely to play very different roles in the regulation of OT and VP neurones and their distinct patterns of physiological activation.

Role of intracellular calcium stores in hair-cell ribbon synapse

Intracellular calcium stores control many neuronal functions such as excitability, gene expression, synaptic plasticity, and synaptic release. Although the existence of calcium stores along with calcium-induced calcium release (CICR) has been demonstrated in conventional and ribbon synapses, functional significance and the cellular mechanisms underlying this role remains unclear. This review summarizes recent experimental evidence identifying contribution of CICR to synaptic transmission and synaptic plasticity in the CNS, retina and inner ear. In addition, the potential role of CICR in the recruitment of vesicles to releasable pools in hair-cell ribbon synapses will be specifically discussed.