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Dendritic spines are the main receptacles of excitatory information in the brain. Their particular morphology, with a small head connected to the dendrite by a slender neck, has inspired theoretical and experimental work to understand how these structural features affect the processing, storage and integration of synaptic inputs in pyramidal neurons (PNs). The activation of glutamate receptors in spines triggers a large voltage change as well as calcium signals at the spine head. Thus, voltage‐gated and calcium‐activated potassium channels located in the spine head likely play a key role in synaptic transmission. Here we study the presence and function of large conductance calcium‐activated potassium (BK) channels in spines from layer 5 PNs. We found that BK channels are localized to dendrites and spines regardless of their size, but their activity can only be detected in spines with small head volumes (≤0.09 μm3), which reduces the amplitude of two‐photon uncaging excitatory postsynaptic potentials recorded at the soma. In addition, we found that calcium signals in spines with small head volumes are significantly larger than those observed in spines with larger head volumes. In accordance with our experimental data, numerical simulations predict that synaptic inputs impinging onto spines with small head volumes generate voltage responses and calcium signals within the spine head itself that are significantly larger than those observed in spines with larger head volumes, which are sufficient to activate spine BK channels. These results show that BK channels are selectively activated in small‐headed spines, suggesting a new level of dendritic spine‐mediated regulation of synaptic processing, integration and plasticity in cortical PNs. Key points: BK channels are expressed in the visual cortex and layer 5 pyramidal neuron somata, dendrites and spines regardless of their size.BK channels are selectively activated in small‐headed spines (≤0.09 μm3), which reduces the amplitude of two‐photon (2P) uncaging excitatory postsynaptic potentials (EPSPs) recorded at the soma.Two‐photon imaging revealed that intracellular calcium responses in the head of 2P‐activated spines are significantly larger in small‐headed spines (≤0.09 μm3) than in spines with larger head volumes.In accordance with our experimental data, numerical simulations showed that synaptic inputs impinging onto spines with small head volumes (≤0.09 μm3) generate voltage responses and calcium signals within the spine head itself that are significantly larger than those observed in spines with larger head volumes, sufficient to activate spine BK channels and suppress EPSPs. [ABSTRACT FROM AUTHOR]

Key points: CAST/ELKS are positive regulators of presynaptic growth and are suppressors of active zone expansion at the developing mouse calyx of Held.CAST/ELKS regulate all three CaV2 subtype channel levels in the presynaptic terminal and not just CaV2.1.The half‐life of ELKS is on the timescale of days and not weeks.Synaptic transmission was not impacted by the loss of CAST/ELKS.CAST/ELKS are involved in pathways regulating morphological properties of presynaptic terminals during an early stage of circuit maturation. Many presynaptic active zone (AZ) proteins have multiple regulatory roles that vary during distinct stages of neuronal circuit development. The CAST/ELKS protein family are evolutionarily conserved presynaptic AZ molecules that regulate presynaptic calcium channels, synaptic transmission and plasticity in the mammalian CNS. However, how these proteins regulate synapse development and presynaptic function in a developing neuronal circuit in its native environment is unclear. To unravel the roles of CAST/ELKS in glutamatergic synapse development and in presynaptic function, we used CAST knockout (KO) and ELKS conditional KO (CKO) mice to examine how their loss during the early stages of circuit maturation impacted the calyx of Held presynaptic terminal development and function. Morphological analysis from confocal z‐stacks revealed that combined deletion of CAST/ELKS resulted in a reduction in the surface area and volume of the calyx. Analysis of AZ ultrastructure showed that AZ size was increased in the absence of CAST/ELKS. Patch clamp recordings demonstrated a reduction of all presynaptic CaV2 channel subtype currents that correlated with a loss in presynaptic CaV2 channel numbers. However, these changes did not impair synaptic transmission and plasticity and synaptic vesicle release kinetics. We conclude that CAST/ELKS proteins are positive regulators of presynaptic growth and are suppressors of AZ expansion and CaV2 subtype currents and levels during calyx of Held development. We propose that CAST/ELKS are involved in pathways regulating presynaptic morphological properties and CaV2 channel subtypes and suggest there is developmental compensation to preserve synaptic transmission during early stages of neuronal circuit maturation. Key points: CAST/ELKS are positive regulators of presynaptic growth and are suppressors of active zone expansion at the developing mouse calyx of Held.CAST/ELKS regulate all three CaV2 subtype channel levels in the presynaptic terminal and not just CaV2.1.The half‐life of ELKS is on the timescale of days and not weeks.Synaptic transmission was not impacted by the loss of CAST/ELKS.CAST/ELKS are involved in pathways regulating morphological properties of presynaptic terminals during an early stage of circuit maturation. [ABSTRACT FROM AUTHOR]

ABSTRACT We examined the synaptic structure, quantity, and distribution of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)- and N-methyl-D-aspartate (NMDA)-type glutamate receptors (AMPARs and NMDARs, respectively) in rat cochlear nuclei by a highly sensitive freeze-fracture replica labeling technique. Four excitatory synapses formed by two distinct inputs, auditory nerve (AN) and parallel fibers (PF), on different cell types were analyzed. These excitatory synapse types included AN synapses on bushy cells (AN-BC synapses) and fusiform cells (AN-FC synapses) and PF synapses on FC (PF-FC synapses) and cartwheel cell spines (PF-CwC synapses). Immunogold labeling revealed differences in synaptic structure as well as AMPAR and NMDAR number and/or density in both AN and PF synapses, indicating a target-dependent organization. The immunogold receptor labeling also identified differences in the synaptic organization of FCs based on AN or PF connections, indicating an input-dependent organization in FCs. Among the four excitatory synapse types, the AN-BC synapses were the smallest and had the most densely packed intramembrane particles (IMPs), whereas the PF-CwC synapses were the largest and had sparsely packed IMPs. All four synapse types showed positive correlations between the IMP-cluster area and the AMPAR number, indicating a common intrasynapse-type relationship for glutamatergic synapses. Immunogold particles for AMPARs were distributed over the entire area of individual AN synapses; PF synapses often showed synaptic areas devoid of labeling. The gold-labeling for NMDARs occurred in a mosaic fashion, with less positive correlations between the IMP-cluster area and the NMDAR number. Our observations reveal target- and input-dependent features in the structure, number, and organization of AMPARs and NMDARs in AN and PF synapses. J. Comp. Neurol. 522:4023-4042, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Key points Spatiotemporal glutamate concentration profile in the cleft is a determinant of the EPSC amplitude and time course., Two key parameters required to describe this profile are the number of glutamate in a vesicle ( NGlu) and its diffusion coefficient ( DGlu), both of which are unestablished., Using the rat calyx of Held synapse as a model, the distribution of AMPA receptors was mapped with SDS-digested freeze fracture replica labelling and their performance as glutamate sensors was evaluated with outside-out patch recordings., Based on these data, synaptic responses were simulated using various combinations of NGlu and DGlu, and an optimal range of the NGlu- DGlu combinations that could reproduce the recordings was determined., Using the estimated profile, we show that release from a single vesicle does not saturate the receptors, glutamate spillover does not affect the synaptic conductance amplitude, and synaptic response increases non-linearly with the number of multivesicular releases., Abstract Establishing the spatiotemporal concentration profile of neurotransmitter following synaptic vesicular release is essential for our understanding of inter-neuronal communication. Such profile is a determinant of synaptic strength, short-term plasticity and inter-synaptic crosstalk. Synaptically released glutamate has been suggested to reach a few millimolar in concentration and last for <1 ms. The synaptic cleft is often conceived as a single concentration compartment, whereas a huge gradient likely exists. Modelling studies have attempted to describe this gradient, but two key parameters, the number of glutamate in a vesicle ( NGlu) and its diffusion coefficient ( DGlu) in the extracellular space, remained unresolved. To determine this profile, the rat calyx of Held synapse at postnatal day 12-16 was studied where diffusion of glutamate occurs two-dimensionally and where quantification of AMPA receptor distribution on individual postsynaptic specialization on medial nucleus of the trapezoid body principal cells is possible using SDS-digested freeze-fracture replica labelling. To assess the performance of these receptors as glutamate sensors, a kinetic model of the receptors was constructed from outside-out patch recordings. From here, we simulated synaptic responses and compared them with the EPSC recordings. Combinations of NGlu and DGlu with an optimum of 7000 and 0.3 μm2 ms−1 reproduced the data, suggesting slow diffusion. Further simulations showed that a single vesicle does not saturate the synaptic receptors, and that glutamate spillover does not affect the conductance amplitude at this synapse. Using the estimated profile, we also evaluated how the number of multiple vesicle releases at individual active zones affects the amplitude of postsynaptic signals. [ABSTRACT FROM AUTHOR]

Among the 20 members in the connexin family of gap junction proteins, only connexin 36 (Cx36) is firmly established to be expressed in neurons and to form electrical synapses at widely distributed interneuronal gap junctions in mammalian brain. Several connexins have recently been reported to interact with the PDZ domain-containing protein zonula occludens-1 (ZO-1), which was originally considered to be associated only with tight junctions, but has recently been reported to associate with other structures including gap junctions in various cell types. Based on the presence of sequence corresponding to a putative PDZ binding motif in Cx36, we investigated anatomical relationships and molecular association of Cx36 with ZO-1. By immunofluorescence, punctate Cx36/ZO-1 colocalization was observed throughout the central nervous system of wild-type mice, whereas labelling for Cx36 was absent in Cx36 knockout mice, confirming the specificity of the anti-Cx36 antibodies employed. By freeze-fracture replica immunogold labelling, Cx36 and ZO-1 in brain were found colocalized within individual ultrastructurally identified gap junction plaques, although some plaques contained only Cx36 whereas others contained only ZO-1. Cx36 from mouse brain and Cx36-transfected HeLa cells was found to coimmunoprecipitate with ZO-1. Unlike other connexins that bind the second of the three PDZ domains in ZO-1, glutathione S-transferase-PDZ pulldown and mutational analyses indicated Cx36 interaction with the first PDZ domain of ZO-1, which required at most the presence of the four c-terminus amino acids of Cx36. These results demonstrating a Cx36/ZO-1 association suggest a regulatory and/or scaffolding role of ZO-1 at gap junctions that form electrical synapses between neurons in mammalian brain. [ABSTRACT FROM AUTHOR]