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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]