The physical distance between presynaptic Ca(2+) channels and the Ca(2+) sensors that trigger exocytosis of neurotransmitter-containing vesicles is a key determinant of the signalling properties of synapses in the nervous system. Recent functional analysis indicates that in some fast central synapses, transmitter release is triggered by a small number of Ca(2+) channels that are coupled to Ca(2+) sensors at the nanometre scale. Molecular analysis suggests that this tight coupling is generated by protein-protein interactions involving Ca(2+) channels, Ca(2+) sensors and various other synaptic proteins. Nanodomain coupling has several functional advantages, as it increases the efficacy, speed and energy efficiency of synaptic transmission.

It is generally thought that transmitter release at mammalian central synapses is triggered by Ca2+ microdomains, implying loose coupling between presynaptic Ca2+ channels and Ca2+ sensors of exocytosis. Here we show that Ca2+ channel subunit immunoreactivity is highly concentrated in the active zone of GABAergic presynaptic terminals of putative parvalbumin-containing basket cells in the hippocampus. Paired recording combined with presynaptic patch pipette perfusion revealed that GABA release at basket cell-granule cell synapses is sensitive to millimolar concentrations of the fast Ca2+ chelator BAPTA but insensitive to the slow Ca2+ chelator EGTA. These results show that Ca2+ source and Ca2+ sensor are tightly coupled at this synapse, with distances in the range of 10-20 nm. Models of Ca2+ inflow-exocytosis coupling further reveal that the tightness of coupling increases efficacy, speed, and temporal precision of transmitter release. Thus, tight coupling contributes to fast feedforward and feedback inhibition in the hippocampal network.

Background-Abnormally elevated blood pressure is the most prevalent risk factor for cardiovascular disease. The large-conductance, voltage-and Ca 2ϩ -dependent K ϩ (BK) channel has been proposed as an important effector in the control of vascular tone by linking membrane depolarization and local increases in cytosolic Ca 2ϩ to hyperpolarizing K ϩ outward currents. However, the BK channel may also affect blood pressure by regulating salt and fluid homeostasis, particularly by adjusting the renin-angiotensin-aldosterone system. Methods and Results-Here we report that deletion of the pore-forming BK channel ␣ subunit leads to a significant blood pressure elevation resulting from hyperaldosteronism accompanied by decreased serum K ϩ levels as well as increased vascular tone in small arteries. In smooth muscle from small arteries, deletion of the BK channel leads to a depolarized membrane potential, a complete lack of membrane hyperpolarizing spontaneous K ϩ outward currents, and an attenuated cGMP vasorelaxation associated with a reduced suppression of Ca 2ϩ transients by cGMP. The high level of BK channel expression observed in wild-type adrenal glomerulosa cells, together with unaltered serum renin activities and corticotropin levels in mutant mice, suggests that the hyperaldosteronism results from abnormal adrenal cortical function in BK Ϫ/Ϫ mice. Conclusions-These results identify previously unknown roles of BK channels in blood pressure regulation and raise the possibility that BK channel dysfunction may underlie specific forms of hyperaldosteronism. 6,7 Recent studies raise the possibility that changes in ␤1 subunit expression contribute to the development of hypertension in rat 8 and that gain of function mutation in the same subunit decreases the prevalence of diastolic hypertension in humans. 9 However, even in the absence of functional ␤1 subunits, the ␣ subunit can still form functional channels, which might be activated at physiological potentials if their voltage and Ca 2ϩ sensitivity are increased by other factors such as endothelial factors 10,11 and/or phosphorylation. 12,13 Thus, functional BK channels may be operative in blood vessels even when the ␤1 subunit is lacking. In addition, BK channels in tissues other than vasculature, such as the adrenal gland, 14 may also influence blood pressure regulation. Therefore, we used mice lacking the BK channel ␣ subunit (BK Ϫ/Ϫ 15 to evaluate the global impact of BK channels on blood pressure regulation. MethodsDetails are given in the online-only Data Supplement. Mice BKϪ/Ϫ mice were generated as described. 15 Wild type (WT) and BK Ϫ/Ϫ mice with the hybrid SV129/C57BL6 background (always F2 generation) were used. Either litter-or age-matched animals were randomly assigned to the experimental procedures undertaken in accordance with the German legislation on protection of animals. Immunohistochemistry of Adrenal GlandFor immunofluorescence, on-slide 5-m cryostat slices from nonfixed WT and BK Ϫ/Ϫ adrenal glands were incubated with anti-BK␣ (674 -1115) . BK...

To determine the number of open Ca(2+) channels necessary for transmitter release at the inhibitory basket cell-granule cell synapse in rat hippocampus, we combined presynaptic Ca(2+) imaging, recording of postsynaptic currents and modeling. We found that that the opening of three or fewer Ca(2+) channels triggered transmitter release. Furthermore, a small number of Ca(2+) channels were able to evoke release with high temporal precision, despite stochastic Ca(2+) channel opening.