Heterogeneous Ca2+ influx along the adult calyx of held: A structural and computational study

Abstract

The calyx of Held is a morphologically complex nerve terminal containing hundreds to thousands of active zones. The calyx must support high rates of transient, sound-evoked vesicular release superimposed on a background of sustained release, due to the high spontaneous rates of some afferent fibers. One means of distributing vesicle release in space and time is to have heterogeneous release probabilities (P r ) at distinct active zones, which has been observed at several CNS synapses including the calyx of Held. P r may be modulated by vesicle proximity to Ca 2+ channels, by Ca 2+ buffers, by changes in phosphorylation state of proteins involved in the release process, or by local variations in Ca 2+ influx. In this study, we explore the idea that the complex geometry of the calyx also contributes to heterogeneous P r by impeding equal propagation of action potentials through all calyx compartments. Given the difficulty of probing ion channel distribution and recording from adult calyces, we undertook a structural and modeling approach based on computerized reconstructions of calyces labeled in adult cats. We were thus able to manipulate placement of conductances and test their effects on Ca 2+ concentration in all regions of the calyx following an evoked action potential in the calyceal axon. Our results indicate that with a non-uniform distribution of Na + and K + channels, action potentials do not propagate uniformly into the calyx, Ca 2+ influx varies across different release sites, and latency for these events varies among calyx compartments. We suggest that the electrotonic structure of the calyx of Held, which our modeling efforts indicate is very sensitive to the axial resistivity of cytoplasm, may contribute to variations in release probability within the calyx. KeywordsSynaptic transmission; release probability; computational neuroscience; auditory brainstem; electrotonic and active action potential propagation; axial resistanceThe calyx of Held, perhaps the largest nerve terminal in the CNS, occupies a pivotal position in binaural circuits that process interaural temporal delay and intensity cues for sound localization (Held, 1893;Stotler, 1953;Harrison and Warr, 1962;Warr, 1966Warr, , 1972Goldberg and Brown, 1969;Guinan et al., 1972;Elverland, 1977). Sound-evoked activity in globular bushy cells, which give rise to the calyx, can exceed 500 spikes/sec and is characterized by high temporal precision that tracks the acoustic fine structure of low frequency sound and Corresponding Author: Paul B. Manis, Ph.D., Department of Otolaryngology/Head and Neck Surgery, 1123 Bioinformatics Res. Bldg. CB#7070, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-7070, e-mail: pmanis@med.unc.edu, tel: (919) 966-8926. NIH Public Access Author ManuscriptNeuroscience. Author manuscript; available in PMC 2010 January 26. Published in final edited form as:Neuroscience. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript amplitude modulation of high frequency sound (Pfe...

Spatial compartmentalization of AMPA glutamate receptor subunits at the calyx of Held synapse

“…In addition, the clearance of released glutamate may be faster at synapses made by swellings, especially in distal parts of calyceal branches compared with synaptic contacts located in stalks and proximal segments, which would be essential to achieve faster kinetics in the adult EPSCs and to accelerate the recovery of postsynaptic AMPA receptors from desensitization (Taschenberger and von Gersdorff, 2000;Iwasaki and Takahashi 2001;Taschenberger et al 2002;Joshi et al, 2004;Koike-Tani et al, 2005. Interestingly and consistent with this idea, the calyces in the cat dorsolateral MNTB transmitting the lowest frequency auditory signals were smaller and showed less branching than calyces more medially located, leading to a more compact electrotonic structure (Spirou et al, 2008).…”

The Micro-Architecture of Mitochondria at Active Zones: Electron Tomography Reveals Novel Anchoring Scaffolds and Cristae Structured for High-Rate Metabolism

“…1 A ) (Spirou et al, 2008), which collectively can envelop the cell body of their postsynaptic target. The calyx-forming axon is of large caliber, on the order of 10 μ m diameter in cats (Spirou et al, 1990), and typically branches into one to several main stalks of up to 4 μ m diameter.…”

“…The calyx-forming axon is of large caliber, on the order of 10 μ m diameter in cats (Spirou et al, 1990), and typically branches into one to several main stalks of up to 4 μ m diameter. Second and third order branches typically consist of ellipsoid swellings often linked by narrow (<1 μ m diameter) necks (Rowland et al, 2000; Spirou et al, 2008). Numerous mitochondria are located in the core of calyx stalks, branches and swellings but also are arranged near the presynaptic membrane, which contains multiple synaptic sites supported by a large number of synaptic vesicles (Fig.…”

“…Details of surgical procedures have been published recently (Spirou et al, 2008) and are briefly summarized here. One animal, colony-raised to be free of ear infection (Harlan Sprague Dawley, Minneapolis, MN), was decerebrated under ketamine (10 mg/kg; Fort Dodge Animal Health, Fort Dodge, IA) anesthesia after receiving xylazine (10 mg; Phoenix Pharmaceuticals, St Joseph, MO) as a tranquilizer.…”

Modulation and control of synaptic transmission across the MNTB

“…Transmitter release is regulated by various presynaptic mechanisms as described above, and influenced by the complex electrotonic structure of the calyx (Spirou et al, 2008) so that the net effect is highly dependent on the incoming AP rates. But glutamate release and presynaptic modulation is only half the story, the evoked EPSC must be integrated with the voltage-gated currents in the target neuron, where other factors come into play (Fig.…”