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Mechanisms of short-term depression at the calyx of Held. a Inactivation of presynaptic Ca 2+ current contributes to synaptic
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The calyx of Held is a large glutamatergic synapse in the mammalian auditory brainstem. By using brain slice preparations, direct patch-clamp recordings can be made from the nerve terminal and its postsynaptic target (principal neurons of the medial nucleus of the trapezoid body). Over the last decade, this preparation has been increasingly employe...
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The study of synaptic transmission in brain slices generally entails the patch-clamping of postsynaptic neurones and stimulation of identified presynaptic axons using a remote electrical stimulating electrode. Although patch recording from postsynaptic neurones is routine, many presynaptic axons take tortuous turns and are severed in the slicing pr...
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... Consistent with our conclusion, manipulations designed to augment opioid-compromised synaptic transmission, for example by either pharmacologically depolarizing presynaptic terminals with specific presynaptic potassium channel blockers, or by allosterically potentiating the open-time kinetics of post-synaptic AMPA receptors with ampakines ), may provide a useful framework and strategy for reversing OIRD. Glutamatergic synaptic transmission is the target of a variety of excitatory neuromodulators (Schneggenburger and Forsythe, 2006;Doi and Ramirez, 2008). The inhibitory action of opioids on glutamatergic synapses, acting in concert with endogenous neuromodulators released in a state-dependent manner, may thus influence the efficacy of in vivo respiratory depression exerted by opioids. ...
Opioid-induced respiratory depression (OIRD) is the major cause of death associated with opioid analgesics and drugs of abuse, but the underlying cellular and molecular mechanisms remain poorly understood. We investigated opioid action in vivo in unanesthetized mice and in in vitro medullary slices containing the preBötzinger Complex (preBötC), a locus critical for breathing and inspiratory rhythm generation. Although hypothesized as a primary mechanism, we found that mu-opioid receptor (MOR1)-mediated GIRK activation contributed only modestly to OIRD. Instead, mEPSC recordings from genetically identified Dbx1-derived interneurons, essential for rhythmogenesis, revealed a prevalent presynaptic mode of action for OIRD. Consistent with MOR1-mediated suppression of presynaptic release as a major component of OIRD, Cacna1a KO slices lacking P/Q-type Ca²⁺ channels enhanced OIRD. Furthermore, OIRD was mimicked and reversed by KCNQ potassium channel activators and blockers, respectively. In vivo whole-body plethysmography combined with systemic delivery of GIRK- and KCNQ-specific potassium channel drugs largely recapitulated these in vitro results, and revealed state-dependent modulation of OIRD. We propose that respiratory failure from OIRD results from a general reduction of synaptic efficacy, leading to a state-dependent collapse of rhythmic network activity.
... Furthermore, the synapses of globular bushy cells onto inhibitory neurons in the MNTB also change. This synapse, called the calyx of Held, has been widely studied as a model for synaptic function and development (von Gersdorff and Borst, 2002;Schneggenburger and Forsythe, 2006). After monaural CHL, calyces increase the number of terminal swellings (Grande et al., 2014), which most likely correspond to synaptic contacts (Wimmer et al., 2006). ...
The effects of traumatic noise-exposure and deafening on auditory system function have received a great deal of attention. However, lower levels of noise as well as temporary conductive hearing loss also have consequences on auditory physiology and hearing. Here we review how abnormal acoustic experience at early ages affects the ascending and descending auditory pathways, as well as hearing behavior.
... Furthermore, APs undergo changes in the amplitude and width during development and repetitive neural activity resulting from inactivation or facilitation of Na + and K + channels 9,22-28 . To systematically study how the diverse waveform of presynaptic APs determines the timing of Ca 2+ influx and transmitter release, we have performed voltage clamp recordings of presynaptic calcium currents (I Ca ) and excitatory postsynaptic currents (EPSC) at the calyx of Held synapse in the mouse auditory brainstem, which is an ideal model for biophysical analysis of synaptic properties 12,14,[29][30][31][32][33][34] . By blocking presynaptic voltage-gated K + and Na + channels with tetraethylammonium (TEA) and tetrodotoxin (TTX) respectively, we find that both channels contribute to the onset timing of I Ca and EPSC by targeting the width and amplitude of APs recorded from the immature and mature synapses. ...
... presynaptic K + and Na + channels control the onset of I Ca and epsC. The calyx of Held synapse is an axosomatic synapse known for its speed and precision in transmitting temporal information in the sound localization pathway 19,31,[35][36][37] . After onset of hearing at postnatal day (P) 12, this synapse undergoes rapid maturation to achieve its functionality. ...
The waveform of presynaptic action potentials (APs) regulates the magnitude of Ca2+ currents (ICa) and neurotransmitter release. However, how APs control the timing of synaptic transmission remains unclear. Using the calyx of Held synapse, we find that Na+ and K+ channels affect the timing by changing the AP waveform. Specifically, the onset of ICa depends on the repolarization but not depolarization rate of APs, being near the end of repolarization phase for narrow APs and advancing to the early repolarization phase for wide APs. Increasing AP amplitude has little effect on the activation but delays the peak time of ICa. Raising extracellular Ca2+ concentration increases the amplitude of ICa yet does not alter their onset timing. Developmental shortening of APs ensures ICa as a tail current and faithful synaptic delay, which is particularly important at the physiological temperature (35 °C) as ICa evoked by broad pseudo-APs can occur in the depolarization phase. The early onset of ICa is more prominent at 35 °C than at 22 °C, likely resulting from a temperature-dependent shift in the activation threshold and accelerated gating kinetics of Ca2+ channels. These results suggest that the timing of Ca2+ influx depends on the AP waveform dictated by voltage-gated channels and temperature.
... In rodents, the expression and maturation of PNs in the SOC coincides with critical morphological and physiological developmental processes and the onset of response to airborne sound (Sonntag et al., 2009). In the MNTB, these changes encompass the (1) transition from multiple inputs to mono-innervation by the large calyx of Held during the first postnatal week (Bergsman, Camilli, & McCormick, 2004;Holcomb et al., 2013;Rodriguez-Contreras, van Hoeve, John Silvio Soria, Habets, Locher, & Borst, 2008; for review see Yu & Goodrich, 2014); (2) the growth and fenestration of the calyx of Held up to the age of P14 (Ford, Grothe, & Klug, 2009;Hoffpauir, Grimes, Mathers, & Spirou, 2006;Kandler & Friauf, 1993;Morest, 1968; for review see Yu & Goodrich, 2014); and (3) the acceleration of synaptic transmission speed, shortening of action potential duration, and increase in synaptic fidelity along with changes in the composition and expression of receptors and channels during the second postnatal week (Sonntag, Englitz, Typlt, & Rubsamen, 2011;Taschenberger & Gersdorff, 2000; for review see Gersdorff & Borst, 2002;Schneggenburger & Forsythe, 2006). Likewise, developmental changes with respect to refinement, pruning, and strengthening of synaptic contacts occur in the MSO and LSO during the second postnatal week, resulting in functional networks around hearing onset (Kandler, Clause, & Noh, 2009). ...
... However, in a recent comparative study it was shown that enzymatic digestion of PNs around neurons in the MNTB and around cortical interneurons results in a similar reduction of excitability in both types of neurons, thus indicating that the function of PNs might be the same irrespective of the type of neuron or transmitter (Balmer, 2016). In addition, especially the huge calyx of Held in the MNTB, which is a unique model synapse for the investigation of glutamatergic synaptic transmission (Gersdorff & Borst, 2002;Schneggenburger & Forsythe, 2006), might greatly help to study the influence of PNs on synaptic transmission. It is currently not possible to elucidate this in the same extent in the cortex and hippocampus as the direct access to the synapses is hampered due to their small sizes. ...
... These channels are inhibited by tetraethylammoniun (TEA) and control the duration of the pre-synaptic action potentials . The reason for this particular synaptic localization is unknown, but it is postulated that this would avoid an accumulation of potassium in the synaptic cleft, where could lead to the depolarization of the terminal during trains of action potentials (Schneggenburger and Forsythe, 2006). Channels containing K v7.5 subunits are also present in the terminal and control the resting membrane potential of the calyx (Huang and Trussell, 2011). ...
The auditory part of the brainstem is composed of several nuclei specialized in the computation of the different spectral and temporal features of the sound before it reaches the higher auditory regions. There are a high diversity of neuronal types in these nuclei, many with remarkable electrophysiological and synaptic properties unique to these structures. This diversity reflects specializations necessary to process the different auditory signals in order to extract precisely the acoustic information necessary for the auditory perception by the animal. Low threshold Kv1 channels and HCN channels are expressed in neurons that use timing clues for auditory processing, like bushy and octopus cells, in order to restrict action potential firing and reduce input resistance and membrane time constant. Kv3 channels allow principal neurons of the MNTB and pyramidal DCN neurons to fire fast trains of action potentials. Calcium channels on cartwheel DCN neurons produce complex spikes characteristic of these neurons. Calyceal synapses compensate the low input resistance of bushy and principal neurons of the MNTB by releasing hundreds of glutamate vesicles resulting in large EPSCs acting in fast ionotropic glutamate receptors, in order to reduce temporal summation of synaptic potentials, allowing more precise correspondence of pre- and post-synaptic potentials, and phase-locking. Pre-synaptic calyceal sodium channels have fast recovery from inactivation allowing extremely fast trains of action potential firing, and persistent sodium channels produce spontaneous activity of fusiform neurons at rest, which expands the dynamic range of these neurons. The unique combinations of different ion channels, ionotropic receptors and synaptic structures create a unique functional diversity of neurons extremely adapted to their complex functions in the auditory processing.
... Several cellular specializations, like fast-conducting midlinecrossing axons (Renier et al., 2010;Ford et al., 2015), and large and fast-releasing excitatory synapses are found at specific sites in the auditory brainstem to enable fast synaptic processing (Trussell, 1999). One example of a fast-processing excitatory connection is the calyx of Held, which connects a globular bushy cell (GBC) in the VCN with a principal neuron in the MNTB via a crossed axon projection (Cant and Benson, 2003;Schneggenburger and Forsythe, 2006;Borst and Soria van Hoeve, 2012). The maturation of functional parameters of synaptic transmission which takes places at the calyx of Held around hearing onset has been studied intensely (Joshi and Wang, 2002;Taschenberger et al., 2002;Takahashi, 2015); hearing onset occurs in rodents at ~ postnatal days (P) 10-14. ...
Large excitatory synapses are found at specific points in the neuronal circuits of the auditory brainstem, to enable fast information transfer and the preservation of acoustic timing information. The extracellular cues and signaling mechanisms that lead to the development of these specialized synaptic connections, exemplified by the calyx of Held in the medial nucleus of the trapezoid body (MNTB), are still largely unknown. Here, we investigate the role of BMP signaling for the early development of the ventral cochlear nucleus (VCN) and MNTB, and for the initial formation of the calyx of Held synaptic connection. We used conditional alleles of two BMP type‐1 receptors in the background of a constitutive BMPR1b knock‐out (KO), or else a conditional allele of SMAD4. The conditional alleles were recombined by the Krox20Cre mouse line that is active around mid‐gestation in rhombomeres (r) 3 and 5 from which the VCN and MNTB are derived; alternatively, virus‐mediated Cre‐expression was performed early postnatally in the VCN. The data shows that embryonic SMAD‐dependent BMP‐signaling in r3 and r5 contributes to the histogenesis of auditory brainstem nuclei. On the other hand, BMP‐receptor signaling early postnatally in presynaptic neurons of the calyx of Held projection is necessary for correct axon branch retraction, which suggests a cell‐autonomous role of presynaptic BMP‐receptors in synapse elimination at the developing calyx of Held. Thus, our work dissects developmentally early and late roles of BMP‐signaling for the formation of auditory brainstem nuclei, and for the development of the highly specialized synaptic connectivity in these structures.
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... The calyx of Held is a giant axon terminal on neurons in the medial nucleus of the trapezoid body (MNTB). This terminal has played a pivotal role in the identification (Guillery, 2005) and understanding of the structure and function of chemical CNS synapses (Borst and Soria van Hoeve, 2012;Schneggenburger and Forsythe, 2006). Compelling images of light and EM reconstructions of the handlike calyx holding the soma of an MNTB neuron have adorned many journal covers in the past decades (Fig. 1). ...
The calyx of Held is the preeminent model for the study of synaptic function in the mammalian CNS. Despite much work on the synapse and associated circuit, its role in hearing remains enigmatic. We propose that the calyx is one of the key adaptations that enables an animal to lateralize transient sounds. The calyx is part of a binaural circuit that is biased toward high sound frequencies and is sensitive to intensity differences between the ears. This circuit also shows marked sensitivity to interaural time differences, but only for brief sound transients (“clicks”). In a natural environment, such transients are rare except as adventitious sounds generated by other animals moving at close range. We argue that the calyx, and associated temporal specializations, evolved to enable spatial localization of sound transients, through a neural code congruent with the circuit's sensitivity to interaural intensity differences, thereby conferring a key benefit to survival. The calyx of Held is a striking and well-studied synaptic specialization in an auditory brainstem circuit, but its functional role is not self-evident. Joris and Trussell provide a new hypothesis on the role of the calyx and its assorted specializations.
... We have studied SNAT3 transporter function in astrocytes located immediately adjacent to the calyx of Held synapse in brain slices from the auditory brainstem of rats and mice. The calyx of Held is a large glutamatergic presynaptic terminal that can be visually identified in brain slices [46]. Astrocytes are in close association with this synapse [47] and heavily express SNAT3 [25]. ...
Astrocytes are glial cells that have an intimate physical and functional association with synapses in the brain. One of their main roles is to recycle the neurotransmitters glutamate and gamma-aminobutyric acid (GABA), as a component of the glutamate/GABA-glutamine cycle. They perform this function by sequestering neurotransmitters and releasing glutamine via the neutral amino acid transporter SNAT3. In this way, astrocytes regulate the availability of neurotransmitters and subsequently influence synaptic function. Since many plasma membrane transporters are regulated by protein kinase C (PKC), the aim of this study was to understand how PKC influences SNAT3 glutamine transport in astrocytes located immediately adjacent to synapses. We studied SNAT3 transport by whole-cell patch-clamping and fluorescence pH imaging of single astrocytes in acutely isolated brainstem slices, adjacent to the calyx of the Held synapse. Activation of SNAT3-mediated glutamine transport in these astrocytes was reduced to 77 ± 6% when PKC was activated with phorbol 12-myristate 13-acetate (PMA). This effect was very rapid (within ~20 min) and eliminated by application of bisindolylmaleimide I (Bis I) or 7-hydroxystaurosporine (UCN-01), suggesting that activation of conventional isoforms of PKC reduces SNAT3 function. In addition, cell surface biotinylation experiments in these brain slices show that the amount of SNAT3 in the plasma membrane is reduced by a comparable amount (to 68 ± 5%) upon activation of PKC. This indicates a role for PKC in dynamically controlling the trafficking of SNAT3 transporters in astrocytes in situ. These data demonstrate that PKC rapidly regulates the astrocytic glutamine release mechanism, which would influence the glutamine availability for adjacent synapses and control levels of neurotransmission.
... The density, proximity and morphology of mitochondria close to the presynaptic active zone is consistent with high metabolic rates at the calyx of Held (Satzler et al. 2002;Perkins et al. 2010). The large size of the calyx presynaptic terminal and its target onto single neurons in the MNTB allows access to both the pre-and the postsynaptic compartments and makes it an ideal preparation for direct investigation of metabolic influence (see von Gersdorff & Borst, 2002;Schneggenburger & Forsythe 2006). This synaptic relay plays an important role in brainstem circuits underlying sound localization (Kopp-Scheinpflug & Forsythe, 2018). ...
... The relationship between presynaptic AP waveform, calcium influx and transmitter release at the calyx of Held is well characterised and detailed (Borst & Sakmann, 1998;Yang & Wang, 2006;Kochubey et al. 2009; for reviews see von Gersdorff Schneggenburger & Forsythe, 2006). Changes in presynaptic AP waveform have been observed following blockade of potassium currents (Wang & Kaczmarek, 1998) in studies of activity-dependent vesicle recycling, and with activity-dependent modulation of presynaptic potassium currents at mossy fibres (Geiger & Jonas, 2000). ...
The superior olivary complex is a group of interconnected brainstem nuclei that receive and integrate binaural auditory input. Each nucleus forms part of local microcircuits subserving multiple complimentary roles in auditory processing, including sound localization, detection of signals in noise, and gap detection. The three nuclei of the trapezoid body (medial, lateral, and ventral) provide indirect inhibitory local projections that are integrated with direct excitatory inputs from the cochlear nuclei at the three output nuclei (the medial and lateral superior olivary nuclei and the superior paraolivary nucleus). Each nucleus expresses a different spectrum of ionic conductances that determine the intrinsic excitability of their principal neurons and adapt how the microcircuit integrates the binaural excitatory and inhibitory synaptic inputs. Specialized synapses, such as the calyx of Held, help maintain temporal information and minimize jitter, while the location of synapses on specific dendrites or somatic regions provides further refinement of the microcircuit. This chapter also includes how the principal neurons of each nucleus express differing densities of ionic conductance by which they exhibit a unique threshold, action potential waveform, and characteristic firing properties. A broad perspective will be provided on how each of these functional elements come together to sculpt the local neuronal microcircuit into performing specific physiological roles for interaural timing discrimination, interaural level discrimination, and gap detection.
... Information processing in the central nervous system (CNS) is mainly achieved by specialized structures called chemical synapses. Synaptic transmission is mediated by one or more neurotransmitter substances, accomplished in the following steps [1][2][3]: (1) action potential triggers opening of voltage-gated calcium channels in the nerve ending; (2) opening of these channels allows an influx of calcium ions into the neuron terminal; (3) on the active zone (AZ) of the cell membrane, calcium ions trigger vesicle fusion and neurotransmitter release into the synaptic cleft; (4) secreted neurotransmitters diffuse into the synaptic cleft, reaching receptors located in the postsynaptic neuron. Postsynaptic excitatory or inhibitory current (I P SC ) or potential (V P SP ) are prompted by neurotransmitters bound to the postsynaptic receptors. ...
The theoretical basis for the neuronal coding, associated with the short term degradation in synaptic transmission, is matter of debate in the literature. In fact, electrophysiological signals are characterized as inversely proportional to stimulus intensity. Among theoretical descriptions for this phenomenon, models based on 1/f-dependency are employed to investigate the biophysical properties of the short term synaptic depression. Thus, considering 1/f-model, as starting point for describing synaptic depletion, we adopt a paradigmatic \textit{q}-differential equation obtain a generalized formalism for investigation of nonextensivity in this specific type of synaptic plasticity. Our analysis reveal nonextensivity in data from electrophysiological recordings, also uncovering a statistical crossover in neurotransmission, which gives additional support to the hypothesis of heterogenous release probability of neurotransmitters. On the other hand, the 1/f-model achieved a satisfatory agreement with data only at lower frequency stimulations. In resume, the present work presents a method to demonstrate that short term depression is ruled by a nonextensive behavior. Our findings also better conciliate morphological and electrophysiological investigations in a coherent biophysical scenario.
