Resistance to action potential depression of a rat axon terminal in vivo

Abstract

The shape of the presynaptic action potential (AP) has a strong impact on neurotransmitter release. Because of the small size of most terminals in the central nervous system, little is known about the regulation of their AP shape during natural firing patterns in vivo. The calyx of Held is a giant axosomatic terminal in the auditory brainstem, whose biophysical properties have been well studied in slices. Here, we made whole-cell recordings from calyceal terminals in newborn rat pups. The calyx showed a characteristic burst firing pattern, which has previously been shown to originate from the cochlea. Surprisingly, even for frequencies over 200 Hz, the AP showed little or no depression. Current injections showed that the rate of rise of the AP depended strongly on its onset potential, and that the membrane potential after the AP (V after ) was close to the value at which no depression would occur during high-frequency activity. Immunolabeling revealed that Na v 1.6 is already present at the calyx shortly after its formation, which was in line with the fast recovery from AP depression that we observed in slice recordings. Our findings thus indicate that fast recovery from depression and an inter-AP membrane potential that minimizes changes on the next AP in vivo, together enable high timing precision of the calyx of Held already shortly after its formation.A ction potentials (APs) are followed by a period of decreased excitability called the refractory period. High-frequency firing thus requires special adaptations to minimize this refractory period and maintain AP stability. The changes in the AP waveform that occur at high firing frequencies are especially relevant in presynaptic terminals, where the shape of the AP critically controls calcium influx via voltage-dependent calcium channels, and thus transmitter release (1, 2). Following the AP, the membrane potential during the recovery period has a large influence on the speed of the recovery from inactivation of voltage-dependent sodium channels and deactivation of voltagedependent potassium channels, which are two major determinants of the refractory period (2). In some terminals, the AP is followed by a depolarizing afterpotential (DAP) (3-9), whereas in others a hyperpolarizing afterpotential (HAP) has been observed (10-14). The sign of this afterpotential depends on the resting potential (6,7,15), suggesting that the membrane potential following the AP (V after ) might be more important than the sign of the afterpotential.The calyx of Held is a glutamatergic axosomatic terminal whose biophysical properties have been well studied (16). Its many release sites enable it to act as an inverting relay synapse within the auditory brainstem that reliably drives its postsynaptic partner, a principal neuron in the medial nucleus of the trapezoid body (MNTB), even at firing frequencies >200 Hz (17). Shortly after its formation, around postnatal day 2 (P2) in rodents (18)(19)(20), it already fires in characteristic high-frequency bursts in vivo (21,22). In slice studi...

Spike activity regulates vesicle filling at a glutamatergic synapse

“…Na + flows into the cytosol during the firing of action potentials, while the location of Na + channels in the calyx of Held is under debate (Huang and Trussell, 2008; Kim et al, 2010; Leao et al, 2005; Sierksma and Borst, 2017). Whole-cell recordings were made at the mouse calyx of Held and the presynaptic Na + changes during action potential (AP) firing were assayed using two-photon laser scanning microscopy (Fig.…”

KCNQ channels enable reliable presynaptic spiking and synaptic transmission at high-frequency

“…In P12–P14 rat brain slices, reliable synaptic transmission was generally possible at frequencies up to 600 Hz for 50 stimuli (Taschenberger and von Gersdorff, 2000). In vivo recordings from mice showed that AP showed little or no (<4%) depression when instantaneous firing frequencies over 200 Hz (Sierksma and Borst, 2017). Here we showed amplitude decrease during 100 Hz firing for 4 s was 3% while 333 Hz firing decreased the AP amplitude by 20% (Fig.…”

“…2), indicating very stable AP waveforms during prolonged high-frequency firing. Two factors appeared to be crucially important for the lack of a change in the AP’s shape: fast recovery of Na + channels from AP depression (Leao et al, 2005) and relative stable while negative membrane potential following the AP (Sierksma and Borst, 2017). The recovery of Na + from inactivation is sharply dependent on the membrane potential, as 20 mV of hyperpolarization double the speed of recovery (Leao et al, 2005).…”

“…Reliability and precision of synaptic transmission are required in the calyx-MNTB synapse in order to encode location of transient sounds in natural environments (Joris and Trussell, 2018). One factor that contributes to this precision is the remarkable stability of the presynaptic waveform during ongoing firing (Sierksma and Borst, 2017). Indeed, calyx of Held terminals show only minor AP waveform change at spike frequencies up to hundreds of Hertz.…”

Structure-function relation of the developing calyx of Held synapsein vivo

“…This switch may be explained by an increase in the currents 333 over the calyceal membrane facing the synaptic cleft or by an increase in the coupling between the 334 pre-and the postsynaptic membranes. The currents depend on apposition area (Hoffpauir et al, 335 2006;Holcomb et al, 2013), time course of the calyceal AP, which accelerates considerably during 336 early development (Taschenberger & von Gersdorff, 2000;Sierksma & Borst, 2017), isochronicity of 337 the presynaptic currents, and the subcellular distribution of presynaptic ion channels. In addition, 338 the coupling depends on synaptic cleft resistivity and postsynaptic admittance (Savtchenko, 2007).…”