Developmental fine-tuning of medial superior olive neurons mitigates their predisposition to contralateral sound sources

Having two ears enables us to localize sound sources by exploiting interaural time differences (ITDs) in sound arrival. Principal neurons of the medial superior olive (MSO) are sensitive to ITD, and each MSO neuron responds optimally to a best ITD (bITD). In many cells, especially those tuned to low sound frequencies, these bITDs correspond to ITDs for which the contralateral ear leads, and are often larger than the ecologically relevant range, defined by the ratio of the interaural distance and the speed of sound. Using in vivo recordings in gerbils, we found that shortly after hearing onset the bITDs were even more contralaterally leading than found in adult gerbils, and travel latencies for contralateral sound-evoked activity clearly exceeded those for ipsilateral sounds. During the following weeks, both these latencies and their interaural difference decreased. A computational model indicated that spike timing-dependent plasticity can underlie this fine-tuning. Our results suggest that MSO neurons start out with a strong predisposition toward contralateral sounds due to their longer neural travel latencies, but that, especially in high-frequency neurons, this predisposition is subsequently mitigated by differential developmental fine-tuning of the travel latencies.</p

A model of the prespike of the calyx of Held synapse in the auditory brainstem

The calyx of Held synapse is a giant axosomatic synapse in the brainstem that functions as a fast and reliable auditory relay. Because of its large size, the presynaptic action potential can be picked up in a recording of the postsynaptic cell. This so-called prespike is caused by the ephaptic coupling of presynaptic membrane currents via the synaptic cleft. We constructed an electric model of the prespike and estimated values for the leak conductance of the synaptic cleft, the capacitance of the release face and the presynaptic calcium current density based on simultaneous pre- and postsynaptic voltage clamp recordings. These values allowed us to estimate an impact of presynaptic capacitive and ionic currents on the cleft potential, and suggested that fenestration of the calyx of Held during development might be important to further reduce the cleft leak conductance of the cleft to minimize its impact on the presynaptic calcium channels

Using ephaptic coupling to estimate the synaptic cleft resistivity of the calyx of Held synapse

At synapses, the pre- and postsynaptic cells get so close that currents entering the cleft do not flow exclusively along its conductance, gcl. A prominent example is found in the calyx of Held synapse in the medial nucleus of the trapezoid body (MNTB), where the presynaptic action potential can be recorded in the postsynaptic cell in the form of a prespike. Here, we developed a theoretical framework for ephaptic coupling via the synaptic cleft, and we tested its predictions using the MNTB prespike recorded in voltage-clamp. The shape of the prespike is predicted to resemble either the first or the second derivative of the inverted presynaptic action potential if cleft currents dissipate either mostly capacitively or resistively, respectively. We found that the resistive dissipation scenario provided a better description of the prespike shape. Its size is predicted to scale with the fourth power of the radius of the synapse, explaining why intracellularly recorded prespikes are uncommon in the central nervous system. We show that presynaptic calcium currents also contribute to the prespike shape. This calcium prespike resembled the first derivative of the inverted calcium current, again as predicted by the resistive dissipation scenario. Using this calcium prespike, we obtained an estimate for gcl of ~1 μS. We demonstrate that, for a circular synapse geometry, such as in conventional boutons or the immature calyx of Held, gcl is scale-invariant and only defined by extracellular resistivity, which was ~75 Ωcm, and by cleft height. During development the calyx of Held develops fenestrations. We show that these fenestrations effectively minimize the cleft potentials generated by the adult action potential, which might otherwise interfere with calcium channel opening. We thus provide a quantitative account of the dissipation of currents by the synaptic cleft, which can be readily extrapolated to conventional, bouton-like synapses

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

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 (Vafter) was close to the value at which no depression would occur during high-frequency activity. Immunolabeling revealed that Nav1.6 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

Structure–function relation of the developing calyx of Held synapse in vivo

International audienceIn adult rodents, a principal neuron in the medial nucleus of the trapezoid (MNTB) is generally contacted by a single, giant axosomatic terminal called the calyx of Held. How this one‐on‐one relation is established is still unknown, but anatomical evidence suggests that during development principal neurons are innervated by multiple calyces, which may indicate calyceal competition. However, in vivo electrophysiological recordings from principal neurons indicated that only a single strong synaptic connection forms per cell. To test whether a mismatch exists between synaptic strength and terminal size, we compared the strength of synaptic inputs with the morphology of the synaptic terminals. In vivo whole‐cell recordings of the MNTB neurons from newborn Wistar rats of either sex were made while stimulating their afferent axons, allowing us to identify multiple inputs. The strength of the strongest input increased to calyceal levels in a few days across cells, while the strength of the second strongest input was stable. The recorded cells were subsequently immunolabelled for vesicular glutamate transporters (VGluT) to reveal axosomatic terminals with structured‐illumination microscopy. Synaptic strength of the strongest input was correlated with the contact area of the largest VGluT cluster at the soma (r = 0.8), and no indication of a mismatch between structure and strength was observed. Together, our data agree with a developmental scheme in which one input strengthens and becomes the calyx of Held, but not with multi‐calyceal competition

Axial currents recordings in mouse retinal ganglion cells

Patch-clamp recordings in mouse retinal ganglion cells performed by Martijn Sierksma. It comprises axial currents recorded in voltage clamp, spontaneous activity and the response to current pulses. The cells were filled with biocytin and subsequently labelled for ankyrin G. The dataset comprises confocal images of the full neurons morphology and their axon initial segments performed by Sarah Goethals

Axial currents recordings in mouse retinal ganglion cells

Patch-clamp recordings in mouse retinal ganglion cells performed by Martijn Sierksma. It comprises axial currents recorded in voltage clamp, spontaneous activity and the response to current pulses. The cells were filled with biocytin and subsequently labelled for ankyrin G. The dataset comprises confocal images of the full neurons morphology and their axon initial segments performed by Sarah Goethals