In vivo matching of postsynaptic excitability with spontaneous synaptic inputs during formation of the rat calyx of Held synapse

Abstract

Key pointsr Neurons in the medial nucleus of the trapezoid body of anaesthetized rats of postnatal day (P)2-6 showed burst firing with a preferred interval of about 100 ms, which was stable, and a second preferred interval of 5-30 ms, which shortened during development.r In 3 out of 132 cases, evidence for the presence of two large inputs was found. r In vivo whole-cell recordings revealed that the excitability of the principal neuron and the size of its largest synaptic inputs were developmentally matched.r At P2-4, action potentials were triggered by barrages of small synaptic events that summated to plateau potentials, while at later stages firing depended on a single, large and often prespike-associated input, which is probably the nascent calyx of Held.r Simulations with a Hodgkin-Huxley-like model, which was based on fits of the intrinsic postsynaptic properties, suggested an essential role for the low-threshold potassium conductance in this transition. AbstractIn the adult, principal neurons of the medial nucleus of the trapezoid body (MNTB) are typically contacted by a single, giant terminal called the calyx of Held, whereas during early development a principal neuron receives inputs from many axons. How these changes in innervation impact the postsynaptic activity has not yet been studied in vivo. We therefore recorded spontaneous inputs and intrinsic properties of principal neurons in anaesthetized rat pups during the developmental period in which the calyx forms. A characteristic bursting pattern could already be observed at postnatal day (P)2, before formation of the calyx. At this age, action potentials (APs) were triggered by barrages of summating EPSPs causing plateau depolarizations. In contrast, at P5, a single EPSP reliably triggered APs, resulting in a close match between preand postsynaptic firing. Postsynaptic excitability and the size of the largest synaptic events were developmentally matched. The developmental changes in intrinsic properties were estimated by fitting in vivo current injections to a Hodgkin-Huxley-type model of the principal neuron. Our simulations indicated that the developmental increases in I h , low-threshold K + channels and leak currents contributed to the reduction in postsynaptic excitability, but that low-threshold K + channels specifically functioned as a dampening influence in the near-threshold range, thus precluding small inputs from triggering APs. Together, these coincident changes help to propagate bursting activity along the auditory brainstem, and are essential steps towards establishing the relay function of the calyx of Held synapse.

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

“…S2), which is in agreement with its generation by the cochlea (25). In contrast to in vivo postsynaptic recordings (22), no fast synaptic transients were observed in voltage-clamp mode (Fig. 1E), which is consistent with the absence of axo-axonal inputs.…”

“…Fourth, the terminals showed a characteristic firing pattern, consisting of minibursts with high firing frequencies ( Fig. 1D) (22,25,26). Its interval distribution resembled auditory nerve activity at this age (Fig.…”

“…2C shows the maximal rate of rise of the AP as a function of the inter-AP interval in a representative whole-cell recording. Whereas the postsynaptic somatic AP typically depresses >40% at the shortest intervals in vivo (22,26,30), the calyceal AP depressed on average only 4% for intervals <5 ms in whole-cell recordings (0.96 ± 0.03; mean ± SEM; n = 9 calyces; Fig. 2 C and D).…”

“…The day of birth was taken as P0. Neonate pups were anesthetized with isoflurane, intubated, and mechanically ventilated, and underwent a ventral approach to expose the right ventral brainstem (22,51). Before the recordings, anesthesia was reduced to 1% isoflurane, which kept the animal areflexive.…”

“…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 studies, a large DAP has been observed (3), to which resurgent sodium currents (23) make a prominent contribution, and which may promote high-frequency firing (15).…”