Neural Activity During The Formation Of A Giant Auditory Synapse

The formation of synapses is a critical step in the development of the brain. During this developmental stage neural activity propagates across the brain from synapse to synapse. This activity is thought to instruct the precise, topological connectivity found in the sensory central nervous system. In the General Introduction some possible hypotheses for establishing topological connectivity in the brain are put forward. In this thesis I demonstrated that during the period when a giant auditory synapse is formed, the calyx of Held synapse, including both the calyx itself and its postsynaptic target, the principal neuron of the medial nucleus of the trapezoid body, displays bursts of neural firing.

With our unique in vivo approach I describe in CHAPTER 2 the patterns of neural activity at the calyx of Held in neonatal rats. Despite the young age of the pup and the early stage of calyx development, very brief intervals between action potentials (APs) were observed with only minor changes in the shape of the calyceal AP. One of the processes that keeps the AP shape invariant was related to the absolute membrane potential attained following the AP, the after-potential. I propose that there might be an essential role for the after-potential to stabilize the AP shape in high frequency firing.

In CHAPTER 3 I focus on the developmental changes in the calyx of Held synapse and the postsynaptic neuron. In a few days a relay synapse emerges that reliably drives postsynaptic activity. The other synaptic inputs become less relevant for postsynaptic firing, because of changes in the intrinsic properties of the postsynaptic neuron. I demonstrate a clear correlation between the postsynaptic excitability and the emergence of the relay synapse, possibly indicating a homeostatic matching of the size of the inputs and the size of the input resistance. In the days prior to the appearance of the relay synapse, the activity of many converging synapses caused a continuous depolarization. By means of modeling I suggest that this depolarization only helps to trigger postsynaptic APs in the period before the relay synapse emerges, as an increase in low-threshold potassium channels precluded the triggering of postsynaptic APs by slowly-rising, prolonged depolarizations at the later developmental stages.

From the many terminals that connect to a principal neuron in neonatal rats, typically a single calyx of Held remains in the adult. During this period I expected that multiple calyces would initially form on a principal cell followed by the selection of a single ‘winner’ calyx of Held, as indicated by some other studies. In CHAPTER 4 I describe in detail that we do not observe electrophysiological evidence for multiple calyceal innervations. Weak inputs were present throughout this period. While the strongest input became stronger, the second strongest input retained its strength. Our attempt to retrieve the morphology of the terminals that are formed on the recorded neuron did not reveal a clear mismatch between our electrophysiological recordings and the terminal. I propose some possible explanations for the apparent discrepancy between previous studies and my findings. In the end, either multiple calyces are rarely formed on single rat principal cells or they are very weak for their giant size.

In the General Discussion I address some possible confounding factors in my research. I return to the propagation of neural activity in the auditory brainstem and the mechanisms that are essential for firing with high frequencies. I elaborate on the role of the after-potential to expand on the idea that the after-potential has a role in stabilizing neural activity that extends beyond the calyx of Held. I also speculate on its possible influence on synaptic neurotransmission which could be an intriguing, new type of plasticity. Next, the neural activity during the formation of the calyx of Held synapse is compared to what is known about the formation of other well-studied synapses. I highlight the similarities in their development with emphasis on a temporal dissociation of synaptic strengthening and synaptic elimination, and the role of synchronized and desynchronized activity. I briefly discuss the hypothesis that a form of cooperation other than synaptic competition might be crucial to the development of the sensory nervous system and continue to suggest what the major next steps will be to understand the formation of the calyx of Held. This thesis will hopefully contribute towards making the calyx of Held synapse a leading research model to investigate the role of neural activity in synapse formation.