The study of axonal physiology in adult cerebellar interneurons – AxonalPhysiology

In the commonly accepted view of synaptic integration, synaptic signals are collected in the somatodendritic domain and summed in the axon initial segment, giving rise to an all-or-none response, the action potential, that is transmitted along the axon and triggers a new wave of transmitter release in presynaptic terminals. In this view, the axon simply behaves as a point-to-point relay. However, recent studies indicate significant deviations from this scheme, particularly in small neurons such as cerebellar interneurons (called MLIs) and suggest that the axon performs rich computations that influence neuronal integration, signal transmission and synaptic efficacy. The development of electrophysiological and microscopy methods has certainly contributed to the recent interest in axons, because they allow to target and record from this small cellular compartment with high temporal resolution.

This project proposes to perform simultaneous electrophysiological recordings from the soma and from presynaptic terminals of a short-axon interneuron of the cerebellar cortex, the MLI. With this approach, we aim at:

a) Assessing effects on synaptic integration ascribable to passive electrical coupling between somatic and axonal compartments (called “analog signaling”). Analog signaling has recently been described in vertebrate neurons and appears in two flavours: the “orthodromic” analog signaling, where subthreshold activity in the dendrites can be passively transmitted to the axon and to the presynaptic terminals and impact on the amount of released neurotransmitter; the “antidromic” analog signaling, where information arising locally in the axon can backpropagate and impact on neuron’s integration. Here, by performing simultaneous intracellular recordings from the soma and the axon combined with laser photolysis of GABA and glutamate, we will quantify the amount of coupling between the different compartments, we will assess the role of the axonally located Ih and INaP conductances, and finally, we will investigate the effect of the physiological (real EPSPs and IPSPs of dendritic origin) coupling on transmitter release.

b) Determining the incidence of axonal ectopic spikes in MLIs and examine their effect on transmitter release. Axonal ectopic spiking has been described in a vast range of pathological conditions, but its incidence in normal conditions has been scarce. Our preliminary results indicate that the axon of MLIs is able to produce spikes independently from the soma. In this proposal we will investigate two fundamental questions in relation to the occurrence of axonal ectopic spikes: how they are generated and what is their impact on transmitter release.

Altogether, our study will improve our understanding of synaptic integration by providing new insights into the cellular mechanisms of analog signaling and axonal ectopic spiking and will potentially be applicable to other, small central neurons. By providing a detailed picture of axonal function, it will also have an impact on our understanding of neurological disorders where axonal excitability and axonal conduction are abnormal.