Optogenetic Visualization of Presynaptic Tonic Inhibition of Cerebellar Parallel Fibers

Area of Science:

• Neuroscience
• Cellular Neuroscience
• Synaptic Physiology

Background:

• Tonic inhibition, mediated by GABAA receptors, influences neuronal excitability.
• Its presence and function in presynaptic terminals, specifically cerebellar parallel fibers, remain less understood.

Purpose of the Study:

• To investigate the presence and characteristics of tonic inhibition in cerebellar granule cell axons (parallel fibers).
• To determine the functional impact of presynaptic tonic inhibition on synaptic transmission and cerebellar circuit function.

Main Methods:

• Utilized transgenic mice expressing the optogenetic chloride indicator, Clomeleon, for imaging chloride concentration.
• Employed electrophysiological recordings in brain slices and in vivo to measure GABAA receptor activity and synaptic transmission.
• Pharmacologically blocked GABAA receptors to assess their role in tonic inhibition.

Main Results:

• Tonic inhibition was detected in cerebellar granule cell bodies and their axons (parallel fibers), contributing significantly to resting chloride concentration.
• Parallel fibers exhibit distinct GABAA receptors: high-affinity for ambient GABA mediating tonic inhibition (chloride influx), and low-affinity causing excitation (chloride efflux).
• Presynaptic tonic inhibition was found to regulate glutamate release, enhancing spontaneous and evoked excitatory postsynaptic currents at the parallel fiber-Purkinje cell synapse.

Conclusions:

• Tonic inhibition in parallel fibers is a key regulator of presynaptic neurotransmitter release.
• This presynaptic signaling mechanism plays a crucial role in modulating information flow within cerebellar circuits.
• The interplay between phasic and tonic signaling represents a general strategy for fine-tuning synaptic circuits.