Synaptotagmin 7 confers frequency invariance onto specialized depressing synapses

Area of Science:

• Neuroscience
• Synaptic Plasticity
• Molecular Biology

Background:

• Most brain synapses exhibit short-term plasticity, dynamically modulating strength with firing frequency.
• Frequency-dependent synaptic changes are crucial for sensory adaptation and neural computations.
• Some synapses, like auditory, vestibular, and cerebellar, maintain constant strength, efficiently encoding firing rates despite synaptic nonlinearities.

Purpose of the Study:

• To investigate the mechanism of frequency-invariant transmission at Purkinje cell to deep cerebellar nuclei and vestibular synapses.
• To determine the role of synaptotagmin 7 (Syt7) in synapses that exhibit overall depression.

Main Methods:

• Studied synapses from Purkinje cells to deep cerebellar nuclei and vestibular synapses in mice.
• Investigated the role of synaptotagmin 7 (Syt7) using wild-type and Syt7 knockout mice.
• Performed presynaptic rescue of Syt7 expression to confirm its function.

Main Results:

• Purkinje cell and vestibular synapses show initial depression followed by frequency-invariant steady-state responses.
• Synaptotagmin 7 (Syt7) supports a normally masked facilitation that counteracts depression, leading to frequency invariance in wild-type mice.
• Syt7 knockout mice exhibit conventional use-dependent depression, with synapses weakening more at higher frequencies; presynaptic Syt7 rescue restores facilitation and frequency invariance.

Conclusions:

• Synaptotagmin 7 (Syt7) has a crucial role in synapses that display overall depression, revealing a novel function.
• Facilitation, supported by Syt7, unexpectedly counteracts depression to achieve frequency-invariant synaptic transmission.
• This mechanism is vital for efficient neural coding in specific brain circuits.