Atypical delayed paired-pulse depression at an identified synapse.

Area of Science:

• Neuroscience
• Synaptic Plasticity
• Cellular Electrophysiology

Background:

• Synaptic plasticity, including facilitation and depression, is modulated by stimulation patterns, presynaptic calcium, and vesicle pool dynamics.
• Synaptic facilitation arises from calcium accumulation, while depression is typically linked to vesicle pool depletion.
• Existing models struggle to explain all observed plasticity phenomena.

Purpose of the Study:

• To investigate an unusual form of paired-pulse delayed depression at the synapse between pressure-sensitive and anterior pagoda neurons in the leech.
• To elucidate the underlying mechanisms of this novel synaptic depression.
• To propose a new model that accounts for the observed plasticity phenomena.

Main Methods:

• Electrophysiological recordings of synaptic transmission between leech neurons.
• Application of varying interstimulus intervals to induce facilitation and depression.
• Experimental manipulation using EGTA-AM to probe calcium dependency.

Main Results:

• Short interstimulus intervals (<700 ms) induced facilitation, consistent with residual calcium.
• Long interstimulus intervals (>700 ms) produced a delayed depression, increasing with interval duration up to 1000 ms.
• This delayed depression was reduced by EGTA-AM and did not correlate with vesicle depletion, deviating from classical models.

Conclusions:

• A novel, calcium-dependent delayed synaptic depression mechanism exists, independent of vesicle pool depletion.
• Classical models of synaptic plasticity are insufficient to explain these findings.
• A new model involving a slow, calcium-dependent mechanism for vesicle removal from the readily releasable pool is proposed.