Ca2+ cooperativity in neurosecretion measured using photolabile Ca2+ chelators

Area of Science:

• Neuroscience
• Cellular Biology
• Biophysics

Background:

• Calcium ions (Ca2+) are critical for neurotransmitter release at synapses.
• The precise mechanism and cooperativity of Ca2+ action remain areas of active investigation.

Purpose of the Study:

• To quantify the cooperativity of Ca2+ in triggering neurotransmitter release at the crayfish motor neuron terminal.
• To investigate the relationship between presynaptic intracellular calcium concentration ([Ca2+]i) and postsynaptic response magnitude.

Main Methods:

• Injected the photolabile Ca2+ chelator DM-nitrophen into crayfish motor neuron terminals.
• Photolyzed DM-nitrophen using light flashes of varying intensity.
• Measured postsynaptic currents under voltage clamp and postsynaptic potentials.
• Utilized computational simulations to model Ca2+ dynamics and transmitter release.

Main Results:

• A phasic postsynaptic response, resembling an excitatory junctional potential, was observed following each light flash.
• Ca2+ cooperativity of approximately 3-4 was inferred from the supralinear dependence of responses on peak [Ca2+]i.
• The time course of flash-evoked transmitter release showed slower kinetics than typical excitatory postsynaptic potentials, including a slow tail.

Conclusions:

• The study quantitatively demonstrates a Ca2+ cooperativity of 3-4 in neurotransmitter release at this synapse.
• The findings provide insights into the molecular mechanisms underlying Ca2+ -dependent exocytosis.
• A model integrating DM-nitrophen photolysis, binding, and Ca2+ activation successfully explained the observed time course of transmitter release.