Reversed State Sodium-Calcium Exchanger Contributes to Synaptic Facilitation in the Crayfish Neuromuscular Junction

Abstract

The sodium-calcium exchanger (NCX) normally transports three Na⁺ into the cell and one Ca²⁺ out of the cell. When the axon depolarizes during stimulation or an action potential, the action of the exchanger reverses (Arao et al. 2007). We studied how the reversal of the sodium-calcium exchanger affected synaptic transmission and facilitation. The drug KB-R7943 prevents the reversal of the NCX with high specificity of action (Arao et al. 2007). We acquired our data using intracellular recording of EPSPs at crayfish neuromuscular junctions while stimulating the associated nerves using paired pulses. We conducted our study by measuring EPSPs resulting from paired pulses in the superficial extensor muscle cells of crayfish using microelectrodes. We found that blocking the reversal of the NCX led to lower facilitation as compared with trials where reversal was not blocked. To support our observations regarding the effects of NCX reversal on facilitation, we repeated the experiment in a low-sodium environment and found the same trend that inhibition of NCX reversal lowered facilitation. This suggests that the calcium pumped into the cell by the NCX in its reversed state plays a role in facilitation. These findings help to understand the physiology of synaptic transmission and can serve as a base for further research aiming to rectify problems in synaptic transmission, such as those found in neurodegenerative diseases.