Abstract
During the last decades it became increasingly evident that electrical synapses are capable of activity-dependent plasticity. However, measuring the actual strength of electrical transmission remains difficult. Usually changes in coupling strength can only be inferred indirectly from measures such as the coupling coefficient and the coupling conductance. Because these are affected by both junctional and non-junctional conductance, plastic changes can potentially be due to both components. Furthermore, these techniques also require the blocking of chemical transmission, so that processes that involve crosstalk between chemical and electrical synapses will be suppressed. To directly examine the magnitude of errors that can occur, we use dual whole-cell current- and voltage-clamp recordings from the soma of the pair of easily accessible, electrically coupled Retzius cells in the leech to simultaneously determine coupling coefficients, coupling conductances and directly measured gap junctional currents. We present the first direct and comparative analysis of gap junction conductance using all three methods and analyze how each method would characterize the response of gap junctions to serotonin. The traditional coupling coefficients showed severe deficits in assessing the symmetry and strength of electrical synapses. These were reduced when coupling conductances were determined and were absent in the direct method. Additionally, both coupling coefficient and coupling conductance caused large and systematic errors in assessing the size and time course of the serotonin-induced changes of gap junctional currents. Most importantly, both measurements can easily be misinterpreted as implying long-term gap junctional plasticity, although the direct measurements confirm its absence. We thus show directly that coupling coefficients and coupling conductances can severely confound plastic changes in membrane and junctional conductance. Wherever possible, voltage clamp measurements should be chosen to accurately characterize the timing and strength of plasticity of electrical synapses. However, we also demonstrate that coupling coefficients can still yield a qualitatively correct picture when amended by independent measurements of the course of membrane resistance during the experiments.
Highlights
Electrical synapses formed by gap junction channels allow the direct flow of electrical currents between coupled neurons
We began by comparing how each of the three methods assessed the symmetry of the electrical synapses, which is typically examined by using the coupling coefficient and the coupling conductance (Haas et al, 2011; Lefler et al, 2014; Wang et al, 2014)
For each R cell pair (n = 17), the strength of electrical coupling was determined in both directions and the neurons with the higher cc, gc, or gj were defined as Retzius cell 1 (R1)
Summary
Electrical synapses formed by gap junction channels allow the direct flow of electrical currents between coupled neurons. In most preparations, gc is likely to allow only a rough estimate of gap junctional strength (Bennett, 1966; Pereda et al, 2013; Shimizu and Stopfer, 2013; Curti and O’Brien, 2016) Both cc and gc are generally used to demonstrate neurotransmitterdependent or activity-dependent regulation of electrical synapses (Landisman and Connors, 2005; Haas et al, 2011; Haas and Landisman, 2012a; Mathy et al, 2014; Wang et al, 2015; Sevetson et al, 2017). To suppress some of the non-junctional changes, cell membranes are often rendered passive by applying cocktails of antagonists and channel blockers (Landisman and Connors, 2005; Wang et al, 2015; Szoboszlay et al, 2016), an approach that clearly comes at the cost of taking out potential signaling pathways that may be involved in the plasticity of electrical synapses
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
