A simple and fast method to image calcium activity of neurons from intact dorsal root ganglia using fluorescent chemical Ca²⁺ indicators

Chemical calcium indicators have been commonly used to monitor calcium (Ca²⁺) activity in cell bodies, i.e., somata, of isolated dorsal root ganglion neurons. Recent studies have shown that dorsal root ganglion somata play an essential role in soma–glia interactions and actively participate in the transmission of nociceptive signals. It is therefore desirable to develop methods to study Ca²⁺ activity in neurons and glia in intact dorsal root ganglia. In our previous studies, we found that incubation of intact dorsal root ganglia with acetoxymethyl dye resulted in efficient Ca²⁺ dye loading into glial cells but limited dye loading into neurons. Here, we introduce a useful method to load Ca²⁺ dyes in intact dorsal root ganglion neurons through electroporation. We found that electroporation greatly facilitated loading of Fluo-4 acetoxymethyl, Oregon green bapta-1-488 acetoxymethyl, and Fluo-4 pentapotassium salt into dorsal root ganglion neurons. In contrast, electroporation did not further facilitate dye loading into glia. Using electroporation followed by incubation of acetoxymethyl form Ca²⁺ dye, we can load acetoxymethyl Ca²⁺ dye well in both neurons and glia. With this approach, we found that inflammation induced by complete Freund’s adjuvant significantly increased the incidence of neuron–glia interactions in dorsal root ganglia. We also confirmed the actions of capsaicin and morphine on Ca²⁺ responses in dorsal root ganglion neurons. Thus, by promoting the loading of Ca²⁺ dye in neurons and glia through electroporation and incubation, Ca²⁺ activities in neurons and neuron–glia interactions can be well studied in intact dorsal root ganglia.