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.