Combined imaging and chemical sensing of L-glutamate release from the foregut plexus of the Lepidopteran, Manduca sexta

A new combined imaging and chemical detection sensor for the measurement of localized L-glutamate release at the insect neuromuscular junction (NMJ) is presented. The sensor is comprised of an L-glutamate-sensitive fluorescent gel, spin-coated onto the tip of an optical imaging fiber. The gel is composed of L-glutamate oxidase (GLOD); a pH-sensitive fluorescent dye, SNAFL; and poly(acrylamide-co-N-acryloxysuccinimide) (PAN). NH₃ is liberated from the interaction of L-glutamate with GLOD, which reversibly reduces the emitted fluorescence signal from SNAFL. This sensor has a spatial resolution of 3-4 μm, and an L-glutamate detection limit of between 10 and 100 μM. L-glutamate release and re-uptake from the foregut plexus of Manduca sexta was detected by the sensor in the presence of the L-glutamate re-uptake blocker dihydrokainate, and the post-synaptic L-glutamate receptor antagonist CNQX.