Laser photolysis of DPNI-GABA, a tool for investigating the properties and distribution of GABA receptors and for silencing neurons in situ

Laser photolysis to release GABA at precisely defined times and locations permits investigation of the distribution of functional GABAA receptors in neuronal compartments, the activation kinetics and pharmacology of GABAA receptors in situ, and the role of individual neurons in neural circuits by selective silencing with low GABA concentrations. We describe the experimental evaluation and applications of a new nitroindoline-caged GABA, DPNI-GABA, modified to minimize the pharmacological interference commonly found with caged GABA reagents, but retaining the advantages of nitroindoline cages. Unlike the 5-methoxycarbonylmethyl-7-nitroindolinyl-GABA tested previously, DPNI-GABA inhibited GABAA receptors with much lower affinity, reducing peak GABA-evoked responses with an IC50 of approximately 0.5 mM. Most importantly, the kinetics of receptor activation, determined as 10–90% rise-times, were comparable to synaptic events and were little affected by DPNI-GABA present at 1 mM concentration, permitting photolysis of DPNI-GABA to mimic synaptic activation of GABAA receptors. With a laser spot of 1 μm applied to cerebellar molecular layer interneurons, the spatial resolution of uncaging DPNI-GABA in dendrites was estimated as 2 μm laterally and 7.5 μm focally. Finally, at low DPNI-GABA concentration, photorelease restricted to the area of the soma suppressed spiking in single Purkinje neurons or molecular layer interneurons for periods controlled by the flash intensity and duration. DPNI-GABA has properties better adapted for fast kinetic studies with laser photolysis at GABAA receptors than previously reported caged GABA reagents, and can be used in experiments where spatial resolution is determined by the dimensions of the laser light spot.