We have developed a method for bimodal control of neuronal membrane potential with subcellular resolution using optically independent two-photon uncaging of glutamate at 720 nm and GABA at 830 nm. Two-color, two-photon uncaging allowed action potentials to be fired and blocked with the two wavelengths of light and may enable optically independent control of many other symbiotic chemical messenger pairs.

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GABA(γ-amino-butylic acid)-mediated inhibition in the dendrites of CA1 pyramidal neurons was characterized by two-photon uncaging of a caged-GABA compound, BCMACM-GABA, and one-photon uncaging of RuBi-GABA in rat hippocampal slice preparations. Although we found that GABAA-mediated currents were diffusely distributed along the dendrites, currents elicited at the branch points of the apical dendritic trunk were approximately two times larger than those elsewhere in the dendrite. We examined the inhibitory action of the GABA-induced currents on Ca2+ transients evoked with a single back-propagating action potential (bAP) in oblique dendrites. We found that GABA uncaging selectively inhibited the Ca2+ transients in the region adjacent (<20 µm) to the uncaging site, and that GABA uncaging was effective only within a short period after bAP (<20 ms). The strength of inhibition was linearly related to the amplitudes of the GABA currents, suggesting that the currents inhibited a sustained, subthreshold after-depolarization without preventing propagation of bAP. GABA uncaging at the dendritic branch points inhibited Ca2+ transients farther into dendritic branches (>20 µm). Our data indicate that GABA inhibition results in spatially confined inhibition of Ca2+ transients shortly after bAP, and suggest that this effect is particularly potent at the dendritic branch points where GABA receptors cluster.

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