Comparative one- and two-photon uncaging of MNI-glutamate and MNI-kainate on hippocampal CA1 neurons

Background The light-induced release of neurotransmitters from caging chromophores provides a powerful means to study the underlying receptors in a physiologically relevant context. Surprisingly, most caged neurotransmitters, including the widely used 4-methoxy-7-nitroindolinyl (MNI)-glutamate, show strong antagonism against GABA-A receptors. Kainate has been shown to exhibit a higher efficacy at glutamate receptors compared to glutamate itself. Thus, uncaging of kainate might allow the application of the caged compound at lower, less antagonistic concentrations. New methods This study provides a detailed comparison of MNI-glutamate and MNI–kainate uncaging by different modes of one- and two-photon irradiation on hippocampal CA1 pyramidal neurons in acute brain slices. Results/Comparison with existing methods Unexpectedly, the data revealed that currents in response to MNI-glutamate uncaging were larger compared to MNI-kainate with local one-photon laser uncaging at the soma and two-photon uncaging at the same spines. Furthermore, the direct comparison demonstrates the influence of type of caged agonist and light delivery conditions used for uncaging on the amplitude and kinetic properties of the current response. Conclusion These findings highlight the importance of experimental design for uncaging experiments and provide a basis for future studies employing one- and two-photon uncaging to understand glutamate-dependent processes. It further provides the first example of two-photon uncaging of kainate at single spines in acute brain slices.