Nitrophenyl-based caged neurotransmitters

Precise 3D modulation of electro-optical parameters during neurotransmitter uncaging experiments with neurons in vitro

Ruthenium–bipyridinetriphenylphosphine–GABA (RuBi–GABA) is a caged compound that allows studying the neuronal transmission in a specific region of a neuron. The inhibitory neurotransmitter γ-aminobutyric acid (GABA) is bound to a caged group that blocks the interaction of the neurotransmitter with its receptor site. Following linear—one-photon (1P)—and non-linear—multi-photon—absorption of light, the covalent bond of the caged molecule is broken, and GABA is released. Such a controlled release in time and space allows investigating the interaction with its receptor in four dimensions (X,Y,Z,t). Taking advantage of this strategy, we succeeded in addressing the modulation of GABA_A in rat cerebellar neurons by coupling the photoactivation process, by confocal or two-photon excitation microscopy, with the electrophysiological technique of the patch-clamp in the whole-cell configuration. Key parameters have been comprehensively investigated and correlated in a temporally and spatially confined way, namely: photoactivation laser power, time of exposure, and distance of the uncaging point from the cell of interest along the X, Y, Z spatial coordinates. The goal of studying specific biological events as a function of controlled physical parameters has been achieved.

EPSPs Measured in Proximal Dendritic Spines of Cortical Pyramidal Neurons

EPSPs occur when the neurotransmitter glutamate binds to postsynaptic receptors located on small pleomorphic membrane protrusions called dendritic spines. To transmit the synaptic signal, these potentials must travel through the spine neck and the dendritic tree to reach the soma. Due to their small size, the electrical behavior of spines and their ability to compartmentalize electrical signals has been very difficult to assess experimentally. In this study, we developed a method to perform simultaneous two-photon voltage-sensitive dye recording with two-photon glutamate uncaging in order to measure the characteristics (amplitude and duration) of uncaging-evoked EPSPs in single spines on the basal dendrites of L5 pyramidal neurons in acute brain slices from CD1 control mice. We were able to record uncaging-evoked spine potentials that resembled miniature EPSPs at the soma from a wide range of spine morphologies. In proximal spines, these potentials averaged 13.0 mV (range, 6.5–30.8 mV; N = 20) for an average somatic EPSP of 0.59 mV, whereas the mean attenuation ratio (spine/soma) was found to be 25.3. Durations of spine EPSP waveforms were found to be 11.7 ms on average. Modeling studies demonstrate the important role that spine neck resistance (R_neck) plays in spine EPSP amplitudes. Simulations used to estimate R_neck by fits to voltage-sensitive dye measurements produced a mean of 179 MΩ (range, 23–420 MΩ; N = 19). Independent measurements based on fluorescence recovery after photobleaching of a cytosolic dye from spines of the same population of neurons produced a mean R_neck estimate of 204 MΩ (range, 52–521 MΩ; N = 34).