Glutamate Uncaging

One of the first glutamate uncaging (p-hydroxyphenacyl-glutamate) experiments on dendrites was done by Kandler on CA1 pyramidal neurons in slices . They reported the long-term depression of glutamate responses after the pairing of uncaging with depolarization of the neuron. A similar result was obtained with bursts of uncaging by Dodt; these authors used an infrared-guided laser stimulation system to uncage glutamate on the apical dendrites of layer 5 neocortical pyramidal neurons.

Uncaging Compounds

RuBi-Glutamate

• caged-glutamate compound
• based on ruthenium photochemistry
• excited with visible wavelengths
• releases glutamate after 1-photon or 2-photon excitation.
• high quantum efficiency
• can be used at low concentrations,
• partly avoids blockade of GABAergic transmission
• 2-photon uncaging of RuBi-Glutamate has a high spatial resolution and generates excitatory responses in individual dendritic spines with physiological kinetics.
• With laser beam multiplexing, two-photon RuBi-Glutamate uncaging can also be used to depolarize and fire pyramidal neurons with single-cell resolution.
• RuBi-Glutamate enables the photoactivation of neuronal dendrites and circuits with visible or two-photon light sources, achieving single cell, or even single spine, precision.

ruthenium-bipyridine complexes can be used as caging compounds. ruthenium is a transition metal with versatile chemistry. polypyridines of ruthenium photorelease entire ligands in a heterolytic fashion, by means of a widely known mechanism in which the initial photoexcited state quickly evolves into a dissociative state, so the photorelease is therefore clean and fast.

Pros: RuBi-Glutamate

• minimal antagonistic effects on GABAergic transmission

Cons: RuBi-Glutamate

• not much prior literature to demo the effects

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MNI-Glutamate

2-photon uncaging of MNI-glutamate has been used successfully to functionally map synaptic receptors , activate individual spines, and individual neurons.

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Two-photon excitation was used to uncage MNI-glutamate while measuring the resulting inward currents at the neuron’s cell body. This method resulted in an incredibly high-resolution view of the dendritic sites that were most sensitive to glutamate. Even the responses of single dendritic spines could be resolved. The pseudocoloring over the dendritic outline corresponds to the magnitude of the inward currents measured, as indicated by the colored scale bar below.

Pros: MNI-glutamate

• lots of prior literature to demo the effects
• antagonist of GABAergic transmission at high concentrations

Cons: MNI-glutamate

• needs to be applied to the tissue at relatively high (mM) concentrations for effective 2-photon uncaging.
• antagonist to GABAergic transmission so can't be used to study GABAergic neurons

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CNB-glutamate

CNB-glutamate

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L-glutamic acid

L-glutamic acid α(4,5-dimethoxy-2-nitrobenzyl) ester (Callaway, Katz, 1993)

REFERENCES

<references>

• Salierno Marceca Peterka Yuste Etchenique 2010 fast ruthenium polypyridine cage complex photoreleases glutamate with visible or IR light in one and two photon regimes J Inorg Biochem

• Callaway Katz 1993 Photostimulation using caged glutamate reveals functional circuitry in living brain slices

• Frick Dodt 2001 Glutamate receptors form hot spots on apical dendrites of neocortical pyramidal neurons

• Pettit Augustine 1997 Chemical twophoton uncaging: a novel approach to mapping glutamate receptors

• Shepherd Svoboda 2003 Circuit analysis of experience-dependent plasticity in the developing rat barrel cortex

• Wieboldt Hess 1994 Photolabile precursors of glutamate: synthesis, photochemical properties, and activation of glutamate receptors on a microsecond time scale.

• Yoshimura Callaway 2005 Excitatory cortical neurons form fine-scale functional networks.