Name: fast micro-iontophoresis of glutamate and gaba: a useful tool to investigate synaptic integration
Uploaded: 2013-07-31T17:00:00
Description: Watch this Scientific Journal Video about Fast Micro-iontophoresis of Glutamate and GABA: A Useful Tool to Investigate Synaptic Integration at JoVE.com

We thank Hans Reiner Polder, Martin Fuhrmann and Walker Jackson for carefully reading the manuscript. The authors received funding that was provided by the ministry of research MIWF of the state Northrhine-Westfalia (S.R.), the BMBF-Projektr&auml;ger DLR US-German collaboration in computational neuroscience (CRCNS; S.R.), Centers of Excellence in Neurodegenerative Diseases (COEN; S.R.), and the University of Bonn intramural funding program (BONFOR; S.R.).

1. System Requirements
<ol>
	<li>Microscope system: Good visualization of the dendrite is crucial. If available, use a two-photon laser scanning microscope system. In our experiments we used a TRIM Scope II, LaVision Biotec, Bielefeld, Germany or an Ultima IV system, Prairie Technologies, Middleton, Wisconsin equipped with a Ti:Sapphire ultrafast-pulsed laser (Chameleon Ultra II, Coherent) and a high NA objective (60X, 0.9 NA, Olympus) to visualize the dendrites which we had filled with a fluorescent dye via the patch ...

<ol>
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Here we explain how to apply fast micro-iontophoresis of neurotransmitters to investigate synaptic integration on dendrites. This technique has been successfully used to investigate glutamatergic and GABAergic synaptic transmission in different brain regions in vitro and in vivo9,20-22. Micro-iontophoresis has been used for more than 60 years, but in early years it was mostly used to either locally apply neurotransmitters and drugs on slow or intermediate timescales23 or for microi...

Neurons in the central nervous system receive a variety of synaptic inputs on their thin and ramified dendritic processes1. There, the majority of the excitatory dendritic inputs are mediated by glutamatergic synapses. These synapses can be activated in a spatially distributed way, resulting in postsynaptic linear integration of excitatory postsynaptic potentials (EPSPs). If the synapses are activated simultaneously and in spatial proximity on the dendrite, these excitatory inputs can be integrated supra-linearly and generate dendritic spikes2-5.
Furthermore, the integration of excitatory inputs depends on the location of the input on the dendritic tree. Signals that arrive at the distal tuft region are much more attenuated than proximal inputs due to cable filtering6. In the hippocampus, distant inputs to the apical tuft dendrites are generated by a different brain region than those on proximal dendrites7. An exciting question is therefore, how synaptic input is processed by different dendritic compartments and if dendritic integration regulates the influence of these layered inputs on neuronal firing in different ways.
Not only the functional properties, morphological features of the dendrite, the location and clustering of the inputs are affecting the dendritic integration of excitatory inputs, also the additional inhibitory inputs from GABAergic terminals crucially determine the efficacy of the glutamatergic synapses8,9. These different aspects of synaptic integration can be ideally investigated using neurotransmitter micro-iontophoresis, which allows spatially defined application of different neurotransmitters to dendritic domains. We demonstrate here how to successfully establish micro-iontophoresis of glutamate and GABA to investigate signal integration in neurons.
For this application, fine-tipped high resistance pipettes filled with concentrated neurotransmitter solutions are used. These pipettes are positioned close to the outer membrane of the cell, where the neurotransmitter receptors are located. A good visualization of the dendritic branches is required. This is best achieved using fluorescent dyes, which are introduced via the patch pipette. Then a very short (&lt;1 msec) current pulse (in the range of 10 - 100 nA) is used to eject the charged neurotransmitter molecules. With these short pulses and effective capacitance compensation, postsynaptic potentials or currents can be evoked with high temporal and spatial precision, which means the location of the excitatory input is precisely known. Glutamate micro-iontophoresis can activate synapses in a defined radius, which is smaller than 6 &mu;m as shown here (Figure 19), but it is also possible to reach single synapse resolution10-12.
Heine, M., et al showed that the spatial resolution of fast micro-iontophoresis can even be adjusted to spot sizes below 0.5 &mu;m, which is smaller than spot sizes regularly achieved with two-photon uncaging of glutamate13. With fast micro-iontophoresis it is easily possible to use two or more iontophoretic pipettes and place them at different, even distant spots on the dendritic tree. In this way, integration of excitatory events, including those from different pathways, can be investigated. It is also possible to use a glutamate and a GABA filled iontophoretic pipette at the same time. In this way the effect of GABAergic inhibition at different locations relative to the excitatory input (on-path, off-path inhibition) can be studied. Also, the impact of inhibition by interneurons targeting specific neuronal domains, like distal dendrites, soma or axons14, can be investigated using GABA iontophoresis. In cultured neurons, fast micro-iontophoresis offers the opportunity to investigate single synapse distribution and the elementary aspects of synaptic communication in neurons in much more detail10,11.
In this article we demonstrate in detail how to establish glutamate and GABA iontophoresis for the use in acute brain slices, which allows investigating synaptic integration of excitatory and inhibitory inputs in dependence of input location, input strengths, and timing, alone or in interplay. We will point out the advantages and limitations of this technique and how to troubleshoot successfully.

<table border="1">
	<tbody>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>Material</td>
		</tr>
		<tr>
			<td>Two-photon laser scanning microscope (TRIM Scope II), and Ultima IV, Prairie Technologies, Middleton, Wisconsin)</td>
			<td>LaVision Biotec, Bielefeld, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Two-photon laser scanning microscope Ultima IV</td>
			<td>Prairie Technologies, Middleton, Wisconsin, USA</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Ti:Sapphire ultrafast-pulsed laser</td>
			<td>Chameleon Ultra II, Coherent</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>60X Objective, NA 0.9</td>
			<td>Olympus</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Zeiss Axioskop 2 FS upright microscope</td>
			<td>TILLPhotonics, Gr&auml;felfing, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Monochromator</td>
			<td>TILLPhotonics, Gr&auml;felfing, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Micro-iontophoresis system MVCS-02</td>
			<td>NPI Electronics, Tamm, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Sutter puller P-97</td>
			<td>Sutter Instrument Company, Novato, CA</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glass filaments (150 GB F 8P)</td>
			<td>Science Products, Hofheim, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>Reagent</td>
		</tr>
		<tr>
			<td>Alexa Fluor 488 hydrazide</td>
			<td>Molecular Probes life technologies</td>
			<td>A-10436</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Alexa Fluor 594</td>
			<td>Molecular Probes life technologies</td>
			<td>A-10438</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Sigma Aldrich</td>
			<td>S7653</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma Aldrich</td>
			<td>P9333</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td>Sigma Aldrich</td>
			<td>S8282</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>Sigma Aldrich</td>
			<td>S6297</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma Aldrich</td>
			<td>S7903</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Sigma Aldrich</td>
			<td>C5080</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>Sigma Aldrich</td>
			<td>M2670</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glucose</td>
			<td>Sigma Aldrich</td>
			<td>G7528</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>K-Gluconate</td>
			<td>Sigma Aldrich</td>
			<td>G4500</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>HEPES-acid</td>
			<td>Sigma Aldrich</td>
			<td>H4034</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Phosphocreatin</td>
			<td>Sigma Aldrich</td>
			<td>P7936</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Sigma Aldrich</td>
			<td>E3889</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glutamic acid</td>
			<td>Sigma Aldrich</td>
			<td>G8415</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>GABA</td>
			<td>Sigma Aldrich</td>
			<td>A5835</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaOH</td>
			<td>Merck</td>
			<td>1.09137.1000</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>HCl</td>
			<td>Merck</td>
			<td>1.09108.1000</td>
			<td>&nbsp;</td>
		</tr>
	</tbody>
</table>

fast micro-iontophoresis of glutamate and gaba: a useful tool to investigate synaptic integration

One of the fundamental interests in neuroscience is to understand the integration of excitatory and inhibitory inputs along the very complex structure of the dendritic tree, which eventually leads to neuronal output of action potentials at the axon. The influence of diverse spatial and temporal parameters of specific synaptic input on neuronal output is currently under investigation, e.g. the distance-dependent attenuation of dendritic inputs, the location-dependent interaction of spatially segregated inputs, the influence of GABAergig inhibition on excitatory integration, linear and non-linear integration modes, and many more.
With fast micro-iontophoresis of glutamate and GABA it is possible to precisely investigate the spatial and temporal integration of glutamatergic excitation and GABAergic inhibition. Critical technical requirements are either a triggered fluorescent lamp, light-emitting diode (LED), or a two-photon scanning microscope to visualize dendritic branches without introducing significant photo-damage of the tissue. Furthermore, it is very important to have a micro-iontophoresis amplifier that allows for fast capacitance compensation of high resistance pipettes. Another crucial point is that no transmitter is involuntarily released by the pipette during the experiment.
Once established, this technique will give reliable and reproducible signals with a high neurotransmitter and location specificity. Compared to glutamate and GABA uncaging, fast iontophoresis allows using both transmitters at the same time but at very distant locations without limitation to the field of view. There are also advantages compared to focal electrical stimulation of axons: with micro-iontophoresis the location of the input site is definitely known and it is sure that only the neurotransmitter of interest is released. However it has to be considered that with micro-iontophoresis only the postsynapse is activated and presynaptic aspects of neurotransmitter release are not resolved. In this article we demonstrate how to set up micro-iontophoresis in brain slice experiments.

A simple approach to determine the spatial spread of iontophoresis is to retract the iontophoretic pipette stepwise from the dendrite, while keeping the ejected glutamate constant. We found that the spatial extent of a micro-iontophoretic stimulation had a diameter of approximately 12 &mu;m (Figure 1 showing radius). How deep in the tissue the iontophoresis can be used depends on the rigidity of the pipette. However, the iontophoretic pipettes needed for experiments in slices (Figure 2),...

Watch this Scientific Journal Video about Fast Micro-iontophoresis of Glutamate and GABA: A Useful Tool to Investigate Synaptic Integration at JoVE.com

Fast Micro-iontophoresis of Glutamate and GABA: A Useful Tool to Investigate Synaptic Integration

In this article we introduce fast micro-iontophoresis of neurotransmitters as a technique to investigate integration of postsynaptic signals with high spatial and temporal precision.

One of the fundamental interests in neuroscience is to understand the integration of excitatory and inhibitory inputs along the very complex structure of ...

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Using an &#945;-Bungarotoxin Binding Site Tag to Study GABA A Receptor Membrane Localization and Trafficking

It is increasingly evident that neurotransmitter receptors, including ionotropic GABA A receptors (GABAAR), exhibit highly dynamic trafficking and cell ...

Induction of an Isoelectric Brain State to Investigate the Impact of Endogenous Synaptic Activity on Neuronal Excitability In Vivo

Induction of an Isoelectric Brain State to Investigate the Impact of Endogenous Synaptic Activity on Neuronal Excitability In Vivo

The way neurons process information depends both on their intrinsic membrane properties and on the dynamics of the afferent synaptic network. In ...

Using Enzyme-based Biosensors to Measure Tonic and Phasic Glutamate in Alzheimer's Mouse Models

Neurotransmitter disruption is often a key component of diseases of the central nervous system (CNS), playing a role in the pathology underlying ...

Culturing In Vivo-like Murine Astrocytes Using the Fast, Simple, and Inexpensive AWESAM Protocol

Culturing In Vivo-like Murine Astrocytes Using the Fast, Simple, and Inexpensive AWESAM Protocol

The AWESAM (a low-cost easy stellate astrocyte method) protocol entails a fast, simple, and inexpensive way to generate large quantities of in vivo-like ...

Combining Optogenetics with Artificial microRNAs to Characterize the Effects of Gene Knockdown on Presynaptic Function within Intact Neuronal Circuits

The purpose of this protocol is to characterize the effect of gene knockdown on presynaptic function within intact neuronal circuits. We describe a ...