Electrophoresis for anatomical and functional imaging
Electrophoresis is a laboratory technique that uses an electric field to move charged particles through a medium, often a gel or a liquid. When it comes to studying neurons, researchers can use electrophoresis to introduce polar fluorescent tracers into the nerve sheath—the protective covering around nerves.
Abstract
The delivery of tracers into populations of neurons is essential to visualize their anatomy and analyze their function. In some model systems genetically-targeted expression of fluorescent proteins is the method of choice; however, these genetic tools are not available for most organisms and alternative labeling methods are very limited. Here we describe a new method for neuronal labelling by electrophoretic dye delivery from a suction electrode directly through the neuronal sheath of nerves and ganglia in insects. Polar tracer molecules were delivered into the locust auditory nerve without destroying its function, simultaneously staining peripheral sensory structures and central axonal projections. Local neuron populations could be labelled directly through the surface of the brain, and in-vivo optical imaging of sound-evoked activity was achieved through the electrophoretic delivery of calcium indicators. The method provides a new tool for studying how stimuli are processed in peripheral and central sensory pathways and is a significant advance for the study of nervous systems in non-model organisms.
Figure 1 - Neuroanatomical imaging of tracers delivered by electrophoresis across the nerve sheath
Electrophoresis is a laboratory technique used to separate and analyze molecules, particularly charged particles like proteins, nucleic acids (DNA and RNA), and other biomolecules. The fundamental principle of electrophoresis is based on the movement of charged molecules in an electric field.
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Basic Principle: When an electric field is applied to a medium containing charged molecules, these molecules will migrate toward the electrode with the opposite charge. For example, negatively charged molecules move towards the positive electrode (anode), while positively charged molecules move towards the negative electrode (cathode).
Medium: Electrophoresis is typically performed in a gel (like agarose or polyacrylamide) or a liquid medium. The gel acts as a molecular sieve, allowing smaller molecules to move faster than larger ones, which helps in separating them based on size.
Applications:
Biochemistry and Molecular Biology: Isolate and analyze proteins, DNA, and RNA for research, diagnostics, and forensic analysis.
Clinical Diagnostics: Used in medical laboratories to assess various diseases by analyzing proteins in blood or urine.
Types of Electrophoresis
Introduction
Neuroanatomical studies and functional imaging, the targeted delivery of dyes and indicators into neuron populations remains a fundamental challenge. In some animals, the gene-targeted expression of fluorescent proteins in specific neuronal populations has become the dominant in-vivo labeling method. However, in most experimental animals, these genetic tools are not available. The classical technique for labeling the central or peripheral projection of neurons involves the diffusion of dyes into cut nerves, but this approach destroys the functional integrity of the nerves, preventing simultaneous labeling in both directions and the recording of neuronal activity.
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These shortcomings prompted researchers to develop an alternative dye delivery method that can be used in a variety of animals and maintains the integrity of the tissue. In this study, they focused on the nervous system of locusts and crickets, which are widely used to study auditory processing.
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Inspired by methods of iontophoretic transdermal medication delivery, in which an external electric field is used to deliver drugs through the skin, the researchers aimed to deliver tracers across the neural sheath. This sheath, known as the neural lamella and the perineurium, forms the outermost layer of connective tissue and glial cells covering nerves and ganglia. They initially focused on the locust auditory nerve, attaching the tip of a suction electrode to the surface of the intact nerve halfway between the metathoracic ganglion and the hearing organ. The electrode was filled with the polar tracers Lucifer yellow or Texas Red-3,000 MW dextran. Whole-nerve field potentials and extracellular spike activity in response to acoustic stimuli indicated a good contact and tight seal between the electrode tip and the surface of the nerve. Pulsing current through the electrode caused electrophoretic transfer of the dye from the pipette through the sheath into the auditory nerve. As the current pulses transiently and locally electroporated the sheath and the axonal membranes of the sensory neurons, these became permeable and the tracers were successfully delivered into the population of auditory afferents. Following the procedure, the specimens were kept at 4 °C and the dye allowed to spread for 24 hours. After dissection and standard histological processing, fluorescent imaging of the auditory organ and the central nervous system demonstrated the simultaneous anterograde and retrograde transport of the tracers. Both the peripheral cell bodies and dendrites of the scolopidial sensory neurons in the auditory organ and their central axonal projections in the auditory neuropils in the metathoracic ganglia were successfully stained.
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With an increase in the total current injection time to 2 minutes, Cobalt ions as well as Alexa-568-10,000 MW dextran were also successfully delivered. With this protocol, dye spread to the mesothoracic ganglion 6 mm from the injection site, though dye concentrations in fibers ascending further became very weak. To investigate the ability of this technique to label neurons over even greater distances, the researchers electrophoretically delivered Neurobiotin into the abdominal connective of the cricket central nervous system and allowed 2 days for the tracer to spread in both directions. After conventional antibody staining against Neurobiotin, which served to enhance tracer detection, they observed numerous stained fibers reaching the entire length of the central nervous system (~20 mm), with the cell body and main dendrites of the cercal medial giant interneuron clearly identifiable in the terminal ganglion as well as its putative axonal arborizations in the brain.
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This technique offers a new tool for studying how stimuli are processed in peripheral and central sensory pathways and is a significant advance for the study of nervous systems in non-model organisms.
Figure 2 - Delivery of Neurobiotin (neurobiotin-antibody staining shown in magenta) into a cricket connective between abdominal ganglia 3 and 4 labelled neuronal projections in the brain and terminal ganglion; the dendrites of the cercal Medial Giant Interneuron (MGI) are indicated. (b) Texas Red-3,000 MW dextran delivered into the locust brain through its surface without removing the sheath (teal dotted-line showing site of pipette attachment), imaged by confocal microscopy.
Credit- Figure 2: Long distance labelling and brain surface staining. | Scientific Reports
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Basic Concept:
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This research focuses on a technique to visualize and study nerve cells (neurons) in the nervous system
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- It uses special fluorescent tracers that can move through nerve sheaths (protective coverings around nerve fibers)
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Key Components:
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Electrophoresis: A method that uses electrical current to move molecules
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Polar fluorescent tracers: Specialized molecules that glow and can be electrically guided
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Nerve sheath: The insulating layer around nerve fibers
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Purpose:
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To map and understand neuronal networks
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To help researchers see how different neurons are connected
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To provide detailed anatomical and functional information about neural structures
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Process:
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Fluorescent tracers are introduced to nerve tissues
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An electrical current helps these tracers move through nerve sheaths
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The tracers highlight specific neuronal populations
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Researchers can then observe and analyze these labeled neurons using imaging techniques
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Significance:
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Helps understand neural connectivity
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Provides insights into brain and nervous system organization
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Potentially useful in studying neurological conditions
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This technique offers a sophisticated way to visualize and study complex neuronal networks.
Calcium indicator Fluo-6 delivered into the locust auditory nerve and imaged in the live metathoracic ganglion by epifluorescence microscopy. The bright white patches in the center-left of the ganglion (asterisk marking largest patch) are artefacts due to reflections in the saline. (b) Map of peak fluorescence change (ΔF/F) of the metathoracic ganglion in response to 5 kHz sound pulses; ROIs drawn for frontal and caudal neuropils. (c) Top, electrophysiological recording of tympanal nerve activity to sound pulses (indicated by light grey bars) of 2–20 kHz as measured by suction electrode prior to dye delivery; traces are rectified average signals (±s.e.m. in light blue, n = 4). Bottom, normalized ΔF/F traces showing frequency-response characteristics of frontal and caudal ROIs assigned to the corresponding auditory neuropils (±s.e.m., n = 6).
(a) Fluo-6 delivered into the anterior auditory neuropil in the cricket brain; site of pipette attachment circled in light blue and region of sound-evoked activity circled in white. (b) Fluorescence change map with ROIs drawn for active cell bodies or neuropil regions. (c) Absolute ΔF/F traces of each ROI showing activation correlated to 5 kHz sound pulses (indicated by light grey bars). Right, average ± s.e.m. of these 5 activations for each ROI
Credit- Figure 4: Sound-evoked activity directly visualized in the cricket brain. | Scientific Reports
Methodology and Application
Animal Selection and Preparation:The researchers used adult locusts (Schistocerca gregaria) and crickets (Gryllus bimaculatus) raised in controlled conditions at 28°C with 12-hour light/dark cycles. They selected insects 1-4 weeks after their final molt. The researchers carefully exposed the necessary nerve tissues through precise dissection techniques.
Experimental Setup: The scientists developed specialized equipment including:
1. Custom suction electrodes made from polypropylene tubing
2. Specialized dye solutions for both anatomical and functional imaging
3. A modified microscope stage for various imaging needs
Methodology:
The researchers employed several key procedures:
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1. Tracer Application:
- Used various fluorescent dyes and tracers
- Applied them through suction electrodes
- Controlled delivery using electrical current pulses
2. Sound Stimulation:
- Generated controlled sound stimuli at 70 dB
- Used specific frequencies (2-20 kHz)
- Delivered sound through calibrated speakers
3. Recording and Imaging:
- Recorded nerve activity using differential amplifiers
- Performed both anatomical and functional imaging
- Used specialized microscopes for different imaging needs
4. Data Collection and Analysis:
- Recorded multiple trials for each experiment
- Used sophisticated imaging software
- Analyzed changes in fluorescence
- Calculated average responses across multiple specimens
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