Electron microscopic analysis of fluorescent neuronal labeling after photoconversion

Area of Science:

• Neuroscience
• Cell Biology
• Microscopy

Background:

• Retrograde neuronal tracing is crucial for understanding neural circuits.
• Visualizing fluorescent tracers at the ultrastructural level presents challenges.
• Photoconversion offers a potential method to enhance tracer visibility.

Purpose of the Study:

• To investigate the efficacy of photo-oxidation for ultrastructural visualization of fluorescent retrograde neuronal labeling.
• To characterize the reaction product formed by photoconversion of common neuronal tracers.
• To assess the specificity and reliability of the photoconversion technique.

Main Methods:

• Neurons were retrogradely labeled using fluorescent tracers: propidium iodide, rhodamine latex microspheres, and Fluorogold.
• Fluorescence was converted to a stable diaminobenzidine (DAB) reaction product via photo-oxidation.
• Ultrastructural analysis was performed using electron microscopy.
• Control samples were used to validate the specificity of the reaction product.

Main Results:

• Photoconversion successfully generated an electron-dense diaminobenzidine reaction product in labeled neurons.
• The reaction product accumulated within lysosomes and was distributed in the cytoplasmic matrix.
• The amount, electron density, and appearance of the product confirmed its specific origin from photoconversion.
• Labeling intensity and photoconversion duration influenced the fine structural features of the product.

Conclusions:

• Photoconversion is an effective technique for ultrastructural studies of fluorescently labeled neurons.
• The specificity of the photoconversion reaction product should be routinely validated for each fluorochrome and tissue type.
• This method enhances the capability to study neuronal connectivity with high resolution.