Q.87. Which ONE of the following techniques can be used for detecting the subcellular localization of
serotonin receptor in intact cells?
(A) Immunoelectron microscopy
(B) SDS-PAGE
(C) Fluorescence in-situ hybridization
(D) Differential centrifugation
The correct answer is: (A) Immunoelectron microscopy.
Correct answer: Immunoelectron microscopy (Option A)
Immunoelectron microscopy combines antigen–antibody specificity with the high resolution of electron microscopy to visualize proteins at the ultrastructural level inside intact cells. Using antibodies against serotonin receptors tagged with electron-dense markers (such as gold particles), this method can pinpoint the receptor on specific membranes and organelles, including plasma membrane, endoplasmic reticulum, and dendrites.
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This technique has been used to map the cellular and subcellular distribution of multiple serotonin receptor subtypes (for example 5‑HT2A and 5‑HT3 receptors) in brain tissue and cultured cells.
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Because it preserves cell architecture, it allows detection of receptor localization in situ in intact cells rather than in lysed or fractionated material.
Therefore, for detecting subcellular localization of serotonin receptors in intact cells, immunoelectron microscopy is the most appropriate technique among the given options.
Why SDS-PAGE is incorrect (Option B)
SDS-PAGE (sodium dodecyl sulfate–polyacrylamide gel electrophoresis) is a protein separation technique based on molecular weight. It denatures proteins and separates them in a polyacrylamide gel, which destroys the spatial and subcellular context of the protein.
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SDS-PAGE can tell whether a receptor of the expected size is present and can be combined with Western blotting for protein detection, but it cannot show where inside the cell the receptor is localized.
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Because cells are lysed and proteins are solubilized, no information about intact cellular architecture or membrane domains is retained, making this method unsuitable for subcellular localization in intact cells.
Thus, SDS-PAGE is useful for molecular weight and expression analysis, not for subcellular localization of serotonin receptors.
Why FISH is incorrect (Option C: Fluorescence in situ hybridization)
Fluorescence in situ hybridization (FISH) detects specific nucleic acid sequences—typically DNA or mRNA—using fluorescently labeled probes that hybridize to complementary sequences. In the context of serotonin receptors, FISH can localize receptor mRNA within cells or tissues but not the receptor protein itself.
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FISH is excellent for mapping gene location on chromosomes or spatial distribution of mRNA transcripts, but subcellular protein localization requires protein-targeted probes or antibodies.
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Since the question specifically asks about subcellular localization of the serotonin receptor (a membrane protein), a nucleic acid–based technique like FISH cannot directly visualize the receptor protein compartments.
Therefore, FISH is not the best choice for localizing serotonin receptor protein within intact cells.
Why differential centrifugation is incorrect (Option D)
Differential centrifugation is a fractionation technique that separates cell components into pellets and supernatants based on size and density by sequential centrifugation at increasing speeds. It can enrich fractions such as nuclei, mitochondria, microsomes, or plasma membrane, which can then be analyzed biochemically.
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Although one might infer approximate localization by detecting the receptor in specific fractions (e.g., plasma membrane fraction), this method does not directly visualize receptors in their native position within intact cells.
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Subcellular architecture is disrupted during homogenization, so spatial relationships between organelles and precise receptor distribution cannot be assessed, unlike microscopy-based immunolabeling.
Hence, differential centrifugation gives indirect biochemical localization but not high‑resolution subcellular localization in intact cells, making it inferior to immunoelectron microscopy for this question.
