Fluorescence in situ hybridization of scarce leptin receptor mRNA using the enzyme-labeled fluorescent substrate method and tyramide signal amplification

To increase the sensitivity of fluorescence in situ hybridization (FISH) for detection of low-abundance mRNAs, we performed FISH on cryostat sections of rat hypothalamus with biotin-labeled riboprobes to leptin receptor (ObRb) and amplified the signal by combining tyramide signal amplification (TSA) and Enzyme-Labeled Fluorescent alkaline phosphatase substrate (ELF) methods. First, TSA amplification was done with biotinylated tyramide. Second, streptavidin-alkaline phosphatase was followed by the ELF substrate, producing a bright green fluorescent reaction product. FISH signal for ObRb was undetectable when TSA or ELF methods were used alone, but intense ELF FISH signal was visible in hypothalamic neurons when the ELF protocol was preceded by TSA. The TSA-ELF was combined with FISH for pro-opiomelanocortin (POMC) and neuropeptide Y (NPY) mRNAs by hybridizing brain sections in a cocktail containing digoxigenin-labeled riboprobes to NPY or POMC mRNA and biotin-labeled riboprobes to ObRb mRNA. Dioxigenin-labeled NPY or POMC mRNA hybrids were subsequently detected first with IgG-Cy3. Then biotin-labeled leptin receptor hybrids were detected with the TSA-ELF method. Combining the ELF and TSA amplification techniques enabled FISH detection of scarce leptin receptor mRNAs and permitted the identification of leptin receptor mRNA in cells that also express NPY and POMC gene products.