biocytin

Biocytin is a biotin derivative used as a histological label and neuronal tracer. It consists of biotin covalently bound to the amino acid lysine, commonly described as biotinyl-L-lysine. The compound is water-soluble and relatively stable under routine histological conditions. Because biotin binds with very high affinity to avidin and streptavidin, biocytin-labeled samples are readily detected with avidin- or streptavidin-conjugated enzymes or fluorophores.

In neuroscience and anatomy, biocytin is widely used to fill neurons for detailed morphological reconstruction and to trace neural connections. It can be introduced into cells through intracellular recording techniques (patch-clamp), via microinjection, or by intracellular diffusion from loaded electrodes. After fixation, tissue is processed with avidin–biotin complex or with streptavidin conjugates to visualize the filled cell or its processes. Biocytin labeling can be combined with immunohistochemical methods to correlate structure with molecular markers, and it is compatible with both light and electron microscopy.

Advantages of biocytin include its high sensitivity and robustness, compatibility with multiple detection methods, and the ability to reveal full neuronal morphology, including fine dendritic and axonal processes. Limitations include potential incomplete diffusion in larger cells and the need for post hoc staining steps. Biocytin is one of several commonly used tracers in neuroanatomy, alongside other biotinylated compounds and non-biotin tracers, each with specific diffusion and detection properties.