Concentration and storage of biotin in the amphibian brain

Prominent displays of endogenous biotin reactivity can be observed at specific locations in histochemical preparations of the forebrain and midbrain in the northern leopard frog (Rana pipiens) and common American toad (Bufo americanus). At the light microscopic level, the biotin reactivity appears in clusters of darkly stained puncta of either spherical or rodlike shape in the olfactory cortex, nucleus isthmi, and hypothalamus. With the electron microscope, the biotin reactive spheres are identified as neuronal varicosities and synaptic boutons and the rods as short segments of axons. Appropriate controls demonstrate that the punctate biotin-reactive structures are sites of concentration of biotin or a biotin analog in the processes of certain neurons. These data represent the first observation on the selective concentration of a vitamin in vertebrate neurons and suggest that biotin may have specialized functions in anatomically delimited areas of the central nervous system. Localization of the densest concentration of the biotin-reactive puncta in the dorsolateral prominence of the olfactory cortex may have relevance to the functional organization of the olfactory system. The distributions of biotin-reactive puncta were observed in laboratory-housed frogs and in wild toads captured in the summer months but were sparse or absent in batches of commercially obtained frogs examined immediately upon arrival in the laboratory. Systemic administration of biotin or biocytin hydrochloride did not alter the appearance or numbers of the biotin-reactive structures either in newly received or laboratory-housed frogs. These findings suggest that the capacity of the biotin-storage mechanism in the amphibian brain may be set by environmental factors and may be readily saturable from natural dietary or enteric sources.