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 Americ
an toad (Bufo americanus). At the light microscopic level, the biotin
reactivity appears in clusters of darkly stained puncta of either sphe
rical or rodlike shape in the olfactory cortex, nucleus isthmi, and hy
pothalamus. With the electron microscope, the biotin reactive spheres
are identified as neuronal varicosities and synaptic boutons and the r
ods 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 d
ata represent the first observation on the selective concentration of
a vitamin in vertebrate neurons and suggest that biotin may have speci
alized functions in anatomically delimited areas of the central nervou
s system. Localization of the densest concentration of the biotin-reac
tive 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 laborat
ory-housed frogs and in wild toads captured in the summer months but w
ere sparse or absent in batches of commercially obtained frogs examine
d immediately upon arrival in the laboratory. Systemic administration
of biotin or biocytin hydrochloride did not alter the appearance or nu
mbers of the biotin-reactive structures either in newly received or la
boratory-housed frogs. These findings suggest that the capacity of the
biotin storage mechanism in the amphibian brain may be set by environ
mental factors and may be readily saturable from natural dietary or en
teric sources. (C) 1996 Wiley-Liss, Inc.