Electron microscopic immunocytochemistry of glutamate-containing nerve fibers in the taste bud of mudpuppy (Necturus maculosus).

The presence of glutamate immunoreactivity (glu-IR) in the nerve fibers of the mudpuppy taste bud was investigated by electron microscopy. Pre-embedding staining with avidin-biotin-peroxidase complex (ABC) and post-embedding staining with 5 nm colloid gold conjugates were used separately to identify immuno-stained structures. We have found the following: 1) the majority of the nerve fibers innervating the mudpuppy taste bud are unmyelinated; 2) about 85% of nerve fibers located at the base of the taste bud and about 60% of the nerve fibers located between the taste cells show glu-IR by pre-embedding staining; 3) there is a preferential staining of the glu-IR in the nerve fibers of the mudpuppy taste bud; and 4) the distribution of the colloidal gold particles in the nerve fibers is 1.5 to 2 times denser than that of the staining in the connective tissue background or cellular profiles of taste cells. From the distribution and pattern of the nerve fibers obtained in the thick and thin sections, we conclude that the mudpuppy taste bud is innervated by glutamate-containing unmyelinated nerve fibers.