Tannic acid enhances staining of microtubule associated proteins, but impairs neuronal physiology

Exposure of the buccal ganglion of the mollusc, Helisoma trivolvis, to tannic acid ringer (TARI), pH 6.8, permits the ultrastructural detection of exocytotically released material within the neuropil. At pH 4.0, tannic acid penetrated cell membranes and resulted in a marked enhancement in the electron density of regularly spaced microtubule associated proteins. Electrophysiological analysis of neurons 19, 4 and 5 exposed to TARI, pH 6.8 and 4.0, demonstrated that the TARI procedure altered the neuronal firing patterns in an irreversible manner. We conclude that the neurotoxic effects of TARi preclude its usefulness in quantitative ultrastructural analysis of exocytotic events, but that at pH 4.0 it is useful for ultrastructural investigations of microtubules and their associated proteins.