CYTOMORPHOLOGICAL ALTERATIONS OF MOSSY FIBER BOUTONS OF HIPPOCAMPUS PRODUCED BY 3-ACETYLPYRIDINE, METHOXYPYRIDOXINE, RESERPINE, AND IPRONIAZID

The hippocampal mossy fiber boutons of the rabbit were studied with phase and electron microscopy. The injection of 3-acetylpyridine, methoxypyridoxine, and reserpine diminishes the conspicuous osmiophilic density of the mossy fiber boutons in comparison to similar regions from nontreated animals as observable in phase microscopy. However, electron micrographs of the same samples show little or no diminution in the number of those synaptic vesicles consisting of a clear homogeneous center (Type I). Treatment with monoamine liberator, reserpine, results in the same cytomorphological appearance of the boutons as with convulsant agents. The number of synaptic dense-core vesicles (Type II) is not altered after treatment with the convulsant agents or reserpine. A certain extra-vesicular substance and a certain granular component of the vesicular membranes of Type I vesicles is progressively reduced after treatment with all of these drugs. It is suggested that this accounts for the decreased density by phase microscopy. The monoamine oxidase inhibitor, iproniazid, increases the density of the extra-vesicular substance as well as the particles attached to the vesicular membranes of Type I vesicles. It is suggested that these osmiophilic particles contain the biogenic monoamines (in this instance probably serotonin and/or histamine) and that in acute experiments the liberation of these neurotransmitters is not related to a disappearence of dense-core vesicles concommitant with a depletion of neurotransmitters but is from particles in the extra-vesicular substance and the granular component of the vesicular of the Type I vesicles. Furthermore, the functional role of zinc in the synaptic vesicles of mossy fiber boutons of the hippocampus is discussed in regard to a possible storage mechanism for biogenic monoamines. © 1969 Springer-Verlag.