Dopamine in the neostriatum is contained in tiny axonal terminals 5, but there is little ultrastructural evidence to distinguish axonal boutons containing dopamine from those with other putative neurotransmitters. H6kfelt and coworkers 9, 1 studied this problem by incubating fresh specimens of rat neostriatum in the presence of the'false transmitters', a-methylnorepinephrine (a-methyl-NE) and 5-hydroxydopamine (5-OH-DA), followed by fixation in permanganate. A dense precipitate, which was due to the chemical reaction of permanganate with the false transmitter, formed in 40-60 nm axonal vesicles and in large granular vesicles, and thus provided a marker or'tag'for boutons which incorporated these false transmitters 9.(For convenience, we will refer to vesicles in which this precipitate is formed as' tagged vesicles' and axonal boutons containing these vesicles as' tagged boutons or varicosities'.) H6kfelt 9 counted over 2500 neostriatal boutons and found that 16.4~(range 10-19~) contained tagged vesicles. Most tagged boutons did not exhibit junctional contacts; only one tagged axospinous asymmetrical synapse was observed and a possible axosomatic synapse was suggested 9.

We have used the technique of incubating fresh tissue slices of neostriatum in medium containing 5-OH-DA to investigate biogenic amine-containing terminals in the rabbit. In addition to confirming the findings of H6kfelt and coworkers 9, 1, we have observed (1) fusion and pit formation between tagged boutons and the axonal membrane in varicosities lacking junctional plaques, and (2) symmetrical junctions on about 2~ of the tagged boutons, some of which synapse with the small neuron in the neostriatum.