Ultrastructure of monoaminergic terminals in the intermediolateral nucleus of the cat spinal cord.

Monoaminergic innervation of the intermediolateral nucleus of the cat spinal cord was investigated by fluorescence histochemistry and electron microscopy. Large numbers of monoaminergic terminals were labeled by prior administration of the false neurotransmitter 5-hydroxydopamine (5-OHDA). Ultrastructurally, 5-OHDA-labeled terminals fell into three types. Type I, which made up 55% of the labeled terminals, contained abundant, large and densely labeled vesicles and only a few small and unlabeled vesicles. This type was "bouton de passage". Type II, which made up 40% of the terminals, made asymmetrical synaptic contacts with typical postsynaptic structures. This type contained many small vesicles, some of which were labeled, and a few large dense-core vesicles. Type III, which made up 5% of the terminals, made close contact with presynaptic nerve endings containing abundant small unlabeled clear vesicles. The type III terminals contained many large and densely labeled vesicles and a few small flattened vesicles, most of which were unlabeled.