Abstract
Using early signs of terminal degeneration caused by direct injection of 6-hydroxydopamine (6-OHDA) as a specific marker for catecholamine (CA) terminal boutons, we studied the fine structure of these terminals in the glutaraldehyde-fixed visual cortex of adult cats. As previously observed in potassium permanganate (KMnO4)-fixed material, only a minority (29%) of the labeled CA boutons was found to be making synaptic contact. This low incidence of synapse-forming boutons was in sharp contrast to the high incidence exhibited by nonaminergic terminals in the same electron micrographs (58%). The former varied somewhat depending on the cortical layer examined, being the highest in layers II and III (40%), the lowest in layer I (22%) and intermediate in the bottom three layers (24-31%).
Dendrites and dendritic spines were the common target of synapse forming CA boutons, but a few axo-somatic contacts were also seen in layer VI. The CAergic synapses were of both types, symmetrical and asymmetrical, with a different frequency depending on the cortical layer. The symmetric type predominated in layers II, III and VI, while in layers IV and V and asymmetric synapses were as numerous as the symmetric ones. The present results further support the earlier suggestions that CA may be released from both synaptic and nonsynaptic axon terminals in the neocortex.
