An ultrastructural study of presynaptic membrane specializations in sympathetic ganglia of 4-aminopyridine treated guinea pigs and rats.

The presynaptic membranes of synapses in sympathetic ganglia of 4-aminopyridine treated guinea pigs and rats were investigated in either freeze-fractured or thin-sectioned material. After freeze-fracture many presynaptic membranes showed dimples in the P face and corresponding crater-like protrusions in the E face. In about 45% of the thin-sectioned synapses small clear omega-shaped invaginations were present at the active zone of the presynaptic membrane. Larger omega profiles with dense cores were sometimes also seen. These membrane features were very rarely observed in non-treated material.