Active zone at Aplysia synapses: organization of presynaptic dense projections.

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Abstract
Aplysia abdominal ganglia were selectively stained with either ethanolic phosphotungstic acid (E-PTA) or Bi-I impregnation followed by uranyl acetate and Pb staining (BIUL). Cytochemically stained active zones display a series of dense projections arising from an amorphous internal coating of the presynaptic membrane. In transversely sectioned junctions, dense projections have the general configuration of small, flattened pyramids approximately 30 nm high and 46 nm wide. These may appear isolated or interconnected along their bases by strands of electron-dense material. En face views at the level of the presynaptic membrane reveal oval-shaped plaques that vary in size. The appearance of dense profiles within these areas suggests that many are discrete projections. In thin sections, the arrangement of dense projections are variable and irregular. Dense projections were found in a regular hexagonal lattice, suggesting that coherent networks similar to the precise grid found in vertebrates exist over at least portions of the presynaptic area in Aplysia. Variation in the size of active zones and in their morphology may be explained by synaptic diversity and by possible differences in functional state and/or degree of maturation. A technique was developed that permits the simultaneous labeling of identified presynaptic terminals with horseradish peroxidase and the visualization of E-PTA-intensified dense projections at their active zones. This technique was applied to identified synapses in Aplysia that mediate simple forms of learning. This approach should prove useful for studies of the cellular role of dense projections in synaptic transmission and their possible contribution to changes in synaptic efficacy as a function of learning, early development and age.