New observations on the substructure of the active zone of brain synapses and motor endplates

This study offers a new concept on the origin and function of the hitherto enigmatic presynaptic dense projections (dps) of neurons and motor endplates. After a deuterium oxide-albumin pretreatment (da), brain tissue and motor endplate of rat and frog reveal an intricate association of smooth endoplasmic reticulum (ser), microtubules (mts) and synaptic vesicles (sv) at the presynaptic grid-active zone of synapses. The ser entwines the mts, which are clothed in svs, and impinges directly onto the presynaptic membrane as sacs or 'tubular-fibrillar' extensions. Since no dps are seen in these sections, whereas they do occur in conventionally processed material (i.e. without da pretreatment), it is suggested that the dps of conventional material may, in part, originate from improperly fixed ser at these points. Thus for the first time we demonstrate an in vivo system of ser which, because its 'finger' processes come into intimate contact with the presynaptic membrane, may be implicated in Ca2+ ion translocation, presumably out of the presynaptic bulb. Since no such tubular ser has been demonstrated in what are claimed to be sophisticated techniques (i.e. high-speed slam-freezing-freeze substitution) the actual sophistication of such methods is questioned.