Presynaptic microtubules: organisation and assembly/disassembly.

With the electron microscope we have examined the organisation of microtubules in synapses and varicosities of central and peripheral axons and their assembly/disassembly in synaptosomes. Following fixation in a tri-aldehyde fixative containing a calcium ion chelating agent, central synapses and autonomic varicosities show up to 10 microtubule profiles. In synapses, these microtubules are found at one or both poles of the terminal. In autonomic varicosities, microtubules occupy a central zone or are located to one side. Synaptosomes also contain microtubules, probably arranged in the form of a coil. The coil lies in an equatorial plane parallel to the active zone and is closely apposed to the plasma membrane. The microtubules in intact synapses, autonomic varicosities and synaptosomes are frequently associated with mitochondria. Serial section analysis shows that the mitochondrion in synaptosomes is horse-shoe shaped and that presynaptic microtubules are continuous with those in the axon. In synaptosomes, presynaptic microtubules disassemble when exposed to colchicine (IC50 = 3 x 10(-4) M) and low temperatures (1 degree C) and reassemble following warming to 37 degrees C. Reassembly occurs with no other apparent effects on the organisation of the presynaptic terminal. The synaptosomes thus retain the capacity to assemble ordered coils of microtubules. The function of presynaptic microtubules is discussed in the light of the finding that there may be a relation between the number of turns in the microtubular coil and the size of the presynaptic terminal. An explanation is proposed for the relative scarcity of these structures at conventionally prepared synapses.