High voltage electron microscopy studies of axoplasmic transport in neurons: a possible regulatory role for divalent cations.

Abstract

Light and high voltage electron microscopy (HVEM) procedures were employed to examine the processes regulating saltatory motion in neurons. Light microscope studies demonstrate that organelle transport occurs by rapid bidirectional saltations along linear pathways in cultured rat neuroblastoma C1300 cell line cells. HVEM stereo images of axons reveal that microtubules (Mts) and organelles are suspended in a countinues latticework of fine microtrabecular filaments and that the Mts and lattice constitute a basic cytoskeletal structure mediating the motion of particles along axons. Particle transport depends on dynamic properties of nonstatic microtrabecular lattice components. Experiments were initiated to determine the effects of changes in divalent cation concentration (Ca2+ and Mg2+) on: the continuation of transport and the corresponding structural properties of the microtrabecular lattice. Transport continues or is stimulated to a limited extent in cells exposed to small amounts of exogenously supplied Ca2+ and Mg2+ ions (< 0.1 mM). Exposure of neurons to increased dosages of Ca2+ and Mg2+ (0.2-1.0 mM) stimulates transport for 2-4 min at 37.degree. C, but after a 5-20-min exposure the saltatory movements of organlles are observed gradually to become shorter in duration and rate until particle motion ceases to occur. HVEM observations demonstrated that Ca2+- and Mg2+-stimulated changes in the organization of the microtrabecular lattice are associated with the cessation of motion. Ca2+-containing solutions produced contractions of the microtrabecular filaments, whereas Mg2+-containing solutions had the opposing effect of stimulating an elongation and assembly (expansion) of microtrabecular. Cycles of Ca2+/Mg2+-coupled contractions and expansions of the microtrabecular lattice probably regulate organelle motion in nerve cells.