Alterations in the cytoskeleton of the goldfish Mauthner cells under various pharmacological treatments.

The ultrastructural changes in the cytoskeleton of the goldfish Mauthner cells (M-cells) at various functional states induced by intracerebral microinjections of biologically active substances were studied. Under the action of kainic acid, a structural analog of the excitatory neurotransmitter of glutamate, the density of the cytoplasmic matrix increased. Cytotoxin II from the cobra toxin, which blocks acetylcholine transmission, had an opposite effect upon the M-cell cytoskeleton. Simultaneously, in some areas of the neural cytoplasm strands of an electron-dense material of various shapes appeared. They had an unique structure which did not resemble any known cytoskeleton element. The molecular composition of the strands is unknown, but similar strands appeared after injections of phalloidin or cytochalasin B, both disturbing the microfilamental component of the cytoskeleton. Decoration with myosin subfragment-1 revealed actin in intact M-cells which was organized as crossing loose filaments and bundles of parallel fibers. The morphology of the fiber bundles resembles the helical part of the strands appearing after the treatment with phalloidin, cytochalasin B, or cytotoxin II. It is suggested that the cytoplasmic matrix of M-cells is a dynamic system which responds to the functional changes by thickening or loosening of its cytoskeletal elements or by formation of new structures.