The effects of diazepam (DZP) on mitosis and the microtubule (MT) cytoskeleton in the live diatoms Hantzschia amphioxys and Surirella robusta were followed using time-lapse video microscopy. Similarly treated cells were fixed and later examined for immunoflouresence staining of MTs or for transmission electron microscopy. DZP treatment (250 microM) had no effect on interphase cells but affected mitosis, resulting in the majority of prometaphase and metaphase chromosomes releasing from one or both spindle poles and collecting irregularly along the central spindle. Chromosomes remaining attached to one pole continued to display slight prometaphase oscillations; however, this activity was never observed in metaphase spindles. Following removal of DZP, some chromosomes still bipolarly attached, immediately released elastically from one pole. Within the first 2 minutes of recovery, all chromosomes recommenced spindle attachment, exhibiting normal prometaphase oscillations and proceeded through mitosis. DZP treatment during anaphase had no detectable effect on chromosome motion or cell cleavage. These results suggest that DZP acts as an anti-MT agent, selectively affecting polar MTs at prophase, prometaphase and metaphase, and thereby weakening kinetochore connection to the poles. From these and other results (unpublished), its mode of action is different to that of most anti-MT agents.

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