SITE OF ACTION OF PYRETHROIDS - REPETITIVE BACKFIRING IN FLIGHT MOTOR UNITS OF HOUSE-FLY

In vivo measurements of flight motor unit repetitive discharges during pyrethroid (tetramethrin) poisoning were reproduced in in vitro nerve-muscle preparations at concentrations down to 10-9 M. Both in vivo and in vitro discharge patterns arose abruptly and were interspersed with normal single spikes. Repetitive discharges originated in the terminal intramuscular portions of motor nerve fibers. Discharges were recorded postsynaptically in muscle fibers and in the main part of axons as they passed antidromically into the central nervous system. Repetitive activity was triggered either by single orthodromic nerve impulses emerging from the thoracic ganglion or occurred spontaneously in the absence of triggering impulses. A mean critical temperature (19°C) was found, below which discharges were abolished in intact flies. At higher temperatures, discharges became more consistent, decreased in duration, and increased in frequency. Since motor unit discharges disappear at lower temperatures while toxicity increases, their role in the events leading to death is dubious. © 1979.