Abstract: The influence of naloxone on respiration impaired by the highly toxic organophosphate nerve agent soman in anaesthetized rats was investigated. Soman, administered in a dose that was ineffective in blocking the electrically induced contractions of the phrenic nerve-diaphragm preparation in situ, induced a complete block of the spontaneous respiratory movements of
the diaphragm, indicating the domination of central over the peripheral effects. Naloxone dose-dependently antagonized the
soman-induced respiratory blockade. Atropine, at a dose that was per se ineffective in counteracting soman-induced respiratory
depression, potentiated the protective effects of naloxone and completely restored respiration. Naloxone remained completely
ineffective in antagonizing respiratory depression induced by the muscarinic receptor agonist the oxotremorine. It is assumed that
naloxone antagonizes soman-induced respiratory inhibition by blocking endogenous opioidergic respiratory control pathways that
are independent of the stimulation of muscarinic receptors.
