The effects of two pyrethroids, cismethrin and deltamethrin, on skeletal muscle and the trigeminal reflex system in the rat

Abstract

The actions of a cyano pyrethroid (deltamethrin) and a non-cyano pyrethroid (cismethrin) upon trigeminal motor reflexes and isolated muscle responses were studied in the rat. Deltamethrin caused a marked facilitation of the muscle response to nerve stimulation in pithed rats at 2.5 μmol kg −1. In intact anaesthetised rats this was associated with abnormal repetitive EMG discharges and, at 4 μmol kg −1 with a suppression of late components of the reflex response to sensory stimuli in the spinal trigeminal nucleus and trigeminal motor nucleus. In contrast cismethrin had no effect on the muscle response to direct nerve stimulation at up to 15 μmol kg −1, but produced abnormal extra responses to sensory stimuli in the trigeminal ganglion, spinal and motor nuclei, and jaw muscles at 9 μmol kg −1. It is concluded that whilst deltamethrin produces reflex excitation within the trigeminal system at a primarily muscular site, cismethrin produces excitation at all stages of the reflex loop. This contrast is consistent with the known difference in duration of sodium current prolongation produced by the two pyrethroids. These findings, together with other known central actions of deltamethrin suggest that it has multiple sites of action in the intact animal, both central and peripheral, whilst most of the simpler symptoms produced by cismethrin may adequately be explained by action at a reflex level.