Abstract: The effects of methomyl on the synaptic transmission of the 6th abdominal ganglion of cockroach (Periplaneta americana ) were studied. The ganglions were partially desheathed and immersed in variousconcentrations of methomyl solution. By stimulation of the bilateral cercal nerves Xl at a rate of 1Hz/sec, evoked excitatory post-synaptic potential (EPSP) and post-synaptic action potential were recorded withmannitol gap method. With weak stimulation only EPSP could be recorded, and under the action of 100 μmol/L methomyl, an increase in the amplitude and duration of the EPSP was first observed, and thenpostsynaptic action potential could be evoked; however, afterwards the amplitude of EPSP reduced progressively until finally disappeared, but the pre-synaptic response remained unchanged. The EPSP could berestored by washing cockroach with saline. With strong stimulation, the post-synaptic action potentials, uperimposed on EPSP, could be recorded. Under the action of methomyl solutions, the synaptic transmission was blocked. The blocking action of methomyl was reversible by washing, however, it still remained some post-effect after 30 min washout. The LD50 for methomyl to adult male cockroach was (3.56+0.01) μg per gram body weight determined by abdominal injection. Based on the action of methomyl on ACHE, the basic characteristics of the blocking process and its killing effect to the insect were discussed.

Key words: methomyl, cockroach ( Periplaneta americana ), synaptic transmission