Abstract: Effects of the insecticides lmidacloprid and Dimehypo on the postsynaptic potential at the cercal-giant fiber synapses located with in the sixth adbominal ganglion of the American cockroach Periplaneta americana were studied by using extracellular electrophysiological recording method. An initial increase in thefrequency of spontaneous postsynaptic potentials and final complete block to the impulse propagation were observed after the application of Imidacloprid (1.73 ×10_-7mol/l) or Dimehypo (1.38×10^-5 mol/L). Blocking action caused by Imidaclopridwas quicker than by Dimehypo, and recovery is more difficult in the normal Ringer solution. It is supposed that the agonistic activity of Imidacloprid is stronger than that of Dimehypo. When a pre-application of Methamidophos (3.37×10^-5mol/L) was conducted, treatment with Dimehypo produced greatly increased spontaneouspostsynaptic firings and high frequency bursts of overshooting. By contrast, no visible changes were brought to the Imidacloprid treatment by Methamidophos pre-application. This study shows that there is some pharmacological difference between Imidacloprid and Dimehypo during their interactions with the postsynaptic acetylcholine receptors.

Key words: Imidacloprid, Dimehypo, American cockroach