IMIDACLOPRID ACTIONS ON INSECT NEURONAL ACETYLCHOLINE-RECEPTORS

The neonicotinoid insecticide Imidacloprid acts at three pharmacologic ally distinct acetylcholine receptor (AChR) subtypes in the cockroach (Periplaneta americana) nervous system, but is ineffective on muscarin ic receptors. Imidacloprid (3-100 mu mol l(-1)) induced dose-dependent depolarizations at cockroach cercal afferent/giant interneurone synap ses. These responses were insensitive to 20 mu mol l(-1) atropine but were completely blocked by the nicotinic antagonist mecamylamine (50 m u mol l(-1)). Similarly, Imidacloprid-induced depolarizations of cultu red cockroach dorsal unpaired median (DUM) neurones dissociated from t he same (terminal abdominal) ganglion were also completely blocked by 100 mu mol l(-1) mecamylamine. However, two components of the response could be distinguished on the basis of their differential sensitiviti es to 0.1 mu mol l(-1) a-bungarotoxin (alpha-BTX), which selectively b locks AChRs with 'mixed' nicotinic/muscarinic pharmacology in this pre paration. This indicates that Imidacloprid affects both AChRs sensitiv e to alpha-BTX and alpha-BTX-insensitive nicotinic acetylcholine recep tors (nAChRs). Thus, in the cockroach, Imidacloprid activates alpha-BT X-sensitive synaptic nAChRs in giant interneurones, alpha-BTX-insensit ive extrasynaptic nAChRs in DUM neurones, and a recently characterized DUM neurone 'mixed' AChR that is sensitive to both nicotinic and musc arinic ligands, Imidacloprid does not act on muscarinic acetylcholine receptors (mAChRs) present on DUM neurone cell bodies and at the cerea l afferent/giant interneurone synapses. This study shows that Imidaclo prid can act on pharmacologically diverse nAChR subtypes.