Nicotine and the Neonicotinoids
The toxicology of nicotine and the neonicotinoid insecticides is discussed, with an emphasis on imidacloprid as it is the most studied neonicotinoid among the seven established members of the class. Neonicotinoids are more effective and safer insecticides than nicotine because of their selective toxicity to insects. Nicotine and the neonicotinoids exert their insecticidal action by binding to the nicotinic acetylcholine receptors (nAChRs) of the postsynaptic membrane of nerve cells in the central nervous system and acting as nAChR agonists. The neonicotinoidsare significantly less toxic to vertebrates because of the combined effects of low affinity for vertebrate nAChRs and relatively poor penetration of the blood-brain barrier compared with nicotine. Nicotine is a potent pharmacologically active drug which has wide-ranging effects mainly involving the nervous, cardiovascular, respiratory and endocrine systems and the gastrointestinal tract. The toxicological characteristics of nicotine and neonicotinoids, while showing some similarities at acute neuropharmacologically active doses, generally display significant differences in overall toxicity. High toxic doses of neonicotinoids lead to reversible neurological signs and behavioural changes but there are rarely any signs of permanent damage to the nervous system in laboratory animal studies. The liver is a common target organ among the neonicotinoids, and hepatic microsomal enzyme induction accompanied by adaptive liver growth is seen with most neonicotinoids. Overall, nicotine and the neonicotinoids do not raise significant concerns with respect to carcinogenicity. Reproductive and developmental effects are seen with both nicotine and the neonicotinoids although they are usually at doses associated with general toxicity.