Recognising nicotine: the neurobiological basis of nicotine discrimination

Drug discrimination methodology makes possible the objective, quantitative study of the perception of psychoactive drug effects in either human or animal subjects. Investigations of the nicotine discriminative stimulus complex have contributed to our present understanding of nicotine psychopharmacology by defining the origin of its effects at specific subtypes of nicotinic receptor and the role of diverse neurotransmitter systems as mediating and modulating mechanisms. The evidence strongly supports central sites as the origins of the nicotine stimulus, and these are likely to be located in the mesocorticolimbic dopaminergic neurons; the medial prefrontal cortex is primarily involved, with the Nucleus accumbens and ventral tegmental area of secondary importance, while another element of the complex stimulus may arise in the dorsal hippocampus. Additionally, it appears that interactions of nicotine with the dopamine, serotonin, cannabinoid and probably glutamate systems all contribute to the final perceived stimulus. The resemblance between the nicotine discriminative stimulus and those of the psychomotor stimulant drugs amphetamine and cocaine contributes to defining the nature of the addictive properties of nicotine. It is particularly interesting that acute and chronic exposure to caffeine produce quantitative and qualitative changes in the characteristics of the nicotine stimulus. Interactions of nicotine with caffeine and cannabinoids strengthen proposals that the use of one substance serves as a "gateway" in sequential shifts of the target substance for drug-seeking behaviour, with profound implications for the human use of the substances concerned. Drug discrimination is also an important standard technique used in assessments of the abuse liability of novel psychoactive compounds, with relevance to attempts to develop novel nicotinic agonists for use as cognitive enhancers.