The reasons why people smoke are varied, but research has demonstrated that genetic influences on various aspects of nicotine addiction are a major factor. There also is a strong genetic influence on measures of nicotine sensitivity in mice. Despite the established contribution of genetics to nicotine sensitivity in mice and humans, no naturally occurring genetic variation has been identified that demonstrably alters sensitivity to nicotine in either species. However, one genetic variant has been implicated in altering nicotine sensitivity in mice is a T529A polymorphism in Chrna4, the gene that encodes the nicotinic receptor (nAChR) α4 subunit. The Chrna4 T529A polymorphism leads to a threonine to alanine substitution at position 529 of the α4 subunit. To more definitively address whether the Chrna4 T529A polymorphism does, in fact, influence sensitivity to nicotine, knockin mice were generated in which the threonine codon at position 529 was mutated to an alanine codon. Compared to Chrna4 T529 littermate controls, the Chrna4 A529 knockin mice exhibited greater sensitivity to the hypothermic effects of nicotine, reduced oral nicotine consumption and did not develop conditioned place preference to nicotine. The Chrna4 A529 knockin mice also differed from T529 littermates for two parameters of acetylcholine-stimulated 86Rb+ efflux in midbrain: maximal efflux and the percentage of α4β2* receptors with high sensitivity to activation by agonists. Results indicate that the polymorphism affects the function of midbrain α4β2* nAChRs and contributes to individual differences in several behavioral and physiological responses to nicotine thought to be modulated by midbrain α4β2* nAChRs.

Inbred mouse strains display significant differences in their levels of brain ␣7 nicotinic acetylcholine receptor (␣7 nAChR) expression, as measured by binding of the ␣7-selective antagonist ␣-bungarotoxin. Variations in ␣-bungarotoxin binding have been shown to correlate with an animal's sensitivity to nicotine-induced seizures and sensory gating. In two inbred mouse strains, C3H/2Ibg (C3H) and DBA/2Ibg (DBA/2), the interstrain binding differences are linked to a restriction length polymorphism in the ␣7 nAChR gene, Chrna7. Despite this finding, the molecular mechanism(s) through which genetic variability in Chrna7 may contribute to ␣7 nAChR expression differences remains unknown. However, studies of the human ␣7 nAChR gene (CHRNA7) previously have demonstrated that CHRNA7 promoter polymorphisms are associated with differences in promoter activity as well as differences in sensory processing. In the present study, a 947-base pair region of the Chrna7 promoter was cloned from both the C3H and DBA/2 inbred mouse strains in an attempt to identify polymorphisms that may underlie ␣7 nAChR differential expression. Sequence analysis of these fragments identified 14 single nucleotide polymorphisms (SNPs). A combination of two of these SNPs affects promoter activity in an in vitro luciferase reporter assay. These results suggest a mechanism through which the Chrna7 promoter genotype may influence interstrain variations in ␣7 nAChR expression.The expression of ␣7 nAChRs, 3 as measured by 125 I-labeled ␣-bungarotoxin (␣-BTX), varies significantly across inbred mouse strains (1, 2), and studies have demonstrated that interstrain differences in ␣7 nAChR expression are genetically regulated. For example, restriction fragment length polymorphisms in Chrna7, the gene that encodes for the mouse ␣7 nAChR subunit, have been shown to be linked to ␣7 nAChR expression in genetically segregating populations derived from the inbred mouse strains C3H and DBA/2 (3). The strain differences in binding appear to be related to differences in ␣7 nicotinic receptor mRNA levels (2, 4), suggesting that transcriptional or posttranscriptional mechanisms may contribute to the variations in ␣-BTX expression levels.Genetically regulated ␣7 nicotinic receptor expression patterns have been associated with individual variations in neurophysiology and neuroanatomy. Interstrain differences in ␣-BTX binding are related to variations in sensitivity to nicotine-induced seizures, acoustic startle response (5), and hippocampal sensory gating (2). In addition, genetic variability in Chrna7 has been implicated in regulating individual differences in cholinergic and GABAergic hippocampal neuroanatomy (6 -8).Variations in ␣-BTX binding have also been associated with differences in neurophysiological function among humans. Schizophrenics, for example, display reduced ␣-BTX binding levels in post-mortem hippocampus (9). Furthermore, promoter polymorphisms in the gene coding for the human ␣7 nAChR subunit are associated with an auditory gating deficit that is common...

Despite the evidence that there is a daily rhythm in smoking behavior and that the effects of drugs of abuse exhibit diurnal variations, very few studies have explored the extent to which sensitivity to the effects of nicotine vary over the course of the day. In the studies described in this report, the melatonin proficient mouse strain C3H/Ibg and the melatonin deficient mouse strains C57BL/6J and DBA/2J were assessed for diurnal variations in sensitivity to the effects of nicotine. Results indicated that there is significant variation in sensitivity to both activity and body temperature depressant effects of nicotine in the melatonin proficient C3H/Ibg strain with maximal sensitivity occurring during the latter third of the light period of the light cycle and minimal sensitivity taking place during the last third of the dark phase of the light cycle. The melatonin deficient strains did not exhibit diurnal differences in sensitivity to the effects of nicotine suggesting a potential role for melatonin in modulating the effects of nicotine. Experiments with knockout mice lacking both the Mtnr1a and Mtnr1b melatonin receptors confirmed that the reduced sensitivity observed during the dark phase is melatonin dependent. Diurnal variation in nicotinic receptor expression also was measured in cortex, hippocampus, hypothalamus and striatum using [125I] α-bungarotoxin and [125I]-epibatidine. [125I] α-bungarotoxin binding in hypothalamus of C3H mice exhibited a diurnal pattern with maximal binding observed in the latter third of the light portion of the light cycle. No other significant differences in binding were detected.