β4-Nicotinic Receptors Are Critically Involved in Reward-Related Behaviors and Self-Regulation of Nicotine Reinforcement

Abstract

Nicotine addiction, through smoking, is the principal cause of preventable mortality worldwide. Human genome-wide association studies have linked polymorphisms in the CHRNA5-CHRNA3-CHRNB4 gene cluster, coding for the a5, a3, and b4 nicotinic acetylcholine receptor (nAChR) subunits, to nicotine addiction. b4*nAChRs have been implicated in nicotine withdrawal, aversion, and reinforcement. Here we show that b4*nAChRs also are involved in non-nicotinemediated responses that may predispose to addiction-related behaviors. b4 knockout (KO) male mice show increased novelty-induced locomotor activity, lower baseline anxiety, and motivational deficits in operant conditioning for palatable food rewards and in reward-based Go/No-go tasks. To further explore reward deficits we used intracranial selfadministration (ICSA) by directly injecting nicotine into the ventral tegmental area (VTA) in mice. We found that, at low nicotine doses, b4KO self-administer less than wild-type (WT) mice. Conversely, at high nicotine doses, this was reversed and b4KO self-administered more than WT mice, whereas b4-overexpressing mice avoided nicotine injections. Viral expression of b4 subunits in medial habenula (MHb), interpeduncular nucleus (IPN), and VTA of b4KO mice revealed dose-and region-dependent differences: b4*nAChRs in the VTA potentiated nicotine-mediated rewarding effects at all doses, whereas b4*nAChRs in the MHb-IPN pathway, limited VTA-ICSA at high nicotine doses. Together, our findings indicate that the lack of functional b4*nAChRs result in deficits in reward sensitivity including increased ICSA at high doses of nicotine that is restored by re-expression of b4*nAChRs in the MHb-IPN. These data indicate that b4 is a critical modulator of reward-related behaviors.

Prolonged nicotine exposure reduces aversion to the drug in mice by altering nicotinic transmission in the interpeduncular nucleus

“…We observed that, on average, WT mice titrate their intake to achieve a consistent nicotine dose, in agreement with previous reports 12,34,35 , and discovered that mutant mice lacking the nAChR β4 subunit did not, resulting in greater nicotine intake, notably at high nicotine concentrations in the drinking solution. These behavioral results are in agreement with the greater intracranial self-administration observed at high nicotine doses in these mice 33 and, conversely, with the results obtained with transgenic TABAC mice overexpressing the β4 subunit at endogenous sites, which avoid nicotine and consequently consume very little 13,33 . Nevertheless, it should be noted that conflicting results have also been reported regarding the role of β4 nAChRs in nicotine consumption.…”

“…Representative currents and average responses following a puff application of nicotine (30 μM, 200 ms) on IPN neurons from β4 −/− mice transduced in the IPN with either lenti-pGK-β4-IRES-GFP (KO-β4 IPN , n = 7 neurons from 2 mice, I = −362 ± 95 pA) or lenti-pGK-GFP (KO-GFP IPN , n = 7 neurons from 1 mouse, I = −71 ± 18 pA; Mann-Whitney test, p = 0.004). C. Average nicotine intake was lower in KO-β4 IPN than in KO-GFP IPN (two-way repeated measure; ANOVA: genotype x dose interaction, F[3, 99] = 6.3, ***p < 0.001; main effect of dose, F[3, 99] = 69.1 ***p < 0.001, effect of genotype, F[1, 33] = 6.637, *p = 0.015). D. Ternary diagram representing each β4 −/− mouse, transduced with either β4 or GFP, and illustrating its nicotine consumption index over its side bias index.…”

“…About a third to half of those who have tried smoking tobacco become regular users 31 . Individual differences in the sensitivities of the VTA and MHb-IPN systems, and in their respective adaptations during chronic tobacco use, could contribute to the vulnerability to nicotine and to the severity of the addiction process 14,[32][33][34][35] . Yet, the neural mechanism that makes individuals more prone to maintain nicotine consumption versus durably stop are unclear.…”

Divergent Roles of α5 and β4 Nicotinic Receptor Subunits in Food Reward and Nicotine-induced Weight Loss in Male Mice

“…Interestingly, central GLP-1 signaling modulates nicotine intake via the habenular–interpeduncular pathway ( 25 ). Given that the habenular–interpeduncular pathway is particularly enriched with α5- and β4-nAChRs ( 34 , 35 ) and site-specific loss- and gain-of-function models have demonstrated key roles for both receptor subunits in governing nicotine consumption ( 17 , 18 ), neural crosstalk between GLP-1R and nAChR signaling might be relevant to this neurobiology. To evaluate this, we exposed α5 KO, β4 KO, and WT mice to a pharmacological treatment study with the long-acting GLP-1R agonist liraglutide ( 36 ).…”

“…Interestingly, the α5-α3-β4 ( CHRNA5-CHRNA3-CHRNB4 ) gene cluster has been linked to smoking heaviness, nicotine dependence ( 17–21 ), and higher body mass index (BMI) ( 22 , 23 ). Animal studies have implicated all individual subunits of the α5-α3-β4 gene cluster in regulating nicotine consumption (titration of reward and aversion) ( 17 , 18 , 24 ). However, despite a clear implication of α5 and β4 in nicotine sensitivity, the role of these subunits in food reward has received very little attention.…”