Reward sensitization: effects of repeated nicotine exposure and withdrawal in mice

Abstract

Tobacco dependence is an addiction with high rates of relapse, resulting in multiple quit attempts in individuals who are trying to stop smoking. How these multiple cycles of smoking and withdrawal contribute to nicotine dependence, long-term alterations in brain reward systems, and nicotine receptor regulation is unknown. Therefore, to evaluate how multiple exposures of nicotine and withdrawal periods modulate rewarding properties of nicotine, we used intracranial self-stimulation to measure alterations in the threshold of brain stimulation reward. In addition, we employed the conditioned place preference (CPP) paradigm to evaluate positive context conditioning following each withdrawal period and measured levels of neuronal nicotinic receptors in cortex, striatum, and hippocampus. We found that repeated nicotine exposure and withdrawal enhanced brain stimulation reward and reward sensitivity to acute injections of nicotine. This increased reward was reflected by enhanced CPP to nicotine. Chronic nicotine is known to up-regulate nAChRs (nicotinic acetylcholine receptors) and we found that this up-regulation was maintained for up to 8 days of withdrawal in the striatum and in the hippocampus, but not in the cortex, of animals exposed to multiple nicotine exposure and withdrawal periods. These results demonstrate that repeated exposures to nicotine, followed by withdrawal, induce a persistent increase in both brain reward function and sensitivity to the hedonic value of nicotine and long-lasting up-regulation of neuronal nicotinic receptors. Together, these data suggest that a continuing increase in brain reward function and enhanced sensitivity to nicotine reward following repeated withdrawal periods may be one reason why smokers relapse frequently.

Figure 1

Brain stimulation-reward thresholds and response latencies expressed as a percentage of pre-nicotine treatment values (mean values±SEM). Dashed line represents basal reward threshold set at 100% of baseline levels. (a) Experimental design. The segments depicted in black refer to the data presented bellow. (b) Asterisks indicate increased brain reward function in animals treated with nicotine minipump (NMP; 24 mg/kg/day) when compared with SMP animals, following multiple withdrawals, as seen during the third cycle of withdrawal (p<0.05). (c) Response latencies. Increase in the latency to respond during chronic treatment in the first cycle was similar in all animals but increased during withdrawal in NMP animals when compared with SMP (as indicated with the asterisk, p<0.05). Asterisks indicate higher latency to respond, in the second and third cycle, in NMP animals in both chronic treatment and withdrawal.

Figure 2

Brain stimulation-reward thresholds and response latencies expressed as a percentage of pre-nicotine treatment values (mean values±SEM). Dashed line represents basal reward threshold set at 100% of baseline levels. (a) Experimental design. The segments depicted in black refer to the data presented bellow. (b) Between-group comparisons. Asterisks indicate decreased reward sensitivity with acute nicotine treatment, during the first and second cycle of withdrawal in NMP and SMP groups when compared with NMP animals injected with saline (p<0.05), this effect was not observed during the third cycle of withdrawal. Asterisk during the third cycle of withdrawal indicates higher sensitivity to reward in both NMP groups, regardless of the injection administered, when compared with SMP groups (p<0.05). (Note: SMP and NMP symbols overlap on day 17). (c) Response latencies. Asterisks indicate lower latencies to respond in SMP animals when compared with NMP animals treated with nicotine or saline during first, second, or third cycle (p<0.05); moreover, nicotine injections reduced the time of response in NMP animals when compared with NMP animals injected with saline during third cycle (p<0.05).

Figure 3

(a) Experimental paradigm. (b) Effects of chronic administration of nicotine (40 mg/kg/day, 14 days) or saline and of spontaneous withdrawal on ICSS thresholds. The asterisk indicates a significant main effect of treatment during withdrawal (p<0.05). (c) Effect of mecamylamine-precipitated withdrawal. Brain reward thresholds are expressed as a percentage of the pretest day values. Asterisk indicates elevations in brain reward thresholds compared with those of the corresponding control group (p<0.05). Data are expressed as mean values±SEM.

Figure 4

(a) Experimental paradigm used for CPP, all groups receive only one set of four nicotine/saline injections administered in 4 consecutive days; 1WD group receives the injections during the first withdrawal, 2WD receives the injections during the second withdrawal, and 3WD group receives the injections during the third withdrawal. 2WD and 3WD groups have one and two withdrawal periods with no treatment, respectively. Saline group animals were treated with SMP and conditioned with nicotine acute injections. (b) Nicotine-induced place conditioning following conditioning with 0.5 mg/kg injection nicotine (s.c.). Animals preferred nicotine-paired environments only after 2 or 3 withdrawals (WD) compared with saline. 3WD animals also showed a higher place preference relative to 1WD animals (^**p<0.01). (b–d) Effects of repeated withdrawals on nAChR modulation. (c) Cortical homogenates failed to show any significant up-regulation following multiple withdrawals. (d) Striatal homogenates revealed progressively higher density of nAChRs. (e) Hippocampal homogenates showed significantly higher levels of [³H]EB binding in samples from the 3WD group only. ^***p<0.0001; ^**p<0.01; ^*p<0.05, compared with saline.