Appetitive memories and relapse to drug-use: investigations on effects of selective disruption of memory mechanisms in a rat model of nicotine dependence.

Tobacco use through cigarette smoking is the leading preventable cause of death in the developed world. The pharmacological effect of nicotine plays a crucial role in tobacco addiction. Nicotine dependence has a huge impact on global health and although several medications are available, including a wide range of nicotine-replacement therapies (NRTs), bupropion, and recently approved nicotinic receptor partial agonist varenicline, at best only about a fifth of smokers are able to maintain long-term (12 months) abstinence with any of these approaches. Thus, there is a need to identify more effective treatment to aid smokers in maintaining long-term abstinence. Several preclinical and clinical studies have underlined the importance of non-pharmacological factors, such as environmental stimuli, in maintaining smoking behaviour and promoting relapse. Initially neutral stimuli that are repeatedly paired with a reinforcing drug (e.g. lighter) acquire a new conditioned value (conditioned stimuli, CS) and become able to elicit craving even in the absence of the drug. Indeed smokers are particularly reactive to smoking/nicotine related CS, this phenomenon is called cue-reactivity and involves a vast array of physiological, psychological and also behavioural response, such as decrease in heart rate and blood pressure and/or increase in skin conductance and skin temperature, increase in craving and urge to smoke and/or mood change, and also change in smoking behaviour (e.g., latency to smoke, cigarette puff volume and frequency, amount of cigarette consumed and relapse to smoking behaviour). Given the importance of the learned association between stimuli and nicotine in the phenomenon of relapse to nicotine-seeking behaviour, it has been proposed that treatment that disrupts the nicotine-associated memories could act as a pro-abstinent and anti-relapse therapy. After learning experience, memories are stored by a process called consolidation. For at least a century it has been a dogma that initially labile memory (short-term memory) are consolidated by the passage of time and become stable and permanent (long-term memory). However converging evidence from animal and human studies have revealed that memories may return to a vulnerable phase during which they can be updated, maintained and even disrupted. The retrieval of memory indeed may destabilize the consolidated memories that require a new process to be maintained. This hypothetical process is called reconsolidation. The disruption of drug-related memories reconsolidation has been proposed as a potential therapeutic target to prevent the CS-induced relapse in ex drug-addicts. Several animal studies have shown that the reconsolidation of drug-related memories can be disrupted by the administration of an amnestic drug contingently upon retrieval of the memory. Unfortunately most of the compound used in animal studies has serious tolerability and safety issues in humans. Recently Monfils at al and Schiller et al have shown that it is possible to disrupt fear memory reconsolidation and consequently prevent the return of fear by providing CS-extinction training shortly after retrieval of the memory. CS-extinction consists in the repeated presentations of CS (e.g. lighter) in the absence of the unconditioned stimulus US (e.g. drug) leading to a decrease of the previously acquired conditioned response (e.g. smoking behaviour). The main objective of the present thesis was to investigated whether it is possible to disrupt nicotine related memories reconsolidation by applying CS-extinction after the retrieval of such as memories, and whether this disruption prevent the relapse to nicotine-seeking behaviour in a rat model of nicotine dependence. Furthermore we investigated also whether nicotine-related memories reconsolidation might be pharmacologically disrupted by administering a drug, that have been shown to disrupt memory reconsolidation in previous literature studies (i.e. propranolol or MK-801), at memory retrieval. The experimental approach used to address this issue was the paradigm of nicotine self-administration in rats, a paradigm based on Pavlovian and operant conditioning to nicotine and nicotine-associated cues. We performed five experiments in which CS-extinction or the pharmacological treatment (i.e. propranolol or MK-801) was associated to different memory retrieval protocols. We therefore assessed the effect of these post-retrieval treatments on relapse to nicotine or food seeking behaviour. Retrieval consists in presenting the CS in the absence of US, a procedure similar to CS- extinction. Since the length of CS exposure (i.e. number of CS presentations) is a crucial factor for reconsolidation or extinction occurrence, different retrieval length (1, 3 or 30 CS presentations) have been presented to retrieve nicotine-related memories. Results showed that CS-extinction applied after a short retrieval (3 CS presentations) reduced the relapse to nicotine seeking behaviour compared to control groups that did not receive CS-extinction, moreover this effect was not observed when CS-extinction was applied without retrieval. These results suggest that the effect of post-retrieval CS-extinction was specifically due to inhibition of nicotine-related memories reconsolidation. To our knowledge, this is the first evidence of post-retrieval CS-extinction effect on drug-seeking behaviour. Considering that this is an indirect demonstration of the occurrence of memory reconsolidation process, we would also consider these findings as the first evidence of nicotine Pavlovian memory reconsolidation. On the other hand, no effect of MK-801 or propranolol on nicotine seeking behaviour has been observed. These results are in contrast with other literature data, however methodological issues might explain the contrasting results. More evidences are needed to confirm that the effect of post-retrieval CS-extinction was due to interference of CS-extinction with reconsolidation process. Further studies will investigate the effect of CS-extinction applied 6 hours after retrieval, a delay time that allows to apply CS-extinction outside the labile phase of memory due to retrieval. Moreover it would be fundamental to identify specific molecular markers of reconsolidation or extinction. To find a selective molecular correlate of reconsolidation will allow to disentangle the point of whether our retrieval protocols are inducing reconsolidation or extinction and will provide further evidence that post-retrieval CS-extinction interfere with reconsolidation of CS-memory. This could also be useful to better understand the lack of effect of MK-801 and propranolol in our experiments. It has been pointed out by Lee & Everitt (2008) that to successfully reactivate a memory acquired instrumentally (as in our experiments) the CS should be presented contingently upon acquired response. We can then hypothesized that presenting the CS contingently upon response during retrieval session, would lead to a more strong retrieval and destabilization of the memories, and to a stronger effect of CS-extinction and of MK-801 on the reconsolidation of that memory. Finally, it would be important to assess whether the effect of post-retrieval CS-extinction on nicotine seeking behaviour is persistent, by repeating the test several week after retrieval-CS-extinction procedure. In conclusion, our findings suggest that the exposure to nicotine CS-extinction, after a short retrieval of the same nicotine CS, may inhibit CS-induced relapse to nicotine-seeking behaviour and may offer a potential co-adjuvant to current therapeutic interventions for smoking cessation and abstinence maintenance.