The present study provides compelling evidence that the enhancement of remote cocaine memory is driven by memory system consolidation. Using an extended cocaine self-administration model in rats, we found that remote cocaine-cue associated memory expression exhibited an enhancement. During this process, we observed a time-dependent strengthening of the response of PrL CaMKⅡ neurons to cocaine-related cues. Chemogenetic suppression also revealed that memory consolidation drove the enhancement of remote cocaine memory through PrL CaMKⅡ neurons, and the early period after training was the major stage for effective intervention. Furthermore, the function of PrL CaMKⅡ neurons in remote memory required the BLA input during the early period of consolidation, which can promote the maturation of PrL dendritic spines, mainly mushroom-shaped spines. These data extend previous findings and showed the contribution of system consolidation to the enhancement of remote cocaine memory through PrL CaMKⅡ neurons receiving the BLA input (Fig. 5n).
Our results indicate that cocaine-cue associated memory was time-dependently enhanced, but not attenuated, over time, which was different from typical remote memory expression. Notably, a similar phenomenon has been reported in previous findings that cue-induced drug craving (i.e., an intense desire for the effects of a drug) progressively increased during long-term abstinence, which was termed as the incubation of cue-induced drug craving31, 32. The incubated drug craving is a critical contributor to drug relapse and generalizes across various drugs33. Many human studies have identified this phenomenon in individuals with a history of taking nicotine34, methamphetamine35, alcohol36, or cocaine37. Emerging evidence support that drug craving stemmed from past experiences about drug use which can be encoded and stored in memory, suggesting that memory serves as the foundation of drug craving2, 10, 38, 39. Here, our results further demonstrate homogeneity between the enhancement of remote drug-cue associated memory and the incubation of drug craving. Thus, we speculate that the stable memory is a prerequisite for persistent drug craving during abstinence, and the enhancement of remote drug-cue memory driven by memory consolidation leads to the increased cue-induced drug craving40. Although direct evidence is still scarce, many indirect studies have shown that disrupting the memory reconsolidation process can attenuate cue-induced drug craving12, 39. Therefore, it is worthy to explore the ways in which the enhancement of drug memories causes an abnormal intensification of drug craving.
Memory system consolidation typically facilitates recent memory to maintain and to turn into stable over time, whereas remote memory tends to become weaker through a process of forgetting1. In contrast, our data reveal that memory consolidation can drive remote cocaine memory to an abnormal state, that is expression enhancement, similar to previous finding5. This kind of remote memory enhancement generalizes to other types of memory, such as the natural reward-cue associated memory41 and fear memory42. Therefore, it is reasonable to suspect that the role of system consolidation in remote memory enhancement is neither attributable to the effect of the additive drug per se, nor correlated with stimulus valence (i.e., appetitive or aversive). One possible factor affecting the function of consolidation is memory strength, in which high-intensity learning would induce stronger expression of remote memory than low-intensity learning43. Here, we applied an extended self-administration training paradigm whereby rats underwent 6-h sessions per day, and every nosepoke can trigger a simultaneous cocaine injection until the maximum number of nosepokes was reached. The strength of association between cocaine and the cue may be a contributor to the increased remote memory expression, due to that the rats which were trained with a rapid infusion rate (5 s), but not a slow (90 s), showed a more pronounced enhancement of remote cocaine memory44. Additional findings showed that in rats cue extinction session (repetitive exposure to drug-related cues without injecting the drug) during abstinence reduced the remote drug memory enhancement45, possibly due to that extinction session weakened the association between the drug and related cues and blocked the consolidation of drug-cue associated memory. Hence, we hypothesize that the over-strengthened cocaine-cue association during learning would be abnormally consolidated over long periods of time, making behavioral responses to related cues gradually increase. This may explain why individuals with substance use disorders or those who have experienced traumatic events have a higher risk of relapse or suffer from post-traumatic stress disorder46, 47. In turn, the early period of consolidation represents a potential time window for blunting remote memory enhancement. Our results showed that chronic inhibition during the early period reduced the cue-induced behavioral response on day 30 post-training. This finding aligns with previous research that the initial 2 weeks post-learning is critical for remote memory24, suggesting the therapeutic value of preventing system consolidation to reduce the drug relapse.
The role of neocortex in remote memory is not fixed but rather undergoes an evolving process during memory system consolidation48. Recent investigations have further revealed a time-dependent maturation process of PrL function in memory which required inputs from kinds of brain regions6, showing a greater influence on remote memory22. In the present study, we found that the function of PrL glutamatergic neurons on remote cocaine memory enhancement was established during the consolidation process. This was supported by two key findings. First, we observed an increase in Ca2+ signals after cue onset from day 1 to day 30 post-training, indicating a gradually greater ability of the PrL to process the information about cocaine related cues. Consistently, previous findings also showed a pronounced increase in the encoding of related cues information after prolonged abstinence from cocaine49. Second, our results revealed that the chronic chemogenetic inhibition of PrL CaMKⅡ neurons during the early period significantly reduced the enhancement of remote cocaine memory, in line with previous research that pharmacological inhibition of the PrL decreased the cue-induced behavioral response to cocaine at remote time point21.
The BLA serves as a focal point for drug memory50, where sensory and emotional signals are processed and integrated51. Our study found an increase in Ca2+ signals in BLA CaMKⅡ neurons following exposure to cocaine-related cues. However, unlike the PrL, BLA activity did not show temporal changes during system consolidation, and the acute inhibition of BLA CaMKⅡ neurons did not affect the expression of cocaine memory. These results imply that although the BLA may mediate the integration of cocaine and related cues, it is not necessary for the behavioral response to cocaine-related cues. Consistent results were also reported, in which pharmacological inhibition of the BLA had no influence on cue-induced cocaine seeking behaviors at remote time 52.
Our findings highlight that the sustained input from the BLA to PrL during the early period of consolidation contributes to enhancement of remote cocaine memory, which is consistent with a previous finding that optogenetic inhibition of the BLA-PrL circuit reduced remote fear memory6. These processes are accompanied by dynamic dendritic spine plasticity on PrL neurons defined by afferent connections29, 53. Here, we found that blocking the BLA glutamatergic projection during the early period of consolidation decreased dendrite spine density in PrL CaMKⅡ neurons receiving BLA input. Among these, the mushroom-shaped spines possibly served as the primary substrate for the elevation of activity of PrL during system consolidation, and appeared to be necessary for the enhancement of remote cocaine memory. This is similar to previous research that PrL neurons displayed more mature and stable synaptic connections during abstinence54. Furthermore, the BLA input to PrL also resulted in the transmission of cocaine-cue association information55. And this transmission possibly has its behavioral significance that the learned object values encoded by the prefrontal cortex can be retained for at least several months and resistant to interference56. Accordingly, we propose that the BLA may function as a cache that integrates the strong association between cocaine and related cues, and during system consolidation BLA can convey this information to PrL for long-term storage. Remarkably, during the early period post-training, more mature and stable dendritic spine plasticity which occurred in PrL neurons receiving BLA input possibly makes the association information abnormally consolidated, inducing stronger response when cue exposure at remote time and leading to enhancement of remote cocaine memory. Following that, the NAcc may be a promising output for investigating the ways in which consolidated cocaine-cue association information flows from the BLA to PrL for persistent storage, and then to downstream regions for directing higher behavioral responses to cues. This is supported by extensive findings that the optogenetic inhibition of NAcc neurons receiving PrL input can attenuate cue-induced drug seeking at remote57.
In summary, we delineated that memory system consolidation drives the enhancement of remote cocaine memory through stepwise mature dendritic spines plasticity and elevated response to cue in PrL neurons receiving BLA input. Importantly, we identified the early period of consolidation as an essential time window for intervention, providing valuable insights into the treatment of drug relapse after prolonged abstinence.