It has been demonstrated that the environmental context of an experience induced by a drugs of abuse is crucial for drug-seeking behavior and relapse in human addicts [64]. Furthermore, learning and memory, particularly contextual memories, are important for the establishment of conditioned responses to drugs of abuse [64, 65]. In our work we described that environmental cues related to drug experience have a direct impact on retrieval of the memory acquired when the DZ withdrawal signs were associated with the context of withdrawal because alteration in the cues presented 24 h after last administration prevented the expression of the anxiety-like behavior when animals were reexposed, 15 days after last administration, to the withdrawal environment. Alterations of contextual cues consisted of a new operator, modified location of the animals in the plus maze test room from the original assessment, and the animals were held with a piece of fabric. These changes constitute a significant alteration in the context of withdrawal and are able to modify conditioned responses previously observed. It is interesting to point out that manipulation of environmental cues during reexposure prevented the expression of anxiety-like behavior in the majority of animals considered to be DZ dependent. Only a small portion of them expressed anxiety during reexposure to the withdrawal environment in spite of the altered contextual cues [112]. This differential susceptibility could be attributed to the fact that the associative component can be particularly strengthened in these animals, and the environmental changes are not enough to disrupt the acquired memory. Moreover, because of the experimental design in which the experimental groups were defined by classification, we cannot rule out the possibility that other selected characteristics, unrelated to anxiety or dependence, may have caused the observed effects.

We have recently demonstrated that DZ dependence is associated with an increased HP synaptic plasticity in both short- [57] and long-term withdrawal [58]. Furthermore, alteration in contextual cues associated with the withdrawal experience previously described, not only impaired the expression of the anxiety-like behavior, but also disrupted the facilitated HP LTP in most of the DZ-dependent animals reexposed to the altered withdrawal environment. Interestingly, the animals that expressed anxiety-like behavior in spite of the altered contextual cues preserved the facilitation in HP LTP [112]. These persistent changes in synaptic strength during synaptic plasticity require new gene expression and protein synthesis [58, 113, 114]. The immediate-early gene activity-regulated cytoskeleton-associated protein (Arc) has been proposed as a marker of neuronal activity linked to synaptic plasticity and to consolidation of spatial memory in the HP, acquiring a key role in the organization of information storage in the CNS [115, 116]. We have reported increased HP Arc protein levels related to the facilitated LTP in DZ-dependent animals during short-term withdrawal [58]. Similar to the observed electrophysiological changes, we observed that the increased HP Arc protein levels were reversed in DZ-dependent animals reexposed to the plus maze test with contextual cue changes. However, only the small proportion of animals that expressed anxiety in spite of the altered contextual cues maintained increased Arc protein levels, indicating that Arc could be responsible, at least in part, for the maintenance of the facilitated HP LTP during this period of withdrawal. These results further support the hypothesis that the HP is a brain structure involved in the associative memory formation and retrieval when contextual cues associated with drug exposure and withdrawal are presented to a dependent subject [58]. Moreover, it has been suggested that Arc promotes LTP consolidation through regulation of actin dynamics [117]. In addition to the maintenance of synaptic potentiation during late-LTP, long-term memories are dependent on the persistent phosphorylation and sustained activation of the brain-specific atypical protein kinase M zeta (PKMζ) [118]. It is also known that actin polymerization is critical for PKMζ synthesis, leading authors to suggest the possibility of a sequential mechanism of LTP maintenance in which Arc-dependent stabilization of F-actin promotes PKMζ synthesis and expression of enduring LTP [117]. We have described that inhibition of PKMζ using the zeta pseudosubstrate inhibitory peptide (ZIP, selective PKMζ inhibitor) prior to reexposure to the withdrawal environment with preserved contextual cues prevented the anxiety expression. Furthermore, LTP facilitation in all DZ-dependent animals was reversed, suggesting that plasticity in the HP has a crucial role in maintenance of these memories that were vulnerable to the amnesic effects of ZIP [112, 118]. These results are in accordance with the hypothesis that the persistently active PKMζ might perpetuate information both at synapses and during long-term memory [119]. Furthermore, LTP was generated in DZ-dependent animals intra-HP infused with ZIP at a similar frequency to control animals, whereas vehicle infusion in DZ-dependent animals did not affect the lower LTP threshold associated with exposure to the contextual cues related to DZ withdrawal experience as previously described. Therefore, inhibition of PKMζ in long-term withdrawal reversed the facilitated LTP generated during DZ chronic administration and maintained through withdrawal, without affecting the mechanisms involved in vitro LTP generation [112].

Abuse of psychostimulants such as cocaine and amphetamines, and misuse of prescribed drugs with or without a prescription represent a significant health and social problem worldwide. A major problem in the treatment of drug addiction is relapse to drug abuse. Then, highlighting common mechanisms underlying persistence of abuse to different drugs could be interesting for the development of strategies of effective treatments. Our work is focused on the study of common mechanisms underlying COC sensitization, as a model of craving and relapse, and DZ dependence. We hypothesize that up regulation of NOS/NO/sGC/cGMP signaling pathway in different brain areas could initiate, contribute, or exacerbate addictive behaviors in humans. These results are of significant importance considering that patients participating in drug detoxification programs or addicts in withdrawal may use other drugs to mitigate withdrawal symptoms or side effects of the primary drug used. In fact, the misuse and recreational use of PDE5i have been described in different human populations [120] and linked to the illicit use of drugs of choice for abuse [121]. Taking our results into consideration, we can speculate that PDE5i may increase vulnerability to drug abuse.

Moreover, our results demonstrate that memories related to drug experience and withdrawal are relevant to the expression of anxiety-like behavior when DZ-dependent animals are reexposed to the contextual cues associated with the withdrawal experience. We further characterize the HP as a pivotal brain structure involved in engagement of learning processes related to drug dependence and withdrawal. These findings contribute to the establishment of the mechanisms by which drugs of choice for abuse can alter brain function that can be used as treatment or prevention of drug dependence and abuse.