Drug addiction causes irreparable damages to the individuals, their families and to the whole society, besides being related to a high cost for the health system. It is know that current pharmacological treatments are only palliative ones and unable to prevent dreaded relapse episodes [28]. This deficiency has stimulated studies in our laboratory, especially the current study, since the tactile stimulation (TS) is a non-invasive approach, which can be used together with conventional pharmacotherapy. To the best of our knowledge, this is the first study to show beneficial TS-induced neuroadaptations in adult rats following amphetamine (AMPH) addiction. We evaluated the TS influence when applied in adult rats on AMPH relapse, occurring together with the evaluation of molecular parameters that are closely related to drug addiction, which were quantified in the Nucleus accumbens (NAc). The evaluation happened as follows: i) animals showed relapse behavior subsequently to preference, withdrawal and re-exposure to drug; ii) during AMPH withdrawal, the TS protocol applied in adult animals was able to prevent AMPH relapse following drug re-exposure; iii) experimental groups did not present memory- or locomotion-impairment in the Y-maze task; and, iv) while AMPH increased D1R, DAT, TH, ΔFosB and MOR immunocontent in the NAc, TS was able to decrease these molecular markers levels.
Previous studies from our laboratory have shown CPP paradigm in experimental protocols related to AMPH addiction [14, 29, 30], besides other addictive substances such as cocaine [16] and morphine [31–33]. Thus, following drug extinction, events such as stress or drug re-exposure can reestablish the addiction, what may be experimentally observed by restoring the drug preference in the CPP paradigm [34]. Our current findings show that subsequently to drug abstinence, AMPH-conditioned animals presented drug-relapse behavior, which was observed through AMPH-CPP, confirming that there is a relation between AMPH-induced hedonic effects and environmental cues, such as those present in the CPP paradigm. Inversely, animals that were submitted to adulthood TS protocol, during the abstinence period, did not show relapse after AMPH re-exposure. In fact, our previous studies have already shown beneficial influences of neonatal TS in the prevention of both AMPH- and cocaine-CPP [15, 16]. In addition, neonatal TS modifies juvenile behavior, attenuates drug-induced behavioral sensitization and reorganizes the brain regions involved in drug addiction [35]. However, our current outcomes show the TS procedure in the treatment form, that is, it was applied in adult animals that previously showed AMPH-CPP, whose behavior indicates addiction. During the abstinence period, these animals were submitted to adulthood TS, which prevented AMPH-relapse behavior after drug re-exposure. As far as we observed, this finding is innovative, since only neonatal TS has been experimentally studied [36] and clinically applied [37]; however, literature data are scarce when TS is applied in adult animals.
In this context, memory impairments could lead to false outcomes, since animals do not keep information connecting environmental clues and hedonic effects [38, 39], making the memory evaluation essential. Similarly, Y-maze task is also able to show locomotor performance in rodents, whose disturbances could cause interpretation mistakes. In this sense, our outcomes did not show any significant difference in the memory and locomotion among the experimental groups, as already described in the literature [40, 41]. These data allow us to propose that the place preference observed in the animals following AMPH re-exposure in the CPP paradigm was directly related to the AMPH-induced hedonic effects, but not to locomotor or memory impairments.
Although AMPH exerts behavioral and cognitive effects by modulating the activity of monoaminergic neurotransmitters, it is known that its reinforcing and euphoric effects are mediate mainly by the dopaminergic system [42]. AMPH are substrates for monoamine vesicular transporters and are absorbed by the neuron cells to increase the non-vesicular dopamine release (DA), this is enhanced by the inhibitory AMPH effect on monoamine oxidase [43]. This DA increase, which ends up reversing the DA transport in presynaptic neurons by the DA transporter (DAT) [44]. DA synthesis and availability are dependent on the enzyme tyrosine hydroxylase activity (TH) [45]. It is known that behavioral changes induced by psychostimulants are mediate by D1 receptor [46].
Connecting these data with our current findings, animals that were conditioned and re-exposure to AMPH, showed increased D1R, DAT and TH immunoreactivity in the NAc, while these events in cascade were inhibited by TS exposure during drug abstinence. Indeed, the increase DA release in the synaptic cleft [43], recruits an increase of the DAT [47], allowing us to propose that the increased level of this transporter in the NAc, as also observed in our findings, was due to increased levels of DA induced by AMPH [48]. Furthermore, DAT exerts influences on the expression and phosphorylation of TH in terminals of the NAc, where decreased levels of DAT were related to a smaller content of TH-protein and DA in the synaptic cleft, indicating that DAT participates in the regulation of DA homeostasis in dopaminergic terminals [49]. Of particular importance for our molecular findings, increased levels of DA in the synaptic cleft has been recognized to increase the D1R density [50] especially in the NAc, as demonstrated in methamphetamine (METH) chronic users [51]. These outcomes allow us propose that adulthood TS modulated DA release in the synaptic cleft, so preventing the activation of the dopaminergic cascade through increasing D1R, DAT and TH immunocontent in the NAc, as observed here.
In fact, TS exposure has promoted an increased DA release in both NAc and tegmental ventral area (VTA) [52]. Until now, the action mechanism involved in the increased DA levels in the synaptic cleft is unknown, but is possible that this increasing evokes the adaptive response preventing AMPH effects on the dopaminergic signaling because TS was able to modulate such releasing. A possible hypothesis would be an increased DAT pre-synaptic activity, thus decreasing DA levels and consequently minimizing reward behaviors linked to AMPH-CPP, as observed here. Unfortunately, at this time, it was not possible to quantify the functionality of the transporter, and it was possible to quantify its levels, allowing us to only give hypothesis, which deserves to be clarified. In fact, we can claim that environmental or behavioral changes could mediate information for the central nervous system neurons (CNS). Besides, the environmental enrichment can be considered a stimulus comparable to TS, demonstrating that environmental manipulations are able to modulate mechanisms underlying the brain plasticity, thus regulating TH amount and increasing neurons generation [53, 54].
In the sequence of these molecular events, AMPH abstinence, following its chronic exposure, has been described as to favor the return of DA basal levels in the NAc [55], while the TS showed itself to able to increase DA levels in the same brain circuit [52]. These evidences allow us to propose that the exposure of adult rats to TS modulated the mesolimbic dopaminergic system. Supposedly, to keep the baseline DA levels moderately increased, while there was an abrupt drop in these levels in abstinent animals that were not re-exposed to TS, since TS was able to kept both D1R and DAT of this pathway in the same levels that were observed in animals that were not exposed to AMPH. These outcomes may be interpreted as an adaptive response to drug abstinence with consequent AMPH relapse what supports our findings obtained in the behavioral evaluations.
The transcription factor ΔFosB (35-37 kDa) was investigated because it is involved in the effects of natural and drug rewards, whose effects supposedly contribute to neuroadaptations in DA-regulated signaling [56, 57]. Besides that consequent binding of dopamine to D1 receptors, signals the activation of the cell-signaling cascade to increase the phosphorylation of factors and transcription in response to cAMP, such as the expression of immediate and long-lasting genes, such as ∆FosB [58]. Here we observe that AMPH increased ΔFosB in the NAc, as already described [59, 60], while TS decreased this chronic transcriptional factor. Besides this, little is known about the specific molecular mechanisms that mediate the TS, especially how it acts on ΔFosB in the Nac. In addition, environmental enrichment has already been shown to reduce ΔFosB in other brain areas related to anxiety, reducing C-Fos expression in the Nac in response to cocaine [61]. This shows that different alternatives related to both environmental and body stimulation can reduce a transcription factor closely linked to drug addiction.
Studies have related the dopaminergic- to opiod-system, suggesting that reward and motivation are closely modulated by these pathways [62–64]. In this sense, µ-opioid receptor (MOR) is an important mediator of rewarding properties and additive behavior [65, 66] and they are located in the GABAergic interneurons [67]. NAc is a brain area with considerable MOR density [68]. Increased MOR resulting from psychostimulant drug administration is related to increased vulnerability to relapse [69], what was also observed in our AMPH-exposed animals. Based on this, our findings show that TS is able to reduce MOR immunoreactivity in AMPH-exposed animals, indicating a possible decreased in impulsivity and vulnerability to AMPH relapse, which was confirmed in our behavioral outcomes. Similar opioid regulation response is shared by physical exercise and it has been seen that it can prevent relapse in rats addicted to amphetamine [14]. This way, we propose that the two treatment methods produce a similar regulation response. It seems that massage treatment induces the release of endogenous opioid peptides, given that results in humans indicate that a massage session is followed by an increase in plasma β-endorphins [70].
In addition, human studies have also shown that an increase in dopamine, improved mood, reduced anxiety and reduced levels of cortisol (stress hormone) even after the first message [71, 72]. Considering that the hypothalamic-pituitary-adrenal (HPA) axis, responsible for controlling the stress response, has a substantial relation with the mesolimbic “reward” pathway, since stressors can activate this pathway [73], and environmental stress is considered a fundamental factor for relapse in addictive drugs [74, 75]. Glucocorticoids increase the DA biosynthesis by increasing the tyrosine hydroxylase activity (TH), the rate-limiting enzyme in DA synthesis [76], and they can regulate DAT expression [77]. TS applied to adult rats has already shown a beneficial influence on both stress and depression-like behaviors, decreased plasma ACTH and corticosterone levels, as well as the ability to decrease the HPA axis activity, minimizing the cascade of corticosteroids, inciting the GR expression [21] whose effect size was equivalent to those observed after conventional treatment with antidepressants [78, 79]. Also according to [16], TS is a form of prevention in face of stressful stimuli throughout life and, considering this, we may propose that it exerts prolonged beneficial influences. Based on this, we may infer that as an initial mechanism, the TS beneficial effects in adult rats may be related to the decrease in stress degree during the withdrawal period, which consequently caused a dopaminergic axis modulation, being sufficient to prevent AMPH relapse.
Considering the TS protocol effectiveness, which was applied in adult animals, and that the behavioral changes in the CPP and the molecular markers that involve DA generation and signaling, we may infer that TS could be stimulating the release of DA regular basis, while the drastic changes caused by the AMPH withdrawal happened. The neural modifications on the AMPH-induced mesolimbic dopaminergic pathway in the NAc may be modulated by TS, as observed in this study.