TBI in general and minimal head injuries in particular constitute a major global health and socioeconomic problem contributing to long-term disability. Medical solutions for immediate treatment after injury and for improving rehabilitation are essential and necessary. To date, there is no pharmacological treatment following impact. Efforts are made to characterize the damage causing processes which occur after a head injury. Previous studies (Wagner et al. 2005a, b; Kobori et al. 2006; Bales et al. 2009; Hutson et al. 2011) suggested that dopamine dysregulation might have a major role in the behavioral deficit after brain injury. This work supports these studies and demonstrates changes in the dopamine pathway following mTBI. The changes are seen in TH and D 2 receptor levels. Additionally, our results suggest that the presence of MDMA prior to mTBI may initiate a protective process that attenuated the deficits in the dopaminergic system after mTBI.

When D 2 was examined in mTBI mice, high levels were seen in the striatum 24 h postinjury. Second, compared with the control group, low TH levels in the cortex together with high TH levels in the striatum were seen. Low cortical TH levels peaked at 24 h postinjury, while the high striatal TH levels were seen 7 and even 30 days after the injury. These results implicate that TH protein levels may be altered differently in a region-specific manner. Since TH is the rate-limiting enzyme in dopamine synthesis, changes in this enzyme suggest dopamine metabolism alterations. Indeed, it is known that changes in normal dopamine activity in both human and animals can cause cognitive deficits (Savitz et al. 2006; Bales et al. 2009; Cools and D’Esposito 2011). In addition, functional changes in D 2 receptors were previously found to underlie a number of mental pathologies (depression, psychosis, and others). These mental conditions were also accompanied by changes in cognitive abilities (Kellendonk et al. 2006; Takahashi et al. 2006). In our previous work, we showed that mTBI alone causes impairments in the cognitive performances of mice (Edut et al. 2011). The poor visual and spatial memory we observed can result from dopamine deficits.

Other studies in rats (Yan et al. 2001, 2007b) demonstrated that TBI produced a late increase in TH protein levels in the frontal cortex. The explanation of Yan et al. (2001) for this phenomenon was a compensatory reaction to the low levels of dopamine that usually occurs after injury (McIntosh et al. 1994). In accordance with these previous suggestions (Yan et al. 2001, 2002, 2007b), we found that after mTBI, there is an immediate/acute decrease in tyrosine hydroxylase levels in the mice cortex. Low cortical dopamine may lead to long-term elevations of TH in the striatum as a compensatory effect. Davis et al. (1991) and Weinberger et al. (1988) found that hyperactivity in the striatum is secondary to a hypofunction of the cortical dopamine system. This phenomenon is usually responsible for the cognitive deficits found in schizophrenia and may explain the cognitive impairments seen after TBI too (Kehagia et al. 2010; Simpson et al. 2010). Another possible reason for cognitive deficits may be changes in norepinephrine levels. TH is also a rate-limiting synthesis enzyme for norepinephrine, and probably, there are alterations in NE levels posttrauma as well. Further investigation on this model of mTBI needs to be conducted in order to verify the contribution of norepinephrine alterations to the deficits observed after injury.

It is known that following mTBI, there is a disruption in the dopaminergic system, which causes a decrease in dopamine release. Many previous studies revealed a mechanism of compensation, an increase in D 2 receptor levels (Tang et al. 1997; Sil’kis 2002; Yan et al. 2004). However, although these elevated D 2 levels may be a compensatory effect, high D 2 receptor levels are also known to cause cognitive impairments. By delaying the Akt pathway, which activates the GSK3 pathway (Beaulieu et al. 2011; Li and Gao 2011), the increased D 2 receptor activity in the striatum was linked to cognitive impairments in working memory tasks (Kellendonk et al. 2006; Li et al. 2011). Here, we found that following mTBI, there is a transient increase in D 2 receptor levels 24 h postinjury. This transient increase of D 2 receptors combined with TH hypofunction in the cortex and overexpression of TH in the striatum may be the basis of the poor visual and spatial memory seen in the mTBI mice. These recent observations may also play a role in mediating posttraumatic secondary damage. In order to establish whether high D 2 receptor levels are part of the repair pathway, or part of the destructive mechanisms in mTBI, further investigation is needed.

MDMA produces a rapid increase in extracellular dopamine with a peak level occurring 1 h after injection (Camarero et al. 2002). In our previous publication (Edut et al. 2011), we demonstrated that mice subjected to MDMA 10 mg/kg prior to mTBI performed better on cognitive and behavioral tests compared with injured mice. In this study, TH levels in MDMD + mTBI mice were normal and as high as the sham group (Fig. 1). This implies that the changes seen in TH levels after mTBI alone were totally abolished by when MDMA was co-administered. Relevantly, studies in both animals and humans have identified a series of temporal alterations in DA neurotransmission and TH levels that occur after TBI (Yan et al. 2001, 2007a, b; Wagner et al. 2005a, b, 2007a, b). The fact that administration of MDMA prior to mTBI reversed this interference leads us to the conclusion that high extracellular dopamine levels provide protection from mTBI damage.

Furthermore, MDMA prior to injury abolished the increase in D 2 receptor level seen earlier in the mTBI group (Fig. 5). The normal D 2 receptor level seen in these mice probably resulted from the lack of decrease in extracellular dopamine level. Because of that, there was no need for a compensation mechanism. Based on previous studies (Ma et al. 2000; Mehta et al. 2005; Yan et al. 2007a, b; Shin et al. 2010; Li et al. 2011) and on our studies, we believe that the alterations found in both TH and D 2 receptors are the basis of the behavioral deficits previously reported in mTBI mice (Edut et al. 2011). Administration of MDMA prior to injury stabilized the dopamine system and protected the mice from the degeneration processes that occur after mTBI. This normal balance of dopamine may explain the improved cognitive performance seen in these mice (Edut et al. 2011). In order to further establish the exact contribution of tyrosine hydroxylase and D 2 receptors to the destructive cascade occurring after our model of mTBI, it will be wise to examine the activation levels of TH and D 2 receptors after the injury. Regarding TH, both the phosphorylation on ser40 and ser19 should be considered. Data about TH and D 2 receptor activation together with the protein expressed levels can give a wider overview on the contribution of TH and D 2 receptor to the deficits seen after mTBI and about the contribution of MDMA to the neuroprotection.

Elevated levels of BDNF are associated with long-term potentiation, learning, and memory (Yamada and Nabeshima 2003; Li et al. 2010). When mice were exposed to MDMA before head injury, significant increases in BDNF levels were seen in the striatum. This suggests that the neuroprotection mechanism of MDMA may run through the BDNF pathway too. Other studies (Griesbach et al. 2009a, b; Kaplan et al. 2010; Tio et al. 2011) found that different neuroprotective drugs or activities can cause elevations in BDNF. These elevated BDNF levels lead to a decrease in brain damage and to better performance in behavioral tests (Meisner et al. 2008; Wu et al. 2008; Cassol et al. 2011; Han et al. 2011). In our study, the relatively high BDNF levels in the cortex and striatum of mice subjected to MDMA before injury may account for the good performance seen in the cognitive tests (Edut et al. 2011).

Additional evidence regarding the involvement of MDMA generally, and D 2 receptors specifically, in protecting the mice from mTBI damage was seen when haloperidol was administered before MDMA. Haloperidol is a well-known dopamine D 2 receptor antagonist. The administration of haloperidol before MDMA caused low performance in the cognitive tests (Figs. 6 and 7). The haloperidol injection reduced the mice’s cognitive abilities and their performance level was similar to the mTBI mice. This suggests that haloperidol blocked the protective effect of MDMA. Indeed, in a study that measured the interaction between MDMA and haloperidol, it was found that the locomotor-activating action of MDMA was blocked by haloperidol (Di Cara et al. 2011). In addition, both Hewitt and Green (1994) and Schmidt et al. (1990) have shown that acute administration of haloperidol together with MDMA provides effective protection against MDMA neurotoxic damage. These studies demonstrate that haloperidol can block MDMA activity. In this study, we show that by blocking the activity of MDMA, we actively canceled the protective effect of it against mTBI deficiencies.

Another possibilty is that the MDMA protection seen in this study is a unique case of preconditioning—a phenomenon where a low dose or a brief exposure to an insult protects the brain from future insults (Zhang et al. 2003; Zhan et al. 2010; Assaf et al. 2012). For example, previous studies showed that acute MDMA treatment induces a robust increase in phospho-ERK. Activation of the ERK pathway has been shown to protect against brain ischemia. Another preconditioning involving MDMA was seen when the latter was injected with lipopolysaccharide (LPS) challenge. Usually after LPS, there is an elevation in inflammatory cytokines like IL-1b and TNF-alpha, and suppression of both IL-1b and TNF-α was observed when MDMA was co-administered with LPS (Connor et al. 2000, 2005). These examples are in line with our study and suggest that when MDMA is administered close to another insult, it can change the outcome.