Extended Methamphetamine Self-Administration in Rats Results in a Selective Reduction of Dopamine Transporter Levels in the Prefrontal Cortex and Dorsal Striatum Not Accompanied by Marked Monoaminergic Depletion [Article Summary]
Abstract
The study sought to unravel whether prolonged self administration of methamphetamine interferes with the level of dopamine and serotonin in rats. The rats were place in a lit chamber in which they were allowed to self administer meth. The researchers regulated the duration of the self administration session to achiever long and short access which the main variables in the experiment. The researchers discovered that long and short access to meth result into a reduction on the levels of dopamine transporter (DAT) protein in the brain.
Extended Methamphetamine Self-Administration in Rats Results in a Selective Reduction of Dopamine Transporter Levels in the Prefrontal Cortex and Dorsal Striatum Not Accompanied by Marked Monoaminergic Depletion [Article Summary]
Introduction
Abusing Methamphetamine (Meth) has multiple effects with the leading and commonly known effects being interference with organic processes in the base of the forebrain as well as the anterior parts of the lobes of the brain. Specifically, abusing Meth interferes with the pumping of neurotransmitter dopamine in most parts of the brain. As indicated in the article, most people abusing Methamphetamine have propensity of developing chronic relapsing disorders as well as several other disorders of affecting neurotransmission as have been observed in several studies involving tomography of the brain.
Studies of the effects of methamphetamine have always used animals that are exposed to various conditions capable of damaging dopamine and serotonin terminals of the brain. The dopamine transporters (DAT) have been established to be greatly involved several cellular mechanisms in the neurotoxicity. Ideally, according to Thomas et al., 2004 and Quinton and Yamamoto, 2006, disrupting the density of DAT by introducing high concentrations of Meth has multiple effects including development of oxidative stress and activation of inflammatory mediators (as cited in the article). Ideally, abusing methamphetamine has several effects that are distributed throughout the brain.
According to the authors of the article, the study of meth-induced intoxication of the nerves in the brains has always employed the use of binge models and observation of the changes that commonly occur in people who abuse Meth. Nonetheless, most of the methods that researcher have used to study meth induced neurotoxicity have always failed to enable the researchers conclusively look at the aspects of Meth addiction as it relates to cognition and motivation. Noting that for any research, validity is an important element, the use of self-administration in the study of the effects of Meth in rodents (also another method) is capable of testing the intended changes caused by meth addiction. As the researchers noted in the article, no study has been conducted to examine how monoaminergic function is normally affected by prolonged meth self administration, the researchers used “short- and long-access Meth self-administration” (Schwendt et. al. 2009, p. 556) in a bid to understand how monoaminergic function is normally affected by extended meth self-administration. The researchers’ was that; “long-access animals may show larger abnormalities in dopamine and serotonin transmission” (Schwendt et. al. 2009, p. 556). To this effect, the researchers were determined to test whether Meth-induced reinstatement would have some effect to monoaminergic function or correspond to changes in the marker in the meth-induced neurotoxicity in the brain.
The study used male Long-Evans rats which were housed in a temperature-and-humidity controlled environment with the light/dark cycle reversed and lasting for 12 hours each. The rats were fed standard rat chow and ad libitum as well as water during the whole duration of the study even though feeding was not done during the first three days of Meth self-administration except for the small grams of standard rat chow which were given to the rats. All the procedures, as the researchers report, met all the required standards. Chronic indwelling catheters were implanted into the rats appropriately through surgery before the catheters were fitted with stylets; this occurred when the rats were still yet to be connected to the infusion pumps. Other measures taken by the researchers include maintenance of catheters potency, and verification of the catheters potency.
The rats were then left to recover from the surgery for about five days before being randomly assigned to Meth or yoked-saline groups (Schwendt et. al., 2009). Necessary data was then collected in every session that always began with the lighting of the chamber. The rats self-administered the meth. The experiment was designed in such a way that pressing a lever in the chamber resulted into a 2 second activation of the infusion pump and a five second presentation of the stimulus complex. Prevent overdose, the researcher included a 20 minute time out during the researcher continued to take data even though the data was not used anywhere in the study. The sessions lasted for one hour daily which went for ten days before the rats began taking 6 hour sessions even though some continued with the one hour sessions. The six hour sessions represented long-access while the one hour session represented access to meth. After division into two groups, the study continued for another two weeks.
Extinction involved subjecting the rats to 1 hour extension sessions for all the rats with the researchers continuing it record the responses; this lasted for ten days. Reinstatement Testing was carried out for two days as described in the article and the researchers are keen to note that the reinstatement method used in the experiment have always been successfully used in laboratories. At end of the experiment, the brains of the rats were removed together with other tissues for analysis with regards to dopamine and serotonin (experiment 1), and their metabolites used to carry out experiment two before being used for experiment three which was basically immunoblotting.
In experiment 1, the level of dopamine and serotonin in the rats brains were determined using high-pressure liquid chromatography (HPLC) as well as electrochemical detection. Appropriate method was used to extract the monoamine from the brains. Protein content of the monoamines was determined using Bradford Assay while the level of serotonin was determined with help of an autosampler. Most importantly the obtained data, expressed in picomoles per milligram of protein, was annealed with the help of an external curve.
In light of immunoblotting, the tissues were prepared appropriately while in frozen state as described in the article. Protein content in the tissues prepared for immunoblotting was then determined; the method used for this determination in reported in article are bicinchoninic acid assay. With in mind that hydrophobic transporter protein is prone to aggregation when not kept in suitable temperatures; the sample was stored (incubated) carefully at 45oC. Another equal amount of the protein was resolved and probes against DAT, acidic protein and glial fibrillary among others using a technique designed by Santa Cruz Biotechnology, Inc. and Wako Pure Chemicals, Osaka. Afterwards (after incubation) the researchers used a technique designed by the Buckinghamshire based GE Healthcare to detect immunoreactive bands in the membranes.
The data obtained by the researchers was subjected to paired Student's t test for evaluation of escalation. Two-way ANOVA was used to evaluate that effect that reinstatement had on lever pressing by the rats in long and short access. This was then followed by Dunnett’s test for evaluation of causal relation between extinction levels of lever pressing. One way ANOVA was also used at some point to evaluate the differences between the tissues of yoked-saline control rats and rats that were subjected to short- and long-access of meth. Again, Dunnett’stest was used for post hoc analysis of the tissues with the tissues from the controls. And just like analysis of differences between the yoked-saline control rats and rats that were subjected to short- and long-access of meth, the data obtained from immunoblotting was analyzed using one way ANOVA and Dunnett’s test. That analyisis was done using a software designed by the SPSS Inc., Chicago called SigmaStat.
Results
With regard to Experiment 1, as the researchers found out, there was great difference between the active and inactive lever responding though the session in which the rats self administered meth. This implies that the animals were able to tell that the active lever when pressed was having a different effect from that effect that the inactive lever had when pressed. However, it was observed that in long-access, there was an inconsistent effect as a result of inactive lever responding. In trying to explain this phenomenon, the researchers asserted that this was probably due to extended access to the lever in the long access. On the same note, the average meth intake in the rats in long access was considerably high in the first three days compared to the average meth intake in the short access. The removal of meth reinforcement resulted in a rapid extinction as all the animals stopped lever pressing. Ideally, when meth is self-administered for extended periods in a day, there levels of DAT protein decrease substantially. The same effect was observed in animals subjected to short access. Because of this, the researchers, in the article, also keenly point out that there is likeliness that the period of access was not an important variable. Probably, this, according to the researchers, is an indication that the decrease in the levels of DAT maybe as a result of lasting consequences that meth might be having on the function of the functioning mechanism on the brain. Besides, the animals subjected to long access were seen to be having propensity to seek for the Meth. Experiment 2 was carried out to the same animals that were used for experiment 1. There was a difference in the total meth intake between the animal subjected to long access and the animals subjected to short access even though long or short access to meth did not seem to have any effect on the levels of dopamine found in tissues of the brain. Slight alterations were however noted in short access. Experiment 3 sought to examine the level of monoamine transporters are affected by self administration of meth. DAT level was affected by short access only; there was no observed effect on the level of serotonin transporters; number of protein markers was also not affected by short or long access.
Discussion
Animals in self administered meth experiments have always proved to have similar administration mechanisms like in humans compared animals in experiments in which meth is administered by the experimenter. Therefore, in concurrence with several other similar experiments, when session of administration was change to 6 hours, there was an increase in the amount of meth intake even though this was not the case with animals that were still left to continue with short access- the animals in long access self administered a huge amount of meth. This, in light of human beings, can be described as transition in which one was previous having very limited access to meth but has all of sudden given an unlimited access to meth; the person naturally tends to take more meth that usual. Ideally, it is reasonable to note that humans always tend to look for drugs aggressively when there improvement from drug addiction relapses (Miller, 2013). The findings of the study are also similar, to some extent, to the chronic cocaine usage. While tests for reinstatement were carried out after extinction using priming doses, there the findings, with regard to reinstatement, was not consistent for long access and short access. According to the researchers, there is still need for this observation to be tested further before any conclusive remarks can be drawn. Though it was apparent that reinstatement in relapse results was considerably different because it was found that it was having a relationship with access.
This study, as the researchers report the study was designed to evaluate how the brain is affected by extended secession of self- dispensed meth. It has been observed severally in the past that lengthened usage of meth affects some system of brain both in humans and animals even though different people hold difference views relating to these changes. It was observed that the animals exhibited modified motivational and cognitive performance, even though this manifestation was not accompanied with any changes in serotonin or dopamine levels in the parts of the brain examined. Notwithstanding, there were significant effects on DAT levels that were found to have been caused by extend Meth; findings that are by all means the same as the findings by other similar studies (Schwendt et. al. 2009). It can therefore been note that DAT level can only be caused by extensive exposure to Meth. Additionally decrease in DAT levels can also be caused by intake of high doses of meth. The researchers also propose that decreased levels of DAT in the examined parts of the brain can be due to adaptation of the brain to the effects that are caused by meth. In this light, the researchers avow to base their future study on the cellular toxicity caused by meth after short duration of withdrawal. Similar assertions are shared by Iversen, Iversen, Dunnett, & Bjorklund (2010).
Concisely, by looking at how the researchers presented their findings and discussion, it warrants saying that the researchers were able to prove that extended self administration of meth does not affect all the dopamine transporters. It is also commendable that the study has opened up an avenue for another study; the researchers noted, any future topic in this topic should delve on cellular toxicity as affected by caused by meth after short duration of withdrawal. Admittedly, the research design was relative complex and made use of various techniques devised by other institutions. Notably, even though the data analysis section was clearly marked experiment 1 to 3, this steps could not be easily drawn from the methodology. DATA levels was observed to have reduced in animal subjected to long and short access even though only long access animals appeared to have been affected behaviorally by Meth. According to the researchers, the finding of the study suggest that prolonged meth use makes one to crave more for the drug upon withdrawal as well as affects the level of DAT dorsal striatum (dSTR) and prefrontal cortex (PFC) parts of the brain. S
References
Schwendt, M., Rocha, A., See, R. E., Pacchioni, A. M., McGinty, J. F., & Kalivas, P. W. (2009). Extended Methamphetamine Self-Administration in Rats Results in a Selective Reduction of Dopamine Transporter Levels in the Prefrontal Cortex and Dorsal Striatum Not Accompanied by Marked Monoaminergic Depletion. J Pharmacol Exp Ther. , 331(2), 555-562.
Iversen, L. L., Iversen, S. D., Dunnett, S., & Bjorklund, A. (2010). Dopamine handbook. Oxford: Oxford University Press.
Miller, P. M. (2013). Biological research on addiction: Comprehensive addictive behaviors and disorders. Amsterdam: Elsevier.
