Research reportDiverging changes in rat striatal extracellular dopamine and DOPAC levels and in frequency-modulated 50-kHz ultrasonic vocalizations rate during repeated amphetamine treatment
Introduction
A characteristic feature of addictive substances is their potential for inducing, after a single exposure, many lasting neurochemical, neuroplastic and other alterations, including behavioral sensitization that can emerge within a few days [[1], [2], [3], [4]]. At its heart are neuroplasticity processes that involve the dopamine systems projecting to the ventral and dorsal striatum [[5], [6], [7], [8]], which are also involved in the emergence of addictions. Not surprisingly, sensitization is frequently explored in studies on substance abuse.
Previous studies focused on the behavioral sensitization related to locomotor activity and stereotypies, and on dopamine release evoked by psychoactive substances in sensitized animals. For instance, Di Chiara’s team proved that repeated administration of small psychostimulant doses evokes an enhanced release of dopamine in the nucleus accumbens core but not shell of sensitized rats in response to a later drug challenge [[9], [10], [11]]. In contrast, Kuczenski et al. demonstrated that repeated high doses of amphetamine evoked a decrease in dopamine release in the striatum but not in the nucleus accumbens core and hence the expression of behavioral sensitization to amphetamine does not require an increase in extracellular striatal dopamine [[12], [13], [14]].
Other researchers reported no considerable differences in baseline extracellular accumbal dopamine level and its response to acute cocaine administration, but a significantly higher baseline extracellular accumbal DOPAC level, in Fischer 344 rats compared to Lewis rats [15]. This observation suggests that extracellular DOPAC can be a more sensitive index of stimulant-induced alterations in dopamine systems. Notably, Lewis rats are more prone to substance abuse than Fischer 344 rats [[16], [17], [18], [19], [20], [21]]. So far, no close relationship was found between stimulant-induced dopamine release and sensitization as assessed from locomotor activation and stereotypies. Results of earlier studies suggested that the sensitization of locomotor activity and frequency-modulated 50-kHz ultrasonic vocalizations (FM 50-kHz USVs) rate reflect different neurobiological phenomena [22,23].
The basis of these differences is not clear, but it likely is due to the fact that FM 50-kHz USVs are mostly dependent on dopaminergic mesolimbic dopamine system activity [24], whereas locomotor activity response depends on the activity of the nigrostriatal dopamine system [25]. No link was found between repeated amphetamine treatment and magnitude of changes in dopamine release in the nucleus accumbens shell [10,11], there is also no link between the changes in the amount of dopamine released in dorsal striatum and locomotor activity sensitization [14]. However, as compared with the time course of changes in locomotor activity or stereotypies reported by, e.g., Kuczenski et al. [26], the time course of changes in non-contingent amphetamine-induced FM 50-kHz USVs rate found in our studies [22,27] showed clearly more resemblance with that in the respective dorsal striatal dopamine release. This similarity should be interpreted cautiously because the time course of the changes, especially of those in USVs emission, may vary depending on drug dose, rat strain, sensitization, or living conditions (e.g., single vs. group housing).
During the last two decades, more and more attention was paid to the role of the dorsal striatum in the development of habit, compulsion and addiction [28,29]. It seemed, therefore, that investigating the relationship between changes in striatal dopamine transmission and sensitization of the FM 50-kHz USVs rate response to amphetamine may help understand the role of the nigrostriatal dopamine system in the development of sensitization. This study aimed at the assessment of the baseline as well as amphetamine challenge-modified extracellular striatal levels of dopamine and its acidic metabolites (DOPAC, HVA) in the context of repeated amphetamine treatment-induced alterations in rat FM 50-kHz USV rate response to the drug.
Section snippets
Rats
Ten adult male Sprague-Dawley rats of about 300 g initial body weight (7 weeks of age), obtained from the stock maintained at the Polish Academy of Sciences Mossakowski Medical Research Centre, Warsaw, Poland, were used for the study. The rats were initially housed five to a clear plastic cage in the local animal facility, in a controlled environment room (20−22 °C and 45–65 % relative humidity) and at 12 h/12 h light/dark day cycle (lights on at 7 a.m.). They had free access to a standard rat
The effect of repeated amphetamine treatment on baseline extracellular striatal dopamine, DOPAC and HVA levels
One-way repeated measure ANOVA showed significant effect of dose number on the baseline dopamine and DOPAC levels (F6,18 = 10.3, p < 0.001, and F6,18 = 5.60, p = 0.007, respectively), but not on the baseline HVA level (F6,18 = 0.88, p = 0.47). Post-hoc Dunnett’s test revealed significantly and markedly increased baseline dopamine level before Amph2, which increase disappeared after the series of six daily amphetamine doses and showed some tendency to recur after the withdrawal from the
Discussion
The amphetamine treatment used in this study began with the regular TIPS protocol [3,22,33,37] aimed at the induction and verification of behavioral sensitization to the drug as assessed based on FM 50-kHz USVs rate response. It was followed first by a series of daily amphetamine doses meant to induce drug tolerance and next by a withdrawal period intended to reverse the tolerance, which ended with the final drug challenge. FM 50-kHz USVs rate and extracellular levels of dopamine, DOPAC and HVA
Author contributions
M.C. performed the surgery, microdialyses and vocalization testing, and assisted in data analysis and manuscript drafting. K.K. habituated the animals, assisted in running experiments, analyzed audiograms and assisted in data analysis. S.J.C. ran statistical analyses, assisted in data interpretation and writing the final version of the manuscript. D.T. was in charge of rat post-surgery care, ran HPLC analyses and analyzed the HPLC data. A.P. assisted in analysis of the study results and
Funding information
This study was supported by the National Science Centre of Poland, Cracow, Poland, grant No. UMO- 2015/19/B/NZ7/03610, and by the Institute of Psychiatry and Neurology, Warsaw, Poland, statutory fund No. 501-40-003-18017-01.
Declaration of Competing Interest
The authors declare that they do not have any conflict of interest related to this work.
Acknowledgment
The authors wish to thank Prof. Dr. Valentina Bassareo of the Department of Biomedical Sciences and the Centre of Excellence on Neurobiology of Addiction, University of Cagliari, Cagliari, Italy, for her skilled advice on preparing microdialysis probes and running microdialysis in rats, for enabling us to get some hands-on experience in preparing microdialysis probes in her lab, and for her kind donation of AN69 Hostal Dasco membrane for our studies.
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