Full length articleTranscriptomic profiling of the ventral tegmental area and nucleus accumbens in rhesus macaques following long-term cocaine self-administration
Introduction
Drug abuse is associated with a number of molecular and cellular effects on the brain including changes in neurocircuitry, gene expression, and epigenetic regulation. These changes are believed to be linked with the transition from substance use to abuse: compulsive drug seeking, loss of restraint, and negative affect (Koob and Volkow, 2010, Luscher and Malenka, 2011). The mesolimbic dopamine system, connecting the ventral tegmental area (VTA) and the nucleus accumbens (NAc), drives the salient effects of reward and has been consistently and repeatedly shown to be the primary site of action of drugs of abuse (Nestler, 2005).
There have been a number of studies in rodents and human post-mortem tissues that focus on gene expression differences associated with addiction both in the mesolimbic pathway as well as other areas of the brain (Zhou et al., 2014b). Microarray studies in rodents have identified differences in immediate-early genes (genes activated rapidly following extracellular stimulation) and dopaminergic pathways (Piechota et al., 2010, Yuferov et al., 2003, Zhang et al., 2005) in accordance with previous findings using Northern blots and immunohistochemistry (Hope et al., 1992). More recent studies using next generation sequencing technologies to study the effect of cocaine on the mouse NAc found differences from controls across multiple neurotransmitter systems including dopaminergic, cholinergic, glutamatergic, GABAergic systems as well as cadherin and Wnt signaling pathways (Eipper-Mains et al., 2013). This has been extended to non-coding RNAs with functional effects imputed using informatics approaches (Bu et al., 2012, Chen et al., 2013).
There have also been a number of microarray studies of gene expression on human cocaine abusers in the NAc (Albertson et al., 2004, Bannon et al., 2005) and midbrain dopaminergic regions (Tang et al., 2003). While confirming many of the dopaminergic findings from rodents, more widespread transcriptional changes were also identified. These findings have been suggested to reflect epigenetic reprogramming resulting from chronic drug exposure (Zhou et al., 2011). The differences observed between rodent and human expression studies may result from methodological differences in the duration of exposure to drug, acute compared to chronic usage (Zhou et al., 2014b). Indeed, transgenic mouse work has shown that histone acetylation is important for response to chronic, but not acute, cocaine exposure (Renthal et al., 2007).
One of the challenges in developing translational animal models is to recapitulate as closely as possible the most salient features of human behavior while maintaining precise experimental control. In addition to greater genetic and neuroanatomical similarities, nonhuman primate models in particular are valuable when modeling aspects of addiction because their consumption and patterns of drug taking so closely reflect that seen in human drug addiction (Platt and Rowlett, 2012). A rhesus macaque model of self-administration provides a unique opportunity to explore the molecular mechanisms involved in the entirety of the neurochemical systems and circuitry associated with the addictive processes (Weerts et al., 2007).
In the present study, we used a cocaine self-administration procedure in rhesus macaques over three months (approximately 100 consecutive days). After this exposure period, transcriptomic analysis was performed on the NAc and the VTA using next generation RNA-seq. This allows for an unbiased and holistic view of the differences between cocaine- and saline-treated animals in these regions and provides a simultaneous assessment of the two brain regions in long-term cocaine consumption.
Section snippets
Ethics statement
Animals were maintained in accordance with the guidelines of the Committee on Animals of Harvard Medical School and the Guide for the Care and Use of Laboratory Animals (8th edition, 2011). Research protocols were approved by the Harvard Medical School Institutional Animal Care and Use Committee.
Animals
Subjects were 10, experimentally naïve, male young adult (4–7 years) rhesus macaques (Macaca mulatta). All animals were raised in shared conditions with identical diet and husbandry; paired animals
Cocaine self-administration
While there is some variability across animals (Supplemental Fig. S1), when cocaine self-administration was averaged across animals, there was evidence of a gradual increase in self-administration over sessions (Fig. 1A,B). Linear trend analysis revealed a significant fit of these data (84 days (n = 5) [F(1,82) = 18.23, p < 0.001]; 100 days (n = 4) [F(1,98) = 25.06, p < 0.001]) with a positive slope, although the relationship of session and number of injections/hour was relatively weak (r2 = 0.18 or 0.20
Discussion
Under conditions of limited access (1 h/day) and long-term exposure (approximately 100 consecutive days), we observed a general increase over time in cocaine self-administration in monkeys. However, while statistically significant, this escalation was modest in size. While there was a limit on the total amount of cocaine that was available for the animals to consume (so as to not lead to overdose), this did not fully account for the magnitude effect. Rather, what was observed was a relatively
Contributors
W.D.Y. and J.K.R. were responsible for study concept and design. N.M.S. and J.K.R. were responsible for collection of animal data. W.D.Y., S.V.M., D.B.G. were responsible for brain and tissue collection. D.B.G. and E.J.V. were responsible for data analysis. E.J.V. and J.K.R. drafted the manuscript. All authors critically reviewed the manuscript and approved the final version for publication.
Role of funding source
Nothing declared.
Conflict of interest
No conflict declared.
Acknowledgement
This work was supported by grants from the NIH: DA021420 (WDY), OD011103.
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