Holly et al. & Vassoler et al.

I really enjoyed both articles this week. The Vassoler article about epigenetic inheritance of a cocaine resistant phenotype was extremely interesting, because it goes against most research that says that drug abuse in parents will lead to drug abuse in children. The authors found that male offspring, but not female offspring of males that were exposed to cocaine have delayed acquisition and maintenance of cocaine self-administration. It is surprising that the cocaine resistant phenotype can be transmitted between generations in male rats, but the same effects aren’t observed in females. I think that the results of the study are really interesting, but I’m not sure how well this information translates to humans because the authors were able to control for environmental factors that wouldn’t be possible in a study with humans.

I liked that the Holly paper looked at the role that stress plays in addiction. Environment has such a profound effect on addiction, I was glad to see it incorporated into a study, unlike in the Vassoler paper. It is important to understand the mechanisms behind addiction, but if we don’t incorporate factors that affect humans, we won’t be able to accurately model addiction in animals, and it will limit our progress in treating humans. Holly et al. mentioned that stress plays a larger role in vulnerability to drug abuse in females than in males. It is interesting that females exposed to social defeat self-administer more cocaine, and I definitely think that the results of this paper are important, because there is so little data pertaining to females. There are clearly sex differences when it comes to addiction, and hopefully we continue to keep this in mind as we develop more personalized treatments in the future.
 

               Holly et al showed significant sex differences in sensitivity to and abuse of cocaine, but these results are somewhat muddied due to the cannulation and catheterization protocol. As this seminar has observed in earlier articles, ketamine has a robust long term effect on mood and anxiety in mice even in low doses. After social defeat and cocaine exposure both stressed and non-stressed rats were anesthetized with 100mg/kg ketamine; a massive dose of a drug that has unknown effects on the dopaminergic systems innervating limbic areas of the brain. This doesn’t undermine the results of the study concerning behavioral sensitivity, but it is reasonable to suspect that ketamine could very well alter the extracellular chemical composition of the NAc in rats of both sexes exposed to social defeat or any anxiolytic stress. Because of this, both the neuronal sensitivity and binge behavior experiments leave me with some doubt. Perhaps the best way to improve on this protocol is to use phenobarbital or some other anesthetic whose long term and short term effects on the mesocorticolimbic system are better understood.

               Vassoler et al supplied astounding evidence for the epigenetic inheritance of cocaine abuse resistance. There are, however, a few ways these results can be expanded upon. First, I’m curious to see the longevity of spermatozoal acetylation of the BDNF promoter region in cocaine exposed males. Perhaps CocSired male rats conceived more than a week after paternal cocaine administration might not receive the altered histone acetylation if the sperm wasn’t produced during or directly subsequent to cocaine exposure. I’m also interested in the epigenetic effects cocaine has on spermatozoal histones specifically on the sex chromosomes of the paternal germ cells. Although I’m unfamiliar with the chromosome structures of male and female rats, it’s reasonable to suspect females did not inherit cocaine resistance due to the absence of a Y chromosome, specifically some spermatozoal histone acetylation of the SRY or nearby genes which may potentiate or otherwise alter BDNF promotion. This would explain why the effects of BDNF promoter acetylation were observed in male but not female progeny of the cocaine exposed sires. 

Effect of Sex Differences and Genetic Inheritance on Addiction

Holly, et al. dives into sex differences and their role on cocaine addiction when paired with episodic social defeat stress. A modified resident intruder paradigm was implemented on both male and female (estrous and non-estrous) rats. Episodic social defeat stress was shown to significantly exacerbate behavioral sensitization to a marginally effective dose of cocaine. This dose isn't enough to produce behavioral changes in control rats but stressed rats were seen to have increased locomotor behavior (walk duration). Female stressed rats had significantly higher walk durations than males while female rats in estrous had an even larger increase in walking duration than non-estrous females. Due to the facilitatory effect of ovarian hormones in female rats, both estrous and non-estrous females had significantly increased sensitization as seen by an increased percentage of dopamine (DA) in the nucleus accumbens (NAc). Stressed females also display significantly higher total cocaine infusions and binge duration when allowed free access to cocaine compared to stressed males (both were significantly higher than their respective controls). These differences seen between sexes can help focus treatment in female cocaine addicts rather than treating them the same way they treat male cocaine addicts since the effect of it is a little different. I thought it was interesting that they misplaced 12 rat brains for the in vivo microdialysis. This was a large amount which could have skewed results. It also makes me wonder how careful they were recording other evidence when they lost 12 brains during the experiment.

Vassoler, et al. discusses effects of parental cocaine use on offspring. Specifically they looked at these effects in male rats. After going into all the differences that occur in female and male rats when sired from cocaine addicted males or dams, they decided that they would look at the effects in males sired by cocaine addicted males. They found increased brain-derived neurotropic factor (BDNF) in the medial prefrontal cortex (mPFC) as well as BDNF promotors. This result also translated to the sperm of these males meaning that they passed down these epigenetic modifications to their male offspring. This produces decreased cocaine reinforcement in their male offspring. This suggests that offspring from that line have a decreased chance of being addicted to cocaine as they are more resistant to it's reinforcing effects. This again highlights the sex difference between male and female rats and their ability to pass down resistance to their offspring,

Holly et al & Vassoler et al

I also really enjoyed the papers this week like many of my peers. And thought it was interesting and important to look into the fact that sex differences are relevant when looking at the mental illnesses. When thinking about mental illnesses, I always felt like it was said or seen that men were affected so it was interesting to see the effect of cocaine on both male and female rats when exposed to social defeat in the Holly et al paper. This paper also looked at the difference between estrous and non-estrous females. I thought it was interesting to see that in any situation the female rat was more vulnerable with the presence of cocaine. This led me to wonder, what is going on exactly within females that would lead them to be more prone to behavioral sensitization especially when stressed. I think an interesting test could be to look at female rats that just gave birth and see if they have the same results as the estrous or non-estrous rats.

The Vassoler et al paper, focused on the epigenetic mechanisms that had an effect on the vulnerability of the rats to cocaine addiction. I thought that this paper was interesting in showing how the offspring of male rats that were exposed to cocaine before had a resistance to cocaine reinforcement but this was not seen for female offspring. However, after reading the paper I was still a bit intrigued on how could this be observed and identified in humans. It did not seem realistic to be able to control and see what happens to the offspring of a man that was exposed to cocaine before having the child. I would love to see if there is a way to connect the two and see if there is an experiment that can be done. 

Holly et al. and Vassoler et al.

I really enjoyed that both of the papers for this week chose to look at sex differences in their studies. Holly et al. and Vassoler et al. both focused on cocaine addiction in rats, looking at the ways that sex, in addition to environmental stimuli, can affect cocaine dependency. One thing that I found particularly interesting in the Holly et al. paper was the effect of stress on addiction. While it is not surprising that high stress can influence the desire to partake in drug use, it was interesting to see that this effect is significantly greater on females than it is on males.

The Vassoler et al. paper was particularly fascinating as it looked at the effect of paternal drug use on the offspring. Going into this paper, I had expected that if any effect was seen, parental drug use would lead to increased drug use in the offspring. The fact that paternal drug use leads to a type of protection against drug seeking behavior in male offspring was very interesting. I think it would be worth looking at human survey data to see if any similar correlation could be found in drug use along paternal lines. Another aspect that would be worth investigating would be the effect of maternal drug use on the offspring. Maternal use of cocaine (as well the use of many other drugs) during pregnancy has been linked with many negative effects on the offspring including premature birth and low birth weight. However it would be worth looking at the effect of cocaine on the developing female germ line. This of course comes with complications as the timing of female primary germ line development has been called into question. It may be necessary to administer cocaine to pregnant dams, and then later follow up on the behaviors of the offspring of the female progeny.

Holly et al. 2012 vs Vassoler et al. 2012

The two papers this week continue to explore the behavioral and biological underpinnings of drug addiction. The Holly et al. paper examines cocaine addiction subsequent to social defeat and places special emphasis on the sex differences apparent in / relevant to cocaine addiction. The Vassoler et al. paper also investigates cocaine addiction; however, the group underscores the role of epigenetics as a modulator of differential addiction susceptibility and addictive behavior. I feel that both papers suit the last class particularly well because they are representative of an integrative experimental approach to studying mental illness.

An introductory (and repeated) point made in the Holly et al. paper was that “females are more vulnerable than males at each phase of cocaine addiction” and that the drug’s effects are “larger and longer lasting in stressed females.” I think that this information would come as a surprise to many given that men are more likely to develop substance abuse disorders. The paper’s findings then seem to suggest that the greater prevalence / incidence of substance abuse in men simply stems from greater likelihood of exposure or other environmental variables. Since I am not too familiar with the literature of this field, I wonder whether the greater susceptibility of females to stress as a whole may partly explain their greater susceptibility to addiction – at least in this stress-paired model.

We have spoken a lot in class about how behavioral models do not accurately reflect human conditions. I think that this social defeat stress model works better than the majority of other models we have seen with respect to that issue. I also appreciate their willingness to study the estrus cycle impacts via intact female animals as I think many would agree that artificial hormone regulation does not accurately reflect human biology. I do have some questions about their characterization of estrous cycle phases. They refer to estrus and proestrus as when “circulating ovarian hormones are higher.” In their discussion the group also states that “females in estrus showed an augmented response compared to rats in metaestrus or diestrus … consistent with the studies in females showing a facilitatory effect of estradiol on behavioral sensitization.” However, it is my understanding that estrogen levels are actually very low in estrous. While it is possible that they are grouping estrogen, progesterone, and LH and then claiming that ‘overall’ ovarian hormones are high despite low estrogen levels, I do not think that explains it. Here is the best chart I could find detailing the levels of ovarian hormones by rodent estrous phase. It appears to show that levels of all three hormones drop sharply between proestrus and estrus. If this is the case, then the reclassification of their data might lead to different findings.

As for the Vassoler paper, I found the discussion of epigenetic factors (and particularly the model employed to study them) interesting since we have barely approached the topic this semester. I appreciated the group’s use of sucrose intake as a comparison for cocaine-intake because introductory classes consistently teach that drugs of addiction ‘hijack’ the brain’s natural reward system. While this serves its purpose as dogma, the sucrose data demonstrates that cocaine (and presumably other drugs) operate via at least partly different regulatory and reward pathways within in the brain. I also thought that the use of molecular biology techniques to assess acetylation was exciting, but I questioned why the group did not conduct similar assessments of methylation.

Finally, in their discussion, Vassoler’s group mentioned that “estrogen enhances the effects of cocaine whereas progesterone has the opposite effects.” I think that this really emphasizes how important it is to better qualify the hormone fluctuations and estrous phase classifications detailed by Holly’s group. Finally, I also believe that progesterone’s stated role points toward to the less-than-translatable nature of OVX studies since typically only estrogen is supplemented. 

Holly et al. and Vassoler et al.

Compared to its predecessors, I especially enjoyed this week’s papers because they studied sex differences in cocaine addiction---something we haven’t really explored. I especially enjoyed the Holley et al. paper because it focused on the increased female vulnerability to stress and addiction.

Although the Holley et al. paper was comprehensive, I did have some complaints regarding its methodology. First off, behavioral sensitization was only tested through the self-administration model. Even though this model is accepted, I think Holley et al. should have confirmed addiction with the conditioned place preference model as well. I know that others might argue that requiring two models to confirm addiction is overkill, but I think the self-administration and conditioned place-preference model draw on different neural circuits. Even may of the same brain regions are involved (the PFC and NAC), the neural circuits are different which is why I think both models should have been used to test for addiction.

Additionally, I didn’t understand certain aspects of the episodic social stress test. The rats were initially placed in a protective cage, which Holley et al. claims allowed for olfactory and visual instigation while preventing tactile. I don’t understand the purpose of this step. If the rats were not initially placed in a protective cage, olfactory, visual, and tactile instigation could have occurred at the same time as in the wild. I think the social stress test would have been more similar to natural dominance displays if the protective cage was not initially installed.

Lastly, I was surprised that Holley et al. did not test to see if the rats were actually stressed after the episodic social stress paradigm. There was no figure or behavioral tests to confirm that the episodic social stress paradigm actually worked. Although the results of the experiment show that there was some difference between the stressed and non-stressed groups, I still think they should have confirmed that the episodic social stress paradigm actually worked by looking for weight gain and changes in corticosterone levels and general behavior (such as grooming). Besides these methodology issues however, I thought the Holley et al. paper was very interesting. I would love to see future studies examine the effect of ovarian hormones on other diseases, since females have increased vulnerability to many cognitive diseases (such as depression and anxiety disorders).

The Vassoler et al. paper was also very thought-provoking, and I would love to see if other diseases involve epigenetic reprogramming of the germline. I would also like to see if addiction recovery can somehow be improved by removing epigenetic reprogramming. Since epigenetics does not involve permanent changes to the genome, it can be reversed which poses a big questions as it whether its reversal has any effect on addiction recovery.