On 2014 Jul 25, Serge Ahmed commented:

Thanks for your comments on my previous comments which, contrary to what you claim, were not based on “incorrect assumptions”. I leave the readers to judge for themselves. Here I just want to persist and sign for the sake of clarification. There is no solid evidence for compulsive cocaine use in your study. Mice gave up easily on cocaine and this apparently, I now learn from your comments, regardless of strain differences. I agree that your study was not designed to directly compare the motivation for food with that for cocaine. But still it shows that mice are able to produce hundreds of responses to obtain certain rewards of importance to them. The fact that mice are able to but do not expand a comparable level of effort to obtain cocaine shows that they are not that motivated for cocaine. More specifically, it shows that even after several weeks of cocaine intake, the motivation for cocaine fails to acquire a degree of intensity that approaches that for food in hungry mice. This is perhaps one of the reasons why mice were trained on a FR1 for cocaine in your study and not on a FR10 like for food. Finally, your operational measure of “perseverative responding” is difficult to interpret because the introduction of the drug-off periods also considerably influences cocaine self-administration during the drug-on periods. Judging from Figure 1, cocaine self-administration rapidly becomes very irregular, with short and long inter-injection intervals. As a result, some of the inter-injection intervals during the drug-on periods are much longer than the drug-off periods themselves, making behavior during the latter periods quite difficult to interpret!

On 2014 Jul 22, Veronica Alvarez commented:

Thanks for your interest in our study. These are interesting comments but based on incorrect assumptions. First, the behavioral score is a combination of breakpoint and responding during the time out period and thus it asses/compounds the individual variability in the incentive motivation for cocaine and the inability to restrain responding when the drug is not available (even when it is an anticipatory response and subjects “are unable to wait until the end of the drug period”). Second, the comparison of breakpoint for cocaine and food is an unfair comparison under these conditions. Mice responding for food were under caloric restriction and must work during the session to earn their remaining daily caloric requirement. This is a very different situation than for cocaine and the incentive motivation for food and cocaine are just incomparable. In addition, mice trained on FR1 for cocaine and FR10 for food which will influence the breakpoint values. The study was not designed to compare breakpoints for cocaine and food. Third, there is no effect of CNO per se. The effect seen corresponds to activation of Gi-coupled DREADDS in indirect-pathway D2-receptor expressing neurons in the NAc which is shown to inhibit the output (GABA transmission) of these neurons and increase the breakpoint for cocaine compared to days the same mice received saline (Fig. 4). The difference in responding between mice in Fig. 1-3 and mice in Fig. 4 and 5 is due to the fact that these are two different strains of genetically engineered mice. Experiments in Fig. 1-3 utilized Drd1a-GFP mice (Swiss Webster background) to identity direct- and indirect-pathway neurons while experiments in Fig. 4 and 5 used Adora2a-Cre-/+ mice (C57Bl6 background) to selectively manipulate the activity of indirect pathway neurons using DREADDs or ChR2. Contrary to the comment’s statement, responding was higher for mice of C57Bl6 background (35±9.5 mg/kg/day) than mice of SW background (18.7±6.1 mg/k/day) but the rate of responding might not be so different are the sessions were longer for C57Bl6 mice and there was no drug-off period.

On 2014 Jan 28, Serge Ahmed commented:

This study is very interesting but it also has several potential problems that may limit its relevance to understand the neural basis of compulsive cocaine use. First, there is no solid evidence for compulsive cocaine use in this study. For instance, mice were clearly able to inhibit cocaine seeking at the onset of the signaled drug-off periods. What they seemed unable to do was to wait until the end of the drug-off periods before resuming responding. This behavior may merely reflect an anticipation of cocaine availability at the end of the drug-off periods and not a compulsion (see: http://www.ncbi.nlm.nih.gov/pubmed/22985696). In addition, PR responding for cocaine was very low in this study and, in fact, 10-30 times lower that PR responding for food (i.e., about 50 versus 1000 responses per PR sessions for cocaine and food, respectively). In other words, mice gave up quickly on cocaine, without spending much effort. Calling this behavior “compulsive” or even “compulsive-like” is excessive. Finally, the facilitatory effects of CNO-induced inhibition of D2-MSNs on PR responding for cocaine should be interpreted with caution. These effects were seen in a small group of mice (n = 6) that had an atypical low baseline (i.e., saline) level of PR responding compared to the other groups from this study (i.e., 18.7±6.1 versus 35±9.5, 41.5±18.2 or 59.2±24.1). In fact, PR responding following CNO-induced inhibition of D2-MSNs did not differ from baseline responding in these other groups.

On 2014 Jan 28, Serge Ahmed commented:

This study is very interesting but it also has several potential problems that may limit its relevance to understand the neural basis of compulsive cocaine use. First, there is no solid evidence for compulsive cocaine use in this study. For instance, mice were clearly able to inhibit cocaine seeking at the onset of the signaled drug-off periods. What they seemed unable to do was to wait until the end of the drug-off periods before resuming responding. This behavior may merely reflect an anticipation of cocaine availability at the end of the drug-off periods and not a compulsion (see: http://www.ncbi.nlm.nih.gov/pubmed/22985696). In addition, PR responding for cocaine was very low in this study and, in fact, 10-30 times lower that PR responding for food (i.e., about 50 versus 1000 responses per PR sessions for cocaine and food, respectively). In other words, mice gave up quickly on cocaine, without spending much effort. Calling this behavior “compulsive” or even “compulsive-like” is excessive. Finally, the facilitatory effects of CNO-induced inhibition of D2-MSNs on PR responding for cocaine should be interpreted with caution. These effects were seen in a small group of mice (n = 6) that had an atypical low baseline (i.e., saline) level of PR responding compared to the other groups from this study (i.e., 18.7±6.1 versus 35±9.5, 41.5±18.2 or 59.2±24.1). In fact, PR responding following CNO-induced inhibition of D2-MSNs did not differ from baseline responding in these other groups.

On 2014 Jul 22, Veronica Alvarez commented:

Thanks for your interest in our study. These are interesting comments but based on incorrect assumptions. First, the behavioral score is a combination of breakpoint and responding during the time out period and thus it asses/compounds the individual variability in the incentive motivation for cocaine and the inability to restrain responding when the drug is not available (even when it is an anticipatory response and subjects “are unable to wait until the end of the drug period”). Second, the comparison of breakpoint for cocaine and food is an unfair comparison under these conditions. Mice responding for food were under caloric restriction and must work during the session to earn their remaining daily caloric requirement. This is a very different situation than for cocaine and the incentive motivation for food and cocaine are just incomparable. In addition, mice trained on FR1 for cocaine and FR10 for food which will influence the breakpoint values. The study was not designed to compare breakpoints for cocaine and food. Third, there is no effect of CNO per se. The effect seen corresponds to activation of Gi-coupled DREADDS in indirect-pathway D2-receptor expressing neurons in the NAc which is shown to inhibit the output (GABA transmission) of these neurons and increase the breakpoint for cocaine compared to days the same mice received saline (Fig. 4). The difference in responding between mice in Fig. 1-3 and mice in Fig. 4 and 5 is due to the fact that these are two different strains of genetically engineered mice. Experiments in Fig. 1-3 utilized Drd1a-GFP mice (Swiss Webster background) to identity direct- and indirect-pathway neurons while experiments in Fig. 4 and 5 used Adora2a-Cre-/+ mice (C57Bl6 background) to selectively manipulate the activity of indirect pathway neurons using DREADDs or ChR2. Contrary to the comment’s statement, responding was higher for mice of C57Bl6 background (35±9.5 mg/kg/day) than mice of SW background (18.7±6.1 mg/k/day) but the rate of responding might not be so different are the sessions were longer for C57Bl6 mice and there was no drug-off period.

On 2014 Jul 25, Serge Ahmed commented:

Thanks for your comments on my previous comments which, contrary to what you claim, were not based on “incorrect assumptions”. I leave the readers to judge for themselves. Here I just want to persist and sign for the sake of clarification. There is no solid evidence for compulsive cocaine use in your study. Mice gave up easily on cocaine and this apparently, I now learn from your comments, regardless of strain differences. I agree that your study was not designed to directly compare the motivation for food with that for cocaine. But still it shows that mice are able to produce hundreds of responses to obtain certain rewards of importance to them. The fact that mice are able to but do not expand a comparable level of effort to obtain cocaine shows that they are not that motivated for cocaine. More specifically, it shows that even after several weeks of cocaine intake, the motivation for cocaine fails to acquire a degree of intensity that approaches that for food in hungry mice. This is perhaps one of the reasons why mice were trained on a FR1 for cocaine in your study and not on a FR10 like for food. Finally, your operational measure of “perseverative responding” is difficult to interpret because the introduction of the drug-off periods also considerably influences cocaine self-administration during the drug-on periods. Judging from Figure 1, cocaine self-administration rapidly becomes very irregular, with short and long inter-injection intervals. As a result, some of the inter-injection intervals during the drug-on periods are much longer than the drug-off periods themselves, making behavior during the latter periods quite difficult to interpret!