On 2016 Apr 22, Emily Ferenczi commented:

Here we expand upon the influence of medial prefrontal cortex (mPFC) stimulation in different hedonic behaviors. As documented in several previous studies, behaviorally relevant sucrose preference changes (for example those induced by stress) are often modulated similarly to the extent we observed (for example: Chaudhury et al., Nature 2013, 493: 532-536; Lim et al., Nature 2012, 487: 183-189; Covington et al., J Neurosci 2010, 30: 16082-16090). Indeed, sucrose has powerful motivational influences (as noted in the earlier commenter’s numerous Pubmed comments and postings relevant to his prior findings, e.g. http://www.ncbi.nlm.nih.gov/pubmed/25120076, http://www.ncbi.nlm.nih.gov/pubmed/23542690) and it would be surprising to achieve complete indifference to sucrose with a mild and distant mPFC modulation, nor is anhedonia absolute in the clinical setting. With respect to the influence of increased excitability of mPFC via SSFO stimulation, we did not claim (or use the word) “indifference” to sucrose (which would presumably mean a sucrose preference of 50%) but rather noted a significant reduction in preference. In line with previous literature, we in fact expected this reduction to be modest and characterized the reduction as a “mild but consistent and reversible reduction in sucrose preference only during days when light-stimulation was delivered”. The decrease in sucrose preference that we observed was in fact not only consistent across days and reversible, but also significantly differed to the behavior of YFP controls (Figure 4J) and accompanied suppressed engagement in another naturally rewarding activity (social interaction), as well. Finally, further support for the neurobiological relevance of this manipulation comes from the observation that it induced differences in fMRI functional connectivity between the medial prefrontal cortex and striatum, which in turn tracked the magnitude of the decrease in sucrose preference at the individual subject level (Fig. 6I). It is interesting to note that recent clinical trials have demonstrated suppression of cocaine use (Terraneo et al., Eur Neuropsychopharmacol 2016, 26: 37-44) and heroin cravings (Shen et al., Biol Psychiatry 2016, doi: 10.1016/j.biopsych.2016.02.006) in patients with addiction following transcranial magnetic stimulation of the prefrontal cortex, perhaps pointing towards a similar principle of cortical regulation of hedonic processing.

In the dual stimulation place preference experiments, rats were allowed to explore three chambers freely for 30 minutes total, and one chamber was paired with stimulation of ventral tegmental area dopamine (VTA-DA) neurons. The rats were exposed to chamber-paired VTA-DA stimulation for the first 10 minutes of the test and quickly began to show a preference for the VTA-DA stimulation side. When SSFO was switched on for the middle 10 minutes of the test, rats no longer preferred the VTA-DA stimulation side (consistent with approximately 50% preference). Once SSFO was switched off, the rats spent another 10 minutes with chamber-paired VTA-DA stimulation only, again preferring the VTA-DA stimulated side. Crucially, in the second 10 minute section of the test when mPFC was concurrently stimulated, it was in a non-chamber paired fashion. Specifically, SSFO stimulation was constantly present throughout the 10 minutes regardless of which chamber rats occupied (as reported in the paper, “10 min of superimposed mPFC activation by SSFO (single 5-s pulse of blue light in mPFC at the start, 10-s pulse of yellow light at the end)”). Thus, the chamber not paired with VTA-DA stimulation acted as an internal control for concurrent SSFO stimulation, and any possible SSFO-mediated aversion would therefore occur in both chambers. Both before and after mPFC stimulation, rats significantly preferred the VTA-DA stimulation chamber (mean percent time spent on stimulation side = 65.1%, 95% CI = 55.0 to 75.1%, p = 0.007) but did not prefer this chamber during mPFC stimulation (mean = 45.1%, 95% CI = 26.7 to 63.4%, p = 0.54). It is possible that a sufficiently aversive condition in which rats are no longer responsive to environmental stimuli could lead to anhedonic behavior, which we would consider to be one of several interesting mechanisms for this effect and would have some clinical relevance, though we note that the mPFC intervention did not alter baseline locomotion, water consumption, initial social interaction, or novel object investigation. While mPFC stimulation reduced the positive hedonic impact of VTA-DA stimulation, we note that the behavioral paradigm can also detect aversive effects, since inhibition of VTA-DA dopamine neurons with eNpHR elicited behavioral avoidance instead (see place preference test in Figure S7).

In summary, the behavioral effects related to mPFC stimulation are significant and coherent across studies, with changes in elicited ofMRI activity linked to individual-subject differences in targeted hedonic behavioral phenotypes. Although much more experimental ground was covered in the article (as noted by the commenter), no claims were made regarding identification of direct top-down anatomical connectivity -- a topic that remains of great future interest. We would be happy to provide any additional clarifications, so if questions remain please directly contact us for more detailed, extensive, or even hands-on productive exchange.

On 2016 Apr 05, Serge Ahmed commented:

There are many different types of data in this ambitious Research Article. I will only comment on the behavioral effects of medial prefrontal (mPFC) activation. Though those manipulations induced large-scale reorganization of both cortical and subcortical brain activities, as measured by fMRI, they had only marginal behavioral repercussions.

Notably, contrary to what the authors seem to imply, there is little or no evidence for anhedonia during mPFC activation (see Figure 4 of the paper). At best, there is a small reduction in preference for a low concentration of sucrose (i.e., from 90 to about 85%) but rats still continued to largely prefer sucrose over water. The behavioral effects of mPFC activation are simply magnified by setting the origin of the Y-axis at 70% - a value well above indifference. The behavioral significance of this tiny decrease in preference, if any, is largely unclear. In addition, there is no direct evidence that it results from a prefrontal top-down control over dopamine-dependent reward signaling in the striatum, as suggested by the authors. Finally, there is no evidence in Figure 5 that rats seek to spend more time in a place associated with stimulation of midbrain DA neurons. Rats are initially globally indifferent. The small, albeit significant, effect of mPFC activation likely reflects a direct aversive effect that might also be observed alone with no concurrent stimulation of midbrain DA neurons – an important control experiment that the authors apparently failed to conduct. It would have been more relevant to test whether and to what extent mPFC activation directly modulates the self-stimulation behavior of midbrain DA neurons reported in Figure 2.

Overall, the behavioral effects reported in this Research Article are too marginal and too disparate to offer a clear picture of the role of mPFC activation in regulating dopamine-dependent reward-seeking behavior.

On 2016 Apr 05, Serge Ahmed commented:

There are many different types of data in this ambitious Research Article. I will only comment on the behavioral effects of medial prefrontal (mPFC) activation. Though those manipulations induced large-scale reorganization of both cortical and subcortical brain activities, as measured by fMRI, they had only marginal behavioral repercussions.

Notably, contrary to what the authors seem to imply, there is little or no evidence for anhedonia during mPFC activation (see Figure 4 of the paper). At best, there is a small reduction in preference for a low concentration of sucrose (i.e., from 90 to about 85%) but rats still continued to largely prefer sucrose over water. The behavioral effects of mPFC activation are simply magnified by setting the origin of the Y-axis at 70% - a value well above indifference. The behavioral significance of this tiny decrease in preference, if any, is largely unclear. In addition, there is no direct evidence that it results from a prefrontal top-down control over dopamine-dependent reward signaling in the striatum, as suggested by the authors. Finally, there is no evidence in Figure 5 that rats seek to spend more time in a place associated with stimulation of midbrain DA neurons. Rats are initially globally indifferent. The small, albeit significant, effect of mPFC activation likely reflects a direct aversive effect that might also be observed alone with no concurrent stimulation of midbrain DA neurons – an important control experiment that the authors apparently failed to conduct. It would have been more relevant to test whether and to what extent mPFC activation directly modulates the self-stimulation behavior of midbrain DA neurons reported in Figure 2.

Overall, the behavioral effects reported in this Research Article are too marginal and too disparate to offer a clear picture of the role of mPFC activation in regulating dopamine-dependent reward-seeking behavior.

On 2016 Apr 22, Emily Ferenczi commented:

Here we expand upon the influence of medial prefrontal cortex (mPFC) stimulation in different hedonic behaviors. As documented in several previous studies, behaviorally relevant sucrose preference changes (for example those induced by stress) are often modulated similarly to the extent we observed (for example: Chaudhury et al., Nature 2013, 493: 532-536; Lim et al., Nature 2012, 487: 183-189; Covington et al., J Neurosci 2010, 30: 16082-16090). Indeed, sucrose has powerful motivational influences (as noted in the earlier commenter’s numerous Pubmed comments and postings relevant to his prior findings, e.g. http://www.ncbi.nlm.nih.gov/pubmed/25120076, http://www.ncbi.nlm.nih.gov/pubmed/23542690) and it would be surprising to achieve complete indifference to sucrose with a mild and distant mPFC modulation, nor is anhedonia absolute in the clinical setting. With respect to the influence of increased excitability of mPFC via SSFO stimulation, we did not claim (or use the word) “indifference” to sucrose (which would presumably mean a sucrose preference of 50%) but rather noted a significant reduction in preference. In line with previous literature, we in fact expected this reduction to be modest and characterized the reduction as a “mild but consistent and reversible reduction in sucrose preference only during days when light-stimulation was delivered”. The decrease in sucrose preference that we observed was in fact not only consistent across days and reversible, but also significantly differed to the behavior of YFP controls (Figure 4J) and accompanied suppressed engagement in another naturally rewarding activity (social interaction), as well. Finally, further support for the neurobiological relevance of this manipulation comes from the observation that it induced differences in fMRI functional connectivity between the medial prefrontal cortex and striatum, which in turn tracked the magnitude of the decrease in sucrose preference at the individual subject level (Fig. 6I). It is interesting to note that recent clinical trials have demonstrated suppression of cocaine use (Terraneo et al., Eur Neuropsychopharmacol 2016, 26: 37-44) and heroin cravings (Shen et al., Biol Psychiatry 2016, doi: 10.1016/j.biopsych.2016.02.006) in patients with addiction following transcranial magnetic stimulation of the prefrontal cortex, perhaps pointing towards a similar principle of cortical regulation of hedonic processing.

In the dual stimulation place preference experiments, rats were allowed to explore three chambers freely for 30 minutes total, and one chamber was paired with stimulation of ventral tegmental area dopamine (VTA-DA) neurons. The rats were exposed to chamber-paired VTA-DA stimulation for the first 10 minutes of the test and quickly began to show a preference for the VTA-DA stimulation side. When SSFO was switched on for the middle 10 minutes of the test, rats no longer preferred the VTA-DA stimulation side (consistent with approximately 50% preference). Once SSFO was switched off, the rats spent another 10 minutes with chamber-paired VTA-DA stimulation only, again preferring the VTA-DA stimulated side. Crucially, in the second 10 minute section of the test when mPFC was concurrently stimulated, it was in a non-chamber paired fashion. Specifically, SSFO stimulation was constantly present throughout the 10 minutes regardless of which chamber rats occupied (as reported in the paper, “10 min of superimposed mPFC activation by SSFO (single 5-s pulse of blue light in mPFC at the start, 10-s pulse of yellow light at the end)”). Thus, the chamber not paired with VTA-DA stimulation acted as an internal control for concurrent SSFO stimulation, and any possible SSFO-mediated aversion would therefore occur in both chambers. Both before and after mPFC stimulation, rats significantly preferred the VTA-DA stimulation chamber (mean percent time spent on stimulation side = 65.1%, 95% CI = 55.0 to 75.1%, p = 0.007) but did not prefer this chamber during mPFC stimulation (mean = 45.1%, 95% CI = 26.7 to 63.4%, p = 0.54). It is possible that a sufficiently aversive condition in which rats are no longer responsive to environmental stimuli could lead to anhedonic behavior, which we would consider to be one of several interesting mechanisms for this effect and would have some clinical relevance, though we note that the mPFC intervention did not alter baseline locomotion, water consumption, initial social interaction, or novel object investigation. While mPFC stimulation reduced the positive hedonic impact of VTA-DA stimulation, we note that the behavioral paradigm can also detect aversive effects, since inhibition of VTA-DA dopamine neurons with eNpHR elicited behavioral avoidance instead (see place preference test in Figure S7).

In summary, the behavioral effects related to mPFC stimulation are significant and coherent across studies, with changes in elicited ofMRI activity linked to individual-subject differences in targeted hedonic behavioral phenotypes. Although much more experimental ground was covered in the article (as noted by the commenter), no claims were made regarding identification of direct top-down anatomical connectivity -- a topic that remains of great future interest. We would be happy to provide any additional clarifications, so if questions remain please directly contact us for more detailed, extensive, or even hands-on productive exchange.