Sophrosyne, the ancient goddess of moderation was considered to be the highest of virtues. Not because
she was very strong or very fast or very powerful - but for the very reason that she was not any one of those. She represented thoughtfulness and balance and (I
think) balance through thoughtfulness. So where is the "moderation center" in the brain? Well, since moderation is rooted in foresight, the prefrontal cortex must play a role in seeing the possible outcomes which are to be considered. That said, the decision to moderate may start in the forebrain but the term is so abstract and applies to so many different behaviors, emotions and cravings that it is surely applied to almost all parts of the brain. It stands to reason therefore that a chemical can be found that induces ubiquitous moderation across all behaviors by agonizing the neural circuitry that evolved to apply it.
In scenarios of human activity that have a tendency to get out of hand - the use of such an agent would decrease the frequency of occurrence of deleterious events born of excess. Theoretically, one could "imbibe civility" and save themselves the damages of extreme behaviors.