Okay, if I've got this right, this is more or less saying that some people's genes vary in such a way that some are more and less receptive to certain neurochemicals that interact with our Oxytocin receptors. It seems VTA plasticity deals with dopamine absorption in related receptors.
This >>5606 appears to note that certain individuals that have issues with Oxytocin receptors may not be as affected by L. reuteri supplementation as others. Such an issue may also affect dopamine absorption in these individuals.