Very interesting work. However, I find it very surprising that you omitted to cite and discuss the paper of Kaczer & Maldonado (2009), also published in Plos One. In that work we comprehensively described the inhibiting effect of OA in an aversive memory in the crab Chasmagnathus, in coincidence with one of the main conclusions of your work. We discussed about the contrasting role of OA, which on the one hand was necessary for appetitive memory formation, and on the other it had a deleterious effect on aversive memory, and we suggested an interaction between appetitive and aversive pathways. In addition, we recently found that OA impairs the reconsolidation phase of this aversive memory (Kaczer et al, 2011). Thus, it has been repeatedly proposed that biogenic amines are not restricted to a single type of learning but instead reveal a much more complex way of action.

References:

* Contrasting role of octopamine in appetitive and aversive learning in the crab Chasmagnathus. Kaczer L, Maldonado H. PLoS One. 2009 Jul 15;4(7):e6223.
* Dissecting mechanisms of reconsolidation: octopamine reveals differences between appetitive and aversive memories in the crab Chasmagnathus. Kaczer L, Klappenbach M, Maldonado H. Eur J Neurosci. 2011 Oct;34(7):1170-8