The anterior medial amygdala transmits sexual odor information to the posterior medial amygdala and related forebrain nuclei.

In Syrian hamsters, reproductive behavior relies on the perception of chemical signals released from conspecifics. The medial amygdala (MEA) processes sexual odors through functionally distinct, but interconnected, sub-regions; the anterior MEA (MEAa) appears to function as a chemosensory filter to distinguish between opposite-sex and same-sex odors, whereas the posterodorsal MEA (MEApd) is critical for generating attraction specifically to opposite-sex odors. To identify how these sub-regions interact during odor processing, we measured odor-induced Fos expression, an indirect marker of neuronal activation, in the absence of either MEAa or MEApd processing. In Experiment 1, electrolytic lesions of the MEAa decreased Fos expression throughout the posterior MEA in male hamsters exposed to either female or male odors, whereas MEApd lesions had no effect on Fos expression within the MEAa. These results indicate that the MEAa normally enhances processing of sexual odors within the MEApd and that this interaction is primarily unidirectional. Furthermore, lesions of the MEAa, but not the MEApd, decreased Fos expression within several connected forebrain nuclei, suggesting that the MEAa provides the primary excitatory output of the MEA during sexual odor processing. In Experiment 2, we observed a similar pattern of decreased Fos expression, using fiber-sparing, NMDA lesions of the MEAa, suggesting that the decreases in Fos expression were not attributable exclusively to damage to passing fibers. Taken together, these results provide the first direct test of how the different sub-regions within the MEA interact during odor processing, and highlight the role of the MEAa in transmitting sexual odor information to the posterior MEA, as well as to related forebrain nuclei.