Alarm pheromone activates posterior medial amygdala-to-bed nucleus of the stria terminalis projections in male rats.

Animals communicate using olfactory signals called pheromones. Alarm pheromones are evolutionarily conserved and alert conspecifics to danger. In rats, a mixture of hexanal and 4-methylpentanal was identified as the main component that evokes anxiety responses by activating the anterior bed nucleus of the stria terminalis (aBNST). In addition, the posteroventral medial amygdala (pvMeA) was proposed to transmit the alarm pheromone signal to the aBNST. Here, we assessed whether the binary mixture activates direct pvMeA-to-aBNST projections or indirect projections via other MeA subdivisions in male rats. In Experiment 1, we virally labeled synaptophysin either in all types of efferent projections (n = 5) or selectively in glutamatergic efferent projections (n = 5) of the pvMeA. When we observed from the main olfactory bulb to the cerebellum, both types of signals were detected across 22 regions, with the aBNST showing the highest densities of both signals. These results suggest that the pvMeA transmits direct, predominantly glutamatergic projections to the aBNST. In Experiment 2, we injected a retrograde tracer into the aBNST and observed Fos expression in response to water (n = 6) or the binary mixture (n = 7) in the medial amygdala. We found that double-labeled cells were increased only in the pvMeA and posterodorsal medial amygdala (pdMeA). These results suggest that the binary mixture activates both the pvMeA-to-aBNST and pdMeA-to-aBNST projections. Taken together, we propose that the posterior medial amygdala transmits the alarm pheromone signal to the aBNST. These findings may support efforts to improve the welfare of livestock animals.