In addition to the main olfactory system that mainly senses volatile, airborne odourants, most species of vertebrate also possess a vomeronasal system, specialised for the detection of non-volatile chemosignals in biological secretions. The traditional, ‘dual olfactory system’ view is that these two chemosensory systems function relatively independently, and has been based largely on the separate anatomical projections of the two systems. Vomeronasal sensory neurons project via the accessory olfactory bulb to the medial amygdala (MeA) and the posteromedial cortical nuclei of the amygdala, which are together often referred to as the vomeronasal amygdala. These areas in turn project to medial regions of the hypothalamus that are involved in the control of reproductive and social behaviour. In contrast, olfactory sensory neurons (OSNs) of the main olfactory system project separately, via the main olfactory bulb (MOB), to the anterior cortical and posterolateral cortical regions of the amygdala. Consequently, main olfactory input had been thought to have a less direct influence on reproductive and social behaviour via intra-amygdala connections with the vomeronasal pathway (Licht & Meredith, 1987). However, recent findings have suggested that functions of the vomeronasal and main olfactory systems are more integrated than previously thought (Zufall & Leinders-Zufall, 2007).
Kang et al. (2009, this issue) have added to this evidence, by using anterograde tracing to demonstrate a previously-neglected, direct projection from the MOB to the MeA in female mice. This projection potentially provides a more direct pathway by which main olfactory input could control reproductive and social behaviour than originally thought. A similar pathway has previously been described in rats (Pro-Sistiaga et al., 2007), but Kang et al. (2009) have gone further, in using retrograde tracing from the MeA to show that these projections arise from a sub-population of mitral and tufted (M/T) neurons located mainly in the ventral region of the MOB. Importantly Kang et al. (2009) have also addressed the function of this pathway using c-fos expression as a marker of mitral/tufted neuron activity. They found that retrogradely labelled M/T neurons in the MOB of female mice responded to chemosignals from male mice, but not to chemosignals from other female mice, or to a predator odour. Interestingly, glomeruli in the ventral region of the mouse MOB have been found to receive input from a distinct population of OSNs that express the transient receptor potential channel M5 (TRPM5) and respond to male-specific volatile chemosignals, such as (methylthio) methanethiol (Lin et al., 2007). An obvious next step will be to investigate whether these TRPM5 OSNs provide the input to the MOB-MeA pathway. It will also be important to determine, whether a similar pathway is present in males and therefore forms a specialised system for assessing chemosignals from the opposite sex, or whether it is a sexually dimorphic pathway that is found only in females. This study provides further evidence for the existence of functionally specialised sub-systems within the MOB, and further evidence for the important role of the main olfactory system in mediating innate pheromonal responses.