Central olfactory pathway in response to olfactory stimulation in rats detected by magnetic resonance imaging.

The olfactory system is known to have important roles in neuroendocrine regulation, emotional responses, reproductive/maternal functions, aggression, food selection and the recognition of conspecifics, predators and prey, etc. (Shipley et al., 1995). Especially in the case of food/fluid selection and intake, olfaction plays very important roles in combination with taste and texture. The pathway of the olfactory system has a unique characteristic relative to other sensory systems in that the signals go directly to the primary sensory cortical areas and limbic system without involvement of the brain stem. The limbic system is related to emotion, learning and memory. Therefore, odors can affect the preference for food and its memory. So far, there is little direct evidence demonstrating the activation of higher-order olfactory centers during olfactory stimulation except for activation in the olfactory bulb (OB) (Yang et al., 1998; Xu et al., 2000, 2003). In the first part of the present study (experiment I), brain activation areas in response to olfactory stimulation were investigated in anesthetized rats using functional magnetic resonance imaging (fMRI). In the second part of the present study (experiment II), neural projections in higher-order olfactory centers, especially projections from several olfactory centers to the medial frontal cortex (MFC), were investigated using manganese-enhanced MRI (MEMRI), a newly developed in vivo neural tracing technique (Pautler et al., 1998, 2003; Van der Linden et al., 2002). The first part of the study suggested that the MFC, which is a part of the limbic system and not considered one of the olfactory centers (Shipley et al., 1995), responded well to olfactory stimulation to a similar extent and with response patterns similar to the primary olfactory cortical areas, i.e. the piriform cortex, olfactory tubercle, etc. (Kondoh et al., 2002, 2003). Injection of Mn2+, a biological calcium analogue and paramagnetic tract tracing agent, allows highlighting specific brain areas that are active in MEMRI.