Enhancement of spatial learning by predator odor in mice: involvement of amygdala and hippocampus.

Olfaction has particular links with learning and memory compared with other sensory cues, due to the interrelations between their neural circuitry. The present study deals with the effects of a putative stressor (i.e. a predator odor) on visuo-spatial learning in mice. Firstly, the results show that a predator odor spread during the Morris water maze task led to learning enhancement. In addition, a stereotaxic approach was used to investigate the involvement of the amygdala in this hippocampus-dependent type of learning. Thus, the performance of mice in visuo-spatial learning under predator odor conditions was dramatically reduced by an ibotenate bilateral amygdala lesion. The involvement of the amygdala was confirmed by a reduced expression of c-fos in the CA1 hippocampus of amygdala-lesioned mice at the end of the learning procedure. Mild exposure to a predator odor during hippocampus-dependent learning therefore leads to an enhancement of performance through the co-activation of the amygdala, probably by a stress mediated mechanism.

Effects of an ambient odor on brain activations during episodic retrieval of objects.

This study investigated the brain areas involved in episodic memory retrieval of pictures depending on the characteristics of the contextual environment during encoding (i.e. presence or absence of an odor). In the first stage, subjects were presented with a series of 32 pictures. Half of the pictures were presented while the subjects smelled an odor (vanillin). No particular odor was associated with the presentation of the other half of the pictures. Two weeks later, a retrieval task was performed in which the same pictures were presented during an fMRI session but without any odor association involved. The results show that both conditions activate a common episodic memory network including the hippocampal formation. Compared with the "encoding without odor" condition, the "encoding with odor" condition shows greater activations in temporal, parietal and frontal cortices, notably within the area of the orbitofrontal cortex which constitutes a main site of the secondary olfactory cortex. No activated areas were observed in the inverse contrast. These results highlight the complexity of the networks involved in episodic memory according to the context during encoding.

Fear-like behavioral responses in mice in different odorant environments: Trigeminal versus olfactory mediation under low doses.

Odors can have repulsive effects on rodents based on two complementary adaptive behaviors: the avoidance of predator odors (potentially dangerous) and the avoidance of trigeminal stimulants (potentially noxious). The present study aimed to compare the behavioral effects on mice of odors according to their trigeminal properties and ecological significance. We used three different odors: 2,4,5-trimethylthiazoline (TMT: a fox feces odor frequently used to elicit fear-induced behaviors), toluene (a strong stimulant of the trigeminal system) and phenyl ethyl alcohol (PEA: a selective stimulant of the olfactory system). First, we checked preference and avoidance behaviors in mice with and without anosmia towards these odors to ensure their olfactory/trigeminal properties. Secondly, we used a standard test (open-field and elevated plus-maze) to assess the behaviors of mice when exposed to these odors. The results show that the anosmic and control mice both avoided TMT and toluene odors. In the open-field and the elevated plus-maze, mice exhibited "anxious" behaviors when exposed to TMT. Conversely, exposure to PEA induced "anxiolytic" effects confirmed by low blood corticosterone levels resulting from completion of the elevated plus-maze. Compared with TMT exposure, toluene exposure induced moderate "anxious" effects.