Imaging unconditioned fear response with manganese-enhanced MRI (MEMRI) Wei Chen, Jeff Tenney, Praveen Kulkarni, and Jean A. King ⁎ University of Massachusetts Medical School, Department of Psychiatry, Center for Comparative Neuroimaging, 55 Lake Avenue North, Worcester, MA 01655, USA Received 24 August 2005; revised 19 April 2007; accepted 3 May 2007 Available online 10 May 2007 Recent use of manganese-enhanced MRI (MEMRI) to assess the neural circuitry involved in autonomic and somatosensory paradigms has been promising. The current study addresses the feasibility of utilizing this technique to assess more complex cognitive and emotional processes. Since olfactory cues are particularly salient to animals, we utilized odorless air, novel/arousing and novel/fear-inducing scents to assess the neural circuitry sub-serving novelty and unconditioned fear. The present imaging data clearly indicate that animals with no prior exposure to a threat-inducing emotional stimulus selectively activated the unconditional fear neuronal pathway, specifically with heightened amygdala and hypothalamic activation. While animals exposed to the novel/arousing compared to fear-inducing odor demonstrated en- hanced uptake in the cingulated and prefrontal cortices. In addition, as expected the hippocampus showed significantly enhanced manganese contrast after novelty exposure. Therefore the current study support the validity of MEMRI in the exploration of highly relevant complex neural circuitries associated with cognition and emotion. © 2007 Elsevier Inc. All rights reserved. Keywords: Manganese; Unconditioned fear; Emotion; MRI; Predator odor; Amygdala; Cognition; Novelty Introduction To understand the neuronal, emotional and cognitive compo- nents of fear researchers have employed several methods, many directed at evaluating the processes associated with fear condition- ing. In such a paradigm a seemingly meaningless stimulus such as tone or light (conditioned stimulus—CS) would be temporally paired with an arousing stimulus (unconditioned stimulus—US). The conditioned stimulus, after several exposures, would retain fear- eliciting properties when presented alone. Several elegant studies utilizing this paradigm have identified brain regions, consistently associated with conditioned fear independent of sensory modality (LeDoux, 2000a,b). Converging evidence indicates that one of the more prominent sites implicated in conditioned fear is the amygdala. Not directly connected to any of the sensory input sites, the amygdala receives sensory information through connections from cortical and sub-cortical sites. The interaction between cortical sites and amygdala facilitates the detection of stimuli from all sensory modalities (LeDoux, 1986, 2000a,b; LeDoux et al., 1990). In addition, sub-cortical routes to the amygdala provide rapid processing and subsequent information update (Li et al., 1996; LeDoux, 1986). Although considerable progress has been made in understanding the neuroanatomy of conditioned fear, unconditioned fear remains relatively under-explored. Early studies examining the effects of amygdala lesions on the rat response to the presentation of a cat (as an unconditioned fear- eliciting stimulus) showed that these lesions blocked the expression of fear-related responses (Blanchard and Blanchard, 1972). How- ever, others have reported that lesions to the amygdala did not impair the expression of unconditioned fear responses (Antoniadis and McDonald, 2001). There are also controversies about the role of the hippocampus in unconditioned fear responses. While some studies show that hippocampal ablation produced a decrement in defensive immobility (Blanchard et al., 1970), others show that the fear responses were not impaired after hippocampal lesions (Antoniadis and McDonald, 2001). These inconsistencies in the data may be dependent on the size and location of the lesion. At least one study has examined the biochemical effects of unconditioned fear stress compared to conditioned fear (Menon, 2001). Menon reports that the dopaminergic system in the amygdala was activated by uncondi- tioned fear but not by conditioned fear stress. For most animals, a primary fear-eliciting event is the possibility of being attacked by a predator. Consequently, predator stress (exposure to predator odor) is currently being simulated under laboratory condi- tions to trigger unconditioned (innate) fear responses in animals (Blanchard et al., 1997). A synthetic compound trimethylthiazoline (TMT) isolated from fox feces has been successfully used as a fear- inducing predator odor (Rosen, 2004), producing increased defensive behaviors and stress hormone release (Morrow et al., 2000; Perrot- Sinal et al., 1999). Interestingly, researchers have also reported that embedded in the fear response is the aspect of the stimulus related to its novelty1 (Williams et al., 2004). Therefore, the theoretical www.elsevier.com/locate/ynimg NeuroImage 37 (2007) 221 – 229 ⁎ Corresponding author. Fax: +1 508 856 8090. E-mail address: Jean.King@umassmed.edu (J.A. King). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2007.05.001