Functional magnetic resonance imaging (fMRI) has become an important method in clinical psychiatry research whereas there are still only few comparable preclinical investigations. Herein, we report that fMRI in rats can provide key information regarding brain areas underlying anxiety behavior. Perfusion as surrogate for neuronal activity was measured by means of arterial spin labeling-based fMRI in various brain areas of high anxiety F344 rats and control Sprague–Dawley rats. In one of these areas, the dorsomedial prefrontal cortex (dmPFC), c-Fos labeling was compared between these two strains with immunolabeling. The effects of a neurotoxic ibotenic acid lesion of the dmPFC in F344 rats were examined in a social approach–avoidance anxiety procedure and fMRI. Regional brain activity of high anxiety F344 rats was different in selective cortical and subcortical areas as compared to that of low anxiety Sprague–Dawley rats; the largest difference (i.e. hyperactivity) was measured in the dmPFC. Independently, c-Fos labeling confirmed that F344 rats show increased dmPFC activity. The functional role was confirmed by neurotoxic lesion of the dmPFC that reversed the high anxiety-like behavior and partially normalized the brain activity pattern of F344 rats. The current findings may have translational value as increased activity is reported in an equivalent cortical area in patients with social anxiety, suggesting that pharmacological or functional inhibition of activity in this brain area should be explored to alleviate social anxiety in patients.
Multiple epidemiological studies have shown that genetics play an important role in anxiety disorders in humans (Hettema et al., 2001 and Leonardo and Hen, 2006). Using inbred mice, this influence was confirmed by identifying several genetic factors that are associated with emotion-related behaviors in various procedures that model anxiety (Clement et al., 2002, Muigg et al., 2009 and O'Mahony et al., 2010). In rats, genetic approaches for studies on anxiety have been limited and only few studies have compared outbred or inbred rat strains. One of the more robust findings is that inbred Fischer 344 (F344) rats display an anxious phenotype in several anxiety-based paradigms, such as the elevated plus maze, the black–white box and the social interaction test (Bert et al., 2002, Berton et al., 1997, Ramos et al., 1997 and Rex et al., 1999), and may be a valid starting point to identify the neuronal pathways underlying high anxiety behavior.
Brain regions of greatest interest in the pathology of anxiety disorders, including social anxiety, are the amygdala, the anterior cingulate cortex, the ventromedial prefrontal cortex (vmPFC), the orbitofrontal cortex, the insula and the temporal areas (Amir et al., 2005, Canteras et al., 2010, Etkin and Wager, 2007, Goldin et al., 2009b, Phan et al., 2005, Shah et al., 2009 and Stein et al., 2007). Multiple preclinical studies using lesions or pharmacological challenges were able to show the involvement of some of these areas in anxiety (Navarro et al., 2004, Rudebeck et al., 2007, Sajdyk et al., 1999 and Sullivan and Gratton, 2002) but the findings are heterogeneous. A potentially valuable approach to better investigate the role of brain areas in high anxiety animals is the use of functional magnetic resonance imaging (fMRI) technology. It allows an indirect measurement of brain activity in many different areas at the same time in a non-invasive manner, with translational value. In contrast to the clinical setting, only few preclinical fMRI studies have examined the neuronal pathways that may be involved in anxiety-like behavior (Ferris et al., 2008, Kalisch et al., 2004 and Nephew et al., 2009).
The main goals in the present study were: 1) to compare the behavior of F344 rats with that of a standard rat strain (Sprague–Dawley, SD) in the social approach–avoidance (SAA) test (Nicolas and Prinssen, 2006), 2) to assess regional brain activity using fMRI in F344 and SD rats. Various brain areas that are associated with anxiety- and stress-related behavioral or autonomic functions were examined as an unbiased approach with the assumption that the brain area showing the largest differences between the strains [i.e. the dmPFC, thought to correspond to the anterior cingulate cortex (ACC) in humans (Seamans et al., 2008 and Uylings et al., 2003)] would play a key role in anxiety, 3) to confirm the hyperactivity of the dmPFC in F344 rats with immunolabeling of c-Fos protein, a marker for transcriptional action, and 4) to examine the effects of a neurotoxic lesion of the dmPFC in F344 rats in both the SAA test and fMRI.
The current study shows that F344 rats demonstrate high anxiety-like behavior that may be related to an ‘abnormal’ basal neurocircuitry of which hyperactivity of the dmPFC is a key element. Our findings, together with clinical observations should provide an impetus to further explore ACC hyperactivity as an imaging biomarker of social anxiety as well as to develop new non-invasive therapeutic approaches that specifically dampen ACC activity in patients with social anxiety (Pallanti and Bernardi, 2009 and Wu et al., 2007). In general, our current findings support the emerging concept that preclinical fMRI research can be of great value in characterizing brain structures and pathways underlying psychiatric diseases, and to help discover innovative treatments.
