Neurobiology of Learning and Memory 87 (2007) 209–217 www.elsevier.com/locate/ynlme 1074-7427/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.nlm.2006.08.009 Methylene blue facilitates the extinction of fear in an animal model of susceptibility to learned helplessness Kathryn M. Wrubel, Douglas Barrett, Jason Shumake, S. Elizabeth Johnson, F. Gonzalez-Lima ¤ Department of Psychology and Institute for Neuroscience, University of Texas at Austin, Austin, TX 78712, USA Received 29 June 2006; revised 22 August 2006; accepted 23 August 2006 Available online 2 October 2006 Abstract The objectives were to (1) extend previous Wndings on fear extinction deWcits in male congenitally helpless rats (a model for susceptibil- ity to learned helplessness) to female congenitally helpless rats, and (2) attempt a therapeutic intervention with methylene blue, a meta- bolic enhancer that improves memory retention, to alleviate the predicted extinction deWcits. In the Wrst experiment, fear acquisition (four tone-shock pairings in operant chamber) was followed by extinction training (60 tones in open Weld). Congenitally helpless rats showed fear acquisition similar to controls but had dramatic extinction deWcits, and did not display the gradual extinction curves observed in con- trols. Congenitally helpless rats demonstrated greater tone-evoked freezing as compared to controls in both the acquisition and extinction contexts one week after extinction training, and also in the extinction probe conducted one month later. In the second experiment (which began one month after the Wrst experiment) congenitally helpless subjects were further exposed to tones for 5 days, each followed by 4 mg/kg methylene blue or saline IP, and had a fear renewal test in the acquisition context. Methylene blue administration improved retention of the extinction memory as demonstrated by signiWcant decreases in fear renewal as compared to saline-administered congenitally helpless subjects. The impaired ability to extinguish fear to a traumatic memory in congenitally helpless rats supports the validity of this strain as an animal model for vulnerability to post-traumatic stress disorder, and this study further suggests that methylene blue may facilitate fear extinction as an adjunct to exposure therapy. © 2006 Elsevier Inc. All rights reserved. Keywords: Extinction; Post-traumatic stress disorder; Congenitally helpless rats; Methylene blue; Fear conditioning; Metabolic enhancer; Animal model; Disease susceptibility; Memory retention; Exposure therapy; Cytochrome oxidase 1. Introduction Inescapable electric shock prevents animals from subse- quently learning an escape response, a phenomenon termed learned helplessness (Overmier & Seligman, 1967). Vulnera- bility to learned helplessness is hereditable and can be enhanced through selective breeding, as evidenced by the creation of a strain of congenitally helpless rats (Henn & Vollmayr, 2005). Studies show that up to 95% of the con- genitally helpless oVspring typically show the learned helpless phenotype, as opposed to the 5–20% of randomly bred rats (Henn, Johnson, Edwards, & Anderson, 1985; Lachman et al., 1992). Since epidemiological studies show that 40–50% of the risk of susceptibility to depression and 30% of the risk of susceptibility to post-traumatic stress disorder (PTSD) is genetic (Fava & Kendler, 2000; Sanders, Detera-Wadleigh, & Gershon, 1999; True et al., 1993), an appropriate animal model of congenital vulnerability to these disorders could be very useful, especially in the devel- opment of therapeutic treatments. A recent paper reported behavioral characteristics of male rats predisposed to learned helplessness (Shumake, Barrett, & Gonzalez-Lima, 2005). Compared to normal rats, congenitally helpless rats demonstrated reduced * Corresponding author. Fax: +1 512 471 4728. E-mail address: gonzalez-lima@mail.utexas.edu (F. Gonzalez-Lima).