Effects of memory age and interval of fear extinction sessions on contextual fear extinction

Importance of home cage condition for contextual fear memory, fear extinction and spontaneous recovery: Cage size and bedding material

Home cage condition influences the central nervous system of experimental animals. However, little is known about the effect of home cage size and bedding material on fear-related behaviors. Thus, in this study, the effects of home cage size (large or small) and/or bedding material (paper or wood) on acquisition, retrieval, extinction, and spontaneous recovery of contextual fear memory were investigated in both male and female mice. The present study demonstrated that males housed in small cages with wood bedding showed a low fear response during fear extinction when compared to males housed in small or large cages with paper bedding. In females, mice housed in small cages with wood bedding showed low fear response during fear conditioning and extinction when compared to mice housed in large cages with paper bedding. Moreover, small cages with wood bedding, but not small or large cages with paper bedding, prevented the spontaneous recovery of fear memory in females. Thus, home cage conditions, and particularly bedding material, influence contextual fear extinction and spontaneous recovery. This finding may help to obtain reproducibility of results by researchers and explain discrepancies of results among research groups.

Effects of swimming exercise on the extinction of fear memory in rats

We investigated the relation between swimming exercise and fear memory extinction. Rats that performed regular swimming exercise over 6 wk underwent fear conditioning. Twenty-eight days later, they were submitted to extinction tests. Swimming rats had enhanced extinction process throughout the 5 days of the extinction test compared with sedentary rats. This suggests that the swimming exercise accelerated the process of aversive memory extinction, reducing the expression of conditioned fear behavior. These results encourage further studies addressing the anxiolytic effects of exercise, with potential implications for anxiety disorders such as posttraumatic stress disorder. NEW & NOTEWORTHY We have shown that rats that performed regular swimming exercise over 6 wk had enhanced extinction process compared with sedentary animals. The swimming exercise may accelerate the process of aversive memory extinction, reducing the expression of conditioned fear behavior.

Sex differences in the effects of adult short-term isolation rearing on contextual fear memory and extinction

Fear conditioning and extinction is a useful tool for understanding the pathogenesis of fear-related disorders including post-traumatic stress disorder (PTSD) and for developing treatments for them. To investigate the role of sub-brain regions or molecular mechanisms in fear conditioning and extinction, neuroscientists have been employing an optogenetic or in vivo recording technique, in which placement of an optical fiber or an electrode into the brain region of a free-moving mouse is essential. These methods require isolation rearing (at least one week) from the brain surgery to the behavioral test. Although such short-term adult rearing has been shown not to influence fear memory and extinction in males, the effect in females remains unclear. In the present study, we investigated the effect on fear memory and fear extinction of adult isolation rearing during the one week before contextual fear conditioning in both male and female mice. This short-term adult isolation rearing increased fear responses in the contextual fear memory test in females but not in males. On the other hand, the rearing showed no effect on fear responses during fear extinction or the recall test in either sex. In summary, adult short-term isolation rearing enhanced only fear memory, and only in females.

Enhancing Endocannabinoid Neurotransmission Augments The Efficacy of Extinction Training and Ameliorates Traumatic Stress-Induced Behavioral Alterations in Rats

Exposure to a traumatic event may result in the development of post-traumatic stress disorder (PTSD). Endocannabinoids are crucial modulators of the stress response, interfere with excessive retrieval and facilitate the extinction of traumatic memories. Exposure therapy, combined with pharmacotherapy, represents a promising tool for PTSD treatment. We investigated whether pharmacological manipulations of the endocannabinoid system during extinction learning ameliorates the behavioral changes induced by trauma exposure. Rats were exposed to inescapable footshocks paired with social isolation, a risk factor for PTSD. One week after trauma, rats were subjected to three spaced extinction sessions, mimicking human exposure therapy. The anandamide hydrolysis inhibitor URB597, the 2-arachidonoylglycerol hydrolysis inhibitor JZL184 or the cannabinoid agonist WIN55,212-2 were administered before or after the extinction sessions. Rats were tested for extinction retention 16 or 36 days after trauma and 24-h later for social interaction. Extinction training alone reduced fear of the trauma-associated context but did not restore normal social interaction. Traumatized animals not exposed to extinction sessions exhibited reductions in hippocampal anandamide content with respect to home-cage controls. Noteworthy, all drugs exerted beneficial effects, but URB597 (0.1 mg/kg) induced the best improvements by enhancing extinction consolidation and restoring normal social behavior in traumatized rats through indirect activation of CB1 receptors. The ameliorating effects remained stable long after treatment and trauma exposure. Our findings suggest that drugs potentiating endocannabinoid neurotransmission may represent promising tools when combined to exposure-based psychotherapies in the treatment of PTSD.

Development of the fear regulation system from early adolescence to young adulthood in female mice

Onset of fear-related disorders such as post-traumatic stress disorder is enhanced from adolescence until adulthood. However, the biological mechanisms underlying this vulnerability remain unclear. Therefore, we investigated contextual fear memory and extinction in 4-, 6-, 8-, 10-, and 15-week-old female mice. We also measured phosphorylation of ERK2 in the medial prefrontal cortex (mPFC) and the dorsal hippocampus following fear conditioning or extinction in 6- and 15-week-old mice. We found that 10- and 15-week-old mice showed stronger fear memory and more resistance to fear extinction than 6-week-old mice. Moreover, 15-week-old mice showed lower ERK2 phosphorylation levels following fear extinction in the mPFC than those 6 weeks old. Our results suggest that female mice acquire strong fear memory and resistance to fear extinction throughout adulthood, which may be related to alteration in ERK2 activation in the mPFC.

Recent developments in the behavioural and pharmacological enhancement of extinction of drug seeking

One of the principal barriers to overcoming addiction is the propensity to relapse, even after months or years of abstinence. Relapse can be precipitated by cues and contexts associated with drug use; thus, decreasing the conditioned properties of these cues and contexts may assist in preventing relapse. The predictive power of drug cues and contexts can be reduced by repeatedly presenting them in the absence of the drug reinforcer, a process known as extinction. The potential of extinction to limit relapse has generated considerable interest and research over the past few decades. While pre-clinical animal models suggest extinction learning assists relapse prevention, treatment efficacy is often lacking when extinction learning principles are translated into clinical trials. Conklin and Tiffany (Addiction, 2002) suggest the lack of efficacy in clinical practice may be due to limited translation of procedures demonstrated through animal research and propose several methodological improvements to enhance extinction learning for drug addiction. This review will examine recent advances in the behavioural and pharmacological manipulation of extinction learning, based on research from pre-clinical models. In addition, the translation of pre-clinical findings-both those suggested by Conklin and Tiffany () and novel demonstrations from the past 13 years-into clinical trials and the efficacy of these methods in reducing craving and relapse, where available, will be discussed. Finally, we highlight areas where promising pre-clinical models have not yet been integrated into current clinical practice but, if applied, could improve upon existing behavioural and pharmacological methods.

Sex differences in fear extinction and involvements of extracellular signal-regulated kinase (ERK)

Stress-related disorders, such as post-traumatic stress disorder (PTSD) and panic disorders, are disproportionately prevalent in females. However, the biological mechanism underlying these sex differences in the prevalence rate remains unclear. In the present study, we examined sex differences in fear memory, fear extinction, and spontaneous recovery of fear. We investigated the presence of sex differences in recent and remote fear memory in mice using contextual fear conditioning, as well as sex differences in spontaneous recovery of fear memory using a consecutive fear extinction paradigm. We examined the number of fear extinction days required to prevent spontaneous recovery of fear in either sex. We investigated whether ovariectomy affected fear extinction and spontaneous recovery. We also measured the activation of extracellular signal-regulated kinase (ERK) 1 and 2 in the dorsal hippocampus and the medial prefrontal cortex following fear extinction sessions. In our results, we found no sex difference in recent or remote fear memory. However, females required more fear extinction sessions compared to males to prevent spontaneous recovery. Within-extinction freezing also differed between males and females. Moreover, females required more extinction sessions than males to increase ERK2 phosphorylation in the dorsal hippocampus. Our data suggest that contextual fear extinction was unstable in females compared to males and that such sex differences may be related to the ERK2 phosphorylation in the hippocampus.