Activation of Nigrostriatal Dopamine Neurons during Fear Extinction Prevents the Renewal of Fear.

Manipulations that increase dopamine (DA) signaling can enhance fear extinction, but the circuits involved remain unknown. DA neurons originating in the substantia nigra (SN) projecting to the dorsal striatum (DS) are traditionally viewed in the context of motor behavior, but growing data implicate this nigrostriatal circuit in emotion. Here we investigated the role of nigrostriatal DA in fear extinction. Activation of SN DA neurons with designer Gq-coupled receptors exclusively activated by designer drugs (Gq-DREADD) during fear extinction had no effect on fear extinction acquisition, but enhanced fear extinction memory and blocked the renewal of fear in a novel context; a pattern of data paralleled by cFos expression in the central amygdala. D1 receptors in the DS are a likely target mediating the effects of SN DA activation. D1-expressing neurons in the medial DS (DMS) were recruited during fear extinction, and Gq-DREADD-induced DA potentiated activity of D1-expressing neurons in both the DMS and the lateral DS (DLS). Pharmacological activation of D1 receptors in the DS did not impact fear extinction acquisition or memory, but blocked fear renewal in a novel context. These data suggest that activation of SN DA neurons and DS D1 receptors during fear extinction render fear extinction memory resistant to the disrupting effects of changes in contextual contingencies, perhaps by recruiting habitual learning strategies involving the DLS. Nigrostriatal DA thus represents a novel target to enhance long-term efficacy of extinction-based therapies for anxiety and trauma-related disorders.

Acute exercise enhances the consolidation of fear extinction memory and reduces conditioned fear relapse in a sex-dependent manner.

Fear extinction-based exposure therapy is the most common behavioral therapy for anxiety and trauma-related disorders, but fear extinction memories are labile and fear tends to return even after successful extinction. The relapse of fear contributes to the poor long-term efficacy of exposure therapy. A single session of voluntary exercise can enhance the acquisition and consolidation of fear extinction in male rats, but the effects of exercise on relapse of fear after extinction are not well understood. Here, we characterized the effects of 2 h of voluntary exercise during the consolidation phase of contextual or auditory fear extinction learning on long-term fear extinction memory and renewal in adult, male and female, Long-Evans rats. Results indicate that exercise enhances consolidation of fear extinction memory and reduces fear relapse after extinction in a sex-dependent manner. These data suggest that brief bouts of exercise could be used as an augmentation strategy for exposure therapy, even in previously sedentary subjects. Fear memories of discrete cues, rather than of contextual ones, may be most susceptible to exercise-augmented extinction, especially in males. Additionally, exercise seems to have the biggest impact on fear relapse phenomena, even if fear extinction memories themselves are only minimally enhanced.

Exercise increases mTOR signaling in brain regions involved in cognition and emotional behavior.

Exercise can enhance learning and memory and produce resistance against stress-related psychiatric disorders such as depression and anxiety. In rats, these beneficial effects of exercise occur regardless of exercise controllability: both voluntary and forced wheel running produce stress-protective effects. The mechanisms underlying these beneficial effects of exercise remain unknown. The mammalian target of rapamycin (mTOR) is a translation regulator important for cell growth, proliferation, and survival. mTOR has been implicated in enhancing learning and memory as well as antidepressant effects. Moreover, mTOR is sensitive to exercise signals such as metabolic factors. The effects of exercise on mTOR signaling, however, remain unknown. The goal of the present study was to test the hypothesis that exercise, regardless of controllability, increases levels of phosphorylated mTOR (p-mTOR) in brain regions important for learning and emotional behavior. Rats were exposed to 6 weeks of either sedentary (locked wheel), voluntary, or forced wheel running conditions. At 6 weeks, rats were sacrificed during peak running and levels of p-mTOR were measured using immunohistochemistry. Overall, both voluntary and forced exercise increased p-mTOR-positive neurons in the medial prefrontal cortex, striatum, hippocampus, hypothalamus, and amygdala compared to locked wheel controls. Exercise, regardless of controllability, also increased numbers of p-mTOR-positive glia in the striatum, hippocampus, and amygdala. For both neurons and glia, the largest increase in p-mTOR positive cells was observed after voluntary running, with forced exercise causing a more modest increase. Interestingly, voluntary exercise preferentially increased p-mTOR in astrocytes (GFAP+), while forced running increased p-mTOR in microglia (CD11+) in the inferior dentate gyrus. Results suggest that mTOR signaling is sensitive to exercise, but subtle differences exist depending on exercise controllability. Increases in mTOR signaling could contribute to the beneficial effects of exercise on cognitive function and mental health.