of exercise. Obesity. 2006;14:345-356. Voluntary physical activity and exercise training can favorably influence brain plasticity by facilitating neurogenerative, neuroadaptive, and neuroprotective processes. At least some of the processes are mediated by neurotrophic factors. Motor skill training and regular exercise enhance executive functions of cognition and some types of learning, including motor learning in the spinal cord. These adaptations in the central nervous system have implications for the prevention and treatment of obesity, cancer, depression, the decline in cognition associated with aging, and neurological disorders such as Parkinson's disease, Alzheimer's dementia, ischemic stroke, and head and spinal cord injury. Chronic voluntary physical activity also attenuates neural responses to stress in brain circuits responsible for regulating peripheral sympathetic activity, suggesting constraint on sympathetic responses to stress that could plausibly contribute to reductions in clinical disorders such as hypertension, heart failure, oxidative stress, and suppression of immunity. Mechanisms explaining these adaptations are not as yet known, but metabolic and neurochemical pathways among skeletal muscle, the spinal cord, and the brain offer plausible, testable mechanisms that might help explain effects of physical activity and exercise on the central nervous system.

The mesolimbic reward pathway is implicated in stress-related psychiatric disorders and is a potential target of plasticity underlying the stress resistance produced by repeated voluntary exercise. It is unknown, however, whether rats find long-term access to running wheels rewarding, or if repeated voluntary exercise reward produces plastic changes in mesolimbic reward neurocircuitry. In the current studies, young adult, male Fischer 344 rats allowed voluntary access to running wheels for 6 weeks, but not 2 weeks, found wheel running rewarding, as measured by conditioned place preference (CPP). Consistent with prior reports and the behavioral data, 6 weeks of wheel running increased ΔFosB/FosB immunoreactivity in the nucleus accumbens (Acb). In addition, semi quantitative in situ hybridization revealed that 6 weeks of wheel running, compared to sedentary housing, increased tyrosine hydroxylase (TH) mRNA levels in the ventral tegmental area (VTA), increased delta opioid receptor (DOR) mRNA levels in the Acb shell, and reduced levels of dopamine receptor (DR)-D2 mRNA in the Acb core. Results indicate that repeated voluntary exercise is rewarding and alters gene transcription in mesolimbic reward neurocircuitry. The duration-dependent effects of wheel running on CPP suggest that as the weeks of wheel running progress, the rewarding effects of a night of voluntary wheel running might linger longer into the inactive cycle thus providing stronger support for CPP. The observed plasticity could contribute to the mechanisms by which exercise reduces the incidence and severity of substance abuse disorders, changes the rewarding properties of drugs of abuse, and facilitates successful coping with stress.

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Background Exposure to uncontrollable stressors often increases anxiety-like behavior in both humans and rodents. In rat, this effect depends upon stress-induced activity within the dorsal raphé nucleus (DRN). However, the role of serotonin in DRN projection regions is largely unknown. The goals of the current study were to 1) determine if DRN activity during a post-stress anxiety test is involved in anxiety-like behavior, 2) assess the effect of uncontrollable stress on extracellular serotonin in the basolateral amygdala during the anxiety test, and 3) determine the role of the serotonin 2C receptor (5-HT2C) in uncontrollable stress-induced anxiety. Method Rats were exposed to tailshocks that were uncontrollable. On the following day anxiety-like behavior was assessed in a JSE test. BLA extracellular serotonin concentrations were assessed during JSE by in vivo microdialysis 24 h after uncontrollable stress, controllable stress or no stress. In separate experiments drugs were administered before the JSE test to inhibit the DRN or to block 5-HT2C receptors. Results Exposure to uncontrollable shock reduced later social exploration. Prior uncontrollable stress potentiated serotonin efflux in the BLA during social exploration, but controllable stress did not. Intra-DRN 8-OH-DPAT and systemic and intra-BLA 5-HT2C receptor antagonist SB 242084 prevented the expression of potentiated anxiety in uncontrollably stressed rats. Intra-BLA injection of the 5-HT2C agonist CP 809101 mimicked the effect of stress. Conclusion These results suggest that the anxiety-like behavior observed after uncontrollable stress is mediated by exaggerated 5-HT acting at BLA 5-HT2C receptors.