Stress-induced deficits in motivation for voluntary wheel running: An investigation of neurophysiological adaptations to major monoamine neurotransmitter systems and striatal transcriptome

Abstract

One of the primary functions of the nervous system is to compel an individual toward reward stimuli that will promote survival, while also encouraging avoidance of stressors, or stimuli that may threaten survival. Reward-seeking and avoidance behaviors are opponent processes, and functionally appear as opposites at the behavioral level. However, the underlying neurophysiology that governs these behaviors is largely shared. Furthermore, it is well known that this shared neurophysiology is extremely plastic, and continuously shaped and remodeled by experience, and is integral to the development of habits and routines. Maintenance of a healthy lifestyle is built around the ability to form habitual behaviors that foster social and physical wellness, such as engaging in social bonding, meeting caloric needs, and regular exercise. Despite the health advantages of habit formation and maintenance, the hyper-plastic nature of the neurobiological mechanisms that govern these processes presupposes risk of developing habits that could be detrimental to health. For example, experiences such as repeated exposure to a reward of high salience (i.e. drugs of abuse) or exposure to a stressor of extreme severity, can be sufficient to commandeer this sensitive neurophysiology to result in maladaptive habit formation, such as compulsive substance abuse, exaggerated avoidance response to threat, and ultimately, impairments in motivation to engage in activities that are beneficial to a healthy lifestyle such as physical activity. We set out to investigate the neurophysiological factors that contribute to preservation of motivation to continue development of healthy lifestyle habits, as well as interrogate how exposure to healthy “reward” behaviors (physical activity) vs. negative life events (severe psychological stress) may disrupt neurophysiology in brain structures critical to normal functioning of motivation related neuropathways. To do this we first asked how exposure to a single episode of traumatic stress (100 uncontrollable tail shocks) may induce adaptations in monoamine neurotransmitter systems that innervate brain areas associated with motivation, reward, and aversion. Following stress-exposure we gave rats 6 weeks of free voluntary running wheel access (vs 6wks no wheel access). We observed stark differences in voluntary wheel running behavior, where stress-exposed rats displayed a 4-fold deficit in running distance throughout the duration of the 6-week study. This was surprising, as voluntary wheel running is a well characterized form of natural reward in rodents, and rats and mice will regularly run long distances (3-10km daily or more). Moreover, this exact stressor has been used to study the effects of stress for over 50 years, and other researchers have reliably reported only transient deficits in measures of anxiety- and depression-like behaviors that recover within 72hrs to 1-week. Interestingly, the most frequently used tests to measure anxiety- and depression-like behaviors of rats exposed to this stressor often included the shuttlebox escape task and forced-swim test, both of which require sustained, high intensity physical exertion. Stress-exposed rats recover to perform equivalent to controls in these physically strenuous tasks, yet, still displayed long term deficits in voluntary wheel-running. To further investigate why stress-exposed rats display these deficits in voluntary wheel running, we repeated this experiment in another cohort of rats, but instead of granting free access to voluntary running wheels, we instead compared stressed vs non-stressed rat performance in a forced treadmill running task “to exhaustion,” 8 days post-shock. We observed no difference in time spent running, similar to what prior researchers had reported when using behavioral tests that forced rats to perform physically as mentioned above. To reduce the compulsory and potentially anxiety-inducing nature of this task, we familiarized a new cohort of rats with the treadmill with daily acclimation trials and repeated administration of the treadmill task 8 days post stress-exposure. With additional familiarization to the treadmill, we observed that stress-exposed rats “gave up” nearly ten minutes sooner than non-stressed rats. This warranted further investigation whereby we repeated this experiment but used the stress-exposed groups average time spent running on the treadmill as a timepoint to collect brain samples from both stressed and non-stressed rats in an effort to compare global brain monoamine neurotransmitter profiles activity at the moment stress-exposed rats would fail to continue running. We observed several stress-induced abnormalities in monoamine content across brain areas including dopamine deficits in the striatum, potentially indicating stress-induced maladaptation to motivation and motor systems. We then performed single-nuclei RNA-sequencing on the striata of rats (stressed vs non-stressed), with or without wheel access to identify changes to gene expression in this tissue that could underlie behavioral deficits in motivation to run on wheels following stress exposure. We were able to characterize 17 unique cell types, including medium spiny neurons, in our striatal samples, and identify alterations to markers that may be indicative of stress-induced maladaptations in MSNs of the direct and indirect pathways which could underlie lasting deficits in motivation to engage in voluntary wheel running, stress-exposed versus non-stressed rats with or without 6-weeks of free access to voluntary running wheels to investigate the transcriptomic adaptations to the striatum that may facilitate motivation to engage in exercise behavior as well as the effects stress-exposure may have on striatal physiology that may underlie stress-induced physical inactivity. Finally, we investigated how moderate daily running wheel access may induce global plasticity in monoamine neurotransmitter systems, and how those neuroadaptations may affect binge-like ethanol drinking behavior in mice. Following four weeks of wheel access, mice drank high amounts of ethanol using the drinking in the dark (DID) model of binge-like ethanol consumption. We found no differences in level of consumption between running vs sedentary mice, however, we were able to detect alterations in monoamine neurochemical levels across brain areas involved in motivation and reward.