Development of Physiological Markers for Tetrabenazine-Induced Motivational Dysfunctions Using Electroencephalography in Male and Female Rats

Depression is a mental illness that is increasingly rampant in our society. With its prevalence, various drugs have become commonplace for treatment. Many of these drugs are serotonin reuptake inhibitors, or SSRIs. These drugs are able to mitigate symptoms of depression such as rumination and anxiety. However, they are not very successful in treating the amotivation and anergia that are seen in these patients. In order to investigate which drugs will be the most successful in treating the symptoms of depression, it is important to develop an animal model that can accurately represent the motivational symptoms of depression in humans. The goal of the present study was to use female and male rats to develop electroencephalography (EEG) markers that can be readily translatable to the pathophysiology of female and male patients with depression. In this study, tetrabenazine (TBZ), a vesicular monoamine transporter-2 (VMAT-2) inhibitor, or a vehicle (VEH) control, was administered to a group of 8 female rats and 7 male rats prior to measuring EEG in the home cage. Recordings were taken from each treatment condition in the medial prefrontal cortex (mPFC), motor (M1/M2), and medial parietal (mParietal) cortices to investigate the effect of TBZ on these cortices and to determine whether they are comparable to the neurological markers associated with depression in humans. It was found that male rats demonstrated a predominant peak frequency at baseline in EEG activity across all three regions in the alpha/theta frequency range (4-12 Hz, with a sharp peak at 6-8.5 Hz frequency range). The sharp 6-8.5 Hz peak was suppressed when TBZ was administered in a 1.0 mg/kg dose. The EEG recordings of the female rats were not significantly affected. These findings can be used as a foundation to develop further EEG studies in behaving rats that can be translated to treatments for clinical depression within humans. Introduction Major depressive disorder affects a large portion of the population, with over 264 million people being diagnosed with this across the globe (WHO, 2020). As stated by the National Institutes of Health, depression is “a common but serious mood disorder. It causes severe symptoms that affect how you feel, think, and handle daily activities” (Depression 2018). As the number of individuals affected by depression has increased over the years, more people have had to grapple with the cognitive, emotional, and physical symptoms that come with this psychopathology (Stahl, 2006; Treadway and Zald, 2011; Fava et al. 2014; Salamone et al. 2016a). One major cognitive impairment of depression is amotivation. Specifically, when given assessments for effort-related decision making, humans with depression are less likely to choose the option that requires more effort (Treadway et al. 2012a; Yang et al. 2014). In humans, this lack of motivation can severely reduce the quality of one’s life, affecting one’s ability to work, attend school, and maintain relationships (WHO 2020). To combat these symptoms and improve quality of life, it is important to develop an understanding of how depression works and determine which drug treatments are the most effective. This necessitates animal models for research, which allow for the use of controlled settings as well as tools and methods that could not be ethically used in humans (Hitzemann 2000). Through the use of animal models, neurochemical changes that lead to effort-related motivation dysfunctions have been wellestablished. For example, interference in the dopamine transmission of the ventral striatum lowers participation in high-effort instrumental activities elicited by conditioned stimuli, whereas low-effort instrumental activities are unaffected (Aberman and Salamone 1999; Salamone and Correa 2012). Within our laboratory, we have used the effort-based choice model on rats to study these motivational dysfunctions in psychopathology. We have conducted investigations in which tetrabenazine (TBZ) was administered to rats. TBZ works by inhibiting the vesicular monoamine transporter 2 (VMAT-2) to deplete dopamine, altering effort-based choice behavior in rats. In comparison with administration of a vehicle control, TBZ reduced the number of lever presses, while the consumption of concurrently available laboratory chow increased (Randall et al. 2014; Yohn et al. 2016a). Thus, similarly to humans, rats demonstrate a low-effort bias when subjected to the pharmacological conditions that have also shown to produce a low-effort bias in humans (Salamone et al. 2016a,b,c; Yohn et al. 2016a,b,c). Moreover, administration of TBZ can induce depressive symptoms in people, including fatigue and amotivation (Salamone et al. 2016a). In terms of the neural basis of depression, there is evidence demonstrating that mesolimbic DA circuits in the forebrain in conjunction with the ventral striatum, prefrontal cortex, and ventral pallidum play a significant role in the selection of high-effort activities (Salamone et al. 1991, 1994; 2007, 2016a,b, 2018; Walton et al. 2003; Floresco and GhodsSharifi 2007; Mingote et al. 2008; Farrar et al. 2008, 2010; Winstanley and Floresco 2016). Still, more research is needed to properly identify and describe the psychopathology of neural circuits in effort-related dysfunctions. Recent studies have suggested that cortical electroencephalographic markers may differ between healthy and depressed individuals. There has been particular emphasis on the frontal cortex, which is primarily responsible for cognitive effort tasks, or CET performance (Winstanley and Floresco 2016). In a study of 37 depressed and 35 nondepressed individuals, the frontal alpha EEG asymmetry of the depressed individuals at rest was significantly increased (Gollan et al. 2014). Further, evidence also shows that depressed people have a reduction of EEG markers in this cortex that are present during the performance of motivated behaviors and in the anticipation of reinforcement (Nelson et al. 2018; Gheza et al. 2019). Specifically, it has been determined that bilateral asymmetry of EEGs in the frontal cortex in humans is an important marker for effort-related motivation (Allen, Coan, & Nazarian, 2004; Coan and Allen 2004). In addition to EEGs, imaging has also been utilized to investigate the neural basis of anergia, psychomotor retardation (Hickie et al. 1999; Capuron et al. 2007), and effort-related decision making (Wardle et al. 2011; Schouppe et al. 2014; Huang et al. 2016; Hogan et al. 2018; Aridan et al. 2019). The results have been consistent with animal studies in that the striatum and frontal cortex are important for effort-based decision making as well as approach motivation, or motivation which is widely prompted by positive stimuli (Winstanley and Floresco 2016; Harmon-Jones et al. 2013). The purpose of the present experiments was to further investigate these findings, comparing the characteristics of the mPFC, M1/M2, and mParietal EEG readings in the left and right hemispheres of awake male and female rats in baseline and TBZ-induced, DA-depleted conditions. The mPFC was the focus of this study due to the abundance of studies drawing connections between executive function, effort-based choice, decision making, and the prefrontal cortex. Additionally, the mPFC is thought to reflect the effects of TBZ on the ventral striatum and striatopallidal circuits that are important for effort-based decision making (Rotolo Ph.D. Dissertation, 2020). EEGs from the mParietal and M1/M2 cortices were recorded to serve as a point of reference for the mPFC while simultaneously ascertaining whether TBZ affected other cortical areas. In summation, this investigation was done to work towards creating physiological markers for altered DA transmission and effort-related dysfunction in preclinical animal models that are translatable to human male and female studies. Materials and Methods