The molecules that move us : evaluating potential genetic and molecular determinants of physical activity motivation
Authors:   , Grigsby Kolter 
Published: April 2021

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Physical activity is at the basis of human history. The fact that we occupy every continent, were built for long distance travel, and that we find reward and value in physical activity, speaks to the notion that we were not designed to be sedentary. That said, physical inactivity remains a major contributor to at least 40 non-communicable diseases, of which heart disease, type-2 diabetes, depression, anxiety, and dementia remain untamed and at the forefront of our public health interests. For the first time in human history, engaging in physical activity is essentially voluntary, a choice that [about] 97% of U.S. adults fail to meet in terms of the U.S. Physical Activity Guidelines. In light of the inactivity epidemic, it follows that finding therapeutics and approaches aimed at reversing or preventing sedentary lifestyles will undoubtedly benefit our global health and well-being. In an attempt to address this possibility, the collective work of this dissertation is made up of three independent studies incorporating wheel-running behavior in a selectively bred rat model of low voluntary running (LVR) behavior. Moreover, these studies address the hypothesis that distinct differences in motivationally-relevant mechanisms within the mesolimbic reward system, in particular the nucleus accumbens (NAc), accompany LVR behavior. In the first study, rats selectively bred for high voluntary running (HVR) or LVR behavior were used to assess inherent differences in both NAc N-methyl-D-aspartate (NMDA) mRNA and protein expression, as well as differences in NMDA-evoked current responses and dopamine release. The data indicate that there were significantly higher levels of mRNA and protein expression of NR1, indicative of higher total NMDA receptor numbers, in HVR compared to LVR rats. There was similarly a greater current response to NMDA in isolated NAc neurons and greater NMDA-evoked dopamine release in NAc tissue from HVR compared to LVR rats. Collectively, these data suggest a likely link between NAc NR1 subunit expression and NMDA function and the predisposition for nightly voluntary running behavior. In study 2, the small kinase inhibitor, Protein Kinase Inhibitor Alpha (PKI[alpha]), was assessed as a potential gene candidate involved in physical activity motivation. Previous work in our lab identified PKl[alpha] as being expressed at low levels in the NAc of LVR compared to HVR rats, with PKl[alpha] expression being positively correlated with running behavior. Due to its key role in dopamine receptor 1 signaling, PKl[alpha] offered a potential target for modulating physical activity motivation. The extent to which PKl[alpha] influences wheel-running behavior and motivation was assessed through site-directed, intra-NAc AAV-mediated overexpression of PKl[alpha]. Following PKl[alpha]overexpression, LVR rats display [about]3-fold higher total running activity over 20 days compared to controls, with no effect of PKl[alpha] overexpression in WT rats. Beyond what appears to be a behavioral resistance in WT rats, there was a noticeably lower dopaminergic transcript expression with PKl[alpha] overexpression in WT rats compared to WT rats expressing the empty-vector control, with no effect seen in LVR rats on dopaminergic expression. From these data, PKl[alpha] appears to be a novel and effective molecular target for reversing low physical activity motivation. To expand knowledge on the critical role of PKl[alpha] in the regulation of dopaminergic signaling, attention shifted to the major downstream transcription factor, CREB. Thus, study 3 addressed the role of NAc CREB in gating voluntary wheel-running motivation, in LVR rats. Results indicate that baseline CREB phosphorylation levels were higher in LVR compared WT rats, suggesting that CREB activation might induce reward and motivational deficits. Further, inducible upstream CREB attenuation via G[i]-coupled hM4Di DREADD or pharmacological inhibition of NAc CREB increased running behavior in LVR but not WT rats. Similarly, environmentally enriched LVR rats exhibit higher running activity compared to socially isolated rats in a CREB-related fashion. To evaluate the role of NAc glutamatergic and dopaminergic pathways in motivational activity, potential genetic and molecular determinants of physical activity motivation were examined. Collectively, findings from these studies suggest that the glutamatergic and dopaminergic pathways play a role in physical activity motivation in the LVR rats, which encourages future efforts to target these pathways to enhance physical activity in humans.