Dopamine Neuron Cycling and the Effect of Nurr1 Deficiency

Abstract

The ventral midbrain (VM) houses two populations of dopamine neurons (DANs) - the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNc). The VTA generally signals the nucleus accumbens and prefrontal cortex to modulate reward, motivation, and focus of attention toward important stimuli required for survival. The SNc functions to signal the basal ganglia for initiation of movement. This makes VM DANs essential for the emotional and physical drive for survival. Pathology associated with VM DANs likewise results in symptoms related to these functions. Parkinson's disease is the result of SNc DANs losing function and ultimately dying off. Diseases that result from dysregulation of the VTA include addiction (reward-seeking behavior), anxiety and depression (motivation-related behaviors), schizophrenia and ADHD (focus of attention). Between these pathologies is a common thread of comorbidity with circadian disorders. It is already known that VM DANs, like many other neuron populations, are regulated by circadian rhythm via the suprachiasmatic nucleus based on light stimulus. On a cellular level, it is also known that Nurr1 is needed in combination with other transcription factors to express genes associated with DANs. Furthermore, Nurr1-deficiency is implicated in all the pathologies mentioned previously. To model Nurr1-deficiency, transgenic Nurr1-null heterozygous (+/-) mice are utilized, which are confirmed to have reduced Nurr1 expression and subsequently reduced expression of DA-associated genes. This study used the Nurr1 +/- mouse model compared with wild-type mice to assess wheel running activity across circadian conditions. Mice were exposed to regular 12:12 light/dark cycles, 24h dark, and light shift conditions as stimuli to assess the subsequent behavioral response of activity. It was found that Nurr1 deficiency shortened the free running period in regular 12:12 light/dark conditions and disrupted circadian cycling of the dopamine neuron phenotype as observed by reduced TH+ neurons at ZT0, the time of typical peak expression. Nurr1-deficient mice also had enhanced photoentrainment to the phase advance in the light shift condition. This collectively showed that Nurr1 deficiency dysregulates the ability of VM DANs to respond to changes in light stimulus. The question was then whether the activity or the light stimulus had the greatest effect on DA phenotype. Using a forced exercise protocol, it could be established whether the time of exercise or the time of sacrifice as a stimulus had the greater impact on DA phenotype. A separate transgenic mouse model was utilized that used a Cre-dependent YFP reporter on the DAT promoter, allowing for a counterstain in IHC/IF to visualize any neuron that had expressed DAT, whether they were actively DANs (also stained for TH) or not. This DATyfp model allowed for visualization of a larger DAN pool that showed fluctuations of active (TH+) DANs with the circadian cycle. What was more, a further staining of Nurr1 with YFP and TH showed a diverse neuron population with more nuance than originally anticipated. These data demonstrate the importance of Nurr1 in turning "on" the DA phenotype with circadian stimuli and raise additional questions about regulation of Nurr1 function. Questions including, "What is the function of 'off' DANs? Is the 'on' population during the general 'off' photoperiod consistent, or do DANs take turns remaining 'on'? What are the mechanisms behind sex differences observed in our experiments?"