The Circadian Rhythm and its Role in the Dynamic Dopamine Neuron Phenotype

Abstract

The circadian rhythm is strongly implicated in many neuropsychiatric and neurodegenerative disorders, all of which are associated with altered dopamine (DA) neurotransmission in the substantia nigra and ventral tegmental area. Progress has been made in elucidating the circadian rhythm-dopaminergic network and its role in the onset of neuropsychiatric and neurodegenerative disorders. Previous research suggests that circadian rhythm transcription factors are responsible for directly regulating the DA phenotype; however, it is currently unknown what the relationship between the circadian rhythm and dopaminergic genes looks like with respect to age and time of day. Using a transgenic mouse model with Cre recombinase expression under control of dopamine transporter (DATCre) and yellow fluorescent protein (YFP) Cre reporter and immunohistochemistry techniques, we are able to characterize sub-populations of DA neurons in the ventral midbrain (VMB) that are responsive to the circadian rhythm. Here, we demonstrate a dynamic DA neuron phenotype, where classic dopaminergic markers, such as dopamine transporter (DAT) and tyrosine hydroxylase (TH) are not detected in dopaminergic neurons, due to regulation by the circadian rhythm. In this study, mice transgenic for DATCre/YFP were analyzed at postnatal day 0 (P0), P21, P35 and adulthood (>P60). Each time point included mice taken at subjective dawn (circadian time 0) and subjective dusk (CT12), excluding P0 mice. Results revealed that between P21 and P35, there was a significant loss of the DA neuron phenotype at CT12, as compared to CT0. There was no statistical difference between P35 and adults at CT0 or CT12. This suggests that between P21 and P35, DA neurons begin to transition to a 'former' phenotype throughout the circadian rhythm. Additionally, qRT-PCR data revealed abnormal dopaminergic gene mRNA levels at P21. Elucidating the molecular characteristics of these DA neurons is crucial to understanding the biological mechanisms behind the dopaminergic-circadian rhythm network, which will have future implications in understanding neuropsychiatric and neurodegenerative disorders.