The Role of CLOCK in Regulation of Dopamine Neurotransmission in the CLOCKdelta19 Mutant Mouse Model
Mice with a mutation in the circadian gene Clock (Clockdelta19) display a behavioral profile which parallels a euphoric manic-like state including hyperactivity, disrupted activity rhythms, increased substance abuse vulnerability, and decreases in anxiety and depression-related behavior. The molecular clock has significant cross-talk with many of the brain’s neurotransmitter systems. The purpose of this dissertation is to characterize the role of CLOCK in regulating dopamine transmission in mood and reward-related circuits. We present a mechanism by which CLOCK regulates dopaminergic activity in the mesoaccumbens circuit and contributes to anxiety-related behavior. In vivo recording of ventral tegmental area (VTA) dopamine cells throughout the 24 hour cycle revealed that firing and bursting was elevated in Clockdelta19 mutants with the most significant deviations early in the light cycle. Mimicking this increase in dopaminergic activity using optogenetic targeting resulted in decreased anxiety-related behavior similar to the Clockdelta19 phenotype. Consistent with the electrophysiological findings, tyrosine hydroxylase (TH) mRNA and protein was elevated in the VTA in a daytime-specific manner leading to increased dopamine synthesis in the nucleus accumbens. CLOCK binding was detected at E-box elements within the TH promoter with greater enrichment observed during the light phase when TH expression is low. These results suggest a negative regulation of TH by CLOCK. To examine alterations in the nigrostriatal dopamine circuit, HPLC measurements of dopamine and metabolites were performed in the dorsal striatum revealing significant increases in DOPAC and HVA. Dopamine receptor agonists and antagonists were used to pharmacologically probe dopamine receptor function. An enhancement of the locomotor suppressing response to dopamine antagonists in Clockdelta19 mice suggested increased dopaminergic tone. Clockdelta19 mice were insensitive to the locomotor stimulating effects of a D1 agonist, but displayed increased levels of D1DR protein. Conversely, the Clockdelta19 mutants displayed enhanced locomotor suppression to a D2 agonist and a coincident increase in D2DR protein. Forskolin stimulation of cAMP resulted in blunted molecular responses in the Clockdelta19 mutants consistent with impairments in D1 signaling and/or enhancements in D2 signaling. In summary, normal CLOCK function appears to be involved in the regulation of dopamine transmission in the striatum.
Circadian Rhythm, CLOCK Proteins, Bipolar Disorder, Dopamine
University of Texas Southwestern Medical Center
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