Unit of Biochemistry, Department of Medicine, Faculty of Science, University of Fribourg
The relationship between the circadian clock and the mesolimbic
dopaminergic signaling system
Gabriele Hampp
The internal clockwork is a self sustained machinery, which is organized by interlocked
transcriptional/translational feedback loops ensuring synchronization of endogenous molecular,
biochemical, physiological and behavioral rhythms to the environmental 24h light-dark cycle. Period2
(Per2) is one of the non-redundant clock genes acting within these interlocked
transcriptional/translational feedback loops of the molecular pacemaker located in the Suprachiasmatic
nuclei (SCN) of the brain. Various biochemical and physiological alterations accompany the circadian
phenotype of mPer2Brdm1-/- mutants, which is caused by the defective internal clock. Hypersensitization to
cocaine and a hyperglutamatergic brain of mPer2Brdm1-/- mutants are the hallmarks of behavioral and
biochemical perturbations related to the brain reward system (Abarca et al. 2002, Spanagel et al. 2005).
Within this signaling system, the neurotransmitter dopamine (DA) acts as major player regulating
motivation and mood.
Differential display analysis revealed that the mouse monoamine oxidase A (mMaoa) gene is less
transcribed in the SCN of Per2Brdm1-/- mutant mice at ZT15 as compared to wild type animals (Holzberg
and Albrecht 2003). mMaoa encodes for MAO-A, which is one of the degrading enzymes of DA in the
monoaminergic signaling pathway.
mMaoa was identified as a direct target of the molecular clockwork - a first order clock controlled gene.
So, the transcription of a 1.1kb fragment of the 5’upstream region of the mouse Maoa gene fused to a
luciferase reporter is induced directly by the mBMAL1/mNPAS2 heterodimer in the neuroblastoma cell
line NG108-15. The mBMAL1/mNPAS2-mediated transcriptional activation of the mouse Maoa gene is
further enhanced by PER2 protein. Supportingly, mBMAL1 was found to bind to the promoter region of
mMaoa and the 1.1K luciferase-reporter construct of mMaoa promoter displays robust bioluminescence
rhythms with a period close to 24 hours in the synaptic conditions of the neuronal cells over several days.
Circadian regulation of transcription of mMaoa is reflected by diurnal oscillations of mMaoa transcript
levels in brain regions of the reward pathway. Amplitude and overall levels of mMaoa transcript are
dampened in Per2Brdm1-/- mutant mice emphasizing an implication of mPER2 in regulation of mMaoa
transcription and in the brain reward system. Diurnal activity of MAO enzyme is decreased in mesolimbic
brain regions of mPer2Brdm1-/- mutant animals probably causing higher DA/DOPAC-ratios in the striatum
of Per2Brdm1-/- mutant mice during the course of the day as detected by HPLC analysis.
Despair based animal models correlate immobility times under conditions without escapism to
neurotransmitter levels in the brain. FST immobility times of mPer2Brdm1-/- mutants are strongly reduced
compared to their wild type littermates indicating altered neurotransmitter levels in these mutants. AMPT,
a specific tyrosine hydroxylase inhibitor, rescues the FST immobility phenotype of Per2Brdm1-/- mutant
mice to wild type immobility times supporting the assumed alterations of dopaminergic respectively
catecholaminergic signaling in the brain of mPer2Brdm1-/- mutants. Application of a
serotonin/norepinephrine-specific reuptake inhibitor results in an equal, dose-dependent decrease of the
immobility times of both genotypes, whereas mPer2Brdm1-/- mutants respond faster to increasing
concentrations of a non-selective MAO inhibitor in TST.
The abrogation of PER2 protein results in increased addictive and less immobile behavior of mPer2Brdm1-/mutant animals helping on a better understanding of the role of the internal clockwork in mood and
motivation and the influence of light on these mental states.
Jury: Prof. Dr. Urs Albrecht (Dissertationsleiter) Universität Freiburg (Schweiz), MathematischNaturwissenschaftliche Fakultät, Abteilung Medizin, Einheit Biochemie; Prof. Dr. Jean-Luc Dreyer (Gutachter),
Universität Freiburg (Schweiz), Mathematisch-Naturwissenschaftliche Fakultät, Abteilung Medizin, Einheit
Biochemie; Prof. Dr. Rainer Spanagel (Gutachter), Universität Heidelberg (Deutschland), Abteilung
Psychopharmakologie, Zentralinstitut für Seelische Gesundheit, Mannheim (Deutschland); Jury-Präsident: Prof.
Dr. Eric M. Rouiller, Universität Freiburg (Schweiz), Mathematisch-Naturwissenschaftliche Fakultät, Abteilung
Medizin, Einheit Physiologie.