Pet-1 Deficiency Alters The Circadian Clock And Its Temporal Organization Of Behavior

The serotonin and circadian systems are two important interactive regulatory networks in the mammalian brain that regulate behavior and physiology in ways that are known to impact human mental health. Previous work on the interaction between these two systems suggests that serotonin modulates photic input to the central circadian clock (the suprachiasmatic nuclei; SCN) from the retina and serves as a signal for locomotor activity, novelty, and arousal to shift the SCN clock, but effects of disruption of serotonergic signaling from the raphe nuclei on circadian behavior and on SCN function are not fully characterized. In this study, we examined the effects on diurnal and circadian behavior, and on ex vivo molecular rhythms of the SCN, of genetic deficiency in Pet-1, an ETS transcription factor that is necessary to establish and maintain the serotonergic phenotype of raphe neurons. Pet-1-/- mice exhibit loss of rhythmic behavioral coherence and an extended daily activity duration, as well as changes in the molecular rhythms expressed by the clock, such that ex vivo SCN from Pet-1-/- mice exhibit period lengthening and sex-dependent changes in rhythmic amplitude. Together, our results indicate that Pet-1 regulation of raphe neuron serotonin phenotype contributes to the period, precision and light/dark partitioning of locomotor behavioral rhythms by the circadian clock through direct actions on the SCN clock itself, as well as through non-clock effects. © 2014 Ciarleglio et al

Main effects of genotype on circadian behavioral characteristics in 12L:12D (a–c) and constant darkness (d–f).

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χ² periodogram analyses of genotype-specific and wheel-dependent effects on overall rhythmic amplitude (power) in LD (<i>top</i>) and DD (<i>bottom</i>).

<p>See inset legend (<i>top-left</i> panel) for genotype. Purple line represents significance at <i>p</i>≤0.05. Periodic component ratios measure the ratio of 12-hour:24-hour components on the wheel. Error bars represent SEM; asterisks (*) denote significance at <i>p</i>≤0.05.</p

Main effects of genotype (and photoperiod) on circadian behavioral phenotypes.

<p>Activity monitored by infrared motion detector and wheel in LD and DD, as denoted. <i>Ex vivo Per1::Luc</i> expression is below. Significance is ascribed at <b><i>p</i></b><b>≤0.05</b>.</p

Main effects of exercise on circadian behavioral phenotypes.

<p>Activity monitored by infrared motion detector in LD and DD. Significance is ascribed at <b><i>p</i></b><b>≤0.05</b>.</p

Wheel open/lock circadian behavioral paradigms.

<p><b>a |</b> Double-plotted graphics representing two equal light cycle paradigms (12L:12D→DD) with opposite order of wheel access in each phase. <b>b |</b> Representative double-plotted actograms showing the monitored activity of mice that are, from <i>left</i> to <i>right</i>, wildtype, heterozygote, or knockout for <i>Pet-1</i>, as monitored by infrared (IR) motion detection (<i>top</i>) and wheel (<i>bottom</i>). Note that there is no wheel activity during wheel-locked phases. The <i>Pet-1</i>^+/+ mouse (<i>left</i>) represents paradigm A, while the <i>Pet-1</i>^+/− and <i>Pet-1</i>^−/− mice (<i>middle</i> and <i>right</i>, respectively) represents paradigm B. Y-axis represents days; X-axis represents time-of-day.</p

Main effects of <i>Pet-1</i> deficiency and running wheel access on circadian behavioral characteristics in 12L:12D (a-b) and constant darkness (c-f) as measured by an infrared motion detector (IR).

<p>a | <i>Pet-1</i>^−/− mice exhibit a significant increase in length of daily activity duration (<i>alpha</i>) in LD. b | Rhythmic power is unaffected by genotype, as measured by χ² periodogram, but exercise leads to a significant increase. c | Free-running period of <i>Pet-1</i>^−/− mice is significantly decreased in DD. Furthermore, locking a wheel significantly decreases this period. d | There is a significant interaction between genotype and wheel-access such that as <i>Pet-1</i> gene dosage decreases, locking the wheel decreases period to an even greater extent. e | As in LD, <i>Pet-1</i>^−/− mice in DD exhibit a greater duration of daily activity. f | <i>Pet-1</i>^−/− mice exhibit reduced rhythmic power, and overall, mice allowed to exercise on a wheel exhibit greater rhythmic power. Error bars represent SEM; asterisks (*) denote significance at <i>p</i>≤0.05.</p

Averaged activity profiles across genotypes as monitored by (a) wheel and (b-d) infrared motion detector (IR) in LD.

<p>Each point represents the average across days and across animals within that particular genotype while in a 12L:12D cycle. Blue points represent <i>Pet-1</i>^+/+ mice; orange points represent <i>Pet-1</i>^+/− mice; black points represent <i>Pet-1</i>^−/− mice. Colored bar (<i>top</i>) represents 24 hour light cycle, where the grey is 12 hours of dark, and the yellow is 12 hours of light. Activity measured in revolutions/hour (a) or counts/hour (b–d). b | Overall IR activity profile of animals regardless of wheel state. These results are further broken-down into IR behavior with a free wheel (c) and with a locked wheel (d). See inset legend (a). Asterisks (*) denote significance at <i>p</i>≤0.05. Note the prominent early morning activity in <i>Pet-1</i>^−/− mice.</p