The elevated "zero-maze" is a modification of the elevated plus-maze model of anxiety in rats which incorporates both traditional and novel ethological measures in the analysis of drug effects. The novel design comprises an elevated annular platform with two opposite enclosed quadrants and two open, removing any ambiguity in interpretation of time spent on the central square of the traditional design and allowing uninterrupted exploration. Using this model, the reference benzodiazepine anxiolytics, diazepam (0.125-0.5 mg/kg) and chlordiazepoxide (0.5-2.0 mg/kg) significantly increased the percentage of time spent in the open quadrants (% TO) and the frequency of head dips over the edge of the platform (HDIPS), and reduced the frequency of stretched attend postures (SAP) from the closed to open quadrants. In contrast, the anxiogenic drug m-chlorophenyl-piperazine (mCPP; 0.25-1.0 mg/kg) induced the opposite effects, decreasing %TO and HDIPS, and increasing SAP. The 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.001-0.1 mg/kg) had no effects on either %TO or HDIPS, but did decrease SAP at 0.01 mg/kg although not at higher or lower doses. Similarly, the 5-HT3 receptor antagonist, ondansetron (0.0001-1.0 mg/kg) decreased SAP and increased %TO at 0.01 mg/kg, but not at other doses. The present data suggest that a combination of the novel "zero-maze" design and a detailed ethological analysis provides a sensitive model for the detection of anxiolytic/anxiogenic drug action.

No abstract

Regulating ribosome number is thought to control cellular growth. Synthesis of ribosomal RNA (rRNA) is a limiting step in ribosome biogenesis and rates of rRNA synthesis are generally altered depending on the growth status of a cell. Although studies in unicellular systems have addressed the mechanisms by which this occurs, few studies have applied a genetic approach to examine growth-dependent control of rRNA synthesis in metazoans. Here, we show that in Drosophila melanogaster Myc (dMyc) is a regulator of rRNA synthesis. Expression of dMyc is both necessary and sufficient to control rRNA synthesis and ribosome biogenesis during larval development. Stimulation of rRNA synthesis by dMyc is mediated through a rapid, coordinated increase in the levels of the Pol I transcriptional machinery. In addition, the growth effects of dMyc in larval wing imaginal discs require de novo rRNA synthesis. We suggest that during animal development, the control of rRNA synthesis and ribosome biogenesis is an essential Myc function.

No abstract

Endocycles are variant cell cycles comprised of DNA Synthesis (S)- and Gap (G)- phases but lacking mitosis1,2. Such cycles facilitate post-mitotic growth in many invertebrate and plant cells, and are so ubiquitous that they may account for up to half the world’s biomass3,4. DNA replication in endocycling Drosophila cells is triggered by Cyclin E/Cyclin Dependent Kinase 2 (CycE/Cdk2), but this kinase must be inactivated during each G-phase to allow the assembly of pre-Replication Complexes (preRCs) for the next S-phase5,6. How CycE/Cdk2 is periodically silenced to allow re-replication has not been established. Here, using genetic tests in parallel with computational modeling, we show that Drosophila’s endocycles are driven by a molecular oscillator in which the E2F1 transcription factor promotes CycE expression and S-phase initiation, S-phase then activates the CRL4Cdt2 ubiquitin ligase, and this in turn mediates the destruction of E2F17. We propose that it is the transient loss of E2F1 during S-phases that creates the window of low Cdk activity required for preRC formation. In support of this model over-expressed E2F1 accelerated endocycling, whereas a stabilized variant of E2F1 blocked endocycling by de-regulating target genes including CycE, as well as Cdk1 and mitotic Cyclins. Moreover, we find that altering cell growth by changing nutrition or TOR signaling impacts E2F1 translation, thereby making endocycle progression growth-dependent. Many of the regulatory interactions essential to this novel cell cycle oscillator are conserved in animals and plants1,2,8, suggesting that elements of this mechanism act in most growth-dependent cell cycles.