Molecular genetic analysis of circadian rhythms in vertebrates and invertebrates

Invertebrate models in addiction research

While drug addiction is a uniquely human problem, most research examining the biological mechanisms of the transition from substance use to addiction is conducted with vertebrate animal models. Many other fields of neuroscience have greatly benefitted from contributions from simple and manipulable invertebrate model systems. However, the potential of invertebrate research has yet to be fully capitalised on in the field of addiction neuroscience. This may be because of the complexity of addiction and the clinical imperative of addiction research. We argue that the homocentric diagnostic criteria of addiction are no more a hindrance to the use of invertebrate models than they are to vertebrate models. We highlight the strengths of the diversity of different invertebrate model systems in terms of neuroanatomy and molecular machinery, and stress that working with a range of different models will aid in understanding addiction and not be a disadvantage. Finally, we discuss the specific advantages of utilising invertebrate animals for addiction research and highlight key areas in which invertebrates are suited for making unique and meaningful contributions to this field.

The dilemmas of the gourmet fly: the molecular and neuronal mechanisms of feeding and nutrient decision making in Drosophila

To survive and successfully reproduce animals need to maintain a balanced intake of nutrients and energy. The nervous system of insects has evolved multiple mechanisms to regulate feeding behavior. When animals are faced with the choice to feed, several decisions must be made: whether or not to eat, how much to eat, what to eat, and when to eat. Using Drosophila melanogaster substantial progress has been achieved in understanding the neuronal and molecular mechanisms controlling feeding decisions. These feeding decisions are implemented in the nervous system on multiple levels, from alterations in the sensitivity of peripheral sensory organs to the modulation of memory systems. This review discusses methodologies developed in order to study insect feeding, the effects of neuropeptides and neuromodulators on feeding behavior, behavioral evidence supporting the existence of internal energy sensors, neuronal and molecular mechanisms controlling protein intake, and finally the regulation of feeding by circadian rhythms and sleep. From the discussed data a conceptual framework starts to emerge which aims to explain the molecular and neuronal processes maintaining the stability of the internal milieu.

PER and TIM inhibit the DNA binding activity of a Drosophila CLOCK-CYC/dBMAL1 heterodimer without disrupting formation of the heterodimer: a basis for circadian transcription

The Drosophila CLOCK (dCLOCK) and CYCLE (CYC) (also referred to as dBMAL1) proteins are members of the basic helix-loop-helix PAS (PER-ARNT-SIM) superfamily of transcription factors and are required for high-level expression of the circadian clock genes period (per) and timeless (tim). Several lines of evidence indicate that PER, TIM, or a PER-TIM heterodimer somehow inhibit the transcriptional activity of a putative dCLOCK-CYC complex, generating a negative-feedback loop that is a core element of the Drosophila circadian oscillator. In this report we show that PER and/or TIM inhibits the binding of a dCLOCK-CYC heterodimer to an E-box-containing DNA fragment that is present in the 5' nontranscribed region of per and acts as a circadian enhancer element. Surprisingly, inhibition of this DNA binding activity by PER, TIM, or both is not accompanied by disruption of the association between dCLOCK and CYC. The results suggest that the interaction of PER, TIM, or both with the dCLOCK-CYC heterodimer induces a conformational change or masks protein regions in the heterodimer, leading to a reduction in DNA binding activity. Together with other findings, our results strongly suggest that daily cycles in the association of PER and TIM with the dCLOCK-CYC complex probably contribute to rhythmic expression of per and tim.

timrit Lengthens circadian period in a temperature-dependent manner through suppression of PERIOD protein cycling and nuclear localization

A fundamental feature of circadian clocks is temperature compensation of period. The free-running period of ritsu (timrit) (a novel allele of timeless [tim]) mutants is drastically lengthened in a temperature-dependent manner. PER and TIM protein levels become lower in timrit mutants as temperature becomes higher. This mutation reduces per mRNA but not tim mRNA abundance. PER constitutively driven by the rhodopsin1 promoter is lowered in rit mutants, indicating that timrit mainly affects the per feedback loop at a posttranscriptional level. An excess of per+ gene dosage can ameliorate all rit phenotypes, including the weak nuclear localization of PER, suggesting that timrit affects circadian rhythms by reducing PER abundance and its subsequent transportation into nuclei as temperature increases.

Drosophila photoreceptors contain an autonomous circadian oscillator that can function without period mRNA cycling

Circadian oscillations in period (per) mRNA and per protein (PER) constitute, in part, a feedback loop that is required for circadian pacemaker function in Drosophila melanogaster. Oscillations in PER are required for oscillations in per mRNA, but the converse has not been rigorously tested because of a lack of measurable quantities of per mRNA and protein in the same cells. This circadian feedback loop operates synchronously in many neuronal and non-neuronal tissues, including a set of lateral brain neurons (LNs) that mediate rhythms in locomotor activity, but whether a hierarchy among these tissues maintains this synchrony is not known. To determine whether per mRNA cycling is necessary for PER cycling and whether cyclic per gene expression is tissue autonomous, we have generated per01 flies carrying a transgene that constitutively expresses per mRNA specifically in photoreceptors, a cell type that supports feedback loop function. These transformants were tested for different aspects of feedback loop function including per mRNA cycling, PER cycling, and PER nuclear localization. Under both light/dark (LD) cycling and constant dark (DD) conditions, PER abundance cycles in the absence of circadian cycling of per mRNA. These results show that per mRNA cycling is not required for PER cycling and indicate that Drosophila photoreceptors R1-R6 contain a tissue autonomous circadian oscillator.

Quantitative analysis of Drosophila period gene transcription in living animals

To determine the in vivo regulatory pattern of the clock gene period (per), the authors recently developed transgenic Drosophila carrying a luciferase cDNA fused to the promoter region of per. They have now carried out noninvasive, high time-resolution experiments allowing high-throughput monitoring of circadian bioluminescence rhythms in individual living adults for several days. This immediately solved several problems (resulting directly from individual asynchrony within a population) that have accompanied previous biochemical experiments in which groups of animals were sacrificed at each time point. Furthermore, the authors have developed numerical analysis methods for automatically determining rhythmicity associated with bioluminescence records from single flies. This has revealed some features of per gene transcription that were previously unappreciated and provides a general strategy for the analysis of rhythmic time series in the study of molecular rhythms.

Mosaic analysis in the Drosophila CNS of circadian and courtship-song rhythms affected by a period clock mutation

The period gene in Drosophila melanogaster controls not only daily rhythms associated with adult emergence and behavior, but also a much higher frequency rhythm that accompanies the male's courtship song. This oscillation in the rate of sound production (normal period, ca. one minute) is either sped up (by perS), slowed down, or eliminated in the three classic per mutants. We have conducted a mosaic analysis in which both lovesong and circadian locomotor cycles were examined in a series of flies that were each part perS and part per+. Consistent with previous studies, the focus for per control of the adult's circadian rhythm of locomotion was found to be in the brain. However, several mosaic individuals were found to exhibit a mutant locomotor rhythm but a wild-type song cycle, or vice-versa, enabling us provisionally to map the song-rhythm focus to the thoracic ganglia. That per is expressed only in glial cells in the thoracic nervous system and, in general, mediates slow (hour-by-hour) fluctuations in the levels of its own products are discussed from the standpoint of the current mosaic mapping results and the renewed focus they bring to the gene's influence on an ultradian rhythm.