The discovery of circadian clock genes and the use of similar strategies to discover unknown genes underlying complex behaviors and brain disorders.

Over just the past few years, tremendous progress has been made in unraveling the molecular basis of the circadian clock in mammals. This success has been primarily due to an approach whereby mutations are induced randomly in the germ line and the offspring of the mutagenized animals are tested for abnormal circadian phenotype. Circadian clock genes have been discovered this way in both fruit flies and mice and it is now clear that most, if not all clock genes show homology between flies and mammals, including humans. This 'forward genetics' approach is a powerful tool for uncovering genes which underline complex behaviors and brain disorders. Even when a complex neural function involves many, many genes, mutating just one of these genes can have pronounced effects on the expressed behavior and can lead to the discovery of other genes, and their protein products, that interact directly or indirectly with the mutated gene.

Metabolic influences on circadian rhythmicity in Siberian and Syrian hamsters exposed to long photoperiods.

Calorie restriction and other situations of reduced glucose availability in rodents alter the entraining effects of light on the circadian pacemaker located in the suprachiasmatic nuclei. Siberian and Syrian hamsters are photoperiodic species that are sexually active when exposed to long summer-like photoperiods, while both species show opposite changes in body mass when transferred from long to short or short to long days. Because metabolic cues may fine tune the photoperiodic responses via the suprachiasmatic nuclei, we tested whether timed calorie restriction can alter the photic synchronization of the light-entrainable pacemaker in these two hamster species exposed to long photoperiods. Siberian and Syrian hamsters were exposed to 16 h:8 h light:dark cycles and received daily hypocaloric (75% of daily food intake) or normocaloric diet (100% of daily food intake) 4 h after light onset. Four weeks later, hamsters were transferred to constant darkness and fed ad libitum. The onset of the nocturnal pattern of locomotor activity was phase advanced by 1.5 h in calorie-restricted Siberian hamsters, but not in Syrian hamsters. The lack of phase change in calorie-restricted Syrian hamsters was also observed in individuals exposed to 14 h:10 h dim light:dark cycles and fed with lower hypocaloric food (i.e. 60% of daily food intake) 2 h after light onset. Moreover, in hamsters housed in constant darkness and fed ad lib., light-induced phase shifts of the locomotor activity in Siberian hamsters, but not in Syrian hamsters were significantly reduced when glucose utilization was blocked by pretreatment with 500 mg/kg i.p. 2-deoxy-D-glucose. Taken together, these results show that the photic synchronization of the light-entrainable pacemaker can be modulated by metabolic cues in Siberian hamsters, but not in Syrian hamsters maintained on long days.

The search for circadian clock and sleep genes.

In recent years, there has been extraordinary progress in elucidating the molecular components of the mammalian circadian clock system. The discovery of circadian clock genes in lower organisms (such as fruit flies and fungi), which show many similarities with clock genes in mammals, together with advances in mouse molecular genetics have led to major new discoveries on the molecular and genetic basis of mammalian circadian rhythms. This article reviews both of these lines of research from an historical perspective and discusses how these lines have merged to provide unique insights into the molecular mechanisms of circadian function. The review also speculates on how the discovery of circadian clock genes may lead directly or indirectly to the discovery of mammalian sleep genes. The determination of the molecular mechanisms via which circadian clock genes (and their protein products) regulate the timing and the need for sleep, and the identification of new genes involved in sleep regulation, may produce new information on the genetic and molecular control of sleep which could ultimately lead to the development of new treatments for sleep disorders.

Chronic circadian desynchronization decreases the survival of animals with cardiomyopathic heart disease.

Shift work is associated with increased cardiovascular morbidity and mortality. Whereas it has been suggested that continuous shifting of the circadian clock/sleep-wake cycle may have negative effects on health, there is very little experimental evidence to support such a hypothesis. Cardiomyopathic Syrian hamsters were either maintained on a fixed light-dark (LD) cycle (n = 31) or were subjected to a 12-h phase shift in the LD cycle on a weekly basis (n = 32). The duration of the life span was recorded for each animal. Chronic reversal of the external LD cycle at weekly intervals resulted in a significant decrease in the survival time in cardiomyopathic hamsters with the median life span being reduced by 11%. Disrupting normal circadian rhythmicity in an animal susceptible to early mortality due to cardiac disease results in a further decrease in longevity. The deleterious effects of the chronic phase shifts in the LD cycle in cardiomyopathic hamsters may be related to reports of increased cardiovascular morbidity and mortality in humans engaged in shift work.

Automated measurement of mouse freezing behavior and its use for quantitative trait locus analysis of contextual fear conditioning in (BALB/cJ x C57BL/6J)F2 mice.

The most commonly measured mouse behavior in fear conditioning tests is freezing. A technical limitation, particularly for genetic studies, is the method of direct observation used for quantifying this response, with the potential for bias or inconsistencies. We report the use of a computerized method based on latency between photobeam interruption measures as a reliable scoring criterion in mice. The different computer measures obtained during contextual fear conditioning tests showed high correlations with hand-scored freezing; r values ranged from 0.87 to 0.94. Previously reported strain differences between C57BL/6J and DBA/2J in context-dependent fear conditioning were also detected by the computer-based system. In addition, the use of computer-scored freezing of 199 (BALB/cJ x C57BL/6J)F2 mice enabled us to detect a suggestive gender-dependent chromosomal locus for contextual fear conditioning on distal chromosome 8 by QTL analysis. Automation of freeze scoring would significantly increase efficiency and reliability of this learning and memory test.