Evidence of circadian rhythm of electric discharge in Eigenmannia virescens system

Cell signaling, receptors, electrical effects and therapy in circadian rhythm

Circadian rhythm has been the object of much attention. This review addresses the aspects of cell signaling, receptors, therapy and electrical effects in a multifaceted fashion. The pineal gland, which produces the important hormones melatonin and serotonin, exerts a prominent influence, in addition to the supraschiasmatic nucleus. Many aspects involve free radicals which have played a widespread role in biochemistry.

Environmental, seasonal, and social modulations of basal activity in a weakly electric fish

The electric organ discharge (EOD) of weakly electric fish encodes information about species, sex, behavioral, and physiological states throughout the lifetime. Its central command is crucial for sensory-motor coordination, and is therefore the target of plastic mechanisms that adapt fish to environmental and social challenges. The EOD waveform of Brachyhypopomus pinnicaudatus is modulated by environmental factors and the neuroendocrine system. In this study we investigate the effects of water temperature and day-night cycle upon EOD rate in this species during the breeding and non-breeding seasons. During the non-breeding season, EOD rate is a linear function of water temperature and exhibits counterclockwise hysteresis. During breeding, a thermal resistance strategy prevents the decrease of EOD rate to cooling. A nocturnal increase of EOD basal rate independent of water temperature and locomotor activity was demonstrated in isolated non-breeding adults and in male-female dyads all year round. An additional increase of nocturnal EOD rate, probably acting as a social courtship signal, was found in breeding dyads. This additional increase of nocturnal EOD rate could not be fully explained by gonadal maturation and was modulated by social stimuli. This study provides novel data on the complex interactions between environment, reproductive cycle, social behavior, and electromotor output in an advantageous model of the vertebrate central nervous system.

Circadian rhythms in electric waveform structure and rate in the electric fish Brachyhypopomus pinnicaudatus

Weakly electric fish have long been known to express day-night oscillations in their discharge rates, and in the amplitude and duration of individual electric organ discharges (EODs). Because these oscillations are altered by social environment and neuroendocrine interactions, electric fish are excellent organisms for exploring the social and neuroendocrine regulation of circadian rhythm expression. Previous studies asserting that these oscillations are circadian rhythms have been criticized for failing to control temperature and randomize feeding regimes, or for running the fish under constant conditions for just 2-3 days. Here we show that the day-night oscillations in the EODs of the neotropical gymnotiform fish Brachyhypopomus pinnicaudatus free-run for over a week under constant photic and thermal conditions, and randomized food provisioning. Sex differences were apparent in strength and magnitude of the circadian oscillations; male oscillations were stronger and larger. All three parameters retain a common oscillation period while differing in the persistence of oscillation strength and magnitude, a difference consistent with proposals by others that declines of behavioral circadian rhythms may result from breakdowns downstream of the central oscillator.

Characterization of diurnal urea excretion in the mangrove killifish,Rivulus marmoratus

An unusual characteristic of nitrogen excretion in the ammoniotelic mangrove killifish Rivulus marmoratus is that urea is excreted(Jurea) in a distinct diurnal pattern, whereas ammonia is excreted (Jamm) at a steady rate. In this study we tested the hypothesis that the diurnal pattern in R. marmoratus is an endogenously generated pattern that is characterized as a circadian rhythm. This hypothesis was tested by measuring Jurea and Jamm following manipulation of feeding or lighting regimes. The diurnal Jurea pattern in food-deprived R. marmoratus had a 24 h periodicity under normal conditions of 12 h:12 h light:dark (12:12 L:D) with 72% more urea excreted during 12:00 h and 18:00 h. In contrast, there was no significant pattern in Jamm. Fed fish (12:12 L:D) demonstrated a diurnal pattern in both Jurea and Jamm with up to an eightfold increase in excretion rates compared with rates obtained from food-deprived fish. Patterns of Jurea were free running with a 24 h period under conditions of continuous darkness (0:24 L:D). Exposure to an inverse photoperiod (12:12 D:L) resulted in entrainment of the Jurea pattern to the new photoperiod, with the highest rates of excretion occurring during midday of the new photoperiod. In contrast to R. marmoratus, nitrogen excretion rates in the zebrafish Danio rerio remained constant over time. The results of this study show that Jurea in R. marmoratus demonstrates the characteristics of a circadian rhythm: a 24 h periodicity, a free-running rhythm in continuous conditions, and entrainment to new photoperiods.