Effect of pigment dispersing hormone on the electrical activity of crayfish visual photoreceptors during the 24-h cycle

A Circadian Rhythm of Visual Sensitivity in the American Lobster, Homarus americanus

AbstractTo determine whether eyes of American lobsters (Homarus americanus) are more sensitive to light at night than during the day, electroretinograms were continuously recorded from 23 adult lobsters for at least 3 days (range: 3 to 9 days) in constant darkness. A green light-emitting diode, mounted 10 cm away from the eyes, was briefly flashed every 2 minutes to evoke the electroretinogram. The average increase in the response to a light flash, between the minimum during the subjective day and the maximum during the subjective night, was 105.6% ± 38.8%; and there was a statistically significant difference between day and night responses. This change in visual sensitivity took place while lobsters were held in constant darkness, suggesting that it was due to the influence of a circadian clock. The average period (tau) for the 10 animals that expressed significant circadian rhythms was 23.4 ± 0.8 hours. Previous studies have demonstrated that lobsters have circadian clocks that influence their locomotor activity; and the present data suggest that this is also true for their eyes, leading to an increase in their visual sensitivity at night, when they are typically most active.

Review: Use of Electrophysiological Techniques to Study Visual Functions of Aquatic Organisms

The light environments of natural water sources have specific characteristics. For the majority of aquatic organisms, vision is crucial for predation, hiding from predators, communicating information, and reproduction. Electroretinography (ERG) is a diagnostic method used for assessing visual function. An electroretinogram records the comprehensive potential response of retinal cells under light stimuli and divides it into several components. Unique wave components are derived from different retinal cells, thus retinal function can be determined by analyzing these components. This review provides an overview of the milestones of ERG technology, describing how ERG is used to study visual sensitivity (e.g., spectral sensitivity, luminous sensitivity, and temporal resolution) of fish, crustaceans, mollusks, and other aquatic organisms (seals, sea lions, sea turtles, horseshoe crabs, and jellyfish). In addition, it describes the correlations between visual sensitivity and habitat, the variation of visual sensitivity as a function of individual growth, and the diel cycle changes of visual sensitivity. Efforts to identify the visual sensitivity of different aquatic organisms are vital to understanding the environmental plasticity of biological evolution and for directing aquaculture, marine fishery, and ecosystem management.

Involvement of Melatonin in the Regulation of the Circadian System in Crayfish

Melatonin (MEL) is an ancient molecule, broadly distributed in nature from unicellular to multicellular species. MEL is an indoleamine that acts on a wide variety of cellular targets regulating different physiological functions. This review is focused on the role played by this molecule in the regulation of the circadian rhythms in crayfish. In these species, information about internal and external time progression might be transmitted by the periodical release of MEL and other endocrine signals acting through the pacemaker. We describe documented and original evidence in support of this hypothesis that also suggests that the rhythmic release of MEL contributes to the reinforcement of the temporal organization of nocturnal or diurnal circadian oscillators. Finally, we discuss how MEL might coordinate functions that converge in the performance of complex behaviors, such as the agonistic responses to establish social dominance status in Procambarus clarkii and the burrowing behavior in the secondary digging crayfish P. acanthophorus.

Desensitization and recovery of crayfish photoreceptors. Dependency on circadian time, and pigment-dispersing hormone

In this work, we studied the characteristics of recovery from desensitization of the light-elicited current of crayfish. Applying a two-flash protocol, we found that the first flash triggers a current that activates with a noticeable latency, reaches a peak value, and thereafter decays along a single exponential time course. In comparison with the first-elicited current, the current elicited by the second flash not only presents an expected smaller peak current, depending on the time between flashes, but it also displays a different latency and decay time constant. Recovery of the first flash values of these current parameters depends on the circadian time at which the experiments are conducted, and on the presence of pigment-dispersing hormone. Our data also suggest the existence of distinctive desensitized states, whose induction depends on circadian time and the presence of pigment-dispersing hormone.

Chronic electromyographic analysis of circadian locomotor activity in crayfish

Animals generally exhibit circadian rhythms of locomotor activity. They initiate locomotor behavior not only reflexively in response to external stimuli but also spontaneously in the absence of any specific stimulus. The neuronal mechanisms underlying circadian locomotor activity can, therefore, be based on the rhythmic changes in either reflexive efficacy or endogenous activity. In crayfish Procambarus clarkii, it can be determined by analyzing electromyographic (EMG) patterns of walking legs whether the walking behavior is initiated reflexively or spontaneously. In this study, we examined quantitatively the leg muscle activity that underlies the locomotor behavior showing circadian rhythms in crayfish. We newly developed a chronic EMG recording system that allowed the animal to freely behave under a tethered condition for more than 10 days. In the LD condition in which the animals exhibited LD entrainment, the rhythmic burst activity of leg muscles for stepping behavior was preceded by non-rhythmic tonic activation that lasted for 1323±488ms when the animal initiated walking. In DD and LL free-running conditions, the pre-burst activation lasted for 1779±31 and 1517±39ms respectively. In the mechanical stimulus-evoked walking, the pre-burst activation ended within 79±6ms. These data suggest that periodic changes in the crayfish locomotor activity under the condition of LD entrainment or free-running are based on activity changes in the spontaneous initiation mechanism of walking behavior rather than those in the sensori-motor pathway connecting mechanoreceptors with leg movements.