Identification of three proteins in the eye of Aplysia, whose synthesis is altered by serotonin (5-HT). Possible involvement of these proteins in the ocular circadian system

Stress-induced thermotolerance of ventilatory motor pattern generation in the locust, Locusta migratoria

Ventilation is a crucial motor activity that provides organisms with an adequate circulation of respiratory gases. For animals that exist in harsh environments, an important goal is to protect ventilation under extreme conditions. Heat shock, anoxia, and cold shock are environmental stresses that have previously been shown to trigger protective responses. We used the locust to examine stress-induced thermotolerance by monitoring the ability of the central nervous system to generate ventilatory motor patterns during a subsequent heat exposure. Preparations from pre-stressed animals had an increased incidence of motor pattern recovery following heat-induced failure, however, prior stress did not alter the characteristics of the ventilatory motor pattern. During constant heat exposure at sub-lethal temperatures, we observed a protective effect of heat shock pre-treatment. Serotonin application had similar effects on motor patterns when compared to prior heat shock. These studies are consistent with previous studies that indicate prior exposure to extreme temperatures and hypoxia can protect neural operation against high temperature stress. They further suggest that the protective mechanism is a time-dependent process best revealed during prolonged exposure to extreme temperatures and is mediated by a neuromodulator such as serotonin.

Translational and transcriptional inhibitors block serotonergic phase advances of the suprachiasmatic nucleus circadian pacemaker in vitro

The mammalian circadian pacemaker is located in the suprachiasmatic nucleus (SCN). Various inputs modulate pacemaker phase, including the serotonergic (5HTergic) input from the midbrain raphe. 5HT phase-advances the SCN pacemaker when applied during mid subjective day. In vitro studies indicate that 5HT advances the mammalian circadian pacemaker through a process that includes stimulating 5HT7 receptors, activating protein kinase A, and opening K+ channels. How these cytoplasmic and membrane events translate into a shift in the molecular core of the circadian oscillator is not known. To further understand this process, the authors investigated whether 5HTergic phase advances require transcription or translation. Using two reversible translational inhibitors, anisomycin and cycloheximide, the authors show that inhibiting protein synthesis blocks 5HTergic phase shifts. The authors further show that the transcriptional inhibitor 5,6-dichloro-1-beta-ribobenzimidazole also blocks 5HTergic phase shifts. These results are similar to those found previously with respect to 5HTergic modulation of the Aplysia ocular circadian clock, and suggest that 5HT may phase-shift the SCN pacemaker through increasing transcription and translation of specific proteins.

The SCN is comprised of a population of coupled oscillators

The circadian clock in the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus exhibits two necessary properties: (1) a mechanism for the generation of autonomous circadian rhythms in individual pacemaker cells, and (2) a means to synchronize the autonomous pacemaker cells. A variety of potential components of the endogenous pacemaker, including ion channels, second messengers, transcriptional factors, and the protein targets of kinases and transcription factors are reviewed. Similarly, reverse transmitter transport, extracellular ion fluxes, small membrane-diffusible molecules, glial regulation, and neural adhesion molecules are considered as possible synchronizing factors. Provisional criteria are suggested for empirical distinction of endogenous pacemaker versus synchronizing mechanisms.

Circadian and ultradian clock-controlled rhythms in unicellular microorganisms

The time structure of a biological system is at least as intricate as its spatial structure. Whereas we have detailed information about the latter, our understanding of the former is still rudimentary. As techniques for monitoring intracellular processes continuously in single cells become more refined, it becomes increasingly evident that periodic behaviour abounds in all time domains. Circadian timekeeping dominates in natural environments. Here the free-running period is about 24 h. Circadian rhythms in eukaryotes and prokaryotes allow predictive matching of intracellular states with environmental changes during the daily cycles. Unicellular organisms provide excellent systems for the study of these phenomena, which pervade all higher life forms. Intracellular timekeeping is essential. The presence of a temperature-compensated oscillator provides such a timer. The coupled outputs (epigenetic oscillations) of this ultradian clock constitute a special class of ultradian rhythm. These are undamped and endogenously driven by a device which shows biochemical properties characteristic of transcriptional and translational elements. Energy-yielding processes, protein turnover, motility and the timing of the cell-division cycle processes are all controlled by the ultradian clock. Different periods characterize different species, and this indicates a genetic determinant. Periods range from 30 min to 4 h. Mechanisms of clock control are being elucidated; it is becoming evident that many different control circuits can provide these functions.

Heat shock proteins and circadian rhythms

Significant circadian rhythms in heat shock gene expression were observed in a prokaryotic species (Synechocystis). In eukaryotes, in contrast, several heat shock genes (constitutive and inducible) were shown to be constantly expressed. A few cases of circadian expression of heat shock proteins (HSPs), however, have been reported. Significant circadian changes of thermotolerance were observed in yeast and several plant species. Higher thermotolerance can be attributed to a higher abundance of HSPs, but also to other adaptive mechanisms. Zeitgeber effects of temperature changes can be explained on the basis of their direct effects on the state variables of the clock gene (per,frq) expression and its negative feedback loop. Effects of increased HSP concentrations, as observed after heat shock, but also after light and serotonin (5HT), appear possible, in particular with respect to nuclear localization of the clock (PER) protein, but these effects have not been documented yet. Thus, the role of HSPs in the circadian clock system is little understood and, from our point of view, deserves more attention.

The use of a reversible transcription inhibitor, DRB, to investigate the involvement of specific proteins in the ocular circadian system of Aplysia

Previously, the effects of 2-h treatments with the reversible transcription inhibitor 5,6-dichloro-1-beta-D-ribobenzimidazole (DRB) on the phase of the circadian rhythm in the eye of Aplysia californica were studied. Here we report a study of the effects of DRB on protein synthesis and a more detailed investigation of the effects of DRB on the phase of the circadian rhythm. Treatments of DRB for 30 min reduced the rate of transcription to about 30% of control values, and this inhibition reversed completely within 2 h after the end of the treatment. A phase-response curve was obtained for 30-min treatments of DRB. Shorter (30 min) treatments with DRB produced phase shifts comparable to those produced by treatments with DRB for 2 h. The phase-response curve obtained using 30-min treatments of DRB was similar to one obtained using 2-h treatments with respect to the phase at which DRB exerts its maximum effect on the rhythm (around circadian time [CT] 6). However, some aspects of the two phase-response curves were different. The effect of DRB on the phase of the rhythm appeared rapidly after removal of DRB treatments given during CT 22-6, but the effects of DRB on the phase of the rhythm appeared more slowly (approximately 10 h) after the treatments given during CT 6-12. Because the effects of DRB on the phase of the overt rhythm appear to be rapid at a particular phase, it is very likely that DRB affects the phase of the rhythm by altering the synthesis of proteins during or shortly after the treatment. Thus we searched for proteins whose synthesis was altered by DRB. Incorporation of labeled amino acids into 2 proteins was found to be altered during the DRB treatment, whereas 15 proteins were affected after the DRB treatment. Among the proteins affected during or shortly after the DRB treatment were four previously identified proteins affected by other treatments that can shift the phase of the eye circadian rhythm. These four proteins are worthy of further study as possible candidates for components of the circadian oscillator.