Entrainment of circadian rhythms to irregular light/dark cycles: A subterranean perspective

Synchronization of biological rhythms to the 24-hour day/night has long been studied with model organisms, under artificial light/dark cycles in the laboratory. The commonly used rectangular light/dark cycles, comprising hours of continuous light and darkness, may not be representative of the natural light exposure for most species, including humans. Subterranean rodents live in dark underground tunnels and offer a unique opportunity to investigate extreme mechanisms of photic entrainment in the wild. Here, we show automated field recordings of the daily light exposure patterns in a South American subterranean rodent, the tuco-tuco (Ctenomys aff. Knighti). In the laboratory, we exposed tuco-tucos to a simplified version of this natural light exposure pattern, to determine the minimum light timing information that is necessary for synchronization. As predicted from our previous studies using mathematical modeling, the activity rhythm of tuco-tucos synchronized to this mostly simplified light/dark regimen consisting of a single light pulse per day, occurring at randomly scattered times within a day length interval. Our integrated semi-natural, lab and computer simulation findings indicate that photic entrainment of circadian oscillators is robust, even in face of artificially reduced exposure and increased phase instability of the synchronizing stimuli.Fil: Flôres, Danilo E. F. L.. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; BrasilFil: Jannetti, Milene G.. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; BrasilFil: Valentinuzzi, Verónica Sandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; ArgentinaFil: Oda, Gisele Akemi. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; Brasi

Modeling natural photic entrainment in a subterranean rodent (Ctenomys aff. knighti), the Tuco-Tuco.

Subterranean rodents spend most of the day inside underground tunnels, where there is little daily change in environmental variables. Our observations of tuco-tucos (Ctenomys aff. knighti) in a field enclosure indicated that these animals perceive the aboveground light-dark cycle by several bouts of light-exposure at irregular times during the light hours of the day. To assess whether such light-dark pattern acts as an entraining agent of the circadian clock, we first constructed in laboratory the Phase Response Curve for 1 h light-pulses (1000lux). Its shape is qualitatively similar to other curves reported in the literature and to our knowledge it is the first Phase Response Curve of a subterranean rodent. Computer simulations were performed with a non-linear limit-cycle oscillator subjected to a simple model of the light regimen experienced by tuco-tucos. Results showed that synchronization is achieved even by a simple regimen of a single daily light pulse scattered uniformly along the light hours of the day. Natural entrainment studies benefit from integrated laboratory, field and computational approaches

Telling the Seasons Underground: The Circadian Clock and Ambient Temperature Shape Light Exposure and Photoperiodism in a Subterranean Rodent

Living organisms anticipate the seasons by tracking the proportion of light and darkness hours within a day—photoperiod. The limits of photoperiod measurement can be investigated in the subterranean rodents tuco-tucos (Ctenomys aff. knighti), which inhabit dark underground tunnels. Their exposure to light is sporadic and, remarkably, results from their own behavior of surface emergence. Thus, we investigated the endogenous and exogenous regulation of this behavior and its consequences to photoperiod measurement. In the field, animals carrying biologgers displayed seasonal patterns of daily surface emergence, exogenously modulated by temperature. In the laboratory, experiments with constant lighting conditions revealed the endogenous regulation of seasonal activity by the circadian clock, which has a multi-oscillatory structure. Finally, mathematical modeling corroborated that tuco-tuco’s light exposure across the seasons is sufficient for photoperiod encoding. Together, our results elucidate the interrelationship between the circadian clock and temperature in shaping seasonal light exposure patterns that convey photoperiod information in an extreme photic environment.Fil: Flôres, Danilo E. F. L.. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; BrasilFil: Jannetti, Milene G.. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; BrasilFil: Carreira Improta, Giovane. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; BrasilFil: Tachinardi, Patricia. Universidade de Sao Paulo, Faculdade de Medicina Veterinária e Zootecnia; BrasilFil: Valentinuzzi, Verónica Sandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Universidad Nacional de Catamarca. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Secretaría de Industria y Minería. Servicio Geológico Minero Argentino. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja. - Provincia de La Rioja. Centro Regional de Investigaciones Científicas y Transferencia Tecnológica de La Rioja; ArgentinaFil: Oda, Gisele Akemi. Universidade de Sao Paulo. Departamento de Fisiología. Instituto de Biociencias; Brasi

Dynamics of the model oscillator under different simulated light-regimens.

<p>The upper-left actogram presents the oscillator in a free-running condition, under a simulated constant darkness (DD). The following graphs show the oscillator under simulated pulse regimens. Except for the DD condition, values over the actograms indicate the duration, in hours, of the time-interval I when the pulses occur. Pulse-times are represented by white dots over the gray background representing a simulated dim background illumination. A consistent 24 h-period is only visualized up to the time-window duration of 12 h. The abscissas for all actograms are the same as indicated under the “I₌12 h” graph.</p

Light-dark regimen experienced by a tuco-tuco in the field enclosure and a simplified model reproduced in computer simulations.

<p>A: Field data obtained by continuous observations of an animal in a semi-natural enclosure for 12 days, during the light-hours of the day (modified from Tomotani et al., 2012) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0068243#pone.0068243-Tomotani1" target="_blank">[3]</a>. In the upper graphic (actogram) white marks indicate the light-exposure episodes (aboveground activity) superimposed on a darkness background (animal underground and/or at night). The time of light exposure in the field varied from day to day. It was however more probable at the beginning of the day, as revealed by the sum of exposure from all days (lower graphic). B: Our simplified model of light-exposure, consisting of a single 1 h-light-pulse per day, whose time changed every day (upper graphic) in a uniformly random fashion (lower graphic), but always restricted to a time-window I (graphic on the right).</p

Photic Phase Response Curve (PRC) of the tuco-tuco.

<p>A: Representative phase-shifts in response to light-pulses. The actograms show three examples of free-running rhythms of tuco-tucos under DD conditions and different phase-shift responses of the rhythms elicited by a single 1 h-light-pulse (1000 lux) on day 28 (open circle). In each graph, the phase of activity onsets after the pulse (lower dashed line) is compared to the phase of activity onsets previous to the pulse (upper dashed line). In response the light stimuli, activity phase was either advanced (left graph), delayed (middle graph) or unshifted (right graph). Values on the upper left of each graph are quantifications, in hours, of the phase-shift responses. The upper right numbers are identification codes for individual animals. B: The magnitudes of phase-shift responses are plotted against the time of the applied light-pulse. The time axis is represented in a standardized time-scale (circadian time). Inset: single values for each of the phase-shift responses. Outer graph: mean phase-shifts ±1 standard deviation for every 3 CT’s. Both graphs illustrate the dependence of the magnitude and sign of phase-shift on the time of the pulse.</p

Synchronization of the wheel-running activity rhythm of a tuco-tuco by light dark cycles LD 1∶23 (A) and LD 12∶12 (B).

<p>The data is depicted in actograms, with times of running-wheel activity (black and dark-gray marks) represented along the days. Dark-gray marks represent free-running rhythms in the first 15 days in DD. White rectangles drawn in the graphics during the LD cycles represent the hours of lights-on. Both light-dark cycles LD 1∶23 and LD 12∶12 synchronize the rhythms of tuco-tucos to a 24 h-period. Figure B was modified from Valentinuzzi et al. (2009) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0068243#pone.0068243-Valentinuzzi1" target="_blank">[1]</a>.</p