Ultraviolet Light-Induced Phase Response Curve for the Locomotor Activity Rhythm of the Field Mouse...

96 Naturwissenschaften 86 (1999) Q Springer-Verlag 1999

Naturwissenschaften 86, 96–97 (1999) Springer-Verlag 1999

Ultraviolet Light-Induced Phase Response Curvefor the Locomotor Activity Rhythm of the FieldMouse Mus boodugaVijay Kumar SharmaChronobiology Laboratory, Evolutionary and Organismal Biology Unit,Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O.,Jakkur, Bangalore 560 064, Karnataka, India

Muniyandi Singaravel1, Ramanujam SubbarajDepartment of Animal Behaviour and Physiology, School of BiologicalSciences, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India

Received: 16 June 1998 / Accepted in revised form: 2 November 1998

Correspondence to: V.K. Sharma1Present address: Department of Zoology,Banaras Hindu University, Varanasi, UttarPradesh, India

Ultraviolet-A (UV-A) light pulses of2.5 W/m2 irradiance (at eye level) and30 min duration were administered tofield mice, Mus booduga (np84), at12 circadian phases, each 2 h apart,and the resulting phase shifts weremeasured. These phase shifts wereused to construct the UV-A lightphase response curve (PRC). Maxi-mum phase delays and maximumphase advances were obtained by ex-posing the animals to UV-A at CT14(circadian time 14) and at CT20.These mice when subjected to UV-Alight-dark cycles of 12 :12 h, confinedtheir locomotor activity to the darkperiod with the onset of activity coin-ciding with dark onset. The results ofour experiments suggest that the cir-cadian timing system of the fieldmouse M. booduga is sensitive tonear-UV light and the sensitivity isphase dependent.Recent evidences indicate that thespectral sensitivity of the retina in ro-dents extends into the ultraviolet(UV) range [1–3]. UV light has beenfound to suppress nocturnal releaseof melatonin in Syrian hamsters [4].Other studies have reported that UVlight evokes phase shifts in the bodytemperature and locomotor activityrhythm in Wistar rats [5], locomotor

activity rhythms in golden hamstersand inbred C57BL/6 J laboratorymice [6, 7]. Eventhough the effect oflight on circadian timing systems hasbeen well studied, the mechanisms ofthe photoreception and transductionremains an enigma [6]. Although UVphotoreception may constitute a ma-jor component of photoentrainmentin mammals, especially during dawnand dusk during which the circadianclock is fine-tuned, there are only afew reports of studies in this area[5–7]. None of the previous studiesreport phase-dependent sensitivity toUV light, a prerequisite for stable en-trainment. We assayed the ability ofUV-A light to induce phase shift inthe locomotor activity rhythm of thenocturnal field mouse Mus boodugausing 30-min pulses of UV-A light (ir-radiance at eye level, 2.5 W/m2).Adult male field mice (age approx.90 days; weight 8–13 g; np120) werecaptured from cultivated fields nearthe Madurai Kamaraj Universitycampus (9758bN, 78710bE). Theywere maintained under laboratoryLD cycles of 12 :12 h (lights on at0600 hours and lights off at1800 hours) for 15 days before beingreleased into continuous darkness(DD). The locomotor activity wasmonitored using an activity-runningwheel (diameter approx. 20 cm) at-tached to a transparent plexiglasscage of dimension 0.07 ! 0.11 !0.09 m, with a small opening of 0.02 mdiameter. Reed-relays attached to the

wheels activated the writing stylets ofan Esterline Angus A620X Event Re-corder when the running mice causedrevolutions of the wheel. The activitypatterns of as many as 18 mice in sep-arate running wheels, placed on openshelves in the experimental room(3.05 ! 2.44 ! 4.01 m) could be as-sayed concurrently. The temperatureand the relative humidity inside theserooms were 257B17 C and 75B5%,respectively. Food (millet and grain)and water were available ad libitum.The room was entered at irregular in-tervals, on average once in 2 days forpurposes of cleaning cages, placingfood and water, and administeringlight pulse etc that seldom lastedbeyond 5–10 min. Red light ofl1640 nm obtained with a combina-tion of red and orange filters wasused inside the cubicle. The individu-al values of t were derived using lin-ear regression on the onsets (CT12)of locomotor activity. Phase shiftswere computed using linear-regres-sion on the two steady states, one pri-or to light pulse administration andanother following it.The light intensity at eye level wasmeasured using a spectroradiometer(IL-700, USA) equipped with a broadband light sensor (type 400). Experi-mental animals (np84) were adminis-tered UV-A light pulses (lamp TL-33/12; Philips, The Netherlands; wavel-ength 325–400 nm) of 30 min durationand 2.5 W/m2 irradiance, at variousphases of their circadian cycle. Con-trol animals (np36) were also trans-ported, at each tested CT, in light-tight containers (wrapped additional-ly in black cloth) in order to establishthat UV light pulses per se, and notthe disturbances associated with han-dling, transfer, and human interfer-ence caused phase shifts. Phase shiftdata were subjected to one-waymixed model analysis of variance(ANOVA) treating light strength as afixed factor and phase as a randomfactor. Multiple comparisons wereperformed using Tukey’s test for sig-nificant difference (with probabilityof type I error fixed at 0.05).The phase shifts evoked by a singleexposure to UV-A were measured atvarious phases of the circadian cyclein the field mouse, M. booduga and aPRC was constructed (Fig. 1). The

Naturwissenschaften 86 (1999) Q Springer-Verlag 1999 97

Fig. 1. PRC evoked by UV-A light pulse of30 min duration and 2.5 W/m2 irradiance forthe circadian rhythm in the locomotor activi-ty of M. booduga. CT0–CT12 (CT: circadiantime): subjective day; CT12–CT24 : subjectivenight. Error bars, 95% confidence intervalabout the mean phase shift for six or sevenanimals Fig. 2. Data for wheel-running activity rhythms of six adult male mice (age (90 days) adminis-

tered UV-A pulse at phases CT14 (left) and CT20 (right) of the circadian cycle

sensitivity to UV-A was found to bephase-dependent. The UV-A PRCshows regions of phase delays duringthe subjective day and early subjec-tive night and regions of phase ad-vances during the late subjectivenight. The ANOVA showed a signifi-cant effect of phase (F7,32p8.9;P~0.001). Multiple comparison ofthe phase shifts of the delay zone ofthe PRC showed that the phase shiftobserved at CT14 was significantlygreater than that at other CTs. How-ever, the phase shifts obtained be-tween the phases CT4 and CT12 didnot differ significantly from each oth-er (Tukey’s test, minimum significantdifferencep0.41, P~0.05). In theUV-A PRC, maximum phase delayresponses of –1.12B0.23 h were ob-tained by exposing the animals toUV-A at CT14 (Fig. 2). Maximumphase advances of 0.72B0.22 h wereobtained by a single exposure to UV-A at CT20 (Fig. 2). The UV-A PRCof M. booduga did not have any lightrefractory zone (Fig. 1). When thesemice were subjected to UV-A light-

dark cycle, the locomotor activityshowed stable entrainment with theonset of locomotor activity coincidingwith the dark onset and remainingconfined to the dark phase (data notshown). The control animals did notundergo any measurable phase shift.Similar to other vertebrates, mam-mals too have been reported to haveextraocular photoreception [5–7].Working with pigmented house mice(M. musculus) Jacobs and co-workers[1] found two sensitivity maxima us-ing 510 nm and 370 nm of monochro-matic light, suggesting that two dis-tinct spectral mechanisms contributetowards spectral sensitivity functions.A separate study observed that a 30-min pulse of UV-light (10 mW/cm2)evoked phase shifts in both the tem-perature and activity rhythms in aphase-dependent manner [5]. Our ex-periments studying the effect of UV-A on the locomotor activity rhythmof the field mouse M. boodugayielded results that are consistentwith other recent observations onmammalian circadian systems.

Financial support from JawaharlalNehru Centre for Advanced ScientificResearch is acknowledged. We alsothank an anonymous referee for sug-gesting improvements to the manu-script.

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