Photoperiodic and light spectral conditions which inhibit circulating concentrations of thyroxine in the male hamster

Life Sciences, Vol. 36, pp. 2183-2188 Printed in the U.S.A.

Pergamon Press

PHOTOPERIODIC AND LIGHT SPECTRALCONDITIONSWHICH INHIBIT CIRCULATING CONCENTRATIONSOF THYROXINE IN THE MALE HAMSTER M.K. Vaughan, G.C. Bralnard and R.J. Reiter Department of Cellular and Structural Biology, The University of Texas Health Science Center at San Antonio, San Antonlo, TX and Department of Neurology, Jefferson Medical College, Philadelphia, PA (Received in final form March 21, 1985)

Summary Adult male Syrian hamsters were exposed daily for 12 weeks to 11 h/day of cool white fluorescent light (350 ± 50 uW/cm2) followed by an addltlonal 3 h of near ultraviolet (339-317 nm), blue (435-500 nm), green (515-550 nm), yellow (558-636 nm) or red (653-668 rim) llght at an irradiance of 0.2 ~W/cm2 or to total darkness. Animals exposed to the wavelengths between 558-668 nm (yellow or red half peak bandwidths) or those receiving a total of 13 h of darkness/day had suppressed circulating levels of thyroxine (T4), a depressed free T4 index (FT41) and a higher T3/T4 ratio compared to animals receiving a total of 14 h of whlte light (350 ± 50 ~W/cm2). These results suggest that speclfic wavelengths of light can affect the neuroendocrine-thyroid axis. Many temperate latitude species u t i l i z e environmental lighting cues to synchronize homeostatic mechanisms whlch best maximize survival of the individual and the species. For wintering animals, seasonal changes must be predicted well in advance to i n i t i a t e those hormonal, metabolic and behavioral mechanisms necessary for the animal to survive. The Syrian hamster (Mesocricetus auratus) is one photosensitive species in which the reproductlve and thyroid hormones react in a predictable fashion when the animal is exposed to natural decreasing or a r t i f l c i a l l y slmulated short daylengths or total darkness. Maintenance of hamsters in less than 12.5 h light/day leads to depression of circulating T4 and reproductive hormones, gonadal atrophy and an increase in body weight (1-4). Such strategies as used by the Syrian hamster are dependent upon its perception of the photoperlod or light and transduction of that information into a neuroendocrine signal. Light is the small portion of the electromagnetlc spectrum to whlch the eye is sensitive. To date, however, no studies have been forthcoming to specifically determine that portion of the spectrum whlch is responsible for the depressive effects that photoperiod can preclpitate on the neuroendocrine-thyroid axis. In the present study, we have concentrated our attentlon on the visible spectrum of electromagnetic energy as well as on the near ultraviolet (uv) range. The rationale behind thls experiment relates to the hamster's Interpretation of 11 h of llght supplemented by 3 h of dim llght of various wavelengths presented as an "extended dusk" period. I f the hamster "sees" the dim light, then i t would interpret the signal as a 14:10 LD photoperiod and T4 levels would be comparable to the 14:10 LD controls. On the other hand, i f the dim colored lights were not perceived, then the animal would interpret i t s photoperiod to be 11:13 LD and circulating T4 levels would be depressed. 0024-3205/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

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Materlals & Methods Anlmals - Adult male Syrian hamsters (M. auratus) weighing 80-100 g were purcha--se'd ~om Lakeview Hamster Colony i n - - N e w - ~ , NJ. The hamsters were housed in clear polycarbonate cages in a windowless room in which temperature (22 ± 2°C) and humidity were automatically regulated. Food and water were supplied ad llbitum. All anlmals were maintained in their respective photoperiodic - c ' 6 n ~ for 12 weeks at which time they were decapitated and a trunk blood sample obtained in heparlnlzed tubes. Plasma samples were stored frozen until the time of assay. Photoperiodic Conditions - The animals were housed in specially equipped cabinets that were illuminated on the lnterlor and well ventilated wlth fans. One cabinet had 5 separate light-proof shelves each equipped with its own fan, light tlmer, ventllation baffle and two fluorescent fixtures. One of these light bulbs provlded the hamsters with 11 h of cool white fluorescent light (Sylvania F40/CW/RS/SS) at an irradiance of 350 ± 50 ~W/cm2 at cage level. The other fluorescent light on each shelf provided the animals with 3 h of dim (0.2 ~W/cmz) light of a restricted bandwidth. These special lamps were designed, manufactured, tested and generously donated by the Durotest Corporation, North Bergen, NJ. Each of the lamps were ordinary fluorescent bulbs that was coated with a special dye to limit the spectral output of the lamp. The half-peak bandwidths of these lamps were: red (668-653 nm), yellow (640-550 nm), green (555-515 nm), blue (505-435 nm) and near ultraviolet (3895-350 nm). The spectral power d~strlbution of the experimental light sources has been published previously (5). Additionally, two groups of 10 hamsters each were maintained in another slmilarly equipped cabinet provldlng elther a 11:13 LD or a 14:10 LD photoperiod wlth a cool white fluorescent light. Assa~ - Plasma Tw and triiodothyronlne (Ts) were determined by radioimmunoassay uslng kits purchased from Diagnostic Products, Los Angeles, CA. An index of the fraction of T4 now bound to plasma proteins was the in vitro T3 uptake (T3U) test (Diagnostic Products). The free hormone Indice-s-T~wI and FT31) were obtained by calculating the product of the respective thyroid hormone and the T3U value. Plasma cholesterol levels were determined by an enzymatic colorimetric assay (Sigma Kit #350). The cholesterol was oxidized to produce H202 which reacted with 4-aminoantipyrine and phenol to yield a quinoeimine dye with a maximum absorbance at 500 nm. Results are expressed as mg cholesterol/dl plasma. All samples were analyzed in a slngle assay for a given variable. Statistlcs - Data were s t a t l s t l c a l l y analyzed by an analysis of variance followed by a Student-Newman-Keuls test for significant differences among multiple means. Results Hamsters exposed to the 11:13 LD photoperiod or to photoperlods extended by the red or yellow half peak bandwldths had depressed circulating levels of T4 (Fig. 1), lower FT, I values (Table I) and an elevated Ts/T4 ratio (Table i) compared to control animals maintained in the 14:10 LD photoperlod; however, their clrculating concentratlons of Ts and cholesterol as well as their FT}I were indistinguishable from the controls (Table 17. Hamsters maintained in photoperlods extended by 3 h/day in ultraviolet, blue or green llght did not have s t a t l s t l c a l l y signiflcant values for any of the varlables tested compared to the controls kept in a 14:10 LD photoperlod.

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TABLE 1

Circulating T3 and Cholesterol Concentratlons, FT41 , FT31 and T3U Values and T3/T4 Ratios

11:10 LD + 3 h of the l i g h t spectra shown

N

FT41

FT3I

White l i g h t Controls

10

2.15 ±0.12

26.2 ±1.6

Near UV 339-371 nm

10

2.04 ±0.06

Blue 435-500 nm

10

Green 515-550 nm Yellow 558-636 nm Red 653-668nm Dark

T3 (ng/dl)

T3U

T3/T4 Ratio

Cholesterol (mg/dl)

56.8 ±3.7

46.3 ±0.4

12.4 ±0.8

187.8 ±5.5

21.8 ±0.9

49.6 ±2.2

44.1 ±0.9

10.7 ±0.4

204.0 ±8.1

1.92 0.06

25.0 ±1.7

52.5 ±5.3

45.1 ±0.7

12.3 ±1.1

190.7 ±5.7

10

1.60 ±0.11

25.6 ±0.7

58.8 ±2.0

43.6 ±0.9

16.7 ±1.3

210.9 ±10.2

9

1.28a ±0.24

19.4 ±2.0

46.1 ±5.3

45.3 ±1.0

20.1 a ±4.7

168.6 ±12.1

10

1.38a ±0.14

24.0 ±1.6

51.8 ±4.3

46.8 ±0.8

19.1a ±2.3

176.1 ±9.8

8

1.39a ±0.16

25.3 ±2.0

55.5 ±4.7

45.8 ±0.9

19.9a ±2.7

172.1 ±11.1

a, p< 0.001 vs. white l i g h t controls Discussion The generation of a nerve impulse from the retina due to light exposure is dependent on the absorption of characteristic wavelengths of l l g h t by specific visual pigments in the retina. Based on the results of this experiment, i t would appear that as far as the neuroendocrine-thyroid axis is concerned, hamsters "see" near u l t r a v i o l e t , blue and green light and interpret red and yellow l i g h t as darkness at the lrradiance of llght used. Thus, visual plgments with peak spectral absorbance between 350-500 nm are most l i k e l y involved in the present response. Two visual plgments f a l l into the above mentioned category: rhodopsin and cyanolabe (blue absorbing pigment). Rods contain rhodopsin, a molecule formed by the combination of retinal (a vltamin A aldehyde) and a protein called opsin. Rhodopsln has four peak absorptive wavelengths (500, 350, 280 and 231 nm) whlle cyanolabe has a peak spectral absorbance of 440 nm. The hamster retlna is composed primarily of rods (6-8) and a few (~3%) cones (6). Whether these cones are rudimentary or functlonal remains unestablished. Thus, rhodopsln would be the photopigment probably involved in the present experiment although the contributlon made by the few cones of the hamster retina cannot be completely discounted at this polnt. Interestingly, for the

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hamsters to perceive the near UV, one might propose that I t was absorbed in the B peak (350 nm) of rhodopsin. Thyroxine

4-

3-

2-

339-371 435-500

515-550 558-836 6 5 3 - 8 M

light Specfm a = p < 0.005 vs controls b = p < 0.001 vs controls

Fig. 1 Circulatlng concentrations of thyroxine (T4) of adult male Syrian hamsters exposed to 3 h of the various light spectra shown in addition to an 11:10 LD photoperlod. Means ± S.E.M. are indicated. Control animals (Controls) were exposed to a 14:10 LD photoperlod at an irradlance level of 350 uW/cm2. Recently, several studies using the Syrian hamster have shown that the plneal gland and one of I t s products, melatonln, exert a modulatory role on the neuroendocrlne-thyroid axis (1,4,8). B11ateral orblta] enucleation, like exposure to short photoperlod, induces a depression in c i r c u l a t i n g concentrations of T4 (4,9,10); however, i f the blinded animals were either plnealectomzed or superior cervical gangllonectomized, then the T4 levels were equivalent to those observed in the 14:10 LD photoperlod controls (9,10). Thus, the neural input whlch signals the length of the photoperlod may relay through the plneal where i t is transduced into an hormonal output which subsequently affects other braln centers involved In the production and/or release of TRH; a l t e r n a t i v e l y , a direct effect on the p i t u i t a r y production and secretion of thyrotropin has not been conclusively ruled out. Unllke the c i r c u l a t i n g concentrations of T4 which are predictably depressed by exposure to hamsters to short photoperiod, T3 levels are not always dmlnished by the reduction in daylength. Thls ms a somewhat perplexlng problem to us slnce the present T3 results confirm some (12,13) but not all (2,4,14) prevlous reports from our laboratory; occasionally, exposure of hamsters to short photoperlod induces a depresslon in T~ (2,4,14). Hormonal changes wrought by the above mentloned procedures are a dynamic process requlring several weeks to be f u l l y expressed. One p o s s i b i l i t y is that in some experlments T3 levels are depressed more qulckly than in other t r i a l s . A l t e r n a t l v e l y , many exogenous and endogenous factors might interact to produce this v a r l a b i l l t y between experiments. Such factors may include residual seasonal s e n s i t i v i t y even in "controlled" conditions, alterations in the

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conversion of T4 to Ts due to endocrine and/or metabollc factors not yet understood or the failure to control some exogenous circumstance of which we are not yet aware. The light irradlances used in the present study to extend the photoperlod (0.2 uW/cm2 for the restricted bandwidths, 350 ~W/cm2 of cool whlte fluorescent light in controls) are deflnltely capable of causing physlological changes in the retina and pineal gland of the Syrian hamster. Recent studies by Bralnard and coworkers (15) revealed that a light irradiance as low as 0.111 uW/cm2 during the night was capable of depressing serotonin N-acetyltransferase actlvity and melatonin content in the plneal gland of the hamster; similarly, irradiances as l i t t l e as 0.222 uW/cm significantly reduced pineal melatonin levels in the Long Evans pigmented rat (16). Indeed, very low Irradiances (0.000001 ~W/cm2) stimulate the hamster visual system (16, 17). Thus, the lrradiance of light used in the present study Is seemingly ample to evoke the hormonal response observed. I t is posslble that greater intensltles of light in the relatlvely ineffectlve wavelengths would have produced a biologlcal response but this was not tested. At the intensity used, there was differential response to different wavelengths of light with 0.2 ~W/cm2 of blue and UV llght as effective as 350 ~W/cm2 of white light. I f melatonln mediates the light-induced thyroid changes in Syrian hamsters, i t would seem reasonable that those specific irradiances and wavelengths of light capable of suppressing melatonin would also block the short photoperiod-induced depression of the thyroid axls. In the present study, irradiances of red or yellow light which do not acutely reduce nocturnal melatonin (5), do not interfere with the short photoperiod depresslon of the thyrold axis. S1mlarly, irradlances of blue or green llght whlch block nocturnal melatonin production, also block short photoperiod suppression of thyroid parameters. Only in the case of near-ultraviolet light is there a lack of parallellsm between the acute effects of light wavelength on the thyrold gland. The reason for a lack In parallelism between acute and chronic exposure is not readlly apparent. Perhaps differences between exposure conditions of the acute and chronlc experiments resulted in different light irradlances at the level of the animals' retina. In the acute studies, near-ultravlolet llght was presented as a direct point source in a black chamber. In contrast, near-ultravlolet light was diffuse and indirect in a whlte chamber in the experiment above. Further study is required to determine why an irradiance of 0.2 uW/cm2 of near-ultraviolet light did not perturb melatonin In a short term exposure whlle I t permitted the thyroid axis to stay active in a long term experiment. Perhaps near UV can act on the thyroid independently of the pineal. In conclusion, this is the f i r s t report which shows that the neuroendocrine-thyroid axis of the hamster is differentially sensltlve to various wavelengths of light. Extenslon of the daylight period by low intensity whte, UV, blue or green light (in terms of human perception, not necessarily to imply color sensatlon in hamsters) presented as a prolonged dusk perlod Is perceived by the hamster as a stimulatory photoperiod whereas the extension of the photoperiod by red or yellow light is perceived as darkness. ACKNOWLEDGEMENTS This work was supported by NSF grant #PCM8003441. The authors w~sh to thank the Durotest Corporation for the donation of the fluorescent light used ~n the present study.

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References I. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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