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Melatonin and lighting condition: Absence of long-term effects on food intake and body weight regulation in the albino rat
Physiology & Behavior, Vo|. 25, pp. 855-857. PergamonPress and Brain Research Publ., 1980.Printed in the U.S.A.
Melatonin and Lighting Condition: Absence of Long-Term Effects on Food Intake and Body Weight Regulation in the Albino R a t a J O H N D A R K 2, L I N D A L. R A Y H A 3, I A N C L A R K - L A N E
AND VICTORIA KIMLER
D e p a r t m e n t o f Psychology, Eastern Michigan University, Ypsilanti, M I 48197 R e c e i v e d 8 M a r c h 1980 DARK, J., L. L. RAYHA, I. CLARK-LANE AND V. KIMLER. Melatonin and lighting condition: Absence of long-term effects on food intake and body weight regulation in the albino rat. PHYSIOL. BEHAV. 25(6) 855-857, 1980.--In contrast to photoperiodic rodents, the nonphotoperiodic laboratory rat's food intake and body weight was unaffected by melatonin treatment (Silastic implants). Constant dark and constant light were equally ineffective as well in disturbing these regulatory behaviors. Body weight regulation
Food intake
Melatonin
T H E putative pineal hormone, melatonin, has been demonstrated to modulate a number of physiological functions in photoperiodic rodents; for example, its ability to induce gonadal regression has been widely studied [9,11]. In addition, melatonin implants have produced a suppression of body weight in the Djungarian hamster (Phodopus sungorus) [4] and a suppression of both food intake and body weight in the white-footed mouse, Peromyscus leucopus [8]. In the albino rat, a nonphotoperiodic rodent, the physiological action of melatonin is less clear. Although it appears ineffective in inducing testicular regression [6,12], daily injections have been reported to suppress rats' food intake and pinealectomy to elevate feeding behavior [5]. This suggests that melatonin can affect food intake, however, there has been no direct evidence for an effect upon body weight [2, 3, 5, 10]. Lighting condition affects food intake of the laboratory rat; placing rats into constant light (LL) suppresses feeding while continuous darkness (DD) has a facilitatory effect ([ 13] Dark and Dark, Manuscript in Preparation). It is not clear at this time whether this is a transitory phenomenon related to a change in food intake independent of body weight, or whether it may reflect an underlying alteration in preferred body weight in different lighting conditions. Also, melatonin synthesis is dark dependent [1]; its production is highest during the dark phase of an LD cycle. Melatonin is hypothesized to inhibit food intake, yet in L L when melatonin levels would be low, there is a suppression of food consumption. Our purpose was two-fold. Primarily, we intended to examine the long-term effects of melatonin administration in the rat. We questioned whether melatonin influenced food
Constant dark
Constant light
Photoperiodism
consumption in a nonphotoperiodic rodent and, if it did, whether there was an effect upon body weight maintenance. In addition, we investigated long-term effects of L L and DD upon food intake and body weight to determine whether there was a lasting or transient perturbation of weight regulating mechanisms in these conditions, and whether there existed an interaction between lighting condition and melatonin treatment. METHOD
Animals Fifty-four male Sprague-Dawley albino rats weighing between 150 and 200 g at the beginning of experimentation were housed in individual stainless steel cages. All animals had food (Purina Laboratory Chow) and tap water available ad lib throughout the procedure.
Procedure After being placed into individual cages within the same experimental room, each animal's total food intake was monitored for a baseline period of one week throughout which all animals were exposed to light at various times of day and for various durations such that there was no consistent cycle of light and dark. Each animal's body weight was taken at the beginning and end of this period. At the end of the week of baseline measurements, the 54 animals were divided into 3 different lighting condition groups of 18 animals each and moved to separate rooms. The first group (LD) was maintained in a 12:12 LD cycle, the
1The authors wish to thank Dr. Don Jackson for his assistance in the analysis of the data, Dr. Phyllis Johnston for her helpful comments on the manuscript, and Darlene Frost for preparing the figures. 2Present address of J. D.: Department of Psychology, University of California, Berkeley, CA 94720. 3Present address of L. L. R.: Department of Psychology, Mackenzie Hall, Wayne State University, Detroit, MI 48202.
C o p y r i g h t © 1980 Brain R e s e a r c h Publications Inc.--0031-9384/80/120855-03502.00/0
S5~
i)ARK E7 A1 5oo
E o
1ii]
0 30
I-Z
l
T
T
20
E J
a 0
0 0 M..
"
M S N M S N FIG. 1. The mean daily food consumption and mean body weight during the final week of testing for the animals receiving melatonin (M), sham-surgery (S), and the normal control group (N) in the LD lighting condition.
second group (LL) in constant light, and the third group (DD) in constant darkness (the room was illuminated at all times by dim red light to allow data collection and general maintenance). The groups remained in these lighting conditions until the termination of the experiment. Each of the lighting condition groups was further subdivided into 3 treatment groups at the same time the lighting conditions were altered. Six animals in each condition (LD-C, LL-C, and DD-C) were left untreated and acted as normal controls. A second 6 animals, a sham-surgery group (LD-S, LL-S, and DD-S), were subcutaneously implanted with 1-3 empty 50 mm Silastic tubing capsules (1.47 mm ID and 1.96 mm OD, Dow Coming) in the interscapular area. The final 6 animals (LD-M, LL-M, and DD-M) were subcutaneously implanted with ten 50 mm (500 mm total length) Silastic capsules containing crystalline melatonin (Sigma Chemical Co.); this dosage having been used previously in the rat [121. Silastic capsule implants were performed under ether anesthesia. A small incision (approximately 2.5 cm) was made in the skin in the interscapular region with a pair of surgical scissors. A blunt probe was then forced under the skin down the midline of the back for a distance of about 7-8 cm to separate the skin from the fascia of the underlying muscle. The Silastic capsules were pushed into the space thus created and the incision closed with several sutures. The weekly total food intake measurements were continued for each animal in each group for 8 weeks. The animals were provided with a measured amount of food 2-3 times/ week and on one day each week (the same day at the same time) the remaining food was removed and measured. Total weekly intake was found from this and the average daily food intake for that week calculated. Each animal was provided with fresh water 2-3 times/week. In addition, each animal's body weight was measured on the day on which the weekly food intake was determined. The mean daily food intake data and the body weight data from the baseline week, the middle week, and the final week were analyzed by a three-way analysis of variance (treatment x lighting × time). RESULTS As previous data suggested, there was no long-term effect of melatonin treatment upon body weight in the albino rat,
0
400
Lid
o
a lO
-=-
-~
20
200
~o
I
I
DD LD
LL
I
'
DD LD
too-,
8
L
FIG. 2. The mean daily food consumption and mean body weight during the final week of testing for the animals maintained in DD, LD, and LL. The data for the different groups are collapsed within each lighting condition.
F(2,45)=0.77, p>0.05 (see Fig. 1). Contrary to what had been reported previously [5], there was also no significant effect of melatonin upon food intake, F(2,45)= 1.08, p>0.05 (see Fig. 1). The ineffectiveness of melatonin in altering body weight or food intake was consistent across time and independent of lighting condition. The overall effect of lighting condition was nonsignificant for both food intake, F(2,45)=0.22, p>0.05; and body weight, F(2,45)=0.37, p>0.05 (Fig. 2). As was indicated above, there was no interaction between the treatment an animal was given and the lighting condition in which it was maintained. DISCUSSION Our data provide evidence indicating that, unlike the Djungarian hamster and the white-footed mouse, treatment with melatonin does not affect the rat's regulation of body weight and, thereby, alter its maintenance of a preferred body weight. Consistent with this, there was no alteration in food intake brought on by the administration of melatonin; a finding which differs from prior work with the rat [5]. There are several possible explanations for the ineffectiveness of melatonin in the present investigation. It is possible that melatonin does suppress food intake and body weight in the rat as in photoperiodic rodents and our discrepant results are due to the method of melatonin administration used (continuous absorption from Silastic capsules). There are some rodent species in which testicular regression can be induced by melatonin injections, but not by release from Silastic capsules (see [14] for a review). Although comparable results have not been reported for body weight regulation or food intake, it is possible that melatonin is capable of affecting the food intake and body weight of the rat only when applied phasically [5]. A tonic application may be of no consequence. An alternative is that melatonin had no or little physiological effect in our study because the laboratory rat is a nonphotoperiodic species. We feel that this last possibility provides the most likely explanation for the discrepancy between our results with the albino rat and those involving photoperiodic species [4, 7, 8], even though it does not explain the contradictory evidence of Ishibashi [5]. This conclusion is supported by data from another nonphotoperiodic
M E L A T O N I N A N D BODY W E I G H T R E G U L A T I O N
species, the golden-mantled ground squirrel (Spermophilus lateralis). Pinealectomy, which removes the putative source of endogenous melatonin, produces no alteration in the amplitude of the circannual rhythm of body weight in ground squirrels (Boshes, Personal Communication). Although melatonin's role in photoperiodic responses is becoming clearer, the physiological function of this hormone in nonphotoperiodic mammals remains to be determined. Despite the suppression of food intake which occurs
857 when rats are put into constant light and the facilitation of feeding seen after the onset of continuous darkness ([13] Dark and Dark, Manuscript in Preparation); our data reveal no long-term alteration in food consumption or body weight maintenance as a result of lighting condition. It appears that the effects produced by constant light and constant dark are transient and that rats in either condition maintain a preferred body weight essentially identical to rats in a light-dark cycle by eating equal amounts of food.
REFERENCES 1. Axelrod, J. The pineal gland: A model to study the regulation of the fl-adrenergic receptor. In: The Nervous System, Vol. 1: The Basic Neurosciences, edited by R. O. Brady. New York: Raven Press, 1975, p. 395. 2. Debeljuk, L. Effect of melatonin on the gonadotrophic function of the male rat under constant illumination. Endocrinology 84: 937-939, 1969. 3. Debeljuk, L., J. A. Vilchez, M. A. Schnitman, O. A. Paulucci and V. M. Feder. Further evidence for a peripheral action of melatonin. Endocrinology 89:1117-1119, 1971. 4. Hoffmann, K. The influence of photoperiod and melatonin on testis size, body weight, and pelage colour in the Djungarian hamster (Phodopus sungorus). J. comp. Physiol. 95: 267-282, 1973. 5. Ishibashi, T., D. W. Hahn, L. Srivastava, P. Kumaresan and C. W. Turner. Effect of pinealectomy and melatonin on feed consumption and thyroid hormone secretion rate. Proc. Soc. exp. Biol. Med. 122: 644-647, 1966. 6. Kinson, G. A. and S. Robinson. Gonadal function of immature male rats subjected to light restriction, melatonin administration and removal of the pineal gland. J. Endocr. 47: 391-392, 1970. 7. Lynch, G. R. and A. L. Epstein. Melatonin induced changes in gonads, pelage, and thermogenic characters in the white-footed mouse, Peromyscus leucopus. Comp. Biochem. Physiol. 53: 67-68, 1976.
8. Lynch, G. R., S. E. White, R. Grundel and M. S. Berger. Effects of photoperiod, melatonin administration and thyroid block on spontaneous daily torpor and temperature regulation in the white-footed mouse, Perornyscus leucopus. J. comp. Physiol. 125: 157-163, 1978. 9. Reiter, R. J., D. E. Blask, L. Y. Johnson, P. K. Rudeen, M. K. Vaughan and P. J. Waring. Melatonin inhibition of reproduction in the male hamster: Its dependency on time of day of administration and on an intact and sympathetically innervated pineal gland. Neuroendocrinology 22: 107-116, 1976. 10. Sorrentino, S., Jr., R. J. Reiter and D. S. Schalch. Hypotrophic reproductive organs and normal growth in male rats treated with melatonin. J. Endocr. 51: 213-214, 1971. 11. Tamarkin, L., W. Westrom, A. Hamill and B. D. Goldman. Effect of melatonin on the reproductive systems of male and female Syrian hamsters: A diurnal rhythm in sensitivity to melatonin. Endocrinology 99: 1534-1541, 1976. 12. Turek, F. W., C. Desjardins and M. Menaker. Differential effects of melatonin on the testes of photoperiodic and nonphotoperiodic rodents. Biol. Reprod. 15: 94-97, 1976. 13. Zucker, I. Light-dark rhythms in rat eating and drinking behavior. Physiol. Behav. 6: 115-126, 1971. 14. Zucker, I., P. G. Johnston and D. Frost. Comparative, physiological and biochronometric analyses of rodent seasonal reproductive cycles. In: Progress in Reproductive Biology: Seasonal Reproduction in Higher Vertebrates, Vol. 5, edited by R. J. Reiter and B. K. Follett. Basel: Karger, 1980.
Melatonin and Lighting Condition: Absence of Long-Term Effects on Food Intake and Body Weight Regulation in the Albino R a t a J O H N D A R K 2, L I N D A L. R A Y H A 3, I A N C L A R K - L A N E
AND VICTORIA KIMLER
D e p a r t m e n t o f Psychology, Eastern Michigan University, Ypsilanti, M I 48197 R e c e i v e d 8 M a r c h 1980 DARK, J., L. L. RAYHA, I. CLARK-LANE AND V. KIMLER. Melatonin and lighting condition: Absence of long-term effects on food intake and body weight regulation in the albino rat. PHYSIOL. BEHAV. 25(6) 855-857, 1980.--In contrast to photoperiodic rodents, the nonphotoperiodic laboratory rat's food intake and body weight was unaffected by melatonin treatment (Silastic implants). Constant dark and constant light were equally ineffective as well in disturbing these regulatory behaviors. Body weight regulation
Food intake
Melatonin
T H E putative pineal hormone, melatonin, has been demonstrated to modulate a number of physiological functions in photoperiodic rodents; for example, its ability to induce gonadal regression has been widely studied [9,11]. In addition, melatonin implants have produced a suppression of body weight in the Djungarian hamster (Phodopus sungorus) [4] and a suppression of both food intake and body weight in the white-footed mouse, Peromyscus leucopus [8]. In the albino rat, a nonphotoperiodic rodent, the physiological action of melatonin is less clear. Although it appears ineffective in inducing testicular regression [6,12], daily injections have been reported to suppress rats' food intake and pinealectomy to elevate feeding behavior [5]. This suggests that melatonin can affect food intake, however, there has been no direct evidence for an effect upon body weight [2, 3, 5, 10]. Lighting condition affects food intake of the laboratory rat; placing rats into constant light (LL) suppresses feeding while continuous darkness (DD) has a facilitatory effect ([ 13] Dark and Dark, Manuscript in Preparation). It is not clear at this time whether this is a transitory phenomenon related to a change in food intake independent of body weight, or whether it may reflect an underlying alteration in preferred body weight in different lighting conditions. Also, melatonin synthesis is dark dependent [1]; its production is highest during the dark phase of an LD cycle. Melatonin is hypothesized to inhibit food intake, yet in L L when melatonin levels would be low, there is a suppression of food consumption. Our purpose was two-fold. Primarily, we intended to examine the long-term effects of melatonin administration in the rat. We questioned whether melatonin influenced food
Constant dark
Constant light
Photoperiodism
consumption in a nonphotoperiodic rodent and, if it did, whether there was an effect upon body weight maintenance. In addition, we investigated long-term effects of L L and DD upon food intake and body weight to determine whether there was a lasting or transient perturbation of weight regulating mechanisms in these conditions, and whether there existed an interaction between lighting condition and melatonin treatment. METHOD
Animals Fifty-four male Sprague-Dawley albino rats weighing between 150 and 200 g at the beginning of experimentation were housed in individual stainless steel cages. All animals had food (Purina Laboratory Chow) and tap water available ad lib throughout the procedure.
Procedure After being placed into individual cages within the same experimental room, each animal's total food intake was monitored for a baseline period of one week throughout which all animals were exposed to light at various times of day and for various durations such that there was no consistent cycle of light and dark. Each animal's body weight was taken at the beginning and end of this period. At the end of the week of baseline measurements, the 54 animals were divided into 3 different lighting condition groups of 18 animals each and moved to separate rooms. The first group (LD) was maintained in a 12:12 LD cycle, the
1The authors wish to thank Dr. Don Jackson for his assistance in the analysis of the data, Dr. Phyllis Johnston for her helpful comments on the manuscript, and Darlene Frost for preparing the figures. 2Present address of J. D.: Department of Psychology, University of California, Berkeley, CA 94720. 3Present address of L. L. R.: Department of Psychology, Mackenzie Hall, Wayne State University, Detroit, MI 48202.
C o p y r i g h t © 1980 Brain R e s e a r c h Publications Inc.--0031-9384/80/120855-03502.00/0
S5~
i)ARK E7 A1 5oo
E o
1ii]
0 30
I-Z
l
T
T
20
E J
a 0
0 0 M..
"
M S N M S N FIG. 1. The mean daily food consumption and mean body weight during the final week of testing for the animals receiving melatonin (M), sham-surgery (S), and the normal control group (N) in the LD lighting condition.
second group (LL) in constant light, and the third group (DD) in constant darkness (the room was illuminated at all times by dim red light to allow data collection and general maintenance). The groups remained in these lighting conditions until the termination of the experiment. Each of the lighting condition groups was further subdivided into 3 treatment groups at the same time the lighting conditions were altered. Six animals in each condition (LD-C, LL-C, and DD-C) were left untreated and acted as normal controls. A second 6 animals, a sham-surgery group (LD-S, LL-S, and DD-S), were subcutaneously implanted with 1-3 empty 50 mm Silastic tubing capsules (1.47 mm ID and 1.96 mm OD, Dow Coming) in the interscapular area. The final 6 animals (LD-M, LL-M, and DD-M) were subcutaneously implanted with ten 50 mm (500 mm total length) Silastic capsules containing crystalline melatonin (Sigma Chemical Co.); this dosage having been used previously in the rat [121. Silastic capsule implants were performed under ether anesthesia. A small incision (approximately 2.5 cm) was made in the skin in the interscapular region with a pair of surgical scissors. A blunt probe was then forced under the skin down the midline of the back for a distance of about 7-8 cm to separate the skin from the fascia of the underlying muscle. The Silastic capsules were pushed into the space thus created and the incision closed with several sutures. The weekly total food intake measurements were continued for each animal in each group for 8 weeks. The animals were provided with a measured amount of food 2-3 times/ week and on one day each week (the same day at the same time) the remaining food was removed and measured. Total weekly intake was found from this and the average daily food intake for that week calculated. Each animal was provided with fresh water 2-3 times/week. In addition, each animal's body weight was measured on the day on which the weekly food intake was determined. The mean daily food intake data and the body weight data from the baseline week, the middle week, and the final week were analyzed by a three-way analysis of variance (treatment x lighting × time). RESULTS As previous data suggested, there was no long-term effect of melatonin treatment upon body weight in the albino rat,
0
400
Lid
o
a lO
-=-
-~
20
200
~o
I
I
DD LD
LL
I
'
DD LD
too-,
8
L
FIG. 2. The mean daily food consumption and mean body weight during the final week of testing for the animals maintained in DD, LD, and LL. The data for the different groups are collapsed within each lighting condition.
F(2,45)=0.77, p>0.05 (see Fig. 1). Contrary to what had been reported previously [5], there was also no significant effect of melatonin upon food intake, F(2,45)= 1.08, p>0.05 (see Fig. 1). The ineffectiveness of melatonin in altering body weight or food intake was consistent across time and independent of lighting condition. The overall effect of lighting condition was nonsignificant for both food intake, F(2,45)=0.22, p>0.05; and body weight, F(2,45)=0.37, p>0.05 (Fig. 2). As was indicated above, there was no interaction between the treatment an animal was given and the lighting condition in which it was maintained. DISCUSSION Our data provide evidence indicating that, unlike the Djungarian hamster and the white-footed mouse, treatment with melatonin does not affect the rat's regulation of body weight and, thereby, alter its maintenance of a preferred body weight. Consistent with this, there was no alteration in food intake brought on by the administration of melatonin; a finding which differs from prior work with the rat [5]. There are several possible explanations for the ineffectiveness of melatonin in the present investigation. It is possible that melatonin does suppress food intake and body weight in the rat as in photoperiodic rodents and our discrepant results are due to the method of melatonin administration used (continuous absorption from Silastic capsules). There are some rodent species in which testicular regression can be induced by melatonin injections, but not by release from Silastic capsules (see [14] for a review). Although comparable results have not been reported for body weight regulation or food intake, it is possible that melatonin is capable of affecting the food intake and body weight of the rat only when applied phasically [5]. A tonic application may be of no consequence. An alternative is that melatonin had no or little physiological effect in our study because the laboratory rat is a nonphotoperiodic species. We feel that this last possibility provides the most likely explanation for the discrepancy between our results with the albino rat and those involving photoperiodic species [4, 7, 8], even though it does not explain the contradictory evidence of Ishibashi [5]. This conclusion is supported by data from another nonphotoperiodic
M E L A T O N I N A N D BODY W E I G H T R E G U L A T I O N
species, the golden-mantled ground squirrel (Spermophilus lateralis). Pinealectomy, which removes the putative source of endogenous melatonin, produces no alteration in the amplitude of the circannual rhythm of body weight in ground squirrels (Boshes, Personal Communication). Although melatonin's role in photoperiodic responses is becoming clearer, the physiological function of this hormone in nonphotoperiodic mammals remains to be determined. Despite the suppression of food intake which occurs
857 when rats are put into constant light and the facilitation of feeding seen after the onset of continuous darkness ([13] Dark and Dark, Manuscript in Preparation); our data reveal no long-term alteration in food consumption or body weight maintenance as a result of lighting condition. It appears that the effects produced by constant light and constant dark are transient and that rats in either condition maintain a preferred body weight essentially identical to rats in a light-dark cycle by eating equal amounts of food.
REFERENCES 1. Axelrod, J. The pineal gland: A model to study the regulation of the fl-adrenergic receptor. In: The Nervous System, Vol. 1: The Basic Neurosciences, edited by R. O. Brady. New York: Raven Press, 1975, p. 395. 2. Debeljuk, L. Effect of melatonin on the gonadotrophic function of the male rat under constant illumination. Endocrinology 84: 937-939, 1969. 3. Debeljuk, L., J. A. Vilchez, M. A. Schnitman, O. A. Paulucci and V. M. Feder. Further evidence for a peripheral action of melatonin. Endocrinology 89:1117-1119, 1971. 4. Hoffmann, K. The influence of photoperiod and melatonin on testis size, body weight, and pelage colour in the Djungarian hamster (Phodopus sungorus). J. comp. Physiol. 95: 267-282, 1973. 5. Ishibashi, T., D. W. Hahn, L. Srivastava, P. Kumaresan and C. W. Turner. Effect of pinealectomy and melatonin on feed consumption and thyroid hormone secretion rate. Proc. Soc. exp. Biol. Med. 122: 644-647, 1966. 6. Kinson, G. A. and S. Robinson. Gonadal function of immature male rats subjected to light restriction, melatonin administration and removal of the pineal gland. J. Endocr. 47: 391-392, 1970. 7. Lynch, G. R. and A. L. Epstein. Melatonin induced changes in gonads, pelage, and thermogenic characters in the white-footed mouse, Peromyscus leucopus. Comp. Biochem. Physiol. 53: 67-68, 1976.
8. Lynch, G. R., S. E. White, R. Grundel and M. S. Berger. Effects of photoperiod, melatonin administration and thyroid block on spontaneous daily torpor and temperature regulation in the white-footed mouse, Perornyscus leucopus. J. comp. Physiol. 125: 157-163, 1978. 9. Reiter, R. J., D. E. Blask, L. Y. Johnson, P. K. Rudeen, M. K. Vaughan and P. J. Waring. Melatonin inhibition of reproduction in the male hamster: Its dependency on time of day of administration and on an intact and sympathetically innervated pineal gland. Neuroendocrinology 22: 107-116, 1976. 10. Sorrentino, S., Jr., R. J. Reiter and D. S. Schalch. Hypotrophic reproductive organs and normal growth in male rats treated with melatonin. J. Endocr. 51: 213-214, 1971. 11. Tamarkin, L., W. Westrom, A. Hamill and B. D. Goldman. Effect of melatonin on the reproductive systems of male and female Syrian hamsters: A diurnal rhythm in sensitivity to melatonin. Endocrinology 99: 1534-1541, 1976. 12. Turek, F. W., C. Desjardins and M. Menaker. Differential effects of melatonin on the testes of photoperiodic and nonphotoperiodic rodents. Biol. Reprod. 15: 94-97, 1976. 13. Zucker, I. Light-dark rhythms in rat eating and drinking behavior. Physiol. Behav. 6: 115-126, 1971. 14. Zucker, I., P. G. Johnston and D. Frost. Comparative, physiological and biochronometric analyses of rodent seasonal reproductive cycles. In: Progress in Reproductive Biology: Seasonal Reproduction in Higher Vertebrates, Vol. 5, edited by R. J. Reiter and B. K. Follett. Basel: Karger, 1980.