Life Sciences Vol . 22, pp . 255-258 Printed in the U .S .A .
Pergamon Press
TIMING OF A SINGLE DAILY MEAL AFFECTS DAILY SERUM PROLACPIN RHYTHM IN GULF KILLIFISH, FUNDULUS GRANDIS Richard E . Spieler , Albert H . Meier t , and Teresa A . Noeske l ' t Milwaukee Public Museum 1 800 W . Wells Milwaukee, Wis . 53233 and Dept . of Zoology and Physiology2 Louisiana State University Baton Rouge, La . 70803 ( Received in final form November 8, 1977) Summary Fish fed at one of two different times of day had differences in mean levels of serum prolactin throughout the day . Although the forms of the daily rhythms from the two groups were similar, they differed at one time of day . Prolactin is a pituitary hormone with a broad spectrum of effects on physiologic processes, i .e ., growth, fattening, reproduction, migration, behaviour, ion regulation, etc . (1) . One of the salient features of prolactin physiology is the temporal dependent effects it produces . Thus, there are significant differences in responses to prolactin injection depending on the time-of-day of injection . These differences include changes in fat stores in fishes, a frog, a lizard, and several species of birds and mammals ; changes in nonlipid growth in fish ; inhibition of amphibian metamorphosis ; promotion of the red eft water drive ; locomotor activity and orientation in birds ; and reproductive responses in fish, birds and mammals (2) . The differences in physiologic responses to prolactin injection appear to parallel seasonal responses in these animals . The results of numerous experiments have led to the conclusion that the temporal effects of prolactin injections are caused by changes in its phase relation with endogenous corticoids and that prolactin injections are mimicking the daily increases in endogenous hormone levels (2) . Apparently, the daily rhythm of circulating prolactin plays a central role in the seasonal integration of some vertebrates with their environment . However, the environmental stimuli which function as entrainers or modifiers of this daily prolactin rhythm have not been fully elucidated . In man the sleep-wake cycle is presumeably the major entrainer of the rhythm (3) . In lower vertebrates, photoperiod (4) and temperature (Spieler, Meier and Noeske, submitted for publication) have been demonstrated to modify circulating prolactin titers . Timing of meal feeding can entrain variations in hormone rhythms (e .g ., corticoids and prolactin) in some vertebrates (5,6) as well as cause differences in seasonally correlated phenomena (i .e ., growth and reproduction) in vertebrates including fish (7) . We investigated the effects of time-of-day of feeding on the daily variation of circulating prolactin in the Gulf killifish, 255
256
Timing of Meal Affecta Prolactin Rhythm
Vol . 22, No . 3,
1977
Fundulus rg andis . Our results indicate that the timing of feeding can significantly alter prolactin titers . Materials and Methods Male Gulf killifish, Fundulus rg andis , were collected from Grand Isle, Louisiana in April . The fish were maintained indoors in 70 liter aquaria which received filtered and aerated water (21 * 2 C, salinity 3 0/oo) and a 13L : 11D photoperiod (approximately the April photoperiod of Grand Isle, La .) . The fish were randomly placed in one of two groups . One group was fed daily at 0900 (light onset) and the other group was fed the same amount (approximately 2 g flake food, TetraMin) at 1600 . Fish were not fed the day before sampling . After 2 weeks the fish were sacrificed at one of six different times of day . Blood was taken by heart puncture, allowed to clot under refrigeration, centrifuged and the serum was assayed with a double antibody technique utilizing an antiserum to pollock prolactin and iodinated ovine prolactin (8,9) . Because killifish prolactin was not available, we were unable to determine absolute levels of prolactin . However, a pooled reference standard was run which allowed us to ascertain relative differences in circulating titers of prolactin . A one-way analysis of variance, Student-Newman-Keuls procedure, and the Student's t test (10) were used as aids in interpretation of data .
TIME OF DAY
FIG .
1
-) Mean serum prolactin of Gulf killifish fed at either 0900 (--+- ) or 1600 ( daily . Sample size is adjacent to the mean ; vertical bars represent the standard error ; horizontal bar the photoperiod .
vol . 22, No . 3, 1977
Timing of Meal Affects Prolactin Rhythm
257
Results The daily variations in circulating prolactin (Fig . 1) were significant (P 4 Mean serum prolactin . 0.01, Analysis of variance) in both groups of fish . levels for the entire day were significantly higher (P < 0.01, Student's in the fish fed at 1600 than those fed at 0900 . With the exception of the fish sampled at 1700 the form of the two rhythms is similar . The highest titers occurred during the latter half of the light period and at approximately the middle of the dark period . Lower levels of prolactin were found at the beginning of the photoperiod. In the group fed at 1600, a significant decrease in prolactin titers is also noted at 1700 . A comparison of differences within the rhythms (Student-Newman-Keuls, P < 0.05) is given below .
D
Fish fed at 0900 : Fish fed at 1600 :
0900, 1300 < 0500, 2100, 1700 0900, 1300, 1700 < 0500,_0100,-2100 Discussion
Our results indicate that the daily rhythm in circulating prolactin is responsive to the time of day of feeding in fish . The decrease to minimal prolactin levels at, or directly after, the time of feeding (0900, 1700) is simi lar to that noted by Bellinger and others (6) in rats . However, our study is apparently the first to indicate that the time-of-day of feeding can cause changes in the mean levels of circulating prolactin throughout a day . The effects of an increase or decrease in mean levels on the physiology of the animal are not known, but a change in mean levels might be expected to elicit physiological differences in processes that are dependent on the daily variation of circulating prolactin . Thus, with higher mean titers, a threshold level would be reached sooner and/or maintained longer at a specific time of day. Conversely, lower mean levels might restrict prolactin's effects at a specific time of day. It would appear that some of the previously cited (7) differences in growth and reproduction caused by meal feeding may be correlated with the time-of-feeding effect on the circulating prolactin rhythm . Should this prove to be the case then the time-of-day of feeding may become an important tool in allowing culturists (i .e ., aquaculture) to manipulate the daily prolactin rhythm and to achieve a greater measure of control over the life cycle of their stock . Acknowledgaments AHM .
This research was supported in part by NSF grant PCM76-06841 to RES and References
T
2. 3. 4. 5. 6. 7. 8.
H. A. BERN and C. S . NICOLL, La S ecificit'e Zoolo i ue des Hormones H ophysaires et de Leurs Activites , pp . 193-203 . Fontaine, e Centre National de la Recherche Scientifique, Paris (1969) . A. H. MEIER, Hormonal Correlates of Behavior , Vol . 2, pp . 469-549 (B . E . Eleftheriou and R. L. Sprott, eds .) Plenum Press, London (1975) . D . F . HORROBIN, Prolactin 1975 , Eden Press, Montreal (1975) . B . A. MCKEOWN and R . E . PETER, Can. J . Zool . 54 1960-1968 (1976) . D . T . KRIEGER, Endocrinology, _95 1195-1201 (1974) . L . L. BELLINGER, G . P . M0BE G and V . E . MENDEL, Horm . Metab . Res . _7 43-45 (1975) . R . E . SPIELER, Proc . World Maricult . Soc. 8 in press (1977) . R . E . SPIELER and A . H . MEIER, J . Fish . Res. Board Can. 33 183-186 (1976) .
258
9. 10 .
Timing of Meal Affects Prolactin Rhythm
Vol . 22, No . 3, 1977
R. E . SPIELER, A . H : FIER and H . LOESCH, Gen. Comp . Endocrinol . 29 156160 (1976) . R. R. SOKAL and F. J. ROHLF, Biometry , W. H. Freeman and Company, San Francisco (1969) .
Pergamon Press
TIMING OF A SINGLE DAILY MEAL AFFECTS DAILY SERUM PROLACPIN RHYTHM IN GULF KILLIFISH, FUNDULUS GRANDIS Richard E . Spieler , Albert H . Meier t , and Teresa A . Noeske l ' t Milwaukee Public Museum 1 800 W . Wells Milwaukee, Wis . 53233 and Dept . of Zoology and Physiology2 Louisiana State University Baton Rouge, La . 70803 ( Received in final form November 8, 1977) Summary Fish fed at one of two different times of day had differences in mean levels of serum prolactin throughout the day . Although the forms of the daily rhythms from the two groups were similar, they differed at one time of day . Prolactin is a pituitary hormone with a broad spectrum of effects on physiologic processes, i .e ., growth, fattening, reproduction, migration, behaviour, ion regulation, etc . (1) . One of the salient features of prolactin physiology is the temporal dependent effects it produces . Thus, there are significant differences in responses to prolactin injection depending on the time-of-day of injection . These differences include changes in fat stores in fishes, a frog, a lizard, and several species of birds and mammals ; changes in nonlipid growth in fish ; inhibition of amphibian metamorphosis ; promotion of the red eft water drive ; locomotor activity and orientation in birds ; and reproductive responses in fish, birds and mammals (2) . The differences in physiologic responses to prolactin injection appear to parallel seasonal responses in these animals . The results of numerous experiments have led to the conclusion that the temporal effects of prolactin injections are caused by changes in its phase relation with endogenous corticoids and that prolactin injections are mimicking the daily increases in endogenous hormone levels (2) . Apparently, the daily rhythm of circulating prolactin plays a central role in the seasonal integration of some vertebrates with their environment . However, the environmental stimuli which function as entrainers or modifiers of this daily prolactin rhythm have not been fully elucidated . In man the sleep-wake cycle is presumeably the major entrainer of the rhythm (3) . In lower vertebrates, photoperiod (4) and temperature (Spieler, Meier and Noeske, submitted for publication) have been demonstrated to modify circulating prolactin titers . Timing of meal feeding can entrain variations in hormone rhythms (e .g ., corticoids and prolactin) in some vertebrates (5,6) as well as cause differences in seasonally correlated phenomena (i .e ., growth and reproduction) in vertebrates including fish (7) . We investigated the effects of time-of-day of feeding on the daily variation of circulating prolactin in the Gulf killifish, 255
256
Timing of Meal Affecta Prolactin Rhythm
Vol . 22, No . 3,
1977
Fundulus rg andis . Our results indicate that the timing of feeding can significantly alter prolactin titers . Materials and Methods Male Gulf killifish, Fundulus rg andis , were collected from Grand Isle, Louisiana in April . The fish were maintained indoors in 70 liter aquaria which received filtered and aerated water (21 * 2 C, salinity 3 0/oo) and a 13L : 11D photoperiod (approximately the April photoperiod of Grand Isle, La .) . The fish were randomly placed in one of two groups . One group was fed daily at 0900 (light onset) and the other group was fed the same amount (approximately 2 g flake food, TetraMin) at 1600 . Fish were not fed the day before sampling . After 2 weeks the fish were sacrificed at one of six different times of day . Blood was taken by heart puncture, allowed to clot under refrigeration, centrifuged and the serum was assayed with a double antibody technique utilizing an antiserum to pollock prolactin and iodinated ovine prolactin (8,9) . Because killifish prolactin was not available, we were unable to determine absolute levels of prolactin . However, a pooled reference standard was run which allowed us to ascertain relative differences in circulating titers of prolactin . A one-way analysis of variance, Student-Newman-Keuls procedure, and the Student's t test (10) were used as aids in interpretation of data .
TIME OF DAY
FIG .
1
-) Mean serum prolactin of Gulf killifish fed at either 0900 (--+- ) or 1600 ( daily . Sample size is adjacent to the mean ; vertical bars represent the standard error ; horizontal bar the photoperiod .
vol . 22, No . 3, 1977
Timing of Meal Affects Prolactin Rhythm
257
Results The daily variations in circulating prolactin (Fig . 1) were significant (P 4 Mean serum prolactin . 0.01, Analysis of variance) in both groups of fish . levels for the entire day were significantly higher (P < 0.01, Student's in the fish fed at 1600 than those fed at 0900 . With the exception of the fish sampled at 1700 the form of the two rhythms is similar . The highest titers occurred during the latter half of the light period and at approximately the middle of the dark period . Lower levels of prolactin were found at the beginning of the photoperiod. In the group fed at 1600, a significant decrease in prolactin titers is also noted at 1700 . A comparison of differences within the rhythms (Student-Newman-Keuls, P < 0.05) is given below .
D
Fish fed at 0900 : Fish fed at 1600 :
0900, 1300 < 0500, 2100, 1700 0900, 1300, 1700 < 0500,_0100,-2100 Discussion
Our results indicate that the daily rhythm in circulating prolactin is responsive to the time of day of feeding in fish . The decrease to minimal prolactin levels at, or directly after, the time of feeding (0900, 1700) is simi lar to that noted by Bellinger and others (6) in rats . However, our study is apparently the first to indicate that the time-of-day of feeding can cause changes in the mean levels of circulating prolactin throughout a day . The effects of an increase or decrease in mean levels on the physiology of the animal are not known, but a change in mean levels might be expected to elicit physiological differences in processes that are dependent on the daily variation of circulating prolactin . Thus, with higher mean titers, a threshold level would be reached sooner and/or maintained longer at a specific time of day. Conversely, lower mean levels might restrict prolactin's effects at a specific time of day. It would appear that some of the previously cited (7) differences in growth and reproduction caused by meal feeding may be correlated with the time-of-feeding effect on the circulating prolactin rhythm . Should this prove to be the case then the time-of-day of feeding may become an important tool in allowing culturists (i .e ., aquaculture) to manipulate the daily prolactin rhythm and to achieve a greater measure of control over the life cycle of their stock . Acknowledgaments AHM .
This research was supported in part by NSF grant PCM76-06841 to RES and References
T
2. 3. 4. 5. 6. 7. 8.
H. A. BERN and C. S . NICOLL, La S ecificit'e Zoolo i ue des Hormones H ophysaires et de Leurs Activites , pp . 193-203 . Fontaine, e Centre National de la Recherche Scientifique, Paris (1969) . A. H. MEIER, Hormonal Correlates of Behavior , Vol . 2, pp . 469-549 (B . E . Eleftheriou and R. L. Sprott, eds .) Plenum Press, London (1975) . D . F . HORROBIN, Prolactin 1975 , Eden Press, Montreal (1975) . B . A. MCKEOWN and R . E . PETER, Can. J . Zool . 54 1960-1968 (1976) . D . T . KRIEGER, Endocrinology, _95 1195-1201 (1974) . L . L. BELLINGER, G . P . M0BE G and V . E . MENDEL, Horm . Metab . Res . _7 43-45 (1975) . R . E . SPIELER, Proc . World Maricult . Soc. 8 in press (1977) . R . E . SPIELER and A . H . MEIER, J . Fish . Res. Board Can. 33 183-186 (1976) .
258
9. 10 .
Timing of Meal Affects Prolactin Rhythm
Vol . 22, No . 3, 1977
R. E . SPIELER, A . H : FIER and H . LOESCH, Gen. Comp . Endocrinol . 29 156160 (1976) . R. R. SOKAL and F. J. ROHLF, Biometry , W. H. Freeman and Company, San Francisco (1969) .
