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Thermotaxis in neonatal rat pups
Physiology & Behavior, Vol. 23, pp. 871-874. Pergamon Press and Brain Research Publ., 1979. Printed in the U.S.A.
Thermotaxis in Neonatal Rat Pups' I N G R I D B. J O H A N S O N
North Carolina Division of Mental Health, Research Section, Raleigh, NC 27611 R e c e i v e d 22 M a r c h 1979 JOHANSON, I. B. Thermotaxis in neonatal rat pups. PHYSIOL. BEHAV. 23(5) 871-874, 1979.--Deprived and nonde-
prived 3- and 6-day-old rat pups were tested on a thermal gradient for thermotaxis. Deprived 3-day olds demonstrated a marked thermotaxic responsiveness that was not observed in nondeprived 3-day olds or in deprived and nondeprived 6-day olds. Deprived 3-day olds did not show a strong thermal orientation immediately, but the distance moved by pups increased with repeated trials on the gradient. These findings demonstrate that neonatal rats are capable of thermotaxis, but that factors of age, deprivation and repeated handling determine pups' responsiveness to thermal gradients. Thermotaxis
Rat pups
Deprivation
T H E ability of young rats to regulate body temperature by physiological, intrinsic means develops in the first 2 weeks after birth [1]. Prior to this time, neonatal rats are dependent on extrinsic methods of thermoregulation. The mother plays an important role in controlling the pups' temperature [9], but the infants themselves help to maintain thermal homeostasis by behavioral thermoregulation. Huddling is a primary means of behavioral thermoregulation in developing rat pups and serves to reduce heat loss when the female is away from the litter [2]. It has also been proposed that infant rats might display thermotaxis, that is, they might move in a directed manner along a thermal gradient to a preferred temperature [1]. Neonatal hamster pups are remarkably sensitive to thermal gradients, showing a rapid, directed locomotion into the warm end of a gradient [10]. While infant rats show greater activity in cool environments than warm environments, suggesting they might be capable of orthokinetic orientation in response to temperature [4,7], there has been no convincing demonstration of thermotaxis in rat pups. One possible reason for the failure to demonstrate thermotaxis in infant rats is the low level of activity in such animals. Pups may be capable of thermotaxis, but are so slow in showing it that it has not been observed. In the current investigation, an attempt was made to elicit thermotaxis in infant rats by heightening their activity. Pups were separated from their mothers, hence deprived of milk and maternal care, a manipulation that results in a marked increase in general activity [12], physiological activation [6] and responsiveness to food stimuli [5]. METHOD
Subjects The subjects were 3- and 6-day-old offspring of Charles River CD strain rats. The dams and litters were housed in plastic cages (20x24x45 cm) with wood chip bedding. Cages
were checked daily at 1700 hr for births, and all new litters found on that check were termed 0 days of age. Two days after birth, litters were culled to 9 pups. The colony room was maintained at 21 to 24°C and a humidity of 40 to 70%, with a 14:10 light:dark cycle. Twenty-four hours prior to testing, 1 pup from each litter was removed and housed in a warm (32.5 +_ 0.5°C), moist (70 to 90% humidity) incubator (lsolette, Air-Shields, Inc.). The pups were placed in the incubator in a small plastic cage (13 x 17x29 cm) that contained fresh bedding. At the time of testing, an additional pup (nondeprived) was removed from each litter. Six litters (12 pups) were tested at 3 days of age and 6 litters at 6 days of age.
Apparatus To generate the thermal gradient, a heating pad (Oster Model 780-01, 120 V, 55 W; Oster Corporation) was placed on a wooden base that measured 36x43 cm. The base and pad sloped at 30 ° away from a level platform constructed of plastic mesh screening on which the pups moved. As a result, points along the platform were increasingly further from the heating pad (see [3], Fig. 1 for a diagram of the apparatus). The platform was 15 cm wide and 38 cm long. Two 6-cm start zones were marked at each end of the platform and one 6-cm start zone was marked in the middle. To increase the rate of temperature change across the gradient, a frozen pack of Blue Ice (Divajex, Santa Ana, CA) was placed 1 cm below the start zone at the cooler end of the gradient. Before testing, the surface temperature on the mesh screening was measured at several points with a thermistor probe (Yellow Springs Instruments, No. 427) attached to a telethermometer (YSI, Model 42SC). The temperature in the middle of the start zone at the cool end of the gradient was approximately 18°C. At the line marking the end of the start zone, the temperature was 25°C and then
~This research was partly Supported by the N. C. Division of Mental Health and NSF Grant BNS-77-23051. I thank W. G. Hall and G. Turkewitz for their comments on the manuscript.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s inc.--0031-9384/79/110871-04502.00/0
872
JOHANSON
rose linearly at a rate of 0.37°C, up to a temperature of 37°C in the middle of the warm zone.
COLD
32
Procedttre
[]
deprived
•
nondeprived
24
Pups were given 3 i-min tests in each of the 3 start zones (cool, moderate, and warm). The order in which the tests were given was random, as was the order in which the pups were tested. The animal being tested was placed on the locomotor surface perpendicular to the gradient. The amount of time spent in the start zone and the number of 5-sec intervals with no movement were noted. From these figures, an activity score was calculated as the percentage of time in the start zone during which the pup was inactive. Finally, the distance from the start line to the pup's final location at the end of 1 min was measured.
RESULTS
Deprived 3-day olds displayed a marked thermotaxic responsiveness not seen in nondeprived 3-day olds or in deprived and nondeprived 6-day olds. When deprived 3day-old pups were started in the cool end of the gradient, they moved (based on an average of 3 trials) almost twice as far as nondeprived 3-day olds and 67% moved across the entire gradient (32 cm) during at least one of the 3 trials (versus 17% of nondeprived pups). Six-day olds, regardless of deprivational state, moved only a short distance outside the cool start zone before ceasing to iocomote (see Fig. 1). The average distance moved was analyzed using a 2 (Age, between block)×2 (Deprivation, within block) split-plot analysis of variance [8]. The analysis revealed a significant Age effect, F(I,10)=I0.4, p<0.01, Deprivation effect, F(I,10)=I0.3, p<0.01, and Age×Deprivation interaction, F(I,10)=5.5, p<0.05. The possibility that the deprived 3-day olds were moving away from the extreme cold and not displaying true thermotaxis is argued against by 2 lines of evidence. First, the behavior of these pups did not suggest that they were simply escaping from the cold. Pups would frequently stop moving for brief periods of time after leaving the start area and then resume locomotion along the gradient. Occasionally, they would turn and move back towards the cold, and after a few centimeters would again turn and move to the warm end of the gradient. Second, thermotaxis was observed in deprived 3-day olds started in the middle of the thermal gradient, where the temperature averaged 30 to 3 I°C. When pups were started in the middle of the gradient, only deprived 3-day olds showed significant movement toward the warm end of the gradient. Deprived 3-day olds moved further than nondeprived 3-day olds and deprived and nondeprived 6-day olds, Age effect, F( 1,10)= 16.2, p<0.005; Deprivation effect, F(1,10)=8.9, p<0.05: and Age×Deprivation interaction, F(1,10)=7.2, p<0.05; see Fig. 1. Pup age and deprivational state had no effect on the distance moved from the warm end of the gradient, with pups in all treatment groups remaining within the start zone (see Fig. 1). To determine whether repeated testing influenced the thermotaxic response shown by 3-day-old pups, the results from the 1st trial in each start zone were compared with the results from the 2nd and 3rd trials. In tests starting from the cool end of the gradient, deprived 3-day olds showed a dramatic increase in the distance moved from the 1st to the 3rd trial (see Fig. 2). A variance analysis of the data from 3-day olds (with trials as a repeated measure) revealed that the
8 16 E
~
8
u
Z IE
16
1
II
in
|
MODERATE
E o
W O Z
4
--
d,
WARM 32
24 E o E o
16
8
5-DAY
OLDS
6-DAY
OLDS
FIG. 1. Distance (in cm) from the start zone at the end of 1 min, for 3- and 6-day-old deprived and nondeprived pups. Error bars indicate the standard error. TABLE 1 M E A N ( -+ S T A N D A R D E R R O R ) P E R C E N T O F T I M E IN E A C H S T A R T ZONE WITH NO MOVEMENT
Cold
Moderate
Warm
3-day olds Deprived Nondeprived
0 _+ 0 0± 0
39.0 + 14.9 59.8 ± 13.6
20.4 _+ 7.6 32.8 _+ 14.2
6-day olds Deprived Nondeprived
0± 0 0± 0
47.8 ± 10.2 45.2 ÷ 4.8
68.0 _+ 8.6 74.3 ± 4.1
Trial effect was significant, F(2,25)=4.0, p<0.05. Deprived 3-day old pups showed a significant increase in distance over trials, F(2,25)=6.33, p<0.01, F-test for simple effects, while there was no significant difference across trials in nondeprived pups (F< 1). There was no significant Trial effect when pups were started in the middle of the gradient, however (F
T H E R M O T A X I S IN RAT PUPS
873
32
28
I-I t r i o l
I
I~1 t r i a l
2
•
3
trial
24
E U v
ZO
I.u o Z I(I)
16
IZ
c~
8
4
DEPRIVED
NONDEPRIVED
FIG. 2. Distance (in cm) from the cool end of the gradient for 3-day-old deprived and nondeprived pups, as a function of trial. Error bars indicate the standard error. *p<0.01, Trial effect.
gradient, F(1,10)=22.7, p<0.001. The lowest activity was observed in 6 - d a y olds (over 65% of the time inactive for both deprived and nondeprived pups) while 3-day olds were more active (less than 35% of the time spent inactive; 6-day old nondeprived pups significantly less active than 3-dayold deprived pups, Tukey, p<0.05). DISCUSSION
Under conditions of deprivation and repeated testing, 3-day-old rat pups show sustained and directed orientation along a thermal gradient. These pups did not simply move out of the cold but continued to move toward the warm end of the gradient, even after frequent pauses or turns. This thermotaxic responding was also observed if pups were started at the more moderate temperatures in the middle of the gradient. In contrast, nondeprived 3-day olds and deprived or nondeprived 6-day olds did not show such marked thermal responsiveness. These pups moved out of the cold, but then ceased movement. They also showed no significant movement from the middle of the gradient. Deprived 3-day olds generally did not show thermotaxis on their 1st trial at the cool end of the gradient, but with repeated trials demonstrated a reliable and rapid orientation along the gradient. These findings that several factors---deprivation, age and
repeated t e s t i n g - - m a y interact to induce thermotaxic responding in young rats, but the underlying mechanisms by which these factors act are not known. Hamster pup thermotaxis appears to be a function of the rate of body temperature loss, so that a minimal rate of heat loss is required for thermotaxis to occur [10]. One effect of deprivation in rat pups is an increased rate of heat loss. Deprived pups show a more rapid drop in body temperature in cool environments (Johanson and Hall, manuscript in preparation) and fail to show the large increase in 02 consumption observed in fed pups in response to cold stress [3]. This enhanced heat loss caused by deprivation may contribute to the thermotaxis observed in deprived 3-day olds. The failure of deprived 6-day-old pups to display thermotaxis may reflect the fact that their increased size and thickening skin render them less susceptible to heat loss. Lack of thermotaxis in the 6-day-old pup might also reflect maturational changes in the olfactory system, which seem to be responsible for the decline in thermotaxis in older hamster pups [11]. The possible contribution of these factors to the loss of thermotaxis in older rat pups needs to be evaluated. Deprivation may also act by enhancing pups' responsiveness to specific types of stimulation. Pups that have been deprived for 24 hr are more responsive to milk odor than nondeprived pups [5]. Furthermore, deprivation alters pups' behavior in a warm environment, in that deprived pups are more active and show a large amount of oral activity (mouthing, probing and licking) when they are placed in a warm environment (Johanson and Hall, manuscript in preparation). These observations, along with the present finding that deprivation enhances thermotaxis, suggest that some aspect of deprivation increases the reactivity of pups to certain forms of sensory stimulation. Deprivation also enhances general activity in pups [12] and may make observation of thermotaxis more likely by maintaining a heightened state of " a r o u s a l " in deprived pups. The failure to observe thermotaxis by deprived 3-day oids on the 1st trial in the cold suggests that a certain level of arousal is required beyond that created by deprivation and that stimulation provided by repeated handling may increase arousal and activity. H o w e v e r , r e p e a t e d testing per se was not sufficient to stimulate thermotaxis, in that nondeprived 3-day olds showed no significant increase in distance with repeated trials. Furthermore, stimulation of activity (by deprivation and handling) did not induce thermotaxis in 6-day olds. In fact, deprived 6-day olds given amphetamine (1 mg/kg IP) to further stimulate activity do not show thermotaxis (Johanson, unpublished observations). These findings suggest that an increased level of arousal may be necessary in young pups, but is not sufficient to stimulate thermotaxis in older rat pups. The heightened thermal responsiveness observed in deprived 3-day-old pups may serve as a behavioral thermoregulatory mechanism, allowing deprived pups to minimize an already rapid rate of heat loss. Further, the thermal sensitivity observed in deprived pups could allow pups displaced from the nest to utilize thermal cues in orienting their homing behavior at ages when orientation using olfactory gradients has not yet developed [7]. However, the deprivation condition used here was extreme and it is not yet known whether pups actually utilize these thermotaxic capabilities in the course of their normal development.
874
JOHANSON REFERENCES
1. Adolph, E. F. Ontogeny of physiological regulations in the rat. Q. Rev. Biol. 32: 89-137, 1957. 2. Alberts, J. R. Huddling by rat pups: Group behavioral mechanisms of temperature regulation and energy conservation. J. comp. physiol. Psychol. 92: 231-245, 1978. 3. Bignall, K. E., F. W. Heggeness and J. E. Palmer. Effects of acute starvation on cold-induced thermogenesis in the preweanling rat. Am. J. Physiol. 277: 1088--1093, 1974. 4. Fowler, S. J. and C. Kellogg. Ontogeny of thermoregulatory mechanisms in the rat. J. comp. physiol. Psychol. 89: 738--746, 1975. 5. Hall, W. G. Feeding and behavioral activation in infant rats. Science 205: 206-209, 1979. 6. Hofer, M. A. The role of nutrition in the physiological and behavioral effects of early maternal separation on infant rats. Psychosom. Med. 35: 350-359, 1973.
7. Johanson, I. B. The development of olfactory and thermal responsiveness in hypothyroid and hyperthyroid rat pups. Devl Psychobiol., in press. 8. Kirk, R. E. Experimental Design: Proced, res for the Behavioral Sciences. Belmont, California: Brooks/Cole, 1968, pp. 237-266. 9. Leon, M., P. G. Croskerry and G. K. Smith. Thermal control of mother-young contact in rats. Physiol. Behav. 21: 793-811, 1978. 10. Leonard, C. M. Thermotaxis in golden hamster pups. J. comp. physiol. Psychol. 86: 458--469, 1974. 11. Leonard, C. M. Maturational loss of thermotaxis prevented by olfactory lesions in golden hamster pups. J. comp. physiol. Psychol. 92: 1084-1094, 1978. 12. Moorcroft, W. H., L. D. Lytle and B. A. Campbell. Ontogeny of starvation-induced behavioral arousal in the rat. J. comp. physiol. Psychol. 75: 56-67, 1971.
Thermotaxis in Neonatal Rat Pups' I N G R I D B. J O H A N S O N
North Carolina Division of Mental Health, Research Section, Raleigh, NC 27611 R e c e i v e d 22 M a r c h 1979 JOHANSON, I. B. Thermotaxis in neonatal rat pups. PHYSIOL. BEHAV. 23(5) 871-874, 1979.--Deprived and nonde-
prived 3- and 6-day-old rat pups were tested on a thermal gradient for thermotaxis. Deprived 3-day olds demonstrated a marked thermotaxic responsiveness that was not observed in nondeprived 3-day olds or in deprived and nondeprived 6-day olds. Deprived 3-day olds did not show a strong thermal orientation immediately, but the distance moved by pups increased with repeated trials on the gradient. These findings demonstrate that neonatal rats are capable of thermotaxis, but that factors of age, deprivation and repeated handling determine pups' responsiveness to thermal gradients. Thermotaxis
Rat pups
Deprivation
T H E ability of young rats to regulate body temperature by physiological, intrinsic means develops in the first 2 weeks after birth [1]. Prior to this time, neonatal rats are dependent on extrinsic methods of thermoregulation. The mother plays an important role in controlling the pups' temperature [9], but the infants themselves help to maintain thermal homeostasis by behavioral thermoregulation. Huddling is a primary means of behavioral thermoregulation in developing rat pups and serves to reduce heat loss when the female is away from the litter [2]. It has also been proposed that infant rats might display thermotaxis, that is, they might move in a directed manner along a thermal gradient to a preferred temperature [1]. Neonatal hamster pups are remarkably sensitive to thermal gradients, showing a rapid, directed locomotion into the warm end of a gradient [10]. While infant rats show greater activity in cool environments than warm environments, suggesting they might be capable of orthokinetic orientation in response to temperature [4,7], there has been no convincing demonstration of thermotaxis in rat pups. One possible reason for the failure to demonstrate thermotaxis in infant rats is the low level of activity in such animals. Pups may be capable of thermotaxis, but are so slow in showing it that it has not been observed. In the current investigation, an attempt was made to elicit thermotaxis in infant rats by heightening their activity. Pups were separated from their mothers, hence deprived of milk and maternal care, a manipulation that results in a marked increase in general activity [12], physiological activation [6] and responsiveness to food stimuli [5]. METHOD
Subjects The subjects were 3- and 6-day-old offspring of Charles River CD strain rats. The dams and litters were housed in plastic cages (20x24x45 cm) with wood chip bedding. Cages
were checked daily at 1700 hr for births, and all new litters found on that check were termed 0 days of age. Two days after birth, litters were culled to 9 pups. The colony room was maintained at 21 to 24°C and a humidity of 40 to 70%, with a 14:10 light:dark cycle. Twenty-four hours prior to testing, 1 pup from each litter was removed and housed in a warm (32.5 +_ 0.5°C), moist (70 to 90% humidity) incubator (lsolette, Air-Shields, Inc.). The pups were placed in the incubator in a small plastic cage (13 x 17x29 cm) that contained fresh bedding. At the time of testing, an additional pup (nondeprived) was removed from each litter. Six litters (12 pups) were tested at 3 days of age and 6 litters at 6 days of age.
Apparatus To generate the thermal gradient, a heating pad (Oster Model 780-01, 120 V, 55 W; Oster Corporation) was placed on a wooden base that measured 36x43 cm. The base and pad sloped at 30 ° away from a level platform constructed of plastic mesh screening on which the pups moved. As a result, points along the platform were increasingly further from the heating pad (see [3], Fig. 1 for a diagram of the apparatus). The platform was 15 cm wide and 38 cm long. Two 6-cm start zones were marked at each end of the platform and one 6-cm start zone was marked in the middle. To increase the rate of temperature change across the gradient, a frozen pack of Blue Ice (Divajex, Santa Ana, CA) was placed 1 cm below the start zone at the cooler end of the gradient. Before testing, the surface temperature on the mesh screening was measured at several points with a thermistor probe (Yellow Springs Instruments, No. 427) attached to a telethermometer (YSI, Model 42SC). The temperature in the middle of the start zone at the cool end of the gradient was approximately 18°C. At the line marking the end of the start zone, the temperature was 25°C and then
~This research was partly Supported by the N. C. Division of Mental Health and NSF Grant BNS-77-23051. I thank W. G. Hall and G. Turkewitz for their comments on the manuscript.
C o p y r i g h t © 1979 B r a i n R e s e a r c h P u b l i c a t i o n s inc.--0031-9384/79/110871-04502.00/0
872
JOHANSON
rose linearly at a rate of 0.37°C, up to a temperature of 37°C in the middle of the warm zone.
COLD
32
Procedttre
[]
deprived
•
nondeprived
24
Pups were given 3 i-min tests in each of the 3 start zones (cool, moderate, and warm). The order in which the tests were given was random, as was the order in which the pups were tested. The animal being tested was placed on the locomotor surface perpendicular to the gradient. The amount of time spent in the start zone and the number of 5-sec intervals with no movement were noted. From these figures, an activity score was calculated as the percentage of time in the start zone during which the pup was inactive. Finally, the distance from the start line to the pup's final location at the end of 1 min was measured.
RESULTS
Deprived 3-day olds displayed a marked thermotaxic responsiveness not seen in nondeprived 3-day olds or in deprived and nondeprived 6-day olds. When deprived 3day-old pups were started in the cool end of the gradient, they moved (based on an average of 3 trials) almost twice as far as nondeprived 3-day olds and 67% moved across the entire gradient (32 cm) during at least one of the 3 trials (versus 17% of nondeprived pups). Six-day olds, regardless of deprivational state, moved only a short distance outside the cool start zone before ceasing to iocomote (see Fig. 1). The average distance moved was analyzed using a 2 (Age, between block)×2 (Deprivation, within block) split-plot analysis of variance [8]. The analysis revealed a significant Age effect, F(I,10)=I0.4, p<0.01, Deprivation effect, F(I,10)=I0.3, p<0.01, and Age×Deprivation interaction, F(I,10)=5.5, p<0.05. The possibility that the deprived 3-day olds were moving away from the extreme cold and not displaying true thermotaxis is argued against by 2 lines of evidence. First, the behavior of these pups did not suggest that they were simply escaping from the cold. Pups would frequently stop moving for brief periods of time after leaving the start area and then resume locomotion along the gradient. Occasionally, they would turn and move back towards the cold, and after a few centimeters would again turn and move to the warm end of the gradient. Second, thermotaxis was observed in deprived 3-day olds started in the middle of the thermal gradient, where the temperature averaged 30 to 3 I°C. When pups were started in the middle of the gradient, only deprived 3-day olds showed significant movement toward the warm end of the gradient. Deprived 3-day olds moved further than nondeprived 3-day olds and deprived and nondeprived 6-day olds, Age effect, F( 1,10)= 16.2, p<0.005; Deprivation effect, F(1,10)=8.9, p<0.05: and Age×Deprivation interaction, F(1,10)=7.2, p<0.05; see Fig. 1. Pup age and deprivational state had no effect on the distance moved from the warm end of the gradient, with pups in all treatment groups remaining within the start zone (see Fig. 1). To determine whether repeated testing influenced the thermotaxic response shown by 3-day-old pups, the results from the 1st trial in each start zone were compared with the results from the 2nd and 3rd trials. In tests starting from the cool end of the gradient, deprived 3-day olds showed a dramatic increase in the distance moved from the 1st to the 3rd trial (see Fig. 2). A variance analysis of the data from 3-day olds (with trials as a repeated measure) revealed that the
8 16 E
~
8
u
Z IE
16
1
II
in
|
MODERATE
E o
W O Z
4
--
d,
WARM 32
24 E o E o
16
8
5-DAY
OLDS
6-DAY
OLDS
FIG. 1. Distance (in cm) from the start zone at the end of 1 min, for 3- and 6-day-old deprived and nondeprived pups. Error bars indicate the standard error. TABLE 1 M E A N ( -+ S T A N D A R D E R R O R ) P E R C E N T O F T I M E IN E A C H S T A R T ZONE WITH NO MOVEMENT
Cold
Moderate
Warm
3-day olds Deprived Nondeprived
0 _+ 0 0± 0
39.0 + 14.9 59.8 ± 13.6
20.4 _+ 7.6 32.8 _+ 14.2
6-day olds Deprived Nondeprived
0± 0 0± 0
47.8 ± 10.2 45.2 ÷ 4.8
68.0 _+ 8.6 74.3 ± 4.1
Trial effect was significant, F(2,25)=4.0, p<0.05. Deprived 3-day old pups showed a significant increase in distance over trials, F(2,25)=6.33, p<0.01, F-test for simple effects, while there was no significant difference across trials in nondeprived pups (F< 1). There was no significant Trial effect when pups were started in the middle of the gradient, however (F
T H E R M O T A X I S IN RAT PUPS
873
32
28
I-I t r i o l
I
I~1 t r i a l
2
•
3
trial
24
E U v
ZO
I.u o Z I(I)
16
IZ
c~
8
4
DEPRIVED
NONDEPRIVED
FIG. 2. Distance (in cm) from the cool end of the gradient for 3-day-old deprived and nondeprived pups, as a function of trial. Error bars indicate the standard error. *p<0.01, Trial effect.
gradient, F(1,10)=22.7, p<0.001. The lowest activity was observed in 6 - d a y olds (over 65% of the time inactive for both deprived and nondeprived pups) while 3-day olds were more active (less than 35% of the time spent inactive; 6-day old nondeprived pups significantly less active than 3-dayold deprived pups, Tukey, p<0.05). DISCUSSION
Under conditions of deprivation and repeated testing, 3-day-old rat pups show sustained and directed orientation along a thermal gradient. These pups did not simply move out of the cold but continued to move toward the warm end of the gradient, even after frequent pauses or turns. This thermotaxic responding was also observed if pups were started at the more moderate temperatures in the middle of the gradient. In contrast, nondeprived 3-day olds and deprived or nondeprived 6-day olds did not show such marked thermal responsiveness. These pups moved out of the cold, but then ceased movement. They also showed no significant movement from the middle of the gradient. Deprived 3-day olds generally did not show thermotaxis on their 1st trial at the cool end of the gradient, but with repeated trials demonstrated a reliable and rapid orientation along the gradient. These findings that several factors---deprivation, age and
repeated t e s t i n g - - m a y interact to induce thermotaxic responding in young rats, but the underlying mechanisms by which these factors act are not known. Hamster pup thermotaxis appears to be a function of the rate of body temperature loss, so that a minimal rate of heat loss is required for thermotaxis to occur [10]. One effect of deprivation in rat pups is an increased rate of heat loss. Deprived pups show a more rapid drop in body temperature in cool environments (Johanson and Hall, manuscript in preparation) and fail to show the large increase in 02 consumption observed in fed pups in response to cold stress [3]. This enhanced heat loss caused by deprivation may contribute to the thermotaxis observed in deprived 3-day olds. The failure of deprived 6-day-old pups to display thermotaxis may reflect the fact that their increased size and thickening skin render them less susceptible to heat loss. Lack of thermotaxis in the 6-day-old pup might also reflect maturational changes in the olfactory system, which seem to be responsible for the decline in thermotaxis in older hamster pups [11]. The possible contribution of these factors to the loss of thermotaxis in older rat pups needs to be evaluated. Deprivation may also act by enhancing pups' responsiveness to specific types of stimulation. Pups that have been deprived for 24 hr are more responsive to milk odor than nondeprived pups [5]. Furthermore, deprivation alters pups' behavior in a warm environment, in that deprived pups are more active and show a large amount of oral activity (mouthing, probing and licking) when they are placed in a warm environment (Johanson and Hall, manuscript in preparation). These observations, along with the present finding that deprivation enhances thermotaxis, suggest that some aspect of deprivation increases the reactivity of pups to certain forms of sensory stimulation. Deprivation also enhances general activity in pups [12] and may make observation of thermotaxis more likely by maintaining a heightened state of " a r o u s a l " in deprived pups. The failure to observe thermotaxis by deprived 3-day oids on the 1st trial in the cold suggests that a certain level of arousal is required beyond that created by deprivation and that stimulation provided by repeated handling may increase arousal and activity. H o w e v e r , r e p e a t e d testing per se was not sufficient to stimulate thermotaxis, in that nondeprived 3-day olds showed no significant increase in distance with repeated trials. Furthermore, stimulation of activity (by deprivation and handling) did not induce thermotaxis in 6-day olds. In fact, deprived 6-day olds given amphetamine (1 mg/kg IP) to further stimulate activity do not show thermotaxis (Johanson, unpublished observations). These findings suggest that an increased level of arousal may be necessary in young pups, but is not sufficient to stimulate thermotaxis in older rat pups. The heightened thermal responsiveness observed in deprived 3-day-old pups may serve as a behavioral thermoregulatory mechanism, allowing deprived pups to minimize an already rapid rate of heat loss. Further, the thermal sensitivity observed in deprived pups could allow pups displaced from the nest to utilize thermal cues in orienting their homing behavior at ages when orientation using olfactory gradients has not yet developed [7]. However, the deprivation condition used here was extreme and it is not yet known whether pups actually utilize these thermotaxic capabilities in the course of their normal development.
874
JOHANSON REFERENCES
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