Psychopharmacology
Psychopharmacology 61,299- 307 (1979)
( by Springer-Verlag 1979
Environmentally Induced Differences in Susceptibility of Rats to CNS Stimulants and CNS Depressants: Evidence Against a Unitary Explanation Dorothy F. Einon* and Barbara J. Sahakian** Psychological Laboratory, University of Cambridge, Downing Street, Cambridge, England
Abstract. It has been suggested that socially isolated rats are more aroused then rats raised in social groups. This hypothesis was tested by examining amphetamineinduced activity and stereotypy in social and isolated rats of both sexes in both the active and inactive phases of their diurnal activity cycle. In socially raised rats it was possible to produce behavioural profiles similar to those of isolated rats by increasing the arousal level of the social rat. However, the complex interaction of housing conditions, diurnal variation and gender with drug dose suggests that one intervening variable such as arousal is too simplistic an explanation. In subsequent experiments, stereotypy was enhanced by a familiar environment, and there was a clear dissociation between the effects of CNS stimulants and CNS depressants. The increased susceptibility of isolates to CNS stimulants depends on social isolation for a short period before 45 days of age; the decreased susceptibility of isolates to CNS depressants may be produced by isolation at any age. We conclude that there is no evidence that isolated rats are hyperaroused. Key words: Social isolation - Arousal - Stereotypy Amphetamine - Barbiturates - Ruts
Increases in non-specific activity are often attributed to changes in levels of behavioural arousal. Although arousal level is a popular explanation (see Hinde, 1970; * Present address and address for offprint requests: Dorothy F. Einon, Psychological Department, Durham University, Science Laboratories, South Road, Durham-DH1 3LE, United Kingdom ** Present address: Laboratory of Neuroendocrine Regulation, Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts
Miezejeski et al., 1976 for reviews) it is notoriously difficult to define. Physical definitions can include altered autonomic or hormonal states; neurological definitions are based on measures of nonspecific activation of the reticular formation; generalized electrocortical activity; and behavioural definitions are generally based on altered levels of motor activity. In this paper we use the term arousal in the sense of a generalized increase in excitability. There is, however, an obvious circularity in attributing differences in the level of activity to differences in the level of a single intervening variable such as arousal. If the term arousal is to be at all useful, we need to show that manipulations posited to affect arousal should affect behaviour in the same way. We shall be concerned with several different ways of increasing non-specific activity in the rat. Our concern is to assess the general usefulness of the term arousal and more particularly to assess the suggestion (Baumel et al., 1969; Defeudis, /975; Walsh and Cummins, 1975) that animals raised in social isolation are more aroused than animals raised in social groups. There is a certain amount of evidence supporting this suggestion: isolates are both hyperactive (Morgan, 1973) and slower to habituate (Einon et al., 1975; Einon and Morgan, 1976). They are more susceptible to CNS stimulants (Sahakian et al., 1975) and less susceptible to CNS depressants (Friedman and Walker, 1969; Einon et al., 1976); and also show increases in tail pinchinduced oral behaviour (Sahakian and Robbins, 1977). If the differences in the behaviour of social and isolated rats can be attributed to the greater arousal potential of stimulation in isolates, it should be possible to produce, in socially reared animals, behavioural profiles similar to those of isolated rats by increasing the arousal level
0033-3158/79/0061/0299/$ 01.80
300 o f the social a n i m a l , t h a t is, socially raised rats tested u n d e r higher levels o f a r o u s a l s h o u l d b e h a v e like isolated rats. In rats, as m a n y other species, there is a d i u r n a l v a r i a t i o n in the level o f activity. R a t s are m o r e active a n d eat m o r e in the n o c t u r n a l phase o f the cycle (Richter, 1922; M o r g a n a n d Einon, 1976). T h e y are also less susceptible to C N S depressants in the n o c t u r nal phase ( E i n o n et al., 1976). W e have used this d i u r n a l v a r i a t i o n in b e h a v i o u r to test the hypothesis t h a t the effects o f social i s o l a t i o n a n d d i u r n a l r h y t h m m i g h t be a t t r i b u t e d to a u n i t a r y intervening variable such as arousal. W e have e x a m i n e d the activity o f socially r e a r e d a n d isolated rats d u r i n g the d a y a n d night phases o f their d i u r n a l cycle. W e have also m a n i p u l a t e d levels o f l o c o m o t o r activity with different doses o f the p s y c h o m o t o r s t i m u l a n t d - a m p h e t a m i n e sulphate to d e t e r m i n e if the differences in l o c o m o t o r activity due to rearing experience or d i u r n a l cycle r e m a i n over the entire d o s e - r e s p o n s e range o f the drug. Since da m p h e t a m i n e induces m o r e intense s t e r e o t y p y at higher doses ( R a n d r u p a n d M u n k v a d , 1970), we also e x a m ined the s t e r e o t y p y o f the d r u g g e d animals to assess whether the degree o f s t e r e o t y p y reflects an u n d e r l y i n g difference in a r o u s a l level. S t e r e o t y p e d b e h a v i o u r m a y be ' r e p e t i t i o n in an i n v a r i a n t p a t t e r n o f certain m o v e ments having no discernable g o a l ' , ( H u t t a n d H u t t , 1970), or, ' b e h a v i o u r w i t h o u t v a r i a t i o n . . , p e r f o r m e d at an increasing rate in a repetitive m a n n e r ' , ( R a n d r u p a n d M u n k v a d , 1970). Both definitions i m p l y t h a t s t e r e o t y p y is an a b n o r m a l p a t h o l o g i c a l type o f behaviour. It has, however, been suggested that drugi n d u c e d s t e r e o t y p y can be seen as an e x a g g e r a t i o n a n d repetition o f selected b e h a v i o u r s o f u n d r u g g e d a n i m a l s ( L y o n a n d R a n d r u p 1972; L y o n a n d R o b b i n s , 1975). T h u s r e p e a t e d sniffing is a characteristic response to m o d e r a t e doses o f a m p h e t a m i n e in the rat, b u t is also a response to n o v e l t y o r to high levels o f a u d i t o r y a n d visual s t i m u l a t i o n (Einon a n d M o r g a n , 1978b). Here we use the w o r d s t e r e o t y p y to refer to a c o n t i n u u m o f b e h a v i o u r f r o m n o r m a l resting to a b n o r m a l repetitive licking a n d gnawing. I f a r o u s a l is c o n s i d e r e d as a u n i t a r y intervening variable, we p r e d i c t t h a t the v a r i o u s w a y s o f m a n i p u l a t ing l o c o m o t o r activity should be additive. Moreover, one o u g h t to be able to p r o d u c e in socially r e a r e d rats b e h a v i o u r a l profiles similar to isolated rats when socially r e a r e d rats are tested u n d e r m o r e a r o u s i n g conditions.
Experiment I The first e x p e r i m e n t examines a m p h e t a m i n e - i n d u c e d activity a n d s t e r e o t y p y in social a n d isolated rats o f
Psychopharmacology 61 (1979) b o t h sexes. Different g r o u p s o f rats are tested in the active and inactive phases o f their d i u r n a l activity cycle.
Materials and Methods Subjects. Sprague Dawley rats (48 male, 48 female, 21 days old) were obtained from Carworth Europe Ltd., Huntingdon. Half (24 males, 24 females) were housed singly in translucent plastic boxes (North Kent Plastics M2; 33 = 15 x 13 cm) and the remainder were housed in groups of six of the same sex in larger (56x 30x 18 cm; NKPGPR 4) cages of similar design. Animals were kept in a temperature controlled room with daily 12-h light/dark cycle. The room was light between 8 a.m. and 8 p.m. When 45 days old, half the animals in each subgroup (12 female isolates, 12female social, 12male isolates, 12 male social) were transferred to an adjoining room where the lighting conditions were reversed (light on between 8 p.m. and 8 a.m.). In the light phase of the diurnal cycle the rooms were lit by two 65 - 80 W strip lights (0.75 log footlamberts, Iog fL): in the dark phase they were illuminated with a dim red light (i.75 log fL). Testing commenced when the animals were 70 days old. All testing was conducted between 12 and 5 p.m. Procedure. Locomotor activity was measured using photocell cages. Each of the 12 wire cages (40 • 25 x 20 cm) had two equally spaced horizontal photocell beams across the short axis. The 96 animals were tested in eight balanced subgroups of 12 animals. To control for lighting differences between home cage and testing situation, four subgroups were tested with dim (i-.75log fL) and four with bright (0.75 log fL) light in the experimental room (nocturnal rats came to the testing room from dim lighting conditions, and daytime rats from bright home cage lighting conditions). For any one animals the subgroup and the lighting conditions were held constant. During each experimental session, the rat was weighed, injected i.p., returned to its carrier cage until all 12 animals had been injected then placed in the photocell cages for the 1-h test trial. The average time between injection and placement was 1.5 rain. Interruptions of the photocell beams were recorded and sterotypy ratings taken for each 5-rain period. Stereotypy was rated on a 0 to 5 point scale (Sahakian et al., 1975). Ratings 0 and 1 describe control behaviour (0inactive, 1 active). Ratings 2 through 5 represent increasing intensities of stereotypy. Rating 2 denotes sporadic bursts of stereotypy (generally discontinuous bouts of repetitive sniffing), while ratings 3 through 5 describe continuous stereotypy: 3, rearing and sniffing over a large portion of the cage; 4, sniffing in one location of the cage; and 5, sniffing in one location with licking and gnawing. Subjects were used as their own controls and were tested once with each dose of the drug, with two intervening days between tests. In the first three sessions, rats received i.p. injections of saline, 0.5 mg/kg d-amphetamine, and 1.5 mg/kg d-amphetamine in a counterbalanced order. Each of the six possible dose orders was received by one animal in each cell of the experimental design. All animals were tested with 5 mg/kg d-amphetamine on the fourth test. The drug d-amphetamine sulphate (Smith, Klein and French) was dissoved in 0.9 ~ saline. 0.9 ~ saline was used for control injections. Doses were expressed in terms of the salt, and administered in a volume of 1 ml/kg body weight.
Results Locomotor Activity. The activity scores for the first three tests were subjected to a factorial analysis o f variance with d r u g dose (0.0, 0.5, a n d 1.5 mg/kg), rearing, d i u r n a l cycle, sex a n d lighting c o n d i t i o n s as m a i n effects. T h e analysis also c o m p a r e d the time course (trials) o f the g r o u p s over the 1-h test period. A s
D. F. Einon and B. J. Sahakian: Environmentally Induced Differences in Drug Potency
--0-- 0.0, -----0-- 0.51.5t d-amphet~ .... 300 -
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-
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5 rain sample Fig. 1. The number of photocell beam interruptions during successive 5-rain samples of the 1-h test session. The figure depicts the activity of social (SOC) and isolated (ISOL) rats under three doses of d-amphetamine (0.0, 0.5, and 1.5 mg/kg)
the 5 mg/kg dose was not part of the randomized design, the activity data for this dose were analysed separately. Individual groups were compared using Newman-Keul tests (Winer, 1971). The analysis of the first three tests revealed significant main effects of rearing: IF (1,18) = 28.28, P < 0.001]; diurnal cycle [F (1,81) = 6.81, P < 0.001]; sex [F (1,81) = 14.41, P
Psychopharmacology 61,299- 307 (1979)
( by Springer-Verlag 1979
Environmentally Induced Differences in Susceptibility of Rats to CNS Stimulants and CNS Depressants: Evidence Against a Unitary Explanation Dorothy F. Einon* and Barbara J. Sahakian** Psychological Laboratory, University of Cambridge, Downing Street, Cambridge, England
Abstract. It has been suggested that socially isolated rats are more aroused then rats raised in social groups. This hypothesis was tested by examining amphetamineinduced activity and stereotypy in social and isolated rats of both sexes in both the active and inactive phases of their diurnal activity cycle. In socially raised rats it was possible to produce behavioural profiles similar to those of isolated rats by increasing the arousal level of the social rat. However, the complex interaction of housing conditions, diurnal variation and gender with drug dose suggests that one intervening variable such as arousal is too simplistic an explanation. In subsequent experiments, stereotypy was enhanced by a familiar environment, and there was a clear dissociation between the effects of CNS stimulants and CNS depressants. The increased susceptibility of isolates to CNS stimulants depends on social isolation for a short period before 45 days of age; the decreased susceptibility of isolates to CNS depressants may be produced by isolation at any age. We conclude that there is no evidence that isolated rats are hyperaroused. Key words: Social isolation - Arousal - Stereotypy Amphetamine - Barbiturates - Ruts
Increases in non-specific activity are often attributed to changes in levels of behavioural arousal. Although arousal level is a popular explanation (see Hinde, 1970; * Present address and address for offprint requests: Dorothy F. Einon, Psychological Department, Durham University, Science Laboratories, South Road, Durham-DH1 3LE, United Kingdom ** Present address: Laboratory of Neuroendocrine Regulation, Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts
Miezejeski et al., 1976 for reviews) it is notoriously difficult to define. Physical definitions can include altered autonomic or hormonal states; neurological definitions are based on measures of nonspecific activation of the reticular formation; generalized electrocortical activity; and behavioural definitions are generally based on altered levels of motor activity. In this paper we use the term arousal in the sense of a generalized increase in excitability. There is, however, an obvious circularity in attributing differences in the level of activity to differences in the level of a single intervening variable such as arousal. If the term arousal is to be at all useful, we need to show that manipulations posited to affect arousal should affect behaviour in the same way. We shall be concerned with several different ways of increasing non-specific activity in the rat. Our concern is to assess the general usefulness of the term arousal and more particularly to assess the suggestion (Baumel et al., 1969; Defeudis, /975; Walsh and Cummins, 1975) that animals raised in social isolation are more aroused than animals raised in social groups. There is a certain amount of evidence supporting this suggestion: isolates are both hyperactive (Morgan, 1973) and slower to habituate (Einon et al., 1975; Einon and Morgan, 1976). They are more susceptible to CNS stimulants (Sahakian et al., 1975) and less susceptible to CNS depressants (Friedman and Walker, 1969; Einon et al., 1976); and also show increases in tail pinchinduced oral behaviour (Sahakian and Robbins, 1977). If the differences in the behaviour of social and isolated rats can be attributed to the greater arousal potential of stimulation in isolates, it should be possible to produce, in socially reared animals, behavioural profiles similar to those of isolated rats by increasing the arousal level
0033-3158/79/0061/0299/$ 01.80
300 o f the social a n i m a l , t h a t is, socially raised rats tested u n d e r higher levels o f a r o u s a l s h o u l d b e h a v e like isolated rats. In rats, as m a n y other species, there is a d i u r n a l v a r i a t i o n in the level o f activity. R a t s are m o r e active a n d eat m o r e in the n o c t u r n a l phase o f the cycle (Richter, 1922; M o r g a n a n d Einon, 1976). T h e y are also less susceptible to C N S depressants in the n o c t u r nal phase ( E i n o n et al., 1976). W e have used this d i u r n a l v a r i a t i o n in b e h a v i o u r to test the hypothesis t h a t the effects o f social i s o l a t i o n a n d d i u r n a l r h y t h m m i g h t be a t t r i b u t e d to a u n i t a r y intervening variable such as arousal. W e have e x a m i n e d the activity o f socially r e a r e d a n d isolated rats d u r i n g the d a y a n d night phases o f their d i u r n a l cycle. W e have also m a n i p u l a t e d levels o f l o c o m o t o r activity with different doses o f the p s y c h o m o t o r s t i m u l a n t d - a m p h e t a m i n e sulphate to d e t e r m i n e if the differences in l o c o m o t o r activity due to rearing experience or d i u r n a l cycle r e m a i n over the entire d o s e - r e s p o n s e range o f the drug. Since da m p h e t a m i n e induces m o r e intense s t e r e o t y p y at higher doses ( R a n d r u p a n d M u n k v a d , 1970), we also e x a m ined the s t e r e o t y p y o f the d r u g g e d animals to assess whether the degree o f s t e r e o t y p y reflects an u n d e r l y i n g difference in a r o u s a l level. S t e r e o t y p e d b e h a v i o u r m a y be ' r e p e t i t i o n in an i n v a r i a n t p a t t e r n o f certain m o v e ments having no discernable g o a l ' , ( H u t t a n d H u t t , 1970), or, ' b e h a v i o u r w i t h o u t v a r i a t i o n . . , p e r f o r m e d at an increasing rate in a repetitive m a n n e r ' , ( R a n d r u p a n d M u n k v a d , 1970). Both definitions i m p l y t h a t s t e r e o t y p y is an a b n o r m a l p a t h o l o g i c a l type o f behaviour. It has, however, been suggested that drugi n d u c e d s t e r e o t y p y can be seen as an e x a g g e r a t i o n a n d repetition o f selected b e h a v i o u r s o f u n d r u g g e d a n i m a l s ( L y o n a n d R a n d r u p 1972; L y o n a n d R o b b i n s , 1975). T h u s r e p e a t e d sniffing is a characteristic response to m o d e r a t e doses o f a m p h e t a m i n e in the rat, b u t is also a response to n o v e l t y o r to high levels o f a u d i t o r y a n d visual s t i m u l a t i o n (Einon a n d M o r g a n , 1978b). Here we use the w o r d s t e r e o t y p y to refer to a c o n t i n u u m o f b e h a v i o u r f r o m n o r m a l resting to a b n o r m a l repetitive licking a n d gnawing. I f a r o u s a l is c o n s i d e r e d as a u n i t a r y intervening variable, we p r e d i c t t h a t the v a r i o u s w a y s o f m a n i p u l a t ing l o c o m o t o r activity should be additive. Moreover, one o u g h t to be able to p r o d u c e in socially r e a r e d rats b e h a v i o u r a l profiles similar to isolated rats when socially r e a r e d rats are tested u n d e r m o r e a r o u s i n g conditions.
Experiment I The first e x p e r i m e n t examines a m p h e t a m i n e - i n d u c e d activity a n d s t e r e o t y p y in social a n d isolated rats o f
Psychopharmacology 61 (1979) b o t h sexes. Different g r o u p s o f rats are tested in the active and inactive phases o f their d i u r n a l activity cycle.
Materials and Methods Subjects. Sprague Dawley rats (48 male, 48 female, 21 days old) were obtained from Carworth Europe Ltd., Huntingdon. Half (24 males, 24 females) were housed singly in translucent plastic boxes (North Kent Plastics M2; 33 = 15 x 13 cm) and the remainder were housed in groups of six of the same sex in larger (56x 30x 18 cm; NKPGPR 4) cages of similar design. Animals were kept in a temperature controlled room with daily 12-h light/dark cycle. The room was light between 8 a.m. and 8 p.m. When 45 days old, half the animals in each subgroup (12 female isolates, 12female social, 12male isolates, 12 male social) were transferred to an adjoining room where the lighting conditions were reversed (light on between 8 p.m. and 8 a.m.). In the light phase of the diurnal cycle the rooms were lit by two 65 - 80 W strip lights (0.75 log footlamberts, Iog fL): in the dark phase they were illuminated with a dim red light (i.75 log fL). Testing commenced when the animals were 70 days old. All testing was conducted between 12 and 5 p.m. Procedure. Locomotor activity was measured using photocell cages. Each of the 12 wire cages (40 • 25 x 20 cm) had two equally spaced horizontal photocell beams across the short axis. The 96 animals were tested in eight balanced subgroups of 12 animals. To control for lighting differences between home cage and testing situation, four subgroups were tested with dim (i-.75log fL) and four with bright (0.75 log fL) light in the experimental room (nocturnal rats came to the testing room from dim lighting conditions, and daytime rats from bright home cage lighting conditions). For any one animals the subgroup and the lighting conditions were held constant. During each experimental session, the rat was weighed, injected i.p., returned to its carrier cage until all 12 animals had been injected then placed in the photocell cages for the 1-h test trial. The average time between injection and placement was 1.5 rain. Interruptions of the photocell beams were recorded and sterotypy ratings taken for each 5-rain period. Stereotypy was rated on a 0 to 5 point scale (Sahakian et al., 1975). Ratings 0 and 1 describe control behaviour (0inactive, 1 active). Ratings 2 through 5 represent increasing intensities of stereotypy. Rating 2 denotes sporadic bursts of stereotypy (generally discontinuous bouts of repetitive sniffing), while ratings 3 through 5 describe continuous stereotypy: 3, rearing and sniffing over a large portion of the cage; 4, sniffing in one location of the cage; and 5, sniffing in one location with licking and gnawing. Subjects were used as their own controls and were tested once with each dose of the drug, with two intervening days between tests. In the first three sessions, rats received i.p. injections of saline, 0.5 mg/kg d-amphetamine, and 1.5 mg/kg d-amphetamine in a counterbalanced order. Each of the six possible dose orders was received by one animal in each cell of the experimental design. All animals were tested with 5 mg/kg d-amphetamine on the fourth test. The drug d-amphetamine sulphate (Smith, Klein and French) was dissoved in 0.9 ~ saline. 0.9 ~ saline was used for control injections. Doses were expressed in terms of the salt, and administered in a volume of 1 ml/kg body weight.
Results Locomotor Activity. The activity scores for the first three tests were subjected to a factorial analysis o f variance with d r u g dose (0.0, 0.5, a n d 1.5 mg/kg), rearing, d i u r n a l cycle, sex a n d lighting c o n d i t i o n s as m a i n effects. T h e analysis also c o m p a r e d the time course (trials) o f the g r o u p s over the 1-h test period. A s
D. F. Einon and B. J. Sahakian: Environmentally Induced Differences in Drug Potency
--0-- 0.0, -----0-- 0.51.5t d-amphet~ .... 300 -
-
ON
ol
os
o L
9 D
oS
Icr 4
9 D
250-
$oc lso[
-
301
XX
~X
XX
200LO
1
I
200 C
o 150-
"5
,
~~
o~ o_ O\o/0
5 rain sample Fig. 1. The number of photocell beam interruptions during successive 5-rain samples of the 1-h test session. The figure depicts the activity of social (SOC) and isolated (ISOL) rats under three doses of d-amphetamine (0.0, 0.5, and 1.5 mg/kg)
the 5 mg/kg dose was not part of the randomized design, the activity data for this dose were analysed separately. Individual groups were compared using Newman-Keul tests (Winer, 1971). The analysis of the first three tests revealed significant main effects of rearing: IF (1,18) = 28.28, P < 0.001]; diurnal cycle [F (1,81) = 6.81, P < 0.001]; sex [F (1,81) = 14.41, P
