Physiology & Behavior, Vol. 19, pp. 811-813. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A.
BRIEF COMMUNICATION Blood Pressure Extremes and Activity in Aging Mice 1 M E R R I L L F. E L I A S
Department o f Psychology, University o f Maine at Orono, Orono, ME 04473 and C L Y D E A. P E N T Z , III
Department o f Psychology, Syracuse University, Syracuse, N Y 13201 (Received 22 July 1977) ELIAS, M. F. AND C. A. PENTZ, III. Blood pressure extremes and activity in aging mice. PHYSIOL. BEHAV. 19(6) 811-813, 1 9 7 7 . - Mice selected genetically for extremes in blood pressure were compared with regard to open field activity. For three younger groups, the high blood pressure line (stock) exhibited significantly lower levels of activity than the low blood pressure line. The low blood pressure stock exhibited a decline in activity from 510 to 750 days of age. Comparisons of extreme blood pressure groups from an F 2 population of mice, derived from an initial cross of high and low blood pressure lines, revealed no differences in activity. Thus, it was concluded that differences in activity for young adult high and low blood pressure mice of similar ages could not be attributed to genetic linkage, pleiotropy or a more direct causal relationship between blood pressure extremes and behavior. Blood pressure
Activity level
Genetic tests
Age differences
G E N E T I C selection provides a useful t o o l for studying the relationship b e t w e e n e x t r e m e physiological traits and behavior [2,11]. There have been a n u m b e r of studies of activity level [9, 12, 14] in the s p o n t a n e o u s l y hypertensive rat (SHR), a strain derived f r o m a K y o t o stock of Wistar albino rats [ 8 ] . The SHR rat is m o r e active in the open field w h e n c o m p a r e d to a variety of n o r m o t e n s i v e strains [9, 12, 14]. The present e x p e r i m e n t (1) extends data on blood pressure e x t r e m e s and open field activity to include Schlager's high and low blood pressure mouse stocks [ 13 ], and (2) compares activity scores for mice that have experienced e x t r e m e s in blood pressure values over a significant p o r t i o n of the life-span. The literature indicates slowing of response and i m p a i r m e n t in cognitive functioning for middle aged and old hypertensive h u m a n s relative to controls (see 2). Thus, it would seem i m p o r t a n t to compare old as well as y o u n g and middle aged high and low blood pressure mice in studies designed to provide i n f o r m a t i o n on the value of hypertensive rodents as models for the study o f hypertension and behavior. Comparisons of high and low blood pressure stocks f r o m
Open field behavior
Hypertension
the same f o u n d a t i o n stock, or high blood pressure mice and ancestral control stocks (e.g., SHR with Kyoto-Wistar), do n o t allow the investigator to conclude that differences in behavior are related to blood pressure values either in a direct causal m a n n e r (e.g., some pathological c o n d i t i o n o f the nervous system) or by virtue of the fact that similar genes influence b o t h blood pressure and behavior via pleiotropy o r linkage. The association b e t w e e n b l o o d pressure and behavior may simply reflect the fact that t w o different genotypes (strains or genetically diverse stocks) differ with respect to blood pressure values and the behavior(s) of concern. In this instance there is a non-causal relationship b e t w e e n blood pressure and behavior. The present study illustrates one of several [4,1 1 ] tests of the non-causal hypothesis. To our knowledge, these tests have n o t been done in those studies which have c o m p a r e d hypertensive and normotensive rat strains. METHOD
Experiment 1 N u m b e r of animals, mean ages, and mean systolic blood
' This research was supported, in part, by a research grant from the National Institute on Aging (AG-00868) to MFE. Blood pressure stocks were generously provided by G. Schlager from his BP I high and low blood pressure stocks. 811
812
ELIAS AND PENTZ
pressure values (mm Hg) are shown in Fig. 1. Mice in Experiment 1 were progeny of matings of generation 14 high and low blood pressure stocks from Schlager's selection program [ 13 ]. Derivation of Schlager's stocks has been described in this journal [4] and elsewhere in detail [1, 2, 5, 13]. Briefly, these stocks resulted from two-way selection from a common foundation stock which was formed from an eight-way cross of eight inbred strains. At generation 7 of selection they differed significantly from each other and from a randomly mated control line [13]. These are not inbred strains as double first cousin and sib matings were avoided throughout most of the selection program (inbreeding coefficients at generation 12 were 30 and 29 percent for the high and low lines respectively). Pulse rate does not differ significantly for the high and low blood pressure mice [ 13 ].
140
I
8
£: ~30 E E
,o
13
High BP ~
120
.igh Bp
I
w
,oo
8 S aa 90 _o dt- 70 ~.) ix
6
Low BP
5O
Exp. I
ExplT
(F 2 )
,2'o
{'_ iO)
215
5,'0
('tO)
MEAN
I
750
(* tO) (-.t 10) AGE IN DAYS (-~ S E M )
//
I 240 (*-20)
FIG. 1. Number of subjects per group and mean systolic (mm Hg) blood pressure values (+_ SEM) for the four age groups of Experiment 1 and the single age group of Experiment 2.
and shipped to our laboratory [5]. Further, the high blood pressure mice exhibit increased protein synthesis in heart and other biochemical evidence for cardiac hypertrophy [6]. All mice were male, and were maintained on a diet of Purina chow and tap water (ad lib) throughout the experiment. Day/night cycle was 12/12 and colony and test room temperature was 24 + 3 ° C. Mean body weights (range = 2 8 - 3 0 g) did not differ significantly across age groups or between blood pressure groups. The open field arena (91.44 x 91.44 × 20.32 cm) was painted flat black and the floors were divided into 14.4 cm squares. Illumination was provided by a 24 W red bulb suspended 85 cm above the center of the field. Mice were tested individually beginning 1.5 hr after the onset of the dark cycle. The number of squares entered with all four paws (activity), time spent in the squares adjacent to the walls (edge time), and boluses were recorded for a single 5 minute trial each day for seven consecutive days. Order of testing was randomized and the field was cleansed thoroughly after each trial.
Experiment 2 Number of animals and mean blood pressure values for the male mice of Experiment 2 are also shown in Fig. 1. These mice (240 + 20 days of age) were extreme blood pressure groups that were obtained as follows. High and low blood pressure mice from Schlager's generation 14 stock were crossed to produce and F[ hybrid which was crossed to produce a segregating F 2 generation of mice with a wide range of blood pressure values. Mice with mean systolic blood pressure values of 110 mm Hg and higher and 75 mm Hg and lower were arbitrarily assigned to high and low blood pressure groups. If pleiotropy, genetic linkage, or some other form of causal relationship explains the differences in activity level between the high and low blood pressure mice of Experiment 1, Experiment 1 findings will be replicated with the extreme blood pressure groups of Experiment 2 [1, 2, 4].
RESULTS Blood pressure records were obtained approximately one month prior to open field testing using the same apparatus (Texas Instrument) and procedure (tail occlusion method in unanesthetized mice) as that used by Schlager [13]. Mice were restrained in a mouse holder mounted on a thermostatically controlled warming plate (37.5 +- 1.0°C), and systolic blood pressure (mm Hg) was recorded by occluding the flow of blood in the tail and detecting the return of the pulse distal to the cuff upon deflation. Six blood pressure measurements were taken on a single day for each mouse and averaged. The validity of the indirect tail-cuff method in unanesthetized hypertensive rats has been established [10] and the reliability of the procedure for Schlager's stocks [ 13], based on simultaneous direct (carotid artery) and indirect measurements of systolic pressure under anesthesia, has been reported. Schlager (unpublished) has obtained correlations of r = .94, N = 11, and r = .97, N = 7, between diastolic and systolic blood pressure recordings for high and low blood pressure mice (BP I). Increased heart/body weight ratios for the high blood pressure mice have been reported for mice bred in our laboratory from Schlager's stock and for mice bred in Schlager's laboratory
The range of scores for boluses and edge time (Experiments 1 and 2) were so restricted that statistical comparisons were meaningless. Activity scores for Experiments 1 and 2 (Fig. 2) were averaged over trials as the Age x Blood Pressure x Trials interactions were nonsignificant (p's>0.05).
Experiment 1 Blood Pressure and Age main effects were significant (p's0.05). Age effects were significant for both blood pressure groups (p's
BRIEF COMMUNICATION Blood Pressure Extremes and Activity in Aging Mice 1 M E R R I L L F. E L I A S
Department o f Psychology, University o f Maine at Orono, Orono, ME 04473 and C L Y D E A. P E N T Z , III
Department o f Psychology, Syracuse University, Syracuse, N Y 13201 (Received 22 July 1977) ELIAS, M. F. AND C. A. PENTZ, III. Blood pressure extremes and activity in aging mice. PHYSIOL. BEHAV. 19(6) 811-813, 1 9 7 7 . - Mice selected genetically for extremes in blood pressure were compared with regard to open field activity. For three younger groups, the high blood pressure line (stock) exhibited significantly lower levels of activity than the low blood pressure line. The low blood pressure stock exhibited a decline in activity from 510 to 750 days of age. Comparisons of extreme blood pressure groups from an F 2 population of mice, derived from an initial cross of high and low blood pressure lines, revealed no differences in activity. Thus, it was concluded that differences in activity for young adult high and low blood pressure mice of similar ages could not be attributed to genetic linkage, pleiotropy or a more direct causal relationship between blood pressure extremes and behavior. Blood pressure
Activity level
Genetic tests
Age differences
G E N E T I C selection provides a useful t o o l for studying the relationship b e t w e e n e x t r e m e physiological traits and behavior [2,11]. There have been a n u m b e r of studies of activity level [9, 12, 14] in the s p o n t a n e o u s l y hypertensive rat (SHR), a strain derived f r o m a K y o t o stock of Wistar albino rats [ 8 ] . The SHR rat is m o r e active in the open field w h e n c o m p a r e d to a variety of n o r m o t e n s i v e strains [9, 12, 14]. The present e x p e r i m e n t (1) extends data on blood pressure e x t r e m e s and open field activity to include Schlager's high and low blood pressure mouse stocks [ 13 ], and (2) compares activity scores for mice that have experienced e x t r e m e s in blood pressure values over a significant p o r t i o n of the life-span. The literature indicates slowing of response and i m p a i r m e n t in cognitive functioning for middle aged and old hypertensive h u m a n s relative to controls (see 2). Thus, it would seem i m p o r t a n t to compare old as well as y o u n g and middle aged high and low blood pressure mice in studies designed to provide i n f o r m a t i o n on the value of hypertensive rodents as models for the study o f hypertension and behavior. Comparisons of high and low blood pressure stocks f r o m
Open field behavior
Hypertension
the same f o u n d a t i o n stock, or high blood pressure mice and ancestral control stocks (e.g., SHR with Kyoto-Wistar), do n o t allow the investigator to conclude that differences in behavior are related to blood pressure values either in a direct causal m a n n e r (e.g., some pathological c o n d i t i o n o f the nervous system) or by virtue of the fact that similar genes influence b o t h blood pressure and behavior via pleiotropy o r linkage. The association b e t w e e n b l o o d pressure and behavior may simply reflect the fact that t w o different genotypes (strains or genetically diverse stocks) differ with respect to blood pressure values and the behavior(s) of concern. In this instance there is a non-causal relationship b e t w e e n blood pressure and behavior. The present study illustrates one of several [4,1 1 ] tests of the non-causal hypothesis. To our knowledge, these tests have n o t been done in those studies which have c o m p a r e d hypertensive and normotensive rat strains. METHOD
Experiment 1 N u m b e r of animals, mean ages, and mean systolic blood
' This research was supported, in part, by a research grant from the National Institute on Aging (AG-00868) to MFE. Blood pressure stocks were generously provided by G. Schlager from his BP I high and low blood pressure stocks. 811
812
ELIAS AND PENTZ
pressure values (mm Hg) are shown in Fig. 1. Mice in Experiment 1 were progeny of matings of generation 14 high and low blood pressure stocks from Schlager's selection program [ 13 ]. Derivation of Schlager's stocks has been described in this journal [4] and elsewhere in detail [1, 2, 5, 13]. Briefly, these stocks resulted from two-way selection from a common foundation stock which was formed from an eight-way cross of eight inbred strains. At generation 7 of selection they differed significantly from each other and from a randomly mated control line [13]. These are not inbred strains as double first cousin and sib matings were avoided throughout most of the selection program (inbreeding coefficients at generation 12 were 30 and 29 percent for the high and low lines respectively). Pulse rate does not differ significantly for the high and low blood pressure mice [ 13 ].
140
I
8
£: ~30 E E
,o
13
High BP ~
120
.igh Bp
I
w
,oo
8 S aa 90 _o dt- 70 ~.) ix
6
Low BP
5O
Exp. I
ExplT
(F 2 )
,2'o
{'_ iO)
215
5,'0
('tO)
MEAN
I
750
(* tO) (-.t 10) AGE IN DAYS (-~ S E M )
//
I 240 (*-20)
FIG. 1. Number of subjects per group and mean systolic (mm Hg) blood pressure values (+_ SEM) for the four age groups of Experiment 1 and the single age group of Experiment 2.
and shipped to our laboratory [5]. Further, the high blood pressure mice exhibit increased protein synthesis in heart and other biochemical evidence for cardiac hypertrophy [6]. All mice were male, and were maintained on a diet of Purina chow and tap water (ad lib) throughout the experiment. Day/night cycle was 12/12 and colony and test room temperature was 24 + 3 ° C. Mean body weights (range = 2 8 - 3 0 g) did not differ significantly across age groups or between blood pressure groups. The open field arena (91.44 x 91.44 × 20.32 cm) was painted flat black and the floors were divided into 14.4 cm squares. Illumination was provided by a 24 W red bulb suspended 85 cm above the center of the field. Mice were tested individually beginning 1.5 hr after the onset of the dark cycle. The number of squares entered with all four paws (activity), time spent in the squares adjacent to the walls (edge time), and boluses were recorded for a single 5 minute trial each day for seven consecutive days. Order of testing was randomized and the field was cleansed thoroughly after each trial.
Experiment 2 Number of animals and mean blood pressure values for the male mice of Experiment 2 are also shown in Fig. 1. These mice (240 + 20 days of age) were extreme blood pressure groups that were obtained as follows. High and low blood pressure mice from Schlager's generation 14 stock were crossed to produce and F[ hybrid which was crossed to produce a segregating F 2 generation of mice with a wide range of blood pressure values. Mice with mean systolic blood pressure values of 110 mm Hg and higher and 75 mm Hg and lower were arbitrarily assigned to high and low blood pressure groups. If pleiotropy, genetic linkage, or some other form of causal relationship explains the differences in activity level between the high and low blood pressure mice of Experiment 1, Experiment 1 findings will be replicated with the extreme blood pressure groups of Experiment 2 [1, 2, 4].
RESULTS Blood pressure records were obtained approximately one month prior to open field testing using the same apparatus (Texas Instrument) and procedure (tail occlusion method in unanesthetized mice) as that used by Schlager [13]. Mice were restrained in a mouse holder mounted on a thermostatically controlled warming plate (37.5 +- 1.0°C), and systolic blood pressure (mm Hg) was recorded by occluding the flow of blood in the tail and detecting the return of the pulse distal to the cuff upon deflation. Six blood pressure measurements were taken on a single day for each mouse and averaged. The validity of the indirect tail-cuff method in unanesthetized hypertensive rats has been established [10] and the reliability of the procedure for Schlager's stocks [ 13], based on simultaneous direct (carotid artery) and indirect measurements of systolic pressure under anesthesia, has been reported. Schlager (unpublished) has obtained correlations of r = .94, N = 11, and r = .97, N = 7, between diastolic and systolic blood pressure recordings for high and low blood pressure mice (BP I). Increased heart/body weight ratios for the high blood pressure mice have been reported for mice bred in our laboratory from Schlager's stock and for mice bred in Schlager's laboratory
The range of scores for boluses and edge time (Experiments 1 and 2) were so restricted that statistical comparisons were meaningless. Activity scores for Experiments 1 and 2 (Fig. 2) were averaged over trials as the Age x Blood Pressure x Trials interactions were nonsignificant (p's>0.05).
Experiment 1 Blood Pressure and Age main effects were significant (p's0.05). Age effects were significant for both blood pressure groups (p's
