Blood pressure extremes and activity in aging mice

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's<0.01). Planned comparisons with the Tukey a test [15] were done for each age group and blood pressure group despite the fact that the Age x Blood Pressure interaction was not significant (p>0.05). Age effects were significant for both blood pressure groups (p's<0.0 I), and significantly greater activity was observed for the low than for the high blood pressure mice (p's<0.05) for the three younger groups but not for the oldest group (p>0.05). Moreover, it was the low blood pressure group rather than the high blood pressure group that exhibited a significant decline in activity when 510 and 750 day old groups were compared (p<0.01).

BLOOD PRESSURE, AGING AND ACTIVITY

813

22C LOW 8P

2OC High BP =E 18C

_z 16C u~ o 14C Exp. [

Exp. 1T (F2)

12C

IO0

I

1-~10|

I

t

(riO)

1"-I0) MEAN

1-*10}

(-'20)

AGE IN D A Y S ( t S E M )

FIG. 2. Mean number of crossings (-+ SEM) for the four age groups of Experiment 1 and the single age group of Experiment 2. Experiment 2 As is apparent f r o m Fig. 2, there was no significant difference in activity level for the high and low b l o o d pressure groups of the F 2 generation. DISCUSSION The lower level of activity exhibited by the y o u n g e r

mice of E x p e r i m e n t 1 is in agreement with data obtained for 380 day old mice from generation 13 of Schlager's selection program that were tested in our laboratory for eight consecutive days under exactly the same conditions as the present e x p e r i m e n t (mean crossing high BP I = 152 13.6; n = 15; mean crossing low BP I = 225 16.5, n = 13). A question m a y be raised as to whether the decline in activity level b e t w e e n 510 and 750 days of age for the low blood pressure mice should p r o m o t e speculation regarding low blood pressure in old age and its relationship to impaired cerebral blood flow in s o m e animals [7]. G i v e n the fact that a n u m b e r of inbred strains exhibit a decline in activity during the latter part of the life span [ 3 ] , it does not seem that this speculation is warranted. Results of E x p e r i m e n t 2 (F 2) indicate that the relationship b e t w e e n blood pressure and activity level for the y o u n g e r groups of E x p e r i m e n t 1 reflects a spurious accident o f selection. This does not imply that differences in activity are unrelated to general differences in g e n o t y p e for the high and low blood pressure stocks of E x p e r i m e n t 1. It does indicate that the higher levels of activity for the younger groups of low blood pressure mice m a y n o t be attributed to pleiotropy, genetic linkage or a more directly apparent causal relationship b e t w e e n blood pressure and activity level for these stocks. Rejection of the causal relationship hypothesis for these mouse stocks does not indicate that differences in activity b e t w e e n hypertensive and n o r m o t e n s i v e rat strains [9, 12, 14] are not due to causal relationships b e t w e e n blood pressure extremes and activity. Rather, t h e y indicate that the presence of causal relationships cannot be assumed in the absence of one o f the available genetic tests of the causality hypothesis [ 11 ] or direct evidence of a physiological nature [ 16 ].

REFERENCES 1. Elias, J. W., M. F. Elias and G. Schlager. Aggressive social interaction in mice genetically selected for blood pressure extremes. Behav. Biol. 13: 155-166, 1975. 2. Elias, M. F. Some contributions ofMus musculus to the study of hypertension and behavior over the life span: methodological considerations and useful directions. In: Genetic Effects on Aging, edited by D. Harrison. White Plains: The National Foundation, in press. 3. Elias, M. F. and P. K. Elias. Drive, motivation and activity. In: The Handbook on the Psychology of Aging, edited by J. E. Birren and K. W. Schaie. New York: Von Nostrand Reinhold, 1977. 4. Elias, M. F. and G. Schlager. Discrimination learning in mice genetically selected for high and low blood pressure: Initial findings and methodological implications. Physiol. Behav. 13: 261-267, 1974. 5. Elias, M. F., R. N. Sorrentino, C. A. Pentz, III, J. R. Florini and G. Schlager. Spontaneously hypertensive mice: A potential genetic model for the study of the relationship between heart size and blood pressure. Exp. Aging R esch. 1: 251-266, 1975. 6. Florini, J. R., S. Geary, Y. Saito and R. N. Sorrentino. Changes in protei n synthesis in heart. In: Explorations in Aging, edited by V. J. Cristofalo, J. Roberts and R. C. Adelman. New York: Plenum, 1975, 149-162. 7. Obrist, W. D. Cerebral ischemia and the senescent electroencephalogram. In: Cerebral Ischemia, edited by E. Simson and T. H. McGavack. Springfield: Thomas Publishing, 1964. 8. Okamoto, K. and K. Aoki. Development of a strain of spontaneously hypertensive rats. Jap. Circ. J. 27: 282-293, 1963.

9. Pappas, B. A., D. A. Peters, M. Saari, S. K. Sobrian and E. Minch. Neonatal 6-hydroxydopamine sympathectomy in normotensive and spontaneously hypertensive rats. Pharmac. Biochem. Behav. 2: 381-386, 1974. 10. Pfeffer, J. M., M. A. Pfeffer and E. D. Frohlich. Validity of an indirect tail-cuff method of determining systolic arterial pressure in unanesthetized normotensive and spontaneoulsy hypertensive rats. J. Lab. Clin. Med. 78: 957-962, 1971. 11. Roderick, T. Y., R. E. Wimer and C. Wimer. Genetic manipulation of neuroanatomical traits. In: Knowing, Thinking and Behaving. A Festschrift for Professor David Krech, edited by L. Petrinovich and J. L. McGaugh. New York: Plenum, 1975. 12. Rosencrans, J. A. and M. D. Adams. Brain 5-hydroxytryptamine correlates of behavior: Studies involving spontaneously hypertensive (SHR) and normotensive Wistar rats. Pharmac. Biochem. Behav. 5: 559-564, 1976. 13. Schlager, G. Selection for blood pressure levels in mice. Genetics 76: 537-549, 1974. 14. Shimamoto, K. and A. Nagaoka. Behavioral and pharmacological characteristics of the spontaneously hypertensive rat. In: Spontaneous Hypertension, edited by K. Okamoto. Tokyo: Igaku Shoin, 1972, pp. 86-88. 15. Winer, B. Statistical Principles in Experimental Design. New York: McGraw-Hill, 1962. 16. Yambe, H., W. DeJong and W. Lovenberg. Further studies on catecholamine synthesis in the spontaneously hypertensive rat: Catechomamine synthesis in the central nervous system. Eur. J. Pharmac. 22: 91-93, 1973.