Cardiovascular Responses to Avoidance Conditioning in the Dog: Effects of Beta Adrenergic Blockade DAVID E. ANDERSON, P H D , AND JOSEPH V. BRADY, P H D
Five laboratory dogs were trained to press a response panel to postpone shocks during 1-hi free-operant avoidance conditioning sessions. In addition, each dog was confined in the experimental environment for 1 hr immediately prior to each avoidance session. During these 2-hr sessions, blood pressure and heart rate were monitored continuously from indwelling catheters. After repeated exposure to the schedule, the onset of the avoidance contingency elicited acute increases in systolic and diastolic pressure and heart rate, which were maintained throughout the session. During the preavoidance interval, systolic and diastolic pressure increased gradually while heart rate decreased. Blockade of beta adrenergic receptor activity in these behaviorallytrained dogs by infusions of propranolol was associated with a significant attenuation of the tachycardia normally maintained during avoidance. Beta blockade did not prevent the rise in blood pressure or fall in heart rate during preavoidance. The results are consistent with the view that the cardiovascular pattern sustained during avoidance is mediated by activation of the sympathetic nervous system, but that the progressive change in cardiovascular activity during preavoidance intervals is mediated by other than sympathetic influences.
culatory adaptations has been assessed in primates (6), and effects of alpha adrenerFree-operant avoidance conditioning has gic blocking agents have been investigated been associated with acute elevations in in avoidance-trained dogs (7). arterial blood pressure in several studies Enclosure of laboratory dogs in the exwith laboratory dogs and primates (1-5). perimental environment for fixed time The pressure elevations occurring under periods immediately preceding avoidance these conditions are mediated by in- sessions has been associated with a precreased heart rate and cardiac output (2,3, paratory cardiovascular pattern, charac5), since total peripheral resistance in- terized by gradual increases in arterial and creases significantly only after long- pulse pressure, mediated solely by progduration experimental sessions (3). The ressive increases in total peripheral resiscontribution of the autonomic nervous tance. Concurrently, heart rate and cardiac system in the development of such cir- output decrease (2). This response has been observed in well-trained animals during preavoidance periods as long as 15 from the Division of Behavioral Biology, Depart- hr (8) but does not emerge preceding sesment of Psychiatry and Behavioral Sciences, The sions in which operant behavior is mainJohns Hopkins University School of Medicine, Baltitained by food reinforcement (9). more, Maryland 21205. Presented in part at the Annual Meeting, American The present study investigates the role Psychosomatic Society, April 7, 1973, Denver, Col- of beta adrenergic receptor activity in the orado. Received for publication July 2,197 5; final revision mediation of blood pressure and heart rate received January 16, 1976 patterns during preavoidance and avoiINTRODUCTION
Psychosomatic Medicine Vol. 38, No. 3 (May-June 1976) Copyright ® 1976 by the American Psychosomatic Society, Inc. Published by American Elsevier Publishing Company, Inc.
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dance periods in laboratory dogs. It is generally agreed (10) that these receptors mediate sympathetic effects upon the heart, while alpha adrenergic receptors mediate sympathetic effects upon the peripheral vessels. In this study, blood pressure and heart rate patterns observed during preavoidance and avoidance periods in dogs whose beta adrenergic receptors were blocked by propranolol infusions were compared with blood pressure and heart rate patterns observed during preavoidance and avoidance in the same animals during nondrug, control sessions. METHODS Subjects and Apparatus Five adult male mongrel dogs, weighing between 13 and 17 kg, served as subjects. Each dog was restrained by a specially designed harness in an experimental chamber (11), which permitted freedom of movement in the chamber but afforded protection for cardiovascular monitoring equipment and shock electrodes. A translucent nose key ( 1 3 x 8 x 1 / 2 mm), mounted on the front chamber wall, activated a microswitch adjusted to approximately 15 g of inward pressure to provide the recorded operant for shock avoidance. Illumination of the response key provided a visual stimulus for performance discrimination. Electrical shock, generated from a constantcurrent source (12), was administered through stainless-steel electrodes taped to a shaved portion of the dog's rear leg. Sound masking and temperature control were provided by a blower and exhaust system mounted on the roof of the chamber. Food and water were freely available to the animals in their home cages following each experimental session.
Behavioral Conditioning Procedure Avoidance conditioning was initiated after the subject was custom fitted to the restraint harness and adapted to the experimental environment. The avoidance procedure required the dog to postpone shocks by pressing the response key within a prespecified interval following a previous response (13). Brief 60-Hz shocks (2-5 mA for 0.5 sec) were programmed
182
every 20 sec unless the dog pressed the response panel within that interval and postponed the shock another 20 sec. The procedure generated stable panel pressing in all animals at rates in excess of 15 per minute, thus avoiding all but an occasional shock (i.e., less than one per hour). The required avoidance performance was scheduled during each of two daily experimental sessions, each of 1-hr duration, signaled by illumination of the response panel. In addition, the dog was installed in the harness 1 hr before the beginning of each session, and no lights or shocks were ever presented during these preavoidance intervals. Sessions were run 7 days per week, and training continued until a behavioral discrimination was established between preavoidance and avoidance segments of each session. Each dog learned to sit quietly during the preavoidance period and to initiate panel-pressing behavior as soon as the panel light was illuminated, signaling the onset of the avoidance contingency. The experimental stimuli were programmed and behavioral responses recorded automatically and remotely through the use of electromechanical relay circuitry.
Cardiovascular Monitoring Procedure Prior to aseptic cardiovascular surgery, each dog was anesthetized with sodium pentobarbital (35 mg /kg), and an 18-gauge polyvinyl catheter was implanted into the aorta via a femoral or carotid artery. A second catheter was implanted into the right atrium via an external jugular vein. The catheters were tunneled under the skin and exited from the body at the nape of the neck. The arterial catheter was filled with a heparin-saline mixture, the venous catheter with saline, and both were obturated and protected by a leather collar. During experimental sessions, the arterial catheter was attached to a Statham strain-gage transducer (P23De) affixed to the front yoke of the restraint harness. Slow infusion of lightly heparinized saline (7 USP units per ml at a rate of 12 ml /hr) via a Harvard peristaltic pump maintained catheter patency during these sessions. A Beckman dynograph (model 382) or, in some cases, a cathode ray oscilloscope, was used to monitor the pressure signal. An electronic system (14) recorded maximal and minimal voltages corresp onding to systolic and diastolic pressure (mm Hg) on a beat-by-beat basis, and printed out averages over successive 10-min intervals. Heart rate (beats /min) was determined from the pressure signal by a Schmitt trigger system and also printed out online.
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Beta Blockade Procedure Each dog was run in two daily experimental sessions (the first at 10 a.m. and the second at 2 p.m.) until consistent behavioral and cardiovascular patterns emerged, as determined by inspection of the data. During the next 7 days, the second daily session was run after the beta adrenergic system had been pharmacologically blocked. Preliminary work had shown that propranolol hydrochloride, when administered in a dose of 1 mg /kg, suppresses for at least 45 min the rise in heart rate elicited by a 0.25-/ng/kg administration of isoproterenol (15). Therefore, a dose of 1 mg /kg was slowly infused into the right atrial catheter 15 min before the beginning of the afternoon experiments. In addition, a second infusion was administered 30 min after the beginning of the preavoidance period, and a third dose 15 min after the beginning of the avoidance period. The second and third doses were administered by a syringe pump attached to an automatic timing system, so as not to interfere with the ongoing behavior of the subject. The patterns of cardiovascular activity observed during preavoidance and avoidance in the seven beta blockade sessions
with each dog were compared with the cardiovascular patterns recorded during preavoidance and avoidance in the seven nondrug, control sessions for that dog. This alternating schedule provided a control for any systematic changes that might take place over time as a result of extended work with each animal.
RESULTS
Cardiovascular Patterns during Control Sessions The solid lines in Fig. 1 show average levels of blood pressure and heart rate over successive 10-min intervals during the 35 nondrug, control sessions for the group of five dogs (i.e., the morning sessions). During the preavoidance hour, systolic pressure increased gradually but significantly (X = 14.3 mmHg;t = 9.4;p 0.01), diastolic pressure also increased gradually but significantly (X = 8.6mmhg;t = 6.5; p 0.01), and heart rate decreased gradually but significantly (X = 4.0bpm;t = 2.72;p 0.01). Table 1 summarizes average changes in blood pressure and heart rate from the beginning to the end of preavoidance for
TABLE 1. Mean Differences in Systolic and Diastolic Pressure and Heart Rate between the First and Last 10 min of the Preavoidance Interval, Averaged for 35 Control and 35 Beta Blockade Experiments for Each Dog Control Subject 1 2 3 4 5 Croup SE
Beta blockade
Systolic
Diastolic
Heart rate
Systolic
Diastolic
Heart rate
16.4 25.8 8.5 12.0 10.8
11.3 14.4 2.5 8.4 3.8
- 4.8 -11.0 - 3.8 3.7 - 5.5
7.8 17.2 4.6 8.3 9.3
4.0 12.7 1.2 5.4 3.0
- 8.3 -10.6 - 5.4 - 5.7 - 6.0
14.3 1.5
8.6 1.3
- 4.0 1.4
9.7 1.2
5.5 1.1
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7.2 0.8
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Systolic
BLOOD
130
PRESSURE
HEART RATE (bpm)
N=35 .
. Control
•—«
Propronolo
TIME IN MINUTES Pre-Avoidonce A
Fig. 1. Mean levels of systolic and diastolic pressure and heart rate during successive 10-min intervals of the preavoidance and avoidance periods, averaged for the group of five dogs over 35 control experiments and 35 experiments in which beta adrenergic activity was blocked by infusion of propranolol. The vertical lines show standard errors.
each dog individually, and shows the consistency with which this effect was observed between subjects. In addition, pulse pressure increased significantly during preavoidance (X = 5.7 nun Hg; t = 7.31; p 0.01). The onset of the avoidance contingency 184
was associated with immediate and significant elevations in both blood pressure and heart rate above levels observed at the end of the preavoidance period (Fig. 1). Systolic pressure was 8.9 mm Hg higher (t = 4.8; p 0.01) and diastolic pressure was 13.5 mm Hg higher (t = 6.5; p 0.01) during
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the first 10 min of avoidance than during the last 10 min of preavoidance. Table 2 presents average differences in each cardiovascular measure between these two intervals, summarized individually for each dog, and shows that blood pressure increases were observed in four dogs, and that heart rate increases were observed in all five. Concurrently, the onset of avoidance was accompanied by a heart rate increase of 28.2 beats per minute above final preavoidance levels (t = 9.6; p 0.01). This acceleration in heart rate was observed in all five subjects (Table 2). Elevations in blood pressure and heart rate were maintained above preavoidance values for the duration of the avoidance period (Fig. 1). Cardiovascular Patterns during Beta Blockade Sessions The dotted lines in Fig. 1 show average levels of systolic and diastolic pressure and heart rate over successive 10-min intervals of preavoidance and avoidance periods for the 35 sessions in which beta activity was suppressed by propranolol administration in all five dogs. Average levels of systolic and diastolic pressure and heart rate during the first 10 min of
preavoidance periods were not significantly different from values observed over this interval in control experiments (Fig. 1). Over the duration of the preavoidance periods, blood pressure gradually increased while heart rate gradually decreased. Table 1 presents differences for each cardiovascular measure between the first and last 10 min of the preavoidance hour in beta blockade sessions, and shows that the average increase in systolic pressure of 9.7 mm Hg (t = 8.1; p < 0.01), the average increase in diastolic pressure of 5.5 mm Hg (t = 4.9; p < 0.01), and the average decrease in heart rate of 7.2 beats per minute (t = 9.1; p < 0.01) were representative of each individual dog. Table 1 also shows that somewhat smaller increases in blood pressure and somewhat larger decreases in heart rate were observed in beta blockade sessions than in control sessions. Inspection of Fig. 1 shows that the rates of change for each measure were comparable for the first 50 min of control and beta blockade sessions. During the last 10 min of preavoidance, however, heart rate increased in control sessions but continued to decrease in beta blockade sessions. Similarly, during the last 10 min of preavoidance, blood pressure continued to increase in control ses-
TABLE 2. Mean Differences in Systolic and Diastolic Pressure and Heart Rate between the last 10 min of Preavoidance and the First 10 min of Avoidance, Averaged for seven control and seven Beta Blockade Sessions for Each Dog Beta blockade
Control 1 2 3 4 5 Croup SE
17.0 18.5 4.1 8.1 - 2.0
20.3 22.6 12.1 14.0 - 0.7
28.2 35.6 28.2 45.2 5.6
20.5 24.1 8.0 14.6 1.6
18.7 25.9 11.8 18.0 1.3
15.7 26.1 13.8 25.3 4.3
8.9 1.9
13.5 2.0
28.2 2.9
13.8 2.2
15.2 2.2
17.2 2.4
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sions but not in beta blockade sessions. This difference was observed in all five dogs and suggests that an additional mechanism may participate in the cardiovascular response toward the end of the preavoidance periods, which tends to amplify the pressure change and reduce the magnitude of bradycardia. The onset of the avoidance contingency in beta blockade sessions was associated with an immediate increase in blood pressure, which was maintained at levels not significantly different from those observed during avoidance in control sessions (Fig. 1). Table 2 presents mean differences for systolic and diastolic levels between the end of the preavoidance and the first 10 min of the avoidance sessions in beta blockade sessions, and shows that the average increases in systolic pressure (X = 13.9; t = 6.4; p < 0.01) and diastolic pressure (X = 15.2; t = 6.8; p < 0.01) were observed consistently in four subjects. The onset of the avoidance contingency in beta blockade sessions, however, was associated with increases in heart rate that were significantly reduced from those observed during control sessions (X = 17.2; t = 7.3; p < 0.01). This attenuation of the avoidance-associated tachycardia was observed consistently in all five subjects (Table 2) and represented, on the average, a 38% diminution in the effect observed under control conditions.
DISCUSSION The results of the experiments show that blockade of beta adrenergic activity in dogs trained on a free-operant avoidance task significantly attenuated the tachycardia accompanying avoidance performances, but did not attenuate the eleva186
tions in blood pressure occurring under these conditions. Moreover, beta blockade did not prevent the occurrence of gradual increases in blood pressure and decreases in heart rate during preavoidance intervals. Heart rate levels at the beginning of propranolol sessions were not significantly different from heart rate levels at the beginning of nondrug control sessions. This finding is consistent with the results of a previous study of propranolol effects upon the circulation of the unanesthetized and chronically-instrumented laboratory dog at rest (16) and indicates that sympathetic tone upon the heart was minimal at the beginning of these experimental sessions. This suggests that the deceleration in heart rate was not mediated by decreased sympathetic activity, but may be accounted for by increases in parasympathetic stimulation. It is well known that decreases in heart rate, which result from baroreceptor reflexes to rising arterial pressure, are mediated by the parasympathetic nervous system (17). However, the vagally-mediated bradycardia during preavoidance may be related to a physiological reflex that facilitates "sensory intake" behavior. During the orienting reflex (18) or the preparatory period of a reaction time task (19,20) somatic motor activity and respiration rate decrease. In addition, "sensory intake" behavior has been associated recently with an increase in peripheral vascular resistance (21). Increases in total peripheral resistance have been reported in dogs during preavoidance intervals (2, 5), and we have observed decreases in respiration rate well below basal levels under these conditions. Respiratory inhibition has been associated in man with an increase in blood pressure, a decrease in heart rate, and a redistribution in blood flow away from the
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skeletal musculature, conserving oxygen for the heart and brain (22,23). This reflex is similar to that observed in the diving reflex, in which marked increases in peripheral resistance occur together with large magnitude reductions in heart rate and cardiac output (24). The cardiac effects during diving are mediated vagally (25), but the pathways mediating the change in peripheral vascular function remain to be determined, as does the mechanism accounting for the similar preavoidance cardiovascular pattern. During avoidance performance periods, heart rates were significantly lower in propranolol experiments than in nondrug, control experiments, suggesting that the tachycardia accompanying avoidance responses was mediated in part by activation of the sympathetic nervous system. A concurrent decrease in parasympathetic activity also may have contributed to the elevations in heart rate during avoidance. Cardiac output measurements were not made during these experiments, but virtually all other studies assessing propranolol effects upon both heart rate and cardiac output have reported parallel effects upon the two measures (for a review, see Ref. 26). Previous work in our laboratory and elsewhere (2, 5) has shown that cardiac output as well as heart rate is elevated during avoidance sessions, and it seems likely that the beta blocking drug decreased the magnitude of elevation in cardiac output associated with avoidance. In this case, it may be inferred that total peripheral resistance was higher during avoidance in beta blocking experiments than in control experiments, since arterial pressure was equally elevated under both conditions. Decreases in cardiac output and increases in total peripheral resistance with variable pressure effects have been reported frequently in acute studies with laboratory
animals (26), and this pattern appears to be a function of propranolol effects upon the periphery as well as the heart. Beta blocking drugs have been shown to change the vasodilator effects of epinephrine to vasoconstrictor effects, and to intensify the vasoconstrictive effects of norepinephrine (27). Moreover, it has been found that cardiac output decreases continue to be observed following beta blockade in subjects whose heart rates are maintained at preblockade levels by ventricular pacing (28). Further studies are needed to differentiate the role of the peripheral circulation from other factors (e.g., strength and speed of myocardial contraction) in determining cardiac output changes associated with beta blockade. In any event, the results of these experiments showing that elevations in blood pressure accompanying avoidance performances are not attenuated by propranolol are consistent with similar experiments with primates (6). The cardiovascular pattern observed during avoidance sessions is similar to that elicited in anesthetized animals by electrical stimulation of areas of the hypothalamus that mediate "fight or flight" behavior (29). The potential role of this "defense alarm" reflex in the pathogenesis of essential hypertension has been discussed extensively (30), although long-term experiments with rodents and primates testing the validity of this hypothesis have met with limited success (30-33). Elevations in blood pressure mediated by increased vascular resistance have been observed less frequently in studies with animals, although a potential role for this response in the pathogenesis of fixed essential hypertension has been suggested (34). It remains for future research to clarify the nature of the physiological systems mediating the cardiac and particularly vascular adaptations
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observed under conditions that elicit acute elevations in peripheral resistance, and to determine the relevance of these factors in the etiology of hypertensive adaptations.
This research was supported by NHLI Grants HL-06945 and HL 17970.
The authors express their appreciation to Dr. Sarjono O. Santoso /or expert pharmacoJogica] advice and to Robert Baer, Judy StribJing, David Siscovick, and John YingJing for helpful technical assistance. We are also grateful to Dr. R. O. Davies of Ayerst Laboratories for the propranolol used in these experiments.
REFERENCES 1. Anderson DE, Brady JV: Pre-avoidance blood pressure elevations accompanied by heart rate decreases in the dog. Science 172:595, 1971 2. Anderson DE, Tosheff JG: Cardiac output and total peripheral resistance changes during pre-avoidance periods in the dog. J Appl Physiol 35:650, 1973 3. Forsyth RP: Regional blood flow changes during 72-hour avoidance schedules in the monkey. Science 173:546, 1971 4. Herd JA, Morse WH, Kelleher RT, et al.: Arterial hypertension in the squirrel monkey during behavioral experiments. Am J Physiol 217:24, 1969 5. Lawler JE, Obrist PA, Lawler KA: Cardiovascular function during pre-avoidance, avoidance, and postavoidance in dogs. Psychophysiology 12:4, 1975 6. Kelleher R, Morse WH, Herd JA: Effects of propranolol, phentolamine, and methyl atropine on cardiovascular function in the squirrel monkey during behavioral experiments. Pharmacol Exp Ther 182:204,1972 7. Anderson DE, Yingling JE, Brady JV: Cardiovascular responses to avoidance conditioning in the dog: Effects of alpha adrenergic blockade. Pavl J Biol Sci, 1975 (in press) 8. Anderson DE, Brady JV: Prolonged pre-avoidance effects upon blood pressure and heart rate in the dog. Psychosom Med 35:4, 1973 9. Anderson DE, Brady JV: Differential preparatory cardiovascular responses to aversive and appetitive behavioral conditioning. Cond Reflex 7:82, 1972 10. Ahlquist RP: A study of the adrenotropic receptors. Am J Physiol 153:586, 1948 11. Anderson DE, Daley LA, Findley JD, et al.: A restraint system for the psychophysiological study of dogs. Behav Res Meth Instr 2:191,1970 12. Swinnen ME, Brady JV, Powell MG: A new device for the application of electric shock. Behav Res Meth Instr 1:184, 1969 13. Sidman M: Avoidance conditioning with brief shock and no exteroceptive warning signal. Science 118:157, 1953 14. Swinnen ME: Blood pressure digitizer. Proc Ann Conf Engr Med Biol 10:184, 1968 15. Hayes A, Cooper RG: Studies on the absorption, distribution and excretion of propranolol in the rat, dog and monkey. J Pharm Exp Ther 176:302, 1971 16. Pitt B, Elliott EC, Khouri EM, et al.: Effect of beta-adrenergic blockade on the coronary hemodynamic response to excitement at fixed ventricular rate in the uanesthetized dog. Physiology 9:267, 1966 17. Berne RM, Levi MN: Cardiovascular Physiology. St. Louis, Mo., C.V. Mosby Co., 1972 18. Sokolov EN: Perception and the Conditioned Reflex. Oxford, Pergamon Press, 1963 19. Obrist PA, Webb RA, Sutterer JR: Heart rate and somatic changes during aversive conditioning and a simple reaction time task. Psychophysiology 5:696, 1969 20. Obrist PA, Wood DM, Perez-Reyes M: Heart rate during conditoning in humans: Effects of UCS intensity, vagal blockade, and adrenergic block of vasomotor activity. J Exp Psychol 70:32, 1965 21. Williams RB, BittkerTE, Buchsbaum MS, Wynne LC: Cardiovascular and neurophysiologic correlates of sensory intake and rejection. I. Effect of cognitive tasks. Psychophysiology 12:427, 1975 22. Schneider EC: Observations on holding the breath. Am J Physiol 94:464, 1930 23. Irving L: Changes in the blood flow through the brain and muscles during the arrest of breathing. Am J Physiol 122:207, 1938 188
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CARDIOVASCULAR RESPONSES TO AVOIDANCE CONDITIONING 24. Andersen HT: Physiological adaptations in diving vertebrates. Physiology 46:212, 1966 25. Lin YC: Autonomic nervous control of cardiovascular response during diving in the rat. Am J Physiol 227:601, 1974 26. Gibson DG: Pharmacodynamic properties of /3-adrenergic receptor blocking drugs in man. Drugs 7:8, 1974 27. Glover WE, Hutchinson KJ: The effect of /3-adrenergic antagonist (propranolol) on the cardiovascular response to intravenous infusions of noradrenaline in man. J Physiol 177:59P, 1965 28. Bloomfield DA, Sowton E: Rate-independent effects of propranolol: The differentiation between chronotropic, inotropic and peripheral vascular responses. Circ Res 21:111, 1967 29. Charvat J, Dell P, Folkow B: Mental factors in cardiovascular disease. Cardiologica 44:121, 1964 30. Folkow B, Neill E: Circulation. New York, Oxford University Press, 1971, p. 569. 31. Findley JD, Brady JV, Robinson WW, Gilliam W: Continuous cardiovascular monitoring in the baboon during long-term behavioral performances. Commun Behav Biol 6:49, 1971 32. Folkow B, Rubinstein EH: Cardiovascular effects of acute and chronic stimulations of the hypothalamic defense area in the rat. Acta Physiol Scand. 68:48, 1966 33. Forsyth RP: Blood pressure responses to long-term avoidance schedules in the restrained rhesus monkey. Psychosom Med 31:300, 1969 34. Eich RH, Peters RJ, Cuddy RP, Sulyan HI, Lyons RH: Hemodynamics in labile hypertension. Am Heart J 63:188, 1962
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Five laboratory dogs were trained to press a response panel to postpone shocks during 1-hi free-operant avoidance conditioning sessions. In addition, each dog was confined in the experimental environment for 1 hr immediately prior to each avoidance session. During these 2-hr sessions, blood pressure and heart rate were monitored continuously from indwelling catheters. After repeated exposure to the schedule, the onset of the avoidance contingency elicited acute increases in systolic and diastolic pressure and heart rate, which were maintained throughout the session. During the preavoidance interval, systolic and diastolic pressure increased gradually while heart rate decreased. Blockade of beta adrenergic receptor activity in these behaviorallytrained dogs by infusions of propranolol was associated with a significant attenuation of the tachycardia normally maintained during avoidance. Beta blockade did not prevent the rise in blood pressure or fall in heart rate during preavoidance. The results are consistent with the view that the cardiovascular pattern sustained during avoidance is mediated by activation of the sympathetic nervous system, but that the progressive change in cardiovascular activity during preavoidance intervals is mediated by other than sympathetic influences.
culatory adaptations has been assessed in primates (6), and effects of alpha adrenerFree-operant avoidance conditioning has gic blocking agents have been investigated been associated with acute elevations in in avoidance-trained dogs (7). arterial blood pressure in several studies Enclosure of laboratory dogs in the exwith laboratory dogs and primates (1-5). perimental environment for fixed time The pressure elevations occurring under periods immediately preceding avoidance these conditions are mediated by in- sessions has been associated with a precreased heart rate and cardiac output (2,3, paratory cardiovascular pattern, charac5), since total peripheral resistance in- terized by gradual increases in arterial and creases significantly only after long- pulse pressure, mediated solely by progduration experimental sessions (3). The ressive increases in total peripheral resiscontribution of the autonomic nervous tance. Concurrently, heart rate and cardiac system in the development of such cir- output decrease (2). This response has been observed in well-trained animals during preavoidance periods as long as 15 from the Division of Behavioral Biology, Depart- hr (8) but does not emerge preceding sesment of Psychiatry and Behavioral Sciences, The sions in which operant behavior is mainJohns Hopkins University School of Medicine, Baltitained by food reinforcement (9). more, Maryland 21205. Presented in part at the Annual Meeting, American The present study investigates the role Psychosomatic Society, April 7, 1973, Denver, Col- of beta adrenergic receptor activity in the orado. Received for publication July 2,197 5; final revision mediation of blood pressure and heart rate received January 16, 1976 patterns during preavoidance and avoiINTRODUCTION
Psychosomatic Medicine Vol. 38, No. 3 (May-June 1976) Copyright ® 1976 by the American Psychosomatic Society, Inc. Published by American Elsevier Publishing Company, Inc.
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DAVID E. ANDERSON AND JOSEPH V. BRADY
dance periods in laboratory dogs. It is generally agreed (10) that these receptors mediate sympathetic effects upon the heart, while alpha adrenergic receptors mediate sympathetic effects upon the peripheral vessels. In this study, blood pressure and heart rate patterns observed during preavoidance and avoidance periods in dogs whose beta adrenergic receptors were blocked by propranolol infusions were compared with blood pressure and heart rate patterns observed during preavoidance and avoidance in the same animals during nondrug, control sessions. METHODS Subjects and Apparatus Five adult male mongrel dogs, weighing between 13 and 17 kg, served as subjects. Each dog was restrained by a specially designed harness in an experimental chamber (11), which permitted freedom of movement in the chamber but afforded protection for cardiovascular monitoring equipment and shock electrodes. A translucent nose key ( 1 3 x 8 x 1 / 2 mm), mounted on the front chamber wall, activated a microswitch adjusted to approximately 15 g of inward pressure to provide the recorded operant for shock avoidance. Illumination of the response key provided a visual stimulus for performance discrimination. Electrical shock, generated from a constantcurrent source (12), was administered through stainless-steel electrodes taped to a shaved portion of the dog's rear leg. Sound masking and temperature control were provided by a blower and exhaust system mounted on the roof of the chamber. Food and water were freely available to the animals in their home cages following each experimental session.
Behavioral Conditioning Procedure Avoidance conditioning was initiated after the subject was custom fitted to the restraint harness and adapted to the experimental environment. The avoidance procedure required the dog to postpone shocks by pressing the response key within a prespecified interval following a previous response (13). Brief 60-Hz shocks (2-5 mA for 0.5 sec) were programmed
182
every 20 sec unless the dog pressed the response panel within that interval and postponed the shock another 20 sec. The procedure generated stable panel pressing in all animals at rates in excess of 15 per minute, thus avoiding all but an occasional shock (i.e., less than one per hour). The required avoidance performance was scheduled during each of two daily experimental sessions, each of 1-hr duration, signaled by illumination of the response panel. In addition, the dog was installed in the harness 1 hr before the beginning of each session, and no lights or shocks were ever presented during these preavoidance intervals. Sessions were run 7 days per week, and training continued until a behavioral discrimination was established between preavoidance and avoidance segments of each session. Each dog learned to sit quietly during the preavoidance period and to initiate panel-pressing behavior as soon as the panel light was illuminated, signaling the onset of the avoidance contingency. The experimental stimuli were programmed and behavioral responses recorded automatically and remotely through the use of electromechanical relay circuitry.
Cardiovascular Monitoring Procedure Prior to aseptic cardiovascular surgery, each dog was anesthetized with sodium pentobarbital (35 mg /kg), and an 18-gauge polyvinyl catheter was implanted into the aorta via a femoral or carotid artery. A second catheter was implanted into the right atrium via an external jugular vein. The catheters were tunneled under the skin and exited from the body at the nape of the neck. The arterial catheter was filled with a heparin-saline mixture, the venous catheter with saline, and both were obturated and protected by a leather collar. During experimental sessions, the arterial catheter was attached to a Statham strain-gage transducer (P23De) affixed to the front yoke of the restraint harness. Slow infusion of lightly heparinized saline (7 USP units per ml at a rate of 12 ml /hr) via a Harvard peristaltic pump maintained catheter patency during these sessions. A Beckman dynograph (model 382) or, in some cases, a cathode ray oscilloscope, was used to monitor the pressure signal. An electronic system (14) recorded maximal and minimal voltages corresp onding to systolic and diastolic pressure (mm Hg) on a beat-by-beat basis, and printed out averages over successive 10-min intervals. Heart rate (beats /min) was determined from the pressure signal by a Schmitt trigger system and also printed out online.
Psychosomatic Medicine Vol. 38, No. 3 (May-June 1976)
CARDIOVASCULAR RESPONSES TO AVOIDANCE CONDITIONING
Beta Blockade Procedure Each dog was run in two daily experimental sessions (the first at 10 a.m. and the second at 2 p.m.) until consistent behavioral and cardiovascular patterns emerged, as determined by inspection of the data. During the next 7 days, the second daily session was run after the beta adrenergic system had been pharmacologically blocked. Preliminary work had shown that propranolol hydrochloride, when administered in a dose of 1 mg /kg, suppresses for at least 45 min the rise in heart rate elicited by a 0.25-/ng/kg administration of isoproterenol (15). Therefore, a dose of 1 mg /kg was slowly infused into the right atrial catheter 15 min before the beginning of the afternoon experiments. In addition, a second infusion was administered 30 min after the beginning of the preavoidance period, and a third dose 15 min after the beginning of the avoidance period. The second and third doses were administered by a syringe pump attached to an automatic timing system, so as not to interfere with the ongoing behavior of the subject. The patterns of cardiovascular activity observed during preavoidance and avoidance in the seven beta blockade sessions
with each dog were compared with the cardiovascular patterns recorded during preavoidance and avoidance in the seven nondrug, control sessions for that dog. This alternating schedule provided a control for any systematic changes that might take place over time as a result of extended work with each animal.
RESULTS
Cardiovascular Patterns during Control Sessions The solid lines in Fig. 1 show average levels of blood pressure and heart rate over successive 10-min intervals during the 35 nondrug, control sessions for the group of five dogs (i.e., the morning sessions). During the preavoidance hour, systolic pressure increased gradually but significantly (X = 14.3 mmHg;t = 9.4;p 0.01), diastolic pressure also increased gradually but significantly (X = 8.6mmhg;t = 6.5; p 0.01), and heart rate decreased gradually but significantly (X = 4.0bpm;t = 2.72;p 0.01). Table 1 summarizes average changes in blood pressure and heart rate from the beginning to the end of preavoidance for
TABLE 1. Mean Differences in Systolic and Diastolic Pressure and Heart Rate between the First and Last 10 min of the Preavoidance Interval, Averaged for 35 Control and 35 Beta Blockade Experiments for Each Dog Control Subject 1 2 3 4 5 Croup SE
Beta blockade
Systolic
Diastolic
Heart rate
Systolic
Diastolic
Heart rate
16.4 25.8 8.5 12.0 10.8
11.3 14.4 2.5 8.4 3.8
- 4.8 -11.0 - 3.8 3.7 - 5.5
7.8 17.2 4.6 8.3 9.3
4.0 12.7 1.2 5.4 3.0
- 8.3 -10.6 - 5.4 - 5.7 - 6.0
14.3 1.5
8.6 1.3
- 4.0 1.4
9.7 1.2
5.5 1.1
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-
7.2 0.8
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Systolic
BLOOD
130
PRESSURE
HEART RATE (bpm)
N=35 .
. Control
•—«
Propronolo
TIME IN MINUTES Pre-Avoidonce A
Fig. 1. Mean levels of systolic and diastolic pressure and heart rate during successive 10-min intervals of the preavoidance and avoidance periods, averaged for the group of five dogs over 35 control experiments and 35 experiments in which beta adrenergic activity was blocked by infusion of propranolol. The vertical lines show standard errors.
each dog individually, and shows the consistency with which this effect was observed between subjects. In addition, pulse pressure increased significantly during preavoidance (X = 5.7 nun Hg; t = 7.31; p 0.01). The onset of the avoidance contingency 184
was associated with immediate and significant elevations in both blood pressure and heart rate above levels observed at the end of the preavoidance period (Fig. 1). Systolic pressure was 8.9 mm Hg higher (t = 4.8; p 0.01) and diastolic pressure was 13.5 mm Hg higher (t = 6.5; p 0.01) during
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the first 10 min of avoidance than during the last 10 min of preavoidance. Table 2 presents average differences in each cardiovascular measure between these two intervals, summarized individually for each dog, and shows that blood pressure increases were observed in four dogs, and that heart rate increases were observed in all five. Concurrently, the onset of avoidance was accompanied by a heart rate increase of 28.2 beats per minute above final preavoidance levels (t = 9.6; p 0.01). This acceleration in heart rate was observed in all five subjects (Table 2). Elevations in blood pressure and heart rate were maintained above preavoidance values for the duration of the avoidance period (Fig. 1). Cardiovascular Patterns during Beta Blockade Sessions The dotted lines in Fig. 1 show average levels of systolic and diastolic pressure and heart rate over successive 10-min intervals of preavoidance and avoidance periods for the 35 sessions in which beta activity was suppressed by propranolol administration in all five dogs. Average levels of systolic and diastolic pressure and heart rate during the first 10 min of
preavoidance periods were not significantly different from values observed over this interval in control experiments (Fig. 1). Over the duration of the preavoidance periods, blood pressure gradually increased while heart rate gradually decreased. Table 1 presents differences for each cardiovascular measure between the first and last 10 min of the preavoidance hour in beta blockade sessions, and shows that the average increase in systolic pressure of 9.7 mm Hg (t = 8.1; p < 0.01), the average increase in diastolic pressure of 5.5 mm Hg (t = 4.9; p < 0.01), and the average decrease in heart rate of 7.2 beats per minute (t = 9.1; p < 0.01) were representative of each individual dog. Table 1 also shows that somewhat smaller increases in blood pressure and somewhat larger decreases in heart rate were observed in beta blockade sessions than in control sessions. Inspection of Fig. 1 shows that the rates of change for each measure were comparable for the first 50 min of control and beta blockade sessions. During the last 10 min of preavoidance, however, heart rate increased in control sessions but continued to decrease in beta blockade sessions. Similarly, during the last 10 min of preavoidance, blood pressure continued to increase in control ses-
TABLE 2. Mean Differences in Systolic and Diastolic Pressure and Heart Rate between the last 10 min of Preavoidance and the First 10 min of Avoidance, Averaged for seven control and seven Beta Blockade Sessions for Each Dog Beta blockade
Control 1 2 3 4 5 Croup SE
17.0 18.5 4.1 8.1 - 2.0
20.3 22.6 12.1 14.0 - 0.7
28.2 35.6 28.2 45.2 5.6
20.5 24.1 8.0 14.6 1.6
18.7 25.9 11.8 18.0 1.3
15.7 26.1 13.8 25.3 4.3
8.9 1.9
13.5 2.0
28.2 2.9
13.8 2.2
15.2 2.2
17.2 2.4
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sions but not in beta blockade sessions. This difference was observed in all five dogs and suggests that an additional mechanism may participate in the cardiovascular response toward the end of the preavoidance periods, which tends to amplify the pressure change and reduce the magnitude of bradycardia. The onset of the avoidance contingency in beta blockade sessions was associated with an immediate increase in blood pressure, which was maintained at levels not significantly different from those observed during avoidance in control sessions (Fig. 1). Table 2 presents mean differences for systolic and diastolic levels between the end of the preavoidance and the first 10 min of the avoidance sessions in beta blockade sessions, and shows that the average increases in systolic pressure (X = 13.9; t = 6.4; p < 0.01) and diastolic pressure (X = 15.2; t = 6.8; p < 0.01) were observed consistently in four subjects. The onset of the avoidance contingency in beta blockade sessions, however, was associated with increases in heart rate that were significantly reduced from those observed during control sessions (X = 17.2; t = 7.3; p < 0.01). This attenuation of the avoidance-associated tachycardia was observed consistently in all five subjects (Table 2) and represented, on the average, a 38% diminution in the effect observed under control conditions.
DISCUSSION The results of the experiments show that blockade of beta adrenergic activity in dogs trained on a free-operant avoidance task significantly attenuated the tachycardia accompanying avoidance performances, but did not attenuate the eleva186
tions in blood pressure occurring under these conditions. Moreover, beta blockade did not prevent the occurrence of gradual increases in blood pressure and decreases in heart rate during preavoidance intervals. Heart rate levels at the beginning of propranolol sessions were not significantly different from heart rate levels at the beginning of nondrug control sessions. This finding is consistent with the results of a previous study of propranolol effects upon the circulation of the unanesthetized and chronically-instrumented laboratory dog at rest (16) and indicates that sympathetic tone upon the heart was minimal at the beginning of these experimental sessions. This suggests that the deceleration in heart rate was not mediated by decreased sympathetic activity, but may be accounted for by increases in parasympathetic stimulation. It is well known that decreases in heart rate, which result from baroreceptor reflexes to rising arterial pressure, are mediated by the parasympathetic nervous system (17). However, the vagally-mediated bradycardia during preavoidance may be related to a physiological reflex that facilitates "sensory intake" behavior. During the orienting reflex (18) or the preparatory period of a reaction time task (19,20) somatic motor activity and respiration rate decrease. In addition, "sensory intake" behavior has been associated recently with an increase in peripheral vascular resistance (21). Increases in total peripheral resistance have been reported in dogs during preavoidance intervals (2, 5), and we have observed decreases in respiration rate well below basal levels under these conditions. Respiratory inhibition has been associated in man with an increase in blood pressure, a decrease in heart rate, and a redistribution in blood flow away from the
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skeletal musculature, conserving oxygen for the heart and brain (22,23). This reflex is similar to that observed in the diving reflex, in which marked increases in peripheral resistance occur together with large magnitude reductions in heart rate and cardiac output (24). The cardiac effects during diving are mediated vagally (25), but the pathways mediating the change in peripheral vascular function remain to be determined, as does the mechanism accounting for the similar preavoidance cardiovascular pattern. During avoidance performance periods, heart rates were significantly lower in propranolol experiments than in nondrug, control experiments, suggesting that the tachycardia accompanying avoidance responses was mediated in part by activation of the sympathetic nervous system. A concurrent decrease in parasympathetic activity also may have contributed to the elevations in heart rate during avoidance. Cardiac output measurements were not made during these experiments, but virtually all other studies assessing propranolol effects upon both heart rate and cardiac output have reported parallel effects upon the two measures (for a review, see Ref. 26). Previous work in our laboratory and elsewhere (2, 5) has shown that cardiac output as well as heart rate is elevated during avoidance sessions, and it seems likely that the beta blocking drug decreased the magnitude of elevation in cardiac output associated with avoidance. In this case, it may be inferred that total peripheral resistance was higher during avoidance in beta blocking experiments than in control experiments, since arterial pressure was equally elevated under both conditions. Decreases in cardiac output and increases in total peripheral resistance with variable pressure effects have been reported frequently in acute studies with laboratory
animals (26), and this pattern appears to be a function of propranolol effects upon the periphery as well as the heart. Beta blocking drugs have been shown to change the vasodilator effects of epinephrine to vasoconstrictor effects, and to intensify the vasoconstrictive effects of norepinephrine (27). Moreover, it has been found that cardiac output decreases continue to be observed following beta blockade in subjects whose heart rates are maintained at preblockade levels by ventricular pacing (28). Further studies are needed to differentiate the role of the peripheral circulation from other factors (e.g., strength and speed of myocardial contraction) in determining cardiac output changes associated with beta blockade. In any event, the results of these experiments showing that elevations in blood pressure accompanying avoidance performances are not attenuated by propranolol are consistent with similar experiments with primates (6). The cardiovascular pattern observed during avoidance sessions is similar to that elicited in anesthetized animals by electrical stimulation of areas of the hypothalamus that mediate "fight or flight" behavior (29). The potential role of this "defense alarm" reflex in the pathogenesis of essential hypertension has been discussed extensively (30), although long-term experiments with rodents and primates testing the validity of this hypothesis have met with limited success (30-33). Elevations in blood pressure mediated by increased vascular resistance have been observed less frequently in studies with animals, although a potential role for this response in the pathogenesis of fixed essential hypertension has been suggested (34). It remains for future research to clarify the nature of the physiological systems mediating the cardiac and particularly vascular adaptations
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observed under conditions that elicit acute elevations in peripheral resistance, and to determine the relevance of these factors in the etiology of hypertensive adaptations.
This research was supported by NHLI Grants HL-06945 and HL 17970.
The authors express their appreciation to Dr. Sarjono O. Santoso /or expert pharmacoJogica] advice and to Robert Baer, Judy StribJing, David Siscovick, and John YingJing for helpful technical assistance. We are also grateful to Dr. R. O. Davies of Ayerst Laboratories for the propranolol used in these experiments.
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CARDIOVASCULAR RESPONSES TO AVOIDANCE CONDITIONING 24. Andersen HT: Physiological adaptations in diving vertebrates. Physiology 46:212, 1966 25. Lin YC: Autonomic nervous control of cardiovascular response during diving in the rat. Am J Physiol 227:601, 1974 26. Gibson DG: Pharmacodynamic properties of /3-adrenergic receptor blocking drugs in man. Drugs 7:8, 1974 27. Glover WE, Hutchinson KJ: The effect of /3-adrenergic antagonist (propranolol) on the cardiovascular response to intravenous infusions of noradrenaline in man. J Physiol 177:59P, 1965 28. Bloomfield DA, Sowton E: Rate-independent effects of propranolol: The differentiation between chronotropic, inotropic and peripheral vascular responses. Circ Res 21:111, 1967 29. Charvat J, Dell P, Folkow B: Mental factors in cardiovascular disease. Cardiologica 44:121, 1964 30. Folkow B, Neill E: Circulation. New York, Oxford University Press, 1971, p. 569. 31. Findley JD, Brady JV, Robinson WW, Gilliam W: Continuous cardiovascular monitoring in the baboon during long-term behavioral performances. Commun Behav Biol 6:49, 1971 32. Folkow B, Rubinstein EH: Cardiovascular effects of acute and chronic stimulations of the hypothalamic defense area in the rat. Acta Physiol Scand. 68:48, 1966 33. Forsyth RP: Blood pressure responses to long-term avoidance schedules in the restrained rhesus monkey. Psychosom Med 31:300, 1969 34. Eich RH, Peters RJ, Cuddy RP, Sulyan HI, Lyons RH: Hemodynamics in labile hypertension. Am Heart J 63:188, 1962
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