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Departments of 1Psychology and 2Pharmacology, and the 3Cardiovascular Center, University of Iowa, Iowa City, Iowa
Submitted 23 January 2004 ; accepted in final form 30 December 2004
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ABSTRACT |
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atropine; phenylephrine; propranolol; reproducibility of results; sodium nitroprusside; validity
Although information regarding resting arterial baroreflex function is useful, the baroreceptor reflex is also essential for maintaining arterial pressure at an appropriate level for adequate perfusion pressure and thus meeting the metabolic demands of an animal during increased levels of activity, vigilance, and stress and while engaging in a number of behavioral activities (e.g., exercise, grooming, exploration, mating, feeding). Because it is difficult to assess baroreceptor reflex function during these activities with presently available methodologies, little is known about arterial baroreflex function during different behaviors and altered physiological states.
There has been increased interest in the assessment of arterial baroreflex function in a noninvasive manner during a wide range of activities and behaviors and across many animal species. Because of these issues, a new technique has recently been employed in which arterial baroreflex function is assessed by correlating beat-to-beat fluctuations in arterial pressure with HR, i.e., evaluation of spontaneous baroreflex function (1, 2, 5, 18). A variation of this spontaneous method has also been described (see Ref. 9) that evaluates particular sequences of data where arterial pressure and pulse interval are changing in the same direction (or, conversely, when arterial pressure and HR are changing in opposite directions) for at least three consecutive beats (herein referred to as the sequence method). With the spontaneous method or the variant sequence method, baroreflex function potentially can be assessed during periods of activity and behavior over a short interval of time and without the potential confounding effects of direct experimental interventions to alter arterial blood pressure. This spontaneous baroreflex method should not be confused with techniques of frequency-domain analyses such as spectral analysis (4, 8).
Importantly, the arterial baroreflex utilizes both the sympathetic and parasympathetic nervous systems to elicit compensation for arterial pressure. The time constant for the sympathetic contribution to HR is relatively longer than that of the vagal component (7). Thus evaluation of the arterial baroreflex over a short time may not appropriately evaluate both sympathetically and parasympathetically mediated compensation for spontaneous fluctuations in arterial pressure. In addition, cardiovascular autonomic tone is known to vary greatly among different animal species. For instance, sympathetic autonomic tone on the cardiovascular system is much greater in rats relative to dogs and humans (11, 12, 15).
Therefore, although the spontaneous method of baroreflex assessment appears to have potential for practical application, this technique may not be applicable for use in some species due to the wide variation in autonomic tone and thus reflex-response time required to elicit appropriate baroreflex responses. In addition, systematic and thorough validation studies of the spontaneous baroreflex technique in rats are limited (15). The present experiments were designed to determine the reliability and validity of the spontaneous baroreflex method by using a number of rigorous validation-assessment protocols. We propose three hypotheses in the present study. First, owing to the predominant basal sympathetic control of cardiovascular regulation in rats combined with the slow time course required of the sympathetic nervous system to respond to changes in arterial pressure, a concern is that the correlation of spontaneous fluctuations in blood pressure and HR may not accurately reflect baroreflex changes. This hypothesis is addressed by examining the relative sympathetic and parasympathetic components of the arterial baroreflex. Second, because the spontaneous method does not include a systematic technique for factoring out fluctuations that are not baroreflex mediated (i.e., "noise"), it is possible that true variations in baroreflex function may not be detected. This hypothesis is addressed by comparing the data generated with the aforementioned sequence method (which systematically removes all data points that are purported to be "noise") with the spontaneous method. Third, owing to the limited ability of spontaneous fluctuations in blood pressure to represent the full range of physiologically relevant blood pressure variations, the correlation of spontaneous fluctuations in blood pressure and HR may not accurately reflect baroreflex control of HR. This hypothesis is addressed by comparing the spontaneous and sequence methods to the conventional technique for assessing baroreflex function (the pharmacological method).
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METHODS |
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Thirteen male Sprague-Dawley rats (Harlan, Indianapolis, IN) weighing 300–400 g were used for the experimental procedures. Rats were allowed 1 wk to acclimate to the surroundings before any experimentation was begun. Animals were housed in individual plastic cages with bedding. Food (Purina Rat Chow 5012) and tap water were available ad libitum for the duration of the experiments. The temperature was maintained at 22 ± 2°C. The light cycle was held at 12:12-h light-dark cycle with lights on at 0600. All experiments were conducted in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals and the American Physiological Society's "Guiding Principles for Research Involving Animals and Human Beings," and were approved by the University of Iowa Animal Care and Use Committee.
Implantation of Femoral Catheters
Surgical procedures for the placement of femoral catheters were conducted using aseptic surgical techniques with rats under halothane anesthesia. Catheters (polyethylene-10 fused to polyethylene-50) were inserted into the aorta and abdominal vena cava via the left femoral artery and vein for measurement of arterial pressure and administration of vasoactive drugs, respectively. Catheters were tunneled subcutaneously and exteriorized at the dorsal cervical region. Catheters were filled with 100 U/ml heparinized saline and capped with an airtight plug until the experiment. Animals were administered subcutaneous fluids and butorphanol (Stadol; 2 mg/kg sc; Bristol Myers Squibb; Princeton, NJ) for postoperative analgesia. After immediate recovery from anesthesia, animals were returned to their cages for 24 h of additional recovery.
Baroreflex Assessment Protocol
Direct arterial pressure and HR were recorded in unrestrained, unanesthetized rats. These recordings were carried out over a 2-day period, during the light period, between the hours of 1100 and 1500. The rats were moved into the testing chamber in their home cages and were allowed to adapt to the surroundings for several hours. Catheters were connected to a pressure transducer (Maxxim Medical, Athens, TX) coupled to a multichannel recorder through a custom-designed amplifier (University of Iowa, Iowa City, IA). The analog input was converted into a digital signal using a PowerLab data acquisition system (ADInstruments, Mountain View, CA). Mean arterial pressure (MAP) was derived electronically using a low-pass filter set at 100 Hz and was calculated using the cyclic mean. Data were acquired at 200 samples/s. HR was determined by measuring the number of heartbeats triggered from the arterial pressure pulse and was calculated online. Once the animal had sufficient time to adapt to the surroundings in the testing chamber, hemodynamic parameters were monitored for 20–40 min to ensure stabilization of MAP and HR. After confirmation of stable hemodynamic parameters, these baseline parameters were continuously recorded for at least 10 min.
After the collection of baseline parameters, arterial baroreflex curves were generated by producing ramp changes in arterial pressure over 
2–3 min. Initially, MAP was increased to 170–180 mmHg within 2–3 min by infusion of the 
1-adrenergic receptor agonist phenylephrine (PE) at increasing rates (2–25 µg·kg–1·min–1 iv). MAP and HR were allowed to return to within 10% of baseline values before we proceeded with the experimental protocol. Arterial pressure was then decreased to 50–60 mmHg within 2–3 min by infusion of the vasodilator sodium nitroprusside (SNP) at increasing rates (10–100 µg·kg–1·min–1 iv). The rate of change of arterial pressure was held constant by observing the recorded pressure alteration and varying the rate of infusion to produce a smooth ramp of pressure increase or decrease. Care was taken to keep the rate of change of arterial pressure similar in all animals, at 1–2 mmHg/s. Volumes infused did not exceed 100 µl. Baroreceptors were always activated first (PE infusion) before unloading (SNP infusion) to minimize any potential effects of reflex release of humoral agents such as vasopressin or angiotensin II on baroreflex function.
Cardiac Autonomic Blockade
The study of HR was performed on a randomly selected subset of animals (n = 5) under conditions of pharmacological blockade of 
-adrenoceptors and muscarinic receptors. MAP and HR were recorded under the following conditions over a 2-day period, between the hours of 1100 and 1500: 1) during -adrenergic receptor blockade with propranolol hydrochloride (2 mg/kg iv); 2) during muscarinic cholinergic receptor blockade with atropine methylbromide (i.e., methylatropine; 1 mg/kg iv); and 3) during blockade with propranolol plus methylatropine. The drug doses were chosen for their ability to effectively block the respective autonomic inputs to the heart according to previous tests of efficacy (16). The order of drug treatment was counterbalanced across all animals such that each animal received either propranolol or methylatropine first followed by the second drug (for -adrenoceptor and muscarinic receptor blockade) on day 1 and the reverse administration on day 2. Thus combined cardiac blockade was performed on both days. Animals were returned to the animal colony between days 1 and 2 of testing.
Data Analysis
Baroreflex control of HR was estimated using the spontaneous method discussed by Burger et al. (2). With the use of a stable portion of at least 700 consecutive arterial pressure pulses of baseline data, systolic blood pressure (SBP) was plotted against HR on the subsequent diastolic beat. The data points were taken from an entire baseline period of blood pressure recording for each rat with the exception of segments during animal movement or during periods of unstable blood pressure. Data points were plotted on a beat-by-beat basis, and HR was recalculated with every heartbeat (2). Figure 1 displays a raw data tracing that shows the relationship of SBP to HR. Linear regression was used to calculate slope, y-intercept, and R2 values for individual rats. Group means were calculated from these parameters.
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Baroreflex control of HR was evaluated using the pharmacological method described by Moffitt et al. (13, 14). Changes in HR were evaluated in response to different levels of MAP during PE and SNP infusion. All data points collected during the experiment were included in this plot. A linear regression analysis was then performed on the linear portion of the data for each animal to directly compare the slope of the linear baroreflex relationship with the spontaneous and sequence methods.
All analyses were performed on data recorded from each animal at approximately the same time of day (between the hours of 1100 and 1500). Group data are listed as means ± SE. The data were analyzed with ANOVA (repeated-measures and single-factor tests) and Student's t-tests or Tukey's honestly significant difference (HSD) where appropriate. A Bonferroni correction was used for all multiple comparisons. A probability value <0.05 was considered to be statistically significant.
Reliability and Validity Analyses
The utility of the spontaneous method was evaluated according to conventional reliability and validity criteria (see, e.g., Ref. 19). The following protocols were used to empirically test the reliability of the spontaneous method (and the sequence method where noted).
Reliability within animal. Linear regression analyses (slope and R2) using the spontaneous method were compared within the same animal on two separate days (data were sampled at the same time on days 1 and 2) and at three separate time points on the same day (three nonoverlapping, 2-min segments of data were randomly selected from an overall 10-min segment of stable data). The sequence method was used to calculate a slope and R2 value on day 2 of testing, and these values were compared with the data obtained from the spontaneous method.
Reliability across animals. Linear regression analyses using the spontaneous method were compared across different rats under the same conditions. Similarly, linear regression analyses using the sequence method were also compared across animals.
The following protocols were used to evaluate the validity of the spontaneous method (and the sequence method where noted).
Face validity. Linear regression analyses using the spontaneous method and the sequence method were compared with the linear portion of the baroreflex function curve generated with the pharmacological method. A regression analysis using the spontaneous method was also performed during pharmacological manipulations in arterial pressure (i.e., a combination of both the spontaneous and pharmacological methods). For this last analysis, the spontaneous method was applied to the data from the linear portion of the baroreflex curve generated with the pharmacological method (i.e., SBP was correlated with HR on the subsequent diastolic beat during PE and SNP infusion).
Predictive validity.
Linear regression analyses using the spontaneous method were performed during selective and combined pharmacological blockade of autonomic inputs to the heart. Specifically, it was predicted that the slopes generated with the spontaneous method would become significantly more negative under -adrenergic receptor blockade with propranolol and would become significantly more positive (less negative) under cholinergic receptor blockade with methylatropine. The HR data generated with the spontaneous method under conditions of -adrenergic, cholinergic, and combined cardiac blockade were compared with the resting baroreflex data generated with this method. In addition, the sequence method was used to determine HR responses to -adrenergic receptor blockade (compared with resting data generated with this method).
Construct validity. To determine the time course necessary for the spontaneous method to accurately reflect baroreflex control of HR, linear regression analyses were performed by plotting SBP against HR on the subsequent through the tenth diastolic beat. Furthermore, the theoretical rationale underlying the spontaneous method and sequence method is addressed (see DISCUSSION).
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RESULTS |
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Mean body weight of the group was 332 ± 9 g. On day 1 of testing, the MAP and HR of the rats were 122 ± 2 mmHg and 368 ± 9 beats/min, respectively. On day 2 of testing, the MAP and HR were 123 ± 2 mmHg and 353 ± 13 beats/min, respectively. Repeated-measures t-tests were performed on each of these parameters between days 1 and 2 and yielded no statistically significant differences (P > 0.05 for both comparisons).
Reliability of Spontaneous Baroreflex Technique
Reliability within animals. SBP was plotted against HR on the subsequent diastolic beat during the baseline period for individual rats. A linear regression function was fit to the data. Within the same animal, the slopes of the SBP-HR relationships were compared on separate occasions using the spontaneous method. These data are presented in Figs. 2– 5. Figure 2 presents the raw data, slope from the linear regression function, and corresponding line equation from a representative rat on day 1 vs. day 2 of testing. The results shown here illustrate that the data generated with the spontaneous method were unreliable when this method was employed on two consecutive days.
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Face validity. Group data from the spontaneous method, the sequence method, the pharmacological method, and a combination of the spontaneous and pharmacological methods were statistically compared in a subset of rats (n = 7) to determine the extent to which the methods were similar in the representation of baroreflex control of HR. With the use of the spontaneous method, the slopes from each individual rat were averaged. A similar calculation was performed on the data using the sequence method. The linear portion of the sigmoidal function curve generated from the administration of PE and SNP was used to evaluate the pharmacologically induced baroreflex responses, and the slopes from these functions were averaged across all animals. With the use of a combination of the spontaneous and pharmacological methods, SBP was plotted against HR on the subsequent diastolic beat from the data generated during the administration of PE and SNP. In other words, the spontaneous method was used to calculate the baroreflex slope while the baroreflex was actively engaged by a forced change in pressure.
Figure 7 shows bar graphs of the mean slope and R2 values using each method alone and a combination of the pharmacological and spontaneous methods. A single-factor repeated-measures ANOVA yielded a main effect of group for the analysis of slope [F(3,24) = 13.41; P < 0.05]. The mean slopes obtained from the spontaneous and pharmacological methods were significantly different [P < 0.05; Tukey’s HSD]; however the slopes obtained using the pharmacological and the combination methods (spontaneous + pharmacological) did not differ (P > 0.05). The slope generated with the sequence method, although not significantly different from that of the spontaneous method alone (P > 0.05; previously described), was significantly different from the slope generated with the pharmacological method alone (P < 0.05 with Tukey's HSD).
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Predictive validity.
Group data from the spontaneous method were compared under conditions of selective and complete autonomic blockade in a subset of animals (n = 5). The slopes obtained from the spontaneous method alone (during rest) were statistically compared with the slope under -adrenergic receptor blockade with propranolol, cholinergic muscarinic receptor blockade with methylatropine, and combined blockade with both agents. Figure 8 displays the mean slopes and R2 values generated with the spontaneous method alone and under selective and combined receptor blockade. An ANOVA yielded no significant main effect of slope (P > 0.05). A priori Student's t-tests confirmed that there were no significant differences in the slopes under any of the conditions (P > 0.05 for all comparisons). Similarly, there were no significant differences in R2 values among the conditions with an ANOVA or a priori Student's t-tests (P > 0.05 for all comparisons). Thus spontaneous baroreflex control of HR was not altered by -adrenoceptor and muscarinic receptor blockade in any predictable manner, as would be expected.
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-adrenergic receptor blockade with propranolol was also compared with resting conditions using the sequence method to determine whether this method would provide different results from the spontaneous method (see Fig. 9). Neither the slope nor the R2 value obtained during -adrenergic receptor blockade was significantly different from the values obtained under resting conditions (P > 0.05 for both comparisons). Thus extracting only beats where SBP and pulse interval were changing in the same direction did not provide different results from data generated with the spontaneous method during -adrenergic receptor blockade.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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An experimental technique must be reliable to have scientific utility. To investigate the reliability of the spontaneous method, we compared the slope and R2 value from the linear regression function on separate occasions under the same conditions in the same animal. The slope values were inconsistent in three out of five rats when they were compared on two consecutive days. Furthermore, in only two rats did the slopes on both days represent the expected inverse relationship between SBP and HR. Similarly, the R2 values were low in all rats (i.e., <0.2), which indicates that the linear regression functions were a poor fit to the actual data. A similar pattern of results was observed when the slope and R2 values were compared in the same rat during three 2-min segments of data sampled from a 10-min period of the arterial pressure recording. It is interesting to note that the slopes from individual rats did not become consistently more negative on day 2 of testing (see Fig. 3), which suggests that increased recovery from surgery did not increase the reliability of the spontaneous method. This finding is mirrored by the resting MAP and HR data, neither of which was different between days 1 and 2 of testing.
The unreliability of the spontaneous method shown here may be confounded by the "noise" that is produced by including for analysis all data points generated during a particular time period. However, when the sequence method was employed to compensate for noise artifact (i.e., excluding all data points that do not conform to the rule of three or more consecutive beats where SBP and pulse interval are changing in the same direction), no additional information was gained. That is, neither the slope nor the R2 value generated with the sequence method differed significantly from the slope or R2 value generated with the spontaneous method. Taken together, these findings suggest that the spontaneous method does not produce reliable data when it is used in normotensive rats, and removing all beats that are putatively not related to an active baroreflex (sequence method) does not add to the reliability of the spontaneous method.
Our results differ from those of a previous study (2) in which a significant negative slope was observed between SBP and HR when the spontaneous baroreflex technique was used. There are two major differences between the present study and the previous study by Burger et al. (2). First, we used male normotensive rats, whereas Burger and colleagues examined the spontaneous baroreflex in female hypertensive rats. Differences in hormonal and cardiovascular status could possibly explain the differences in the data. However, the present study attempted to validate the spontaneous baroreflex technique in a controlled situation in which differences in these types of variables are held at a minimum to determine the applicability of the spontaneous baroreflex technique in rats in general (rather than for a specific disease state). In addition, although Burger et al. allowed an extra day of recovery from surgery vs. the present protocol, these authors used injected anesthetics. In the present study, we used inhalant anesthetics, which allow for quicker recovery. As discussed above, MAP and HR did not differ between the 2 days of testing in our study, which indicates that animals were of a similar cardiovascular status on day 1 vs. day 2 of testing. However, there is still a possibility that the reliability results in the present protocol have been affected by surgical stress owing to the fact that animals were tested 24 and 48 h after surgery. Additional studies are required to investigate the reliability of the spontaneous baroreflex technique (and other methods) by performing direct comparisons of different periods of recovery after administration of various anesthetics.
The utility of the spontaneous method (and the variant sequence method) is in part influenced by its ability to accurately evaluate baroreceptor reflex function. For instance, the spontaneous method should generate valid data that contain certain characteristics including data that are unambiguously interpreted and based on a sound theoretical rationale (construct validity), an inverse relationship between SBP and HR, which indicates that the baroreflex is operating properly to control HR (face validity), and predictable changes in slope when autonomic inputs to the heart are altered or removed (predictive validity). The construct validity of the spontaneous method may be compromised because inherent in this method is the assumption that the autonomic nervous system can respond within one heartbeat to either a rise or a fall in blood pressure. Given the basal sympathetic dominance in rats (relative to dogs and humans; Refs. 11, 15), the relationship between SBP and HR may become apparent slightly or even significantly later than one heartbeat. However, when we investigated this possibility by correlating SBP with HR on the subsequent through the tenth heartbeat to determine whether there was a systematic point at which SBP was negatively correlated with HR, we were unable to find a consistent relationship between SBP and HR across animals.
Another test of construct validity employed in the present study was an evaluation of the variant of the spontaneous method, the sequence method, which assumes that spontaneous fluctuations in blood pressure and HR are not exclusively baroreflex mediated, and therefore all sequences of spontaneous data that are putatively not related to an active baroreflex should be excluded from analysis. When this criterion was applied to the data generated from spontaneous fluctuations in blood pressure and HR, it provided no additional information regarding the utility of the spontaneous method.
To determine the face validity of the spontaneous method (and the sequence method), spontaneous variations in blood pressure and HR were recorded alone and while blood pressure was being pharmacologically manipulated with PE and SNP. The resulting mean data from the spontaneous method alone, the sequence method, the pharmacological method alone, and the spontaneous and pharmacological methods in combination were systematically compared. As expected, the pharmacological method produced a negative relationship between SBP and HR, whereas the spontaneous and sequence methods produced significantly different slopes (both with less negativity) than the pharmacological method. Because both the spontaneous method and the variant sequence method rely on spontaneous fluctuations in SBP and HR, it is possible that neither can reflect baroreflex function as accurately as the pharmacological method. However, when the spontaneous changes in blood pressure were plotted against HR during pharmacological manipulations in pressure (i.e., spontaneous and pharmacological methods combined), a negative relationship between blood pressure and HR was observed. It can be concluded from these data that it is necessary to evaluate baroreflex function at a time when the baroreflex is actively engaged without confounding influence.
With regard to predictive validity, it was predicted that the slope representing the correlation between SBP and HR would become significantly more negative under pharmacological sympathetic blockade because changes in HR would be exclusively mediated by fast-conducting myelinated fibers of the vagus nerve. Given the inadequate amount of time allowed by the sympathetic nervous system to respond to a change in arterial pressure in only one heartbeat, we further hypothesized that the relationship of SBP to HR should become less negative during parasympathetic blockade due to the exclusive reliance on the sympathetic nervous system. Contrary to our predictions, neither -adrenergic receptor blockade nor cholinergic receptor blockade altered the spontaneous relationship between SBP and HR. Similarly, -adrenergic receptor blockade also did not produce a more negative slope when the sequence method was used to analyze these data. The present data are in contrast to findings from Oosting et al. (15), which show that baroreflex sensitivity was altered under both parasympathetic blockade with methylatropine and combined blockade with metoprolol and methylatropine when assessed by spontaneous changes in arterial pressure and HR. In fact, contradictory to sound physiological rationale, a negative relationship between SBP and HR was observed after combined blockade. Thus it appears that the spontaneous baroreflex technique and the sequence method have little if any predictive validity when used to evaluate baroreflex function in normotensive rats at rest.
When the findings from the present investigation are considered together, they suggest that the spontaneous method (as well as its variant sequence method) may be problematic when used to assess baroreflex function in male normotensive rats. Our first hypothesis, that the predominant sympathetic tone in rats and the relatively slow response rate of the sympathetic nervous system would prevent the spontaneous method from accurately reflecting baroreflex function, was not confirmed in these experiments. There was no change in the relationship between SBP and HR when cardiac sympathetic inputs were removed. Furthermore, allowing more time for the sympathetic nervous system to respond to changes in blood pressure by correlating SBP with later HR responses (i.e., on the second through the tenth heartbeats after the systolic beat) did not increase the accuracy of the spontaneous method.
Our second hypothesis, that the data generated with the spontaneous method are confounded by extraneous noise that may interfere with the reliability and validity of this technique, was also not confirmed in the present study. When the data from the present experiments were reanalyzed using the sequence method, which systematically excludes noise, neither the reliability nor the validity was improved.
Our third hypothesis, that the limited range of variability obtained by recording spontaneous blood pressure fluctuations would decrease the accuracy of the spontaneous method, was confirmed by the present experiments. When the slope and R2 values obtained with the spontaneous method were directly compared with those values obtained from pharmacological manipulation of blood pressure in the same rat, these two methods yielded vastly different results. However, when spontaneous fluctuations in SBP and HR were correlated during pharmacological manipulation of blood pressure, the spontaneous method was capable of producing a negative relationship between the variables as well as a desirable fit to the actual data (i.e., high R2 value). The present experiments demonstrate that when the baroreflex is actively engaged through a range of pressures (both high and low), the correlation of spontaneous fluctuations in SBP and HR produces valid and useful data.
Thus it is our conclusion that utility of the spontaneous baroreflex method and the variant sequence method in male normotensive rats may be achieved when they are used in combination with pharmacologically manipulated changes in blood pressure. The requirement of using vasoactive drugs, however, interferes with the advantage of the spontaneous method to be employed noninvasively, within a short duration of time, and without the confounding effects of experimenter-induced alterations in arterial pressure. However, techniques that involve the correlation of spontaneous fluctuations in arterial pressure and HR have been employed under particular conditions in rats such as in the spontaneously hypertensive rat during exercise (2), which may provide insight into the potential utility of these methods. Furthermore, with the increasing popularity of radiotelemetry techniques in rodents (for instance, Ref. 3), it is now possible to monitor hemodynamic and cardiovascular variables in long-term studies and under various behavioral conditions. This may prove to be a useful technique by which spontaneous changes in blood pressure and HR can be assessed without the confounding influence of recent surgical stress. However, because the cost of radiotelemetry is prohibitive for many laboratories, future studies are still needed to develop an appropriate and useful method for determining baroreflex function during a wide range of behaviors and activity levels of varying time duration in rats.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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