Exp Physiol 90.1 pp 141–150 141

Experimental Physiology

Differential cardiovascular responses to stressorsin hypertensive and normotensive rats

Stuart J. McDougall, Andrew J. Lawrence and Robert E. Widdop

Department of Pharmacology, Monash University, Victoria 3800, Australia

The aim of this study was to determine to what extent stress-induced cardiovascular responsesdepend upon rat strain and/or stressor. Spontaneously hypertensive rats (SHRs), Wistar-Kyotorats (WKYs) and Sprague-Dawley (SD) rats were implanted with telemetry probes in order tomeasure heart rate and blood pressure changes when exposed to a stressor. The stress protocolsemployed included handling, air-jet and restraint, where each stressor was repeated over 10consecutive days. In addition, a heterologous protocol was established whereby the experimentalgroups having experienced 10 days of air-jet stress were then immediately exposed to 10consecutive days of restraint. Each stressor caused graded tachycardic and pressor responses in allstrains. For all strains, the magnitude and duration of heart rate and blood pressure increases weregreatest in the restraint-based protocols while handling and air-jet caused submaximal changes. Acomparison between strains indicated that SHRs exhibited prolonged pressor responses to each ofthe stressor types tested as compared to the normotensive strains. In addition, repeated exposureover 10 days to handling and air-jet in SHRs caused tachycardic and/or pressor responses to adaptto ‘normotensive-like’ levels. Heterologous restraint stress caused sensitization of cardiovascularresponses upon first exposure, predominantly in normotensive strains. Collectively these datashow that the magnitude and duration of the tachycardia and pressor responses evoked by thestressors were different within the strains and were also modified by prior experience. In addition,the cardiovascular profiles presented in this study demonstrate that, within each strain, the heartrate response during stress is graded according to the type of stressor encountered.

(Received 13 June 2004; accepted after revision 20 October 2004; first published online 12 November 2004)Corresponding author R. E. Widdop: Department of Pharmacology, Monash University, Victoria 3800, Australia.Email: robert.widdop@med.monash.edu.au

A stress response involves vast physiological andbehavioural changes via increased activation of thehypothalamo-pituitary-adrenal axis and the sympatho-adrenal system. Included within this repertoire arealmost instant alterations within the cardiovascularsystem. Recognition and generation of a stress responseto psychological stressors is first thought to occurin the higher centres of the central nervous system(CNS), from where the sympathetic and parasympatheticloops are modulated to control the cardiovascularcomponents of the response (Huether, 1996). Fosimmmunohistochemistry studies indicate that differentcombinations of central nuclei are activated in any mannerof circumstances that would all fall under the umbrella ofstress (Chowdhury et al. 2000; Thrivikraman et al. 2000;Dayas et al. 2001). However, the extent to which such CNS

specificity actually contributes to differences, if any, in theresulting cardiovascular response remains unclear.

Tachycardic and pressor responses have been recordedin various rat strains subjected to a number ofmanipulations including common animal husbandryand intraperitoneal injections (Schnecko et al. 1998);and in various stressor protocols including novelenvironment (van den Buuse et al. 2001), acousticarousal (Bao et al. 1999), conditioned fear (Carrive,2000, 2002), immobilization, air-jet (Ely, 1995) andrestraint (Chen & Herbert, 1995). Indeed many of thesemanipulations including handling, air-jet and restraintalso cause hypothalamo-pituitary-adrenal axis activationas indicated by increases in plasma adrenocorticotropinand corticosterone concentrations (Dobrakovova et al.1993; Engelmann et al. 1996; Chung et al. 2000).

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142 S. J. McDougall and others Exp Physiol 90.1 pp 141–150

We have previously shown that the cardiovascularcomponent of the stress response in spontaneouslyhypertensive rats (SHRs) differs from that of Wistar-Kyotorats (WKYs) when subjected to restraint stress (McDougallet al. 2000). Furthermore there was a difference inthe adaptation of the cardiovascular response betweenthe strains, where the duration of the restraint-inducedtachycardia observed in SHRs normalized with repeatedexposure to ‘WKY-like’ levels. In the present study, wedetermined whether the cardiovascular responses of WKYsand SHRs would differ in response to other psychologicalstressors as we had found with restraint stress (McDougallet al. 2000).

To this end, we have characterized the cardiovascularstress response to repeated stress in SHRs, utilizingthree common experimental stressors: handling, air-jet and restraint (McDougall et al. 2000). In addition,a heterologous protocol was included, whereby ratsexperienced 10 days of air-jet stress followed immediatelyby 10 consecutive days of restraint stress. This protocolwas designed to determine what effect repeated mild stresshas upon the cardiovascular response to a subsequentmore severe stressor (i.e. heterologous stress). We havecompared these effects in SHRs with those in WKYsand Sprague-Dawley (SD) rats, representing inbred andoutbred normotensive strains.

Methods

All experimental procedures were carried out inaccordance with the guidelines of the National Healthand Medical Research Council of Australia for animalexperimentation and approved by the Monash UniversityAnimal Ethics Committee. WKYs and SHRs wereobtained from the Austin Hospital (Heidelberg, Victoria)and SD rats from Monash University Central AnimalServices (Clayton, Victoria). All rats were maintained on a12-h light cycle with access to standard rat chow and waterad libitum.

Experimental protocol

Groups of adult male SD, WKY and SHR were implantedwith telemetry probes (TA11-PAC40; Data SciencesInternational) under general anaesthesia (ketamine andxylazine, 67 mg kg−1 and 7 mg kg−1, respectively, i.p.) aspreviously described (McDougall et al. 2000). All rats wereleft to recover for a 10-day period.

Each group/strain was handled at the same time eachday for 10 consecutive days; this involved the experimenterlifting and holding the rat by the tail and torso from itshome cage, such that contact lasted less than 30 s. Thisprotocol was based upon a routine that has been previouslydescribed (McDougall et al. 2000), and was performed sothat all groups would be treated exactly the same before

exposure to either air-jet or restraint protocols. Telemetrydata were sampled for 10 s every minute for each rat overthe daily 2-h experimental period.

For air-jet stress, the rats were exposed to a 2-ms burst ofair (pressure regulated at 150 kPa) directed at the forehead,of sufficient force to part the fur, every minute for 1 h day−1

for 10 consecutive days. The rats remained in their homecage throughout the stress challenge. Data were sampledfor 4 s every 30 s, with the air-jet burst timed to coincidewith sampling on the minute interval.

The day after the final air-jet session, the threegroups experienced restraint stress, which involvedconfinement within a Perspex tube (diameter, 6 cm; PlasticLaboratories) for 1 h day−1 for 10 days where data weresampled for 10 s every min for the 2-h experimental period.This portion of the regime we termed ‘heterologousrestraint’. A separate group of SD rats were put throughthe restraint (only) protocol, as we described for WKYand SHR groups in our previous study (McDougall et al.2000), in order for data comparisons to be made. All stressprocedures were carried out between 08.00 and 12.00 h tominimize diurnal variation.

From these protocols, a total of 10 strain/stressorcombinations were obtained: SD rats exposed to repeatedhandling (n = 8), air-jet (n = 7), restraint (n = 8) andthe heterologous restraint (n = 7) stress protocols andWKYs and SHRs exposed to repeated handling (n = 5per group), air-jet (n = 10 per group) and heterologousrestraint (n = 5 and 6 for WKYs and SHRs, respectively)stress protocols. For comparison of ‘within strain’ and‘between strain’ stressor effects, as summarized in Tables2 and 3, data from our previous study describing the effectof repeated restraint on WKYs (n = 5) and SHRs (n = 5)rats were utilized (McDougall et al. 2000), therefore 12strain/stressor combinations are discussed.

Statistical analysis

All data are represented as mean ± s.e.m. except in Fig. 1where only mean values are shown to better illustratethe actual response patterns. Baseline values were definedas the average of the 30 min immediately prior to agiven stress session. Maximum change, % change andthe duration of the heart rate (HR) and mean arterialpressure (MAP) responses were determined, taking thebasal values into account, for each stress period for all stressprotocols. To obtain an indication of the duration andintensity of the HR and MAP responses for the 1-h periodafter the beginning of stress, an area under curve (AUC)calculation was used (GraphPad Prism). AUC data werecompared by one-way ANOVA with repeated measuresfor analysis within each of the stress protocols in eachstrain or by t-test as appropriate. A two-way ANOVA foranalysis of the temporal responses was also employed forstrain and stress protocol comparisons. Post hoc testing

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with a Dunnett’s or Newman–Keuls test was performedas appropriate. Maximum change (as % change) in HRand MAP responses were analysed similarly. In all casesP < 0.05 was considered significant.

Results

Basal parameters

SHRs consistently exhibited elevated resting MAP asmeasured via telemetry (Figs 1 and 2). The SD groupexhibited higher resting HR values than those of both theWKY and SHR groups on day 1 of the air-jet protocol(Fig. 1); however, this day seemed to be an exceptionbecause resting HR across the groups were not significantlydifferent on days 2–10 (Fig. 2).

Cardiovascular response patterns

Figure 1 illustrates the mean HR and MAP responsepatterns evoked within each strain as caused by acute(day 1) exposure to handling, air-jet and heterologous

Figure 1. Cardiovascular stressresponse patterns as evoked by acutestress exposureRadiotelemetry recordings of average HR(A, C and E) and average MAP (B, D and F)responses (S.E.M. not shown) to acute (day1) exposure of handling (A and B), air-jet(C and D) and heterologous restraint (E andF) in Sprague-Dawley (SD) rats (�),Wistar-Kyoto rats (WKYs) (♦) andspontaneously hypertensive rats (SHRs) (�).Arrow indicates point in time whenhandling occurred (A and B); dashed barrepresents air-jet period (C and D),continuous bar represents restraint period(E and F). SD handling, n = 8; SD air-jet,n = 7; SD heterologous restraint, n = 7;WKY handling, n = 5; WKY air-jet, n = 10;WKY heterologous restraint, n = 5; SHRhandling, n = 5; SHR air-jet, n = 10; SHRheterologous restraint, n = 6.

restraint. For all strains, handling generally evoked anincrease in HR (30–121 beats min−1 or 12–38%, day 1)and MAP (11–24 mmHg or 11–19%, day 1) that initiallypeaked with contact; thereafter most of the resultingcardiovascular response was apparently due to the animalexploring its home cage as indicated by activity counts(movement across the horizontal plane; data not shown),during which time the HR and MAP fell back to basal levels(Fig. 1).

Air-jet stress (sampled twice per minute) caused apulsatile-like change in HR in each strain. An overall rise inHR (61–78 beats min−1 or 17–28%, day 1) and MAP (19–41 mmHg or 19–31%, day 1), compared to basal levels,was measured as the air puffs were repeated over the hourperiod (Fig. 1).

The restraint (only) and heterologous restraint responsepatterns were similar in SD rats, where each stressorelicited the largest changes in HR (149–185 beats min−1

or 43–59%, day 1) and MAP (31–42 mmHg or 31–46%,day 1) compared to handling and air-jet stress exposure(Figs 1 and 4). Similarly in WKYs and SHRs, heterologous

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restraint caused the largest changes in HR (209–214 beats min−1, or 59–85% day 1) and MAP (47–53 mmHg or 44–50%, day1) as compared to the handlingand air-jet protocols (Fig. 1, Table 1). During the restraintand heterologous restraint sessions, the duration of thetachycardia and pressor responses varied, as indicated byAUC measurements, and was dependent upon the strainand/or day of exposure.

Stressor comparison within strains

Sprague-Dawley rats. Acute exposure (day 1) to eachof the stress protocols caused differential degrees ofcardiovascular activation in SD rats. For example,upon exposure to each of the stressors for the firsttime (day 1), SD rats exhibited increases in HRand MAP of 61 ± 24 beats min−1 (17 ± 7%) and19 ± 6 mmHg (19 ± 6%) to air-jet; 91 ± 12 beats min−1

(28 ± 4%) and 11 ± 1 mmHg (11 ± 1%) to handling;149 ± 21 beats min−1 (43 ± 7%) and 31 ± 3 mmHg

Figure 2. Group analysis of change inHR and MAP in response to differentstressors in different strainsFor group data analysis, the change in (filledbars) HR (A, C and E) and MAP (B, D and F)was used as a principle measure forcomparison of handling (A and B), air-jet(C and D) and heterologous restraint stressprotocols (E and F) in SD rats, WKYs andSHRs. On each day, the average of the30-min period immediately before (days1–10), and on the day preceding (day 0)stressor exposure was used to define restingHR and MAP levels (open bars). Themaximum HR and MAP value reached afterstressor exposure was recorded as being thepeak (open + filled bars) response.Statistical information is shown for thechange in (peak minus basal) HR and MAPonly (filled bars). †P < 0.05 for individualday versus day 1 within same group(one-way ANOVA).

(31 ± 4%) to restraint; and 185 ± 11 beats min−1

(59 ± 4%) and 43 ± 3 mmHg (46 ± 4%) to heterologousrestraint, respectively, (Figs 2 and 4, Table 1). Analysisof the maximum changes and duration of HR and MAPfor the overall stress responses (days 1–10 combined,Figs 2–4) showed that heterologous restraint and thenrestraint (only) were more effective stressors in SD ratscompared to handling and air-jet (Table 2). There wereno differences in the cardiovascular responses to each ofthe stressors when repeated on days 2–10 as compared tothe acute (day 1) response, indicating that SD rats did notadapt their cardiovascular responses to handling, air-jetor restraint stress protocols.

The heterologous restraint group (i.e. those SD ratsthat had previously experienced 10 days of air-jet stress),had a significantly heightened and a longer-lasting pressorresponse when first exposed to heterologous restraintcompared to day 1 of the restraint (only) group (Fig. 4).Moreover, the overall HR and MAP effects (both changeand duration over 10 days) in the heterologous restraint

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Table 1. Percentage change in (∆)heart rate (HR) and mean arterial pressure (MAP)from baseline values on each day of the stressor protocols

% � HR % � MAPHandling Handling

day SD WKY SHR day SD WKY SHR

1 28 ± 4 38 ± 6 12 ± 7 1 11 ± 1 19 ± 6 19 ± 52 30 ± 4 47 ± 7 22 ± 7 2 10 ± 2 21 ± 3 17 ± 43 27 ± 4 40 ± 3 22 ± 4 3 9 ± 1 21 ± 3 18 ± 44 29 ± 3 43 ± 7 19 ± 12 4 11 ± 1 22 ± 4 27 ± 55 20 ± 4 47 ± 5 23 ± 11 5 7 ± 2 22 ± 3 26 ± 26 29 ± 4 40 ± 8 26 ± 4 6 7 ± 1 23 ± 5 23 ± 37 25 ± 4 37 ± 5 20 ± 9 7 8 ± 1 21 ± 7 17 ± 48 22 ± 2 32 ± 5 17 ± 7 8 2 ± 1† 16 ± 3 22 ± 49 26 ± 2 38 ± 6 27 ± 7 9 7 ± 1 21 ± 6 14 ± 6

10 29 ± 2 39 ± 5 18 ± 2 10 9 ± 3 25 ± 4 30 ± 4

Air-jet Air-jet

day SD WKY SHR day SD WKY SHR

1 17 ± 7 20 ± 4 28 ± 5 1 19 ± 6 21 ± 4 31 ± 42 26 ± 8 22 ± 3 19 ± 4 † 2 24 ± 2 5 ± 10 30 ± 33 21 ± 6 20 ± 3 17 ± 2 † 3 12 ± 2 15 ± 3 30 ± 34 18 ± 8 21 ± 3 15 ± 2 † 4 13 ± 4 13 ± 3 23 ± 25 22 ± 4 20 ± 3 12 ± 3 † 5 14 ± 3 13 ± 2 21 ± 56 18 ± 7 28 ± 3 10 ± 2 † 6 11 ± 3 17 ± 2 15 ± 3 †7 18 ± 4 17 ± 3 10 ± 1 † 7 13 ± 2 8 ± 3 16 ± 2 †8 18 ± 7 23 ± 2 11 ± 1 † 8 15 ± 4 14 ± 3 15 ± 2 †9 14 ± 5 22 ± 4 13 ± 2 † 9 14 ± 4 13 ± 2 17 ± 2 †

10 37 ± 11 21 ± 4 14 ± 6 † 10 18 ± 4 10 ± 2 7 ± 3 †

Heterologous restraint Heterologous restraint

day SD WKY SHR day SD WKY SHR

1 59 ± 4 85 ± 8 79 ± 3 1 46 ± 4 50 ± 4 44 ± 52 55 ± 4 83 ± 7 74 ± 5 2 37 ± 3 47 ± 4 43 ± 63 46 ± 8 80 ± 13 75 ± 8 3 31 ± 3 42 ± 8 42 ± 54 63 ± 4 81 ± 9 84 ± 9 4 34 ± 4 36 ± 3 48 ± 75 63 ± 7 66 ± 9 81 ± 8 5 31 ± 2 31 ± 3 49 ± 36 72 ± 8 76 ± 11 96 ± 6 6 41 ± 6 38 ± 3 45 ± 57 77 ± 3 72 ± 9 96 ± 6 7 43 ± 1 46 ± 7 52 ± 58 74 ± 5 61 ± 10 59 ± 18 8 39 ± 3 42 ± 8 42 ± 109 81 ± 4 71 ± 2 90 ± 5 9 37 ± 3 33 ± 3 45 ± 7

10 76 ± 6 62 ± 5 81 ± 8 10 37 ± 4 41 ± 2 43 ± 3

†P < 0.05 for individual day versus day 1 within same group (one-way ANOVA).

group were significantly greater than those of the restraint(only) group (Fig. 4 and Table 2).

Wistar-Kyoto rats. The overall change in HR and MAPand duration of tachycardic and pressor responses inWKYs showed that heterologous restraint and restraint(only) were equally effective followed by handling andair-jet (Figs 2 and 3, Table 2). There was no adaptationevident in the maximum cardiovascular responses (Fig. 2and Table 1) to handling and air-jet, as there were nodifferences between cardiovascular responses of day 1 andsubsequent days (2–10) in each of these protocols; a resultsimilar to that demonstrated with the restraint (only)protocol (McDougall et al. 2000). However, the duration oftachycardia (AUC data) due to acute (day 1) heterologousrestraint exposure was significantly greater than that

of the acute restraint (only) (McDougall et al. 2000)response. This tachycardic response was not maintainedon subsequent exposure to repeated heterologous restraintas the duration of the tachycardic response decreased,indicating adaptation within the WKY heterologousrestraint group (Fig. 3). This adaptive response resultedin days 3–10 being significantly different from day 1 andsimilar to levels previously measured in a restraint (only)WKY group (McDougall et al. 2000).

Spontaneously hypertensive rats. The overall change inHR and MAP and duration of tachycardic and pressorresponses in SHRs showed that heterologous restraintand restraint (only) were more effective then handlingand air-jet (Figs 2 and 3, Table 2). Interestingly, SHRsdemonstrated an ability to adapt certain aspects of their

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Figure 3. Group analysis of duration ofHR and MAP responses to differentstressors in different strainsFor group data analysis, the duration of HR(A, C and E) and MAP (B, D and F) was usedas a principle measure for comparison ofhandling (A and B), air-jet (C and D) andheterologous restraint stress protocols(E and F) in SD rats, WKYs and SHRs. Valueswere calculated as area under curve (AUC)measurements for the 60-min period afterthe beginning of stressor exposure wherethe 30-min period immediately before (days1–10, Fig. 2) exposure was used to definebasal HR and MAP levels. †P < 0.05 forindividual day versus day 1 within the samegroup (one-way ANOVA).

Figure 4. Mean HR and MAP responsesto restraint (n = 8) and heterologousrestraint (n = 7) stress protocols in SDratsChange in (�)(A and B) and duration of(AUC)(C and D) HR (A and C) and MAP(B and D) responses to repeatedrestraint-only (open bars) and heterologousrestraint (filled bars) stress. Change in andAUC values calculated as in Figs 2 and 3,respectively. †P < 0.05 for individual dayversus day 1 within same group (one-wayANOVA). #P < 0.05 versus day 1 ofrestraint (only) group (t test). ∗ P < 0.05 foroverall effect (days 1–10) versus restraintgroup (two-way ANOVA).

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Table 2. Rank order of effect for maximum change in (∆) and duration of (AUC)tachycardic and pressor responses as evoked by each stressor protocol withineach strain

Strain �HR �MAP

SD Het Rest > Rest > Hand > Air-jet Het Rest > Rest > Air-jet > HandWKY Het Rest = Rest > Hand > Air-jet Het Rest = Rest > Hand > Air-jetSHR Het Rest = Rest > Hand = Air-jet Het Rest = Rest = Hand > Air-jet

Strain AUC HR AUC MAP

SD Het Rest > Rest > Hand = Air-jet Het Rest > Rest > Hand = Air-jetWKY Het Rest = Rest > Hand > Air-jet Het Rest = Rest > Hand = Air-jetSHR Het Rest = Rest > Hand = Air-jet Het Rest > Rest > Hand = Air-jet

Hand, Handling; Rest. Restraint; Het Rest, heterologous restraint. >, significantdifferences between stressors as indicated (two-way ANOVA); = , non-significantdifferences between stressors as indicated (two-way ANOVA). Data for the changein HR and MAP temporal responses are from Figs 2 and 4. Data for AUC temporalresponse comparisons are from Figs 3 and 4. Data for the restraint (only) responsesin WKYs and SHRs were obtained from our previous study (McDougall et al. 2000).

cardiovascular response to repeated exposure for all threestressor types. In response to handling, the SHRs exhibiteda limited tachycardic response (Figs 1–3), yet a substantialpressor response was produced (Fig. 2), the duration ofwhich decreased with repeated exposure (Fig. 3). In theair-jet group, the maximum change and percentage changein HR and MAP, and the duration of the tachycardicresponse, significantly decreased with repeated exposureto air-jet, while a trend for a similar pattern of adaptationwas observed in the duration of the pressor response(Figs 2 and 3, Table 1). It is interesting that there was noevidence of adaptation in duration of tachycardia (AUCHR) in response to repeated heterologous restraint inSHRs. Heterologous restraint caused the duration of thetemporal pressor responses (AUC MAP) to be greater thanthat exhibited to repeated restraint (only) (McDougallet al. 2000). However, no other differences between therestraint (only) and heterologous restraint groups werefound (Table 2).

Stressor comparison between strains

Analysis of raw data indicates that generally SHRs arehyper-responsive with respect to changes in MAP inresponse to all stressors when compared to the twonormotensive strains; that is, SHRs had significantlygreater pressor responses that persisted longer than bothWKYs and SD rats for all stressor protocols. Normalizationof the maximum change data shows that these responseswere not dissimilar from those in the normotensive strains(Table 1), although clearly peak pressor responses weregreater in SHRs. In addition, the maximum change in, andduration of, tachycardic responses was lower in SHRs forhandling and air-jet than that of WKYs and SD rats but thisrank order of strains was reversed for the restraint-basedprotocols (Table 3). There were also a number of significant

differences between the normotensive strains; for examplethe duration of the tachycardic response in SD rats wasmaintained after handling and during air-jet for longerthan that of the WKY strain. In addition, WKYs and SDrats had similar changes in, and duration of, tachycardicresponses to restraint alone; however, for the heterologousrestraint protocol, the response of WKYs was significantlylower than that of the SD rats (and SHRs).

Of the stressors presented, the restraint-based protocolsclearly evoke the greatest change in cardiovascular activity,yet submaximal responses were evoked in response tohandling and air-jet. This difference, or grading, of thecardiovascular response based on the stressor presentedwas common to all three strains tested.

Discussion

This study provides the first comparison of thecardiovascular responses, as measured via telemetry,to repeated handling, air-jet and restraint in threecommonly used rat strains. Furthermore a heterologousstress protocol was employed whereby those groups thathad experienced repeated air-jet were then immediatelyexposed to repeated restraint over the subsequent 10 days.The major findings of the present study were that (1) theCNS graded or gated the cardiovascular component ofthe stress response, (2) SHRs exhibit prolonged pressorresponses to both mild (handling and air-jet) and severe(restraint only and heterologous restraint) forms ofstress. In addition, SHRs adapted to all stressors withrepeated exposure except to the heterologous restraintstress protocol, and (3) repeated mild stress (air-jet) innormotensive strains sensitized the cardiovascular systemto a subsequent heterologous stressor (restraint), which inturn adapted with repeated heterologous stress.

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Table 3. Rank order of effect for maximum change in (∆) andduration of (AUC) tachycardic and pressor responses as evokedby each stressor protocol between the strains

Stressor �HR �MAP

Handling WKY > SD > SHR SHR > WKY > SDAir-jet SD = WKY > SHR SHR > WKY = SDRestraint SHR > WKY = SD SHR > WKY > SDHet Rest SHR = SD > WKY SHR > WKY = SD

Stressor AUC HR AUC MAP

Handling SD > WKY > SHR SHR > WKY > SDAir-jet SD > WKY > SHR SHR > WKY = SDRestraint SHR > WKY = SD SHR > WKY = SDHet Rest SHR = SD > WKY SHR > SD > WKY

>, significant differences between strains as indicated (two-wayANOVA); = , non-significant differences between strains asindicated (two-way ANOVA). Data for the change in HR andMAP temporal responses are from Figs 2 and 4. Data for AUCtemporal response comparisons are from Figs 3 and 4. Data forthe restraint (only) temporal responses in WKYs and SHRs wereobtained from our previous study (McDougall et al. 2000).

An initial aim of this study was to assess the effectof various stressor protocols on the cardiovascularcomponent of the stress response and it was observedthat each rat strain graded their cardiovascular responsewhen exposed to each of the stressor protocols. Forexample, the maximum changes in HR on the first dayof exposure to air-jet, handling and restraint in SD ratswere 61 ± 24 (17 ± 7%), 91 ± 12 (28 ± 4%), 149 ± 21(43 ± 7%) beats min−1, respectively. Such grading of thecardiovascular response to stressful stimuli is noteworthyin the context of patterned sympathetic responses,as recently reviewed (Morrison, 2001; Saper, 2002),where evidence for differential autonomic outflow inresponse to varying physiological demands is strong.However, there is limited evidence of different patterns ofsympathetic activity with exposure to different stressorsof a psychological nature. Indeed, the data presentedhere suggest a level of control with respect to thecardiovascular system such that tachycardic responses aregraded according to stressor type. Such a notion wouldbe consistent with the integration of information byhigher centres of the CNS yielding a series of gradedcardiovascular outputs.

A number of studies have shown central specificitybetween what may be broadly classified as ‘physical’ and‘psychological’ stressors, such that the central circuitryinvolved in producing the final stress response is distinctbetween these categories of stressors (Thrivikraman et al.2000; Dayas et al. 2001). Based on these and other studies,restraint, air-jet and handling can be classified as thepsychological-type of stressors; however, we have clearlyshown that each of these stressors caused a differentialcardiovascular response. Thus, these results would suggest

that a gating mechanism is operative in the CNS thatsubsequently determines the degree of cardiovascularactivation, according to the perceived threat, althoughthe precise anatomical and/or functional location(s) isnot known. Restraint and immobilization stress, havepreviously been shown to cause differential levels of Fosimmunoreactivity (a marker of neuronal activation) inthe paraventricular nucleus of the hypothalamus, locuscoeruleus and central and medial nuclei of the amygdala(Chowdhury et al. 2000), implicating those regions insuch a gating mechanism within the ‘psychological stress’circuitry. Other regions possibly involved may include thedorsomedial hypothalamus, as inhibition of this regionwith muscimol significantly attenuates ‘air stress’-inducedcardiovascular responses (Stottpotter et al. 1996). Giventhat the aforementioned nuclei are known to be ableto modulate central cardiovascular control (Dampney,1994) it seems likely that such nuclei may at least bepartly responsible for mediating the central processingthat resulted in the differential cardiovascular responsesto handling, air-jet and restraint observed in this study.

We have previously shown that the cardiovascularresponse to repeated restraint differed between WKYsand SHRs (McDougall et al. 2000), in that SHRs hadheightened pressor responses that remained elevated forlonger when compared to WKYs. In addition, SHRshad longer lasting tachycardic responses that decreasedwith repeated restraint. In the present study we havedemonstrated that such differences occur with otherpsychological stressors as SHRs produced significantlygreater peak pressor responses (both change in andduration) to handling, air-jet, restraint and heterologousrestraint compared to those of the normotensive SDand WKY strains. Our raw data are consistent withother studies indicating that SHRs have significantlygreater changes in blood pressure to open field (novelenvironment) (van den Buuse et al. 2001), 15-minimmobilization and 30-s air-jet (Ely, 1995) as comparedto WKYs. On the other hand, no difference was reportedbetween SHRs and WKYs in peak MAP or pressor durationto an airpuff startle protocol (Palmer & Printz, 1999),although the pattern of the HR response to individualairpuff stimuli differed between the strains. However,normalization of the maximum change in HR andMAP data indicates negligible differences between thestrains although duration of pressor responses (AUC) wasprolonged in SHRs. Thus, as SHRs exhibited extremeabsolute peak pressure for a prolonged period duringexposure to each of the stressors as compared to thenormotensive WKY and SD strains, this in turn may havean impact on their cardiovascular status particularly ifthe effects of the stressors are maintained.

In SHRs there was evidence of adaptation of thecardiovascular responses to repeated handling and air-jet, as we have seen previously with restraint (McDougall

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et al. 2000). The adaptation seen in the cardiovascularresponse of SHRs was associated with an exaggerated initialresponse compared with responses in either WKYs or SDrats. Subsequent repeated exposure results in the responsesof SHRs being modified to levels similar (or even lower) tothose measured in the normotensive SD and WKY strains.In the SHR restraint (only) group, this occurred by day 7of the 10-day regimen with respect to the duration of thetachycardia response (McDougall et al. 2000), while in thepresent study the change in MAP due to air-jet reached‘normotensive-like’ levels by day 6. With handling, theduration of the MAP response in SHRs was comparable tothat of the responses in WKYs and SD rats by the secondexperience. The number of exposures it takes for SHRsto ‘normalise’ may in turn be related to the magnitude ofthe initial stress-induced cardiovascular response. On theother hand, WKYs and SD rats did not show any signsof cardiovascular adaptation to each of the stressors alonein the present study, although the tachycardic response torepeated restraint did adapt in a different normotensiverat strain (Listar) (Chen & Herbert, 1995; Chung et al.2000).

Finally, the heterologous restraint protocol was includedto elucidate what impact repeated air-jet exposure has onthe responses of a subsequent heterologous stressor, in thisinstance restraint. There are few studies that have focusedon the cardiovascular system with heterologous protocolexposure, and those usually present the heterologousstressor once (Bhatnagar & Dallman, 1998; Bhatnagar etal. 1998), not repeatedly, as in this study. This techniquerevealed specific differences between the strains. Theblood pressure response (both change in and duration)in SD rats was increased in response to heterologousrestraint as compared to the restraint (only) group onday 1. In addition, the HR and MAP responses overthe 10-day period in the heterologous restraint groupwere significantly greater than that of the restraint (only)group. In WKYs, the duration of the tachycardic responsewas elevated on day 1 in the heterologous restraintgroup as compared to restraint alone; however, withrepeated exposure to the heterologous stressor (restraint) asubsequent decrease in the tachycardic response in WKYswas recorded which returned to restraint-alone levels byday 3. In SHRs, there were minimal differences between theheterologous restraint and restraint (only) stress-evokedcardiovascular responses. Collectively, the heterologousstress protocol indicates that repeated mild stress causesrats to exhibit exaggerated cardiovascular reactivity, at leasttransiently in normotensive strains, when exposed to asecondary stressor.

In conclusion, the present study provides a comparisonof the cardiovascular profiles obtained as a result ofrepeated exposure to differing stressors. SHRs exhibitprolonged cardiovascular responses to all stressor typestested but show an ability to adapt those responses

to ‘normotensive-like’ levels with repeated exposure.Normotensive strains show some degree of sensitizedcardiovascular responses to heterologous stress, wheresubsequent repeated exposure leads to strain-specificresponses. The differential tachycardia responses inducedwithin each of the strains as evoked by different stressorsreflect specificity in the regulation of the cardiovascularsystem. These data clearly support the hypothesis thatunder different stressful scenarios higher brain centrescan modulate the cardiovascular system to produce gradedresponses.

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Acknowledgements

This work was supported in part by grants from the NationalHealth and Medical Research Council of Australia.

C© The Physiological Society 2004