Circadian rhythms of blood pressure, heart rate, and locomotor activity in spontaneously hypertensive rats as measured with radio-telemetry

Telemetry as a Method for Measuring the Impact of Housing Conditions on Rats' Welfare

Various tools have been developed over previous years to study the welfare of laboratory animals. These include preference tests, which are commonly used to evaluate housing environments. Preference tests, however, have some pitfalls: they supply information only on the animals’ present preferences, and they allow the animal the choice only between the options offered. Other methods based upon the collection of clinico-chemical data require handling of the animals, which can be stressful in itself. An alternative may be to use telemetry to measure the changes in physiological parameters caused by different environmental conditions. The aim of this study was to use telemetry to evaluate the short-term impact of housing conditions on rodents. We monitored heart rate, blood pressure and body temperature in rats kept on three different types of flooring — bedding, grid floors and plastic floors. The study revealed significant differences in systolic and diastolic blood pressure, heart rate and body temperature between rats housed in the three conditions, indicating that both grid floors and plastic floors are more stressful for the animals than bedding. The observed differences did not diminish over the two-week observation period. The grid-floor housing induced elevations in blood pressure and heart rate. Blood pressure remained elevated even when the animals were returned to standard bedding, whereas the heart rate declined back to its original value immediately in response to this shift. This study shows that telemetry is a very effective tool but that it needs integrating with other methods: in addition, a greater understanding of the biological significance of the changes in cardiovascular parameters is required before the hypothesis that these changes represent an indication of distress can be accepted.

Circadian rhythms in blood pressure in free-ranging three-toed sloths (Bradypus variegatus)

Blood pressure (BP) profiles were monitored in nine free-ranging sloths (Bradypus variegatus) by coupling one common carotid artery to a BP telemetry transmitter. Animals moved freely in an isolated and temperature-controlled room (24 degrees C) with 12/12-h artificial light-dark cycles and behaviors were observed during resting, eating and moving. Systolic (SBP) and diastolic (DBP) blood pressures were sampled for 1 min every 15 min for 24 h. BP rhythm over 24 h was analyzed by the cosinor method and the mesor, amplitude, acrophase and percent rhythm were calculated. A total of 764 measurements were made in the light cycle and 721 in the dark cycle. Twenty-four-hour values (mean +/- SD) were obtained for SBP (121 +/- 22 mmHg), DBP (86 +/- 17 mmHg), mean BP (MBP, 98 +/- 18 mmHg) and heart rate (73 +/- 16 bpm). The SBP, DBP and MBP were significantly higher (unpaired Student t-test) during the light period (125 +/- 21, 88 +/- 15 and 100 +/- 17 mmHg, respectively) than during the dark period (120 +/- 21, 85 +/- 17 and 97 +/- 17 mmHg, respectively) and the acrophase occurred between 16:00 and 17:45 h. This circadian variation is similar to that observed in cats, dogs and marmosets. The BP decreased during "behavioral sleep" (MBP down from 110 +/- 19 to 90 +/- 19 mmHg at 21:00 to 8:00 h). Both feeding and moving induced an increase in MBP (96 +/- 17 to 119 +/- 17 mmHg at 17:00 h and 97 +/- 19 to 105 +/- 12 mmHg at 15:00 h, respectively). The results show that conscious sloths present biphasic circadian fluctuations in BP levels, which are higher during the light period and are mainly synchronized with feeding.

A new inbred Wistar-Kyoto rat substrain exhibiting apparent salt sensitivity and borderline hypertension

The normotensive Wistar-Kyoto (WKY) rat strain is a traditional control for the spontaneously hypertensive rat (SHR). We found trait differences between two inbred normotensive WKY strains, derived originally from different vendors, and compared these two strains from La Jolla-Taconic Farms (WKY/lj-tf) and La Jolla-Charles River (WKY/lj-cr) with the inbred SHR/lj-cr for cardiovascular, diurnal, and activity traits under normal and high (8%) NaCl diets. Marked genetic diversity was found between the two vendor-derived WKY. By using an extended study design and radiotelemetry, we compared WKY/lj-cr, WKY/lj-tf, and SHR/lj-cr with the following results: systolic pressure (120 +/- 1, 133 +/- 1, 168 +/- 3 mmHg, respectively); diurnal variation in heart rate (DeltaHR: 46 +/- 3, 71 +/- 4, 57 +/- 2 beats/min, respectively); and salt sensitivity of arterial pressure (Deltasystolic: 10 +/- 1, 21 +/- 1, 20 +/- 1 mmHg, respectively). The WKY/lj-tf genotype apparently results in compromised control of arterial pressure and heart rate, especially during high NaCl intake, and greater susceptibility to high pressure (i.e., high NaCl-induced secondary changes). WKY/lj-tf thus constitutes a new inbred borderline hypertensive WKY substrain offering unique opportunities for genomic studies into the development of genetic hypertension.

Effects of angiotensins on day-night fluctuations and stress-induced changes in blood pressure

In this study we evaluated by telemetry the effects of ANG II and ANG-(1-7) infusion on the circadian rhythms of blood pressure (BP) and heart rate (HR) and on the cardiovascular adjustment resulting from restraint stress in rats. ANG II or ANG-(1-7) or vehicle were infused subcutaneously for 7 days. Restraint stress was carried out before, during, and after infusion at 7-day intervals. Parallel with an increase in MAP, ANG II infusion produced an inversion of MAP circadian rhythm with a significant MAP acrophase inversion. It also produced bradycardia during the first 3 days of infusion. Thereafter, HR progressively increased, reaching values similar to or above those of the control period at the end of the infusion period. HR circadian variation was not changed by ANG II infusion. Strikingly, ANG II significantly attenuated the increase in MAP induced by restraint stress without altering the HR response. ANG-(1-7) infusion produced a slight but significant decrease in MAP restricted to the daytime period. No significant changes in the MAP acrophase were observed. In addition, ANG-(1-7) infusion produced a small but significant sustained bradycardia. ANG-(1-7) did not change cardiovascular responses to restraint stress. These data indicate that ANG II can influence the activity of brain areas involved in the determination of stress-induced or circadian-dependent variations of blood pressure without changing HR fluctuations. A significant modulatory influence of ANG-(1-7) on basal MAP and HR is also suggested.

The circadian pattern of breathing in conscious adult rats

Recently, a circadian oscillation in pulmonary ventilation (VE) was reported in conscious, undisturbed rats [Respir. Physiol. 120 (2000) 179], with a pattern similar to that of body temperature (Tb), oxygen consumption (V(O(2))) and activity. The present study explored the relationship between the daily VE pattern and these rhythms. Adult rats (n=23) were instrumented for measurements of Tb and activity by telemetry, and placed in a chamber for measurement of VE by the barometric method and V(O(2)) by an open-flow method. Simultaneous recordings were made continuously for 3 consecutive days in a 12-h light:12-h dark (L:D) cycle. All variables showed substantial daily oscillations, with significantly higher values in the D phase, and approximately proportionate changes in VE and V(O(2)). Daily changes in tidal volume (VT) relative to inspiratory time (TI), rather than in TI relative to total breath duration, accounted for the oscillation in VE. The VT rhythm was phase-advanced relative to those of V(O(2)) and Tb. L-D differences in VE persisted when comparison between the phases was made for the same level of either very low or very high activity. We conclude that the oscillation in VE does not depend on the daily changes in activity. Rather, the daily pattern of VE is likely shaped by the oscillations of multiple physiological variables, two of which may be Tb and V(O(2)).

Sodium-induced rise in blood pressure is suppressed by androgen receptor blockade

Our objective was to test the hypothesis that 1) a high Na (HNa, 3%) diet would increase blood pressure (BP) in male Wistar-Kyoto (WKY) and spontaneously hypertensive Y chromosome (SHR/y) rat strains in a territorial colony; 2) sympathetic nervous system (SNS) blockade using clonidine would lower BP on a HNa diet; and 3) prepubertal androgen receptor blockade with flutamide would lower BP on a HNa diet. A 2 x 4 factorial design used rat strains (WKY, SHR/y) and treatment [0.3% normal Na (NNa), 3% HNa, HNa/clonidine, and HNa/flutamide]. BP increased in both strains on the HNa diet (P < 0.0001). There was no significant decrease in BP in either strain with clonidine treatment. Androgen receptor blockade with flutamide significantly decreased BP in both strains (P < 0.0001) and normalized BP in the SHR/y colony. Neither heart rate nor activity could explain these BP differences. In conclusion, a Na sensitivity was observed in both strains, which was reduced to normotensive values by androgen blockade but not by SNS blockade.

Cardiovascular and behavioural responses to psychological stress in spontaneously hypertensive rats: effect of treatment with DSP-4

We used a model of psychological stress combining exposure to an open-field novel environment, radio-telemetric measurement of blood pressure and heart rate, and behavioural tracking analysis of behavioural parameters. All rats showed significant increases in blood pressure and heart rate for the duration of open-field exposure, with spontaneously hypertensive rats (SHR) showing markedly greater pressor responses and tachycardia when compared to either Wistar-Kyoto (WKY) or Sprague-Dawley rats (SD rats). Behavioural responses in the open-field were unrelated to the magnitude of cardiovascular responses. Open-field exposure on 4 consecutive days induced similar pressor responses and tachycardia on each day. By contrast, behavioural responses were reduced from the second day of open-field exposure. Treatment of SHR and WKY rats with DSP-4, to deplete central noradrenaline levels, did not affect cardiovascular responses in SHR, whereas WKY rats showed a trend towards inhibition. However, in WKY rats, but not SHR, DSP-4 treatment caused a marked reduction in behavioural activity in the open-field. In conclusion, these data show that: (1) SHR display marked cardiovascular hyperreactivity to psychological open-field stress when compared to two normotensive rat strains; (2) unlike behavioural responses, cardiovascular stress responses do not habituate upon repeated stress exposure; and (3) noradrenergic projections from the locus coeruleus do not appear to play a major role in cardiovascular stress responses in SHR or WKY rats, although they may be involved in behavioural responses in WKY rats.

Overexpression of eNOS in NTS causes hypotension and bradycardia in vivo

The role of nitric oxide (NO) in the brain in the control of blood pressure and the sympathetic nervous system is debated. This study examined the effect of overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarii (NTS) on blood pressure in conscious rats. Adenovirus vectors encoding either eNOS (AdeNOS) or ss-galactosidase were transfected into the NTS in vivo. In the AdeNOS-treated rats, the local expression of eNOS in the NTS was confirmed by immunohistochemical staining and Western blot analysis for the eNOS protein and by increased production of nitrite/nitrate in the NTS measured by in vivo microdialysis. Blood pressure and heart rate, monitored by the use of a radiotelemetry system in a conscious state, were significantly decreased in the AdeNOS-treated group at day 5 to day 10 after the gene transfer. Urinary norepinephrine excretion also was decreased at day 7 after the gene transfer in the AdeNOS-treated group. Our results indicate that overexpression of eNOS in the NTS decreases blood pressure, heart rate, and sympathetic nerve activity in conscious rats.

Investigatory behavior of a novel conspecific by Wistar Kyoto, Wistar and Sprague-Dawley rats

The assumption was made that investigatory behaviors (i.e., ano-genital and general body sniffing) of a female conspecific by a mature male rat, has positive hedonic characteristics. Because reduced interest in pleasurable events (i.e., anhedonia) is diagnostically related to depressive behavior, the hypothesis was advanced that less investigatory behavior would be observed in an animal model of depression, namely the Wistar Kyoto (WKY) rat strain. In Experiment 1, WKY, Wistar and Sprague-Dawley male rats were subjected, in the first test series, to three consecutive 2-min exposures to one intruder stimulus female, followed later by another three consecutive 2-min exposures to a second stimulus intruder female. On the second test series, 24 h later, the male rats were exposed to one female for 2 min, followed 6 min later to another 2-min exposure to another stimulus female. Half the male subjects were subjected to tail shock stress 2 h before the first test series. All males demonstrated a habituation of the investigatory response to the same stimulus female, but a dishabituation when subsequently exposed to a new stimulus female. Only WKY rats, exposed to prior stress, revealed a significant reduction in investigatory behavior. In Experiment 2, using only WKY and Wistar rats, a factorial design was used to observe any differences between two stressors, namely tail shock and water restraint, and also to observe possible differences in investigatory behavior towards male vs. female intruder rats. Restraint stress and shock stress elicited significant reductions in investigatory behavior for WKY rats, but not Wistar rats, when confronted with female intruder rats. Male intruders elicited more freezing behavior, as well as aggressive defensive fighting behavior from resident male rats. The results are interpreted to suggest that the significant decrease in investigatory behavior towards a female intruder, which was observed primarily in stressed WKY males, reflects the presence of anhedonia in stressed WKY rats, and reinforces our assertion that the WKY rat strain represents a useful animal model of depressive behavior.

A novel procedure for daily measurements of hemodynamical, hematological, and biochemical parameters in conscious unrestrained rats

Accurate and chronic measurements of various parameters in conscious animals are fundamental for depicting pathological chronic conditions and their etiology in many experimental models, but they are often difficult to achieve. The aim of the present work was to develop and describe step-by-step a reproducible surgical procedure and daily manipulations for continuous, chronic use of conscious rats as models towards a better understanding of various cardiovascular and renal diseases and the testing of novel pharmacological drugs. The complete apparatus involved the use of a series of specialized devices (harness, rotating swivel, revolving arm) supporting a flexible, permanently implanted vascular catheter into the left femoral artery up to the abdominal aorta connected to a miniaturized individual peristaltic pump for delivering fluid at a constant rate. Such a set-up also enabled easy, quick, and reproducible daily blood sampling for the evaluation of more than 20 parameters, including the monitoring of heart rate (HR) and blood pressure in freely moving conscious rats. The overall success and survival rate reached 98% over 14 days and could be extended further. This model represents a much needed and valuable advance in surgical research techniques to evaluate the hemodynamic, hematological, biochemical, pharmacokinetic, and toxicological profile of any new drugs over time in conscious animal models such as rats. What makes this procedure satisfactory is the long-term reliable arterial access and reproducibility of the methodological approach for accurate and continuous measurements, minimizing the stress or invasiveness associated with the use of currently employed systems.

Effects of photoperiod reduction on rat circadian rhythms of BP, heart rate, and locomotor activity

The effects of a photoperiod reduction in the entrainment of circadian rhythms of systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), and spontaneous locomotor activity (SLA) were determined in conscious Wistar rats by using radiotelemetry. Two groups of seven rats were maintained in a 12:12-h light-dark (12L/12D) photoperiod for 11 wk and then placed in a reduced photoperiod of 8:16-h light-dark (8L/16D) by advancing a 4-h darkness or by advancing and delaying a 2-h darkness for 6 wk. Finally, they were resynchronized to 12L/12D. Advancing a 4-h dark phase induced a 1-h advance of acrophase for SBP, DBP, and HR, but not for SLA. The percent rhythm, amplitude, and the 12-h mean values of all parameters were significantly decreased by the photoperiod reduction. When symmetrically advancing and delaying a 2-h dark phase, a 1 h 20 min delay of acrophases and a decrease in percent rhythms and amplitudes of SBP, DBP, HR, and SLA were observed. Only the 12-h mean values of HR and SLA were decreased. Our findings show that the cardiovascular parameters differ from SLA in phase-shift response to photoperiod reduction and that the adjustment of circadian rhythms to change from 12L/12D to 8L/16D photoperiod depends on the direction of the extension of the dark period.

SHR Y chromosome enhances the nocturnal blood pressure in socially interacting rats

Our objective was to test the hypothesis that nocturnal mean arterial pressure (MAP), heart rate (HR), and activity would be increased in 1) colony over individually caged rats and 2) the spontaneously hypertensive rat (SHR) Y chromosome strain (SHR/y colony) compared with Wistar-Kyoto (WKY) rats. MAP, HR, and activity were monitored using radiotelemetry. The nocturnal MAP rise expressed as the percentage change in MAP from light to dark was increased (P < 0.05) in the SHR/y colony. The SHR Y chromosome increased MAP in both the colony and caged groups compared with WKY (P < 0.001). The SHR/y colony animals spent 23% of a 24-h period at a MAP >120 mmHg, whereas the WKY colony animals spent 2% of a 24-h period in this range. The MAP of the SHR/y colony on clonidine was reduced (P < 0.001) to WKY baseline values. Activity but not HR was increased (P < 0.01) in the WKY and SHR/y colonies compared with caged animals. In conclusion, colony housing and the SHR Y chromosome increased MAP compared with individually caged housing.

Short-term and long-term blood pressure and heart rate variability in the mouse

Knowledge on murine blood pressure and heart rate control mechanisms is limited. With the use of a tethering system, mean arterial pressure (MAP) and pulse interval (PI) were continuously recorded for periods up to 3 wk in Swiss mice. The day-to-day variation of MAP and PI was stable from 5 days after surgery. Within each mouse (n = 9), MAP and PI varied by 21+/-6 mm Hg and 17+/-4 ms around their respective 24-h averages (97+/-3 mm Hg and 89+/-3 ms). Over 24-h periods, MAP and PI were bimodally distributed and clustered around two preferential states. Short-term variability of MAP and PI was compared between the resting (control) and active states using spectral analysis. In resting conditions, variability of MAP was mainly confined to frequencies <1 Hz, whereas variability of PI was predominantly linked to the respiration cycle (3-6 Hz). In the active state, MAP power increased in the 0.08- to 3-Hz range, whereas PI power fell in the 0.08- to 0.4-Hz range. In both conditions, coherence between MAP and PI was high at 0.4 Hz with MAP leading the PI fluctuations by 0.3-0.4 s, suggesting that reflex coupling between MAP and PI occurred at the same frequency range as in rats. Short-term variability of MAP and PI was studied after intravenous injection of autonomic blockers. Compared with the resting control state, MAP fell and PI increased after ganglionic blockade with hexamethonium. Comparable responses of MAP were obtained with the alpha-blocker prazosin, whereas the beta-blocker metoprolol increased PI similarly. Muscarinic blockade with atropine did not significantly alter steady-state levels of MAP and PI. Both hexamethonium and prazosin decreased MAP variability in the 0.08- to 1-Hz range. In contrast, after hexamethonium and metoprolol, PI variability increased in the 0.4- to 3-Hz range. Atropine had no effect on MAP fluctuations but decreased those of PI in the 0.08- to 1-Hz range. These data indicate that, in mice, blood pressure and its variability are predominantly under sympathetic control, whereas both vagal and sympathetic nerves control PI variability. Blockade of endogenous nitric oxide formation by N(G)-nitro-L-arginine methyl ester increased MAP variability specifically in the 0.08- to 0.4-Hz range, suggesting a role of nitric oxide in buffering blood pressure fluctuations.

Interaction between lifetime captopril treatment and NaCI-sensitive hypertension in spontaneously hypertensive rats and Wistar-Kyoto rats

DESIGN

Previous studies that were based on daytime arterial pressure recordings indicate that lifetime treatment with captopril exacerbates the hypertensive response to a high NaCl diet in spontaneously hypertensive rats (SHR) but has no such effect in normotensive Wistar-Kyoto (WKY) rats. The present study used 24-h recording methods to examine the hypothesis that during the normal waking hours of rats (night-time) the hypertensive response to a high NaCl diet is exacerbated in SHR and induced in WKY rats treated with lifetime captopril.

METHODS

SHR and WKY rats were (1) untreated, (2), lifetime captopril treated or (3) lifetime captopril treated but removed from the treatment 2 weeks prior to exposure to a high (8%) NaCl diet

RESULTS

Compared to untreated SHR, in SHR that were continuously treated with captopril, the high NaCl diet caused a more rapid and greater rise in arterial pressure. Discontinuation of the captopril treatment did not significantly diminish this NaCl-sensitivity. In untreated WKY rats, the high NaCl diet did not alter mean arterial pressure, but in the lifetime captopril-treated WKY rats the high NaCl diet induced a rapid rise in arterial pressure. In WKY rats, discontinuation of the lifetime captopril treatment did not diminish this NaCl-induced rise in arterial pressure, even though baseline mean arterial pressure in this group is similar to that in untreated WKY rats.

CONCLUSIONS

Lifetime captopril treatment accelerates the hypertensive response to a high NaCl diet in SHR, and it induces a similar response in WKY rats. In both strains, the lifetime captopril treatment causes a change in the response that is not dependent on concurrent administration of the drug. This finding further suggests that lifetime captopril treatment causes a long-term resetting of cardiovascular response mechanisms.

Diurnal variations of blood pressure in dogs

Using the telemetry system, we measured the blood pressure (BP) invasively in seven adult mongrels while unanesthetized and unbound. Post-operative BP after implanting the telemetry BP transmitter showed temporarily high values due to the invasive nature of the surgery. It was, however, observed that BP gradually decreased thereafter, and showed settled trends from the eighth day post-operatively. When we took the average of the systolic, mean and diastolic BP at hourly intervals for each of the dogs once their BP had settled, a twin peak diurnal variation (at 8:00 and 19:00) was observed. Moreover, significantly high values (p < 0.05) were identified in active state compared with when sleeping or at rest. The 24 hr BP measured by the telemetry system in seven normal dogs resulted in the following values: systolic 123.4 +/- 7.9 mmHg, mean 91.1 +/- 5.6 mmHg, and diastolic 74.5 +/- 4.9 mmHg.

Cardiovascular changes during morphine administration and spontaneous withdrawal in the rat

Morphine maintenance doses of 10 mg kg(-1) day(-1), 20 mg kg(-1) day(-1) and 30 mg kg(-1) day(-1) were administered to three groups of rats via miniosmotic pumps for 7 days to induce physical dependence. They were then allowed to undergo spontaneous withdrawal. Radiotelemetric blood pressure measurements showed that morphine increased systolic and diastolic blood pressure on the first day of morphine treatment and produced a dose dependent decrease in heart rate, systolic and diastolic blood pressure thereafter. After the peak depressive effect, development of tolerance to morphine was observed in systolic and diastolic blood pressure, but not in the heart rate. During spontaneous withdrawal, both systolic and diastolic blood pressure increased beyond pre-morphine levels for all doses and there was a rebound increase in heart rate at the 30 mg kg(-1) day(-1) dose. These results suggest that the improved sensitivity of telemetric measures combined with the use of minipumps for morphine treatment provide an animal model of spontaneous opioid withdrawal.

Telemetry for cardiovascular monitoring in a pharmacological study: new approaches to data analysis

Radio-telemetry systems offer the ability to measure blood pressure and heart rate in experimental models of hypertension without the stress artifacts induced by some other methods. We therefore aimed to develop improved, nonparametric regression methods for radio-telemetry data and to use these to assess the effects of pharmacological interventions on cardiac and vascular hypertrophy in the stroke-prone spontaneously hypertensive rat. One control group and 5 groups treated either with losartan (alone or in combination with NG-nitro-L-arginine methyl ester [ L-NAME]), perindopril (also alone or in combination with L-NAME), or hydralazine plus hydrochlorothiazide were monitored for 4 weeks. Cardiac hypertrophy was assessed by the left ventricle plus septum weight to body weight ratio and vascular hypertrophy by flow-cytometry analysis of vascular smooth muscle cell polyploidy. Hemodynamic series were split into trend and cyclic components by the seasonal and trend decomposition procedure based on Loess and compared between groups by Loess regression modeling. Systolic and diastolic blood pressures were reduced systematically by losartan and perindopril (P<10(-10)) but to a lesser extent by hydralazine plus hydrochlorothiazide (P<10(-8)), and diurnal variation was reduced in the latter group (P<10(-6)). L-NAME significantly reduced the hypotensive effect only of losartan. Vascular and cardiac hypertrophy were significantly attenuated with losartan or perindopril, but were unchanged with other treatments. The new analysis proposed here identifies differential effects on trends and cyclic variation and associations with regression of end-organ damage for losartan and perindopril compared with hydralazine plus hydrochlorothiazide. The method offers a powerful tool for detailed investigation of radio-telemetry data.

Effects of melatoninergic agonists on light-suppressed circadian rhythms in rats

This study addressed the question whether light-suppressed circadian rhythms in cardiovascular parameters in rats could be restored by melatonin and a synthetic analogue. Blood pressure, heart rate, and locomotor activity were monitored by radiotelemetry in six Sprague-Dawley rats. After synchronization to a 12:12 light/dark (LD) schedule (lights on at 0700 hours, 100 lux), rats were kept in constant light (LL) of low intensity (5-10 lux) for 11 weeks. After 3 weeks of LL, rats received daily intraperitoneal (i.p.) injections at 1900 hours of vehicle, the melatonin agonist S-21767 (5 mg/kg) and melatonin (1 mg/kg). Spectral power, 24-h amplitudes and the differences between day and night means were calculated as measures of circadian rhythmicity. During LL a lengthening of the endogenous period to 26 h was observed, which was accompanied by a continuous decrease in circadian amplitude in all parameters monitored until, in the third week of LL, circadian rhythmicity was almost abolished. Neither vehicle, S-21767 nor melatonin were able to restore circadian rhythms in blood pressure and locomotor activity. In contrast, both agonists induced circadian rhythmicity in heart rate in two out of six rats. The day/night difference in heart rate of all animals was significantly increased by S-21767 and, to a smaller extent, by melatonin, whereas the circadian amplitude was not affected. In conclusion, melatonin and the synthetic agonist were able to partially synchronize circadian rhythmicity in heart rate during constant light, but could not restore circadian rhythms in blood pressure.

Chronopharmacological dependence of antihypertensive effects of the imidazoline-like drugs in stroke-prone spontaneously hypertensive rats

In the present study we investigated the chronopharmacological dependence of dose-dependent hypotensive and cardiochronotropic effects of the imidazoline-like drugs (clonidine, rilmenidine and moxonidine) in stroke-prone spontaneously hypertensive rats (SHR-SP), using radio-telemetric system (Data Sciences, USA). The 24-h blood pressure, heart rate and locomotor activity profiles showed peak values during the rats' active phase during the night period. The degree of hypotensive and bradycardic effects of all drugs were most evident at this time and occurred in the absence of a change in locomotor activity. These studies show that clonidine, rilmenidine and moxonidine decrease blood pressure and heart rate in a time-dependent manner in SHR-SP. It was demonstrated that the degree and duration of hypotensive action of imidazoline-like drugs vary with the time of drug administration.

Size constraints of telemeters in rats

This study was designed to determine the maximum-size subcutaneous telemeter that would enable long-term and multichannel data collection in a 170-g rat for 90 days. In phase 1, rats with implants weighing 5 (2.5 cm3), 15 (7.5 cm3), 25 (12.5 cm3), 35 (17.5 cm3), or 45 (22.5 cm3) g were compared with sham-operated (SOC) and nonoperated (NOC) control animals. Severe skin lesions, seromas, and lower growth rates were observed in rats having implants >/=35 g. Thus, in phase 2, rats implanted with 23.5 g (17.5 cm3; 11-g active telemeter and 12.5-g implant) were compared with rats implanted with 11 g (6 cm3; telemeter only) and with the SOC and NOC groups. No differences were found among implanted groups in mean arterial pressure (MAP), heart rate (HR), subcutaneous temperature, or spontaneous activity under standard housing conditions. All groups were more active and had a higher MAP during the dark than the light phase of the daily cycle. During 2 h of cold exposure (3 degreesC), both telemetered groups exhibited similar changes in HR, MAP, temperature, and activity levels. Adrenal glands were larger in the 23.5-g group (51 +/- 1.6 mg) than in the SOC (46 +/- 1.0 mg) and the NOC groups (41 +/- 2.0 mg). No other significant differences were found in organ, muscle, or bone weights. These data verify the feasibility of using 23.5-g (17.5 cm3) subcutaneous telemeters for chronic recordings in young adult rats.

Radiotelemetric versus externalized catheter monitoring of blood pressure: effect of vasopressin in spontaneous hypertension

The changes in arterial pressure that follow withdrawal of a 3-h intravenous infusion of arginine vasopressin (AVP; 20 ng/kg/min) in spontaneously hypertensive rats (SHR) were monitored by radiotelemetry or conventional externalized femoral arterial catheters connected to pressure transducers. Baseline control arterial pressure was lower in the telemetry group compared to the externalized group. After withdrawal of the AVP infusion, blood pressure fell below preinfusion levels in both groups but the decrease was much less in the telemetry group. Strikingly, absolute blood pressure values recorded both during and after the vasopressin infusion were remarkably similar in the two groups. Responses in rats with externalized catheters implanted 7 days before infusion of AVP, a protocol similar to the telemetry group, were similar to those in rats with catheters implanted 24 h earlier. Blood pressure remained decreased in SHR infused with AVP for several days with complete recovery requiring 6-7 days. In contrast, physical activity decreased only on the first day following withdrawal of the infusion. Thus, the mechanism accounting for the blood pressure decrease must be of a long duration and unrelated to a change in gross physical activity. The results emphasize the value of radiotelemetry for recording blood pressure responses.

Influence of neonatal focal cerebral hypoxia-ischemia on cardiovascular and neurobehavioral functions in adult Wistar rats

No experimental studies looked at the disturbances appearing after a neonatal focal cerebral hypoxia-ischemia (HI) when animals become adults. Using radiotelemetry, we examined the effects of neonatal focal cerebral HI on blood pressure (BP), heart rate (HR), locomotor activity (LA), body temperature (BT) levels and circadian rhythm parameters of unrestrained adult Wistar rats. At 15 weeks of age, we continuously recorded the cardiovascular and neurobehavioral parameters of HI (n = 6) and sham-operated (n = 6) rats. In adult rats, HI induced persistent hypertensive effects associated with alteration in BP circadian rhythms and pronounced decreases in mesor and percent rhythm of LA. HR and BT parameters were not significantly modified. Therefore, our results suggest that the rat cardiovascular and behavioural circadian control systems may involve several structures which present selective vulnerability to early cerebral HI.

Nicorandil affects diurnal rhythms of body temperature, heart rate and locomotor activity in rats

The effects of nicorandil, a K+ channel opener with a potent vasodilator action, on diurnal rhythms of body temperature, heart rate and locomotor activity were assessed in rats. Transmitters were intraperitoneally implanted under ether anaesthesia. After recovery from surgery, body temperature, heart rate and locomotor activity were recorded during control, saline or nicorandil (10 mg x kg(-1) administered orally) treatment and for 5 days after treatment. For each period, Fourier analysis determined the predominant rhythmicity for body temperature, heart rate and locomotor activity while cosinor analysis assessed the corresponding mesors, acrophases and amplitudes and maxima and minima were directly plotted from raw data. The results indicated: (1) loss of the diurnal rhythmicity for all three rhythms after implantation; (2) stress-induced modifications of almost all the characteristics of the three rhythms after saline and (3) a loss of diurnal rhythmicity of heart rate after nicorandil, an effect that was not observed after saline and which was reversed when nicorandil administration was stopped. In conclusion, nicorandil perturbed the diurnal rhythmicity of heart rate while the rhythmicity of body temperature and locomotor activity was not affected.

The effects of voluntary running on cardiac mass and aortic compliance in Wistar-Kyoto and spontaneously hypertensive rats

OBJECTIVE

To investigate the effects of voluntary running exercise from 4-20 weeks of age on aortic compliance in Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR).

DESIGN

For each species we made comparisons between rats housed with an exercise wheel locked (10 rats) and unlocked (10 rats).

METHODS

Rats were killed using CO2 asphyxia and the aorta and heart of each rat were rapidly removed. The heart was dissected and weighed. A 4 mm descending proximal aortic ring was mounted on wires in an organ bath for determination of static compliance from the slope of the diameter-pressure relationship derived using Laplace's equation.

RESULTS

During the final 2 weeks of training WKY rats ran an average of 7.9 +/- 1.0 km/24 h compared with 1.0 +/- 0.2 km/24 h for SHR. Body weights of WKY rats and SHR and of animals housed with locked and unlocked exercise wheels did not differ. The septum, left ventricle and total heart weights and left ventricular:body weight ratios of sedentary SHR were greater than those of sedentary WKY rats. Trained WKY rats had significantly higher atrial, left and right ventricular and total heart weights and left ventricular:body weight ratios than did untrained WKY rats. Aortic compliance was higher in trained than it was in sedentary WKY rats (12.3 +/- 0.4 versus 14.2 +/- 0.5 microm/mmHg, P < 0.05). There was no difference between heart weights and aortic compliances of SHR housed with exercise wheels locked and unlocked.

CONCLUSION

Exercise-trained WKY rats had greater intrinsic aortic compliance when it was measured statically in vitro, which supports results of previous human work revealing a blood-pressure-independent component in the elevation of arterial compliance with training. The lower physical activity of the SHR strain used in this study could contribute to their higher blood pressures and lack of change in aortic compliance with exercise training.

Vagal activation in novelty-induced tachycardia during the light phase in the rat

The effects of repeated exposure to a novel test box on cardiac and behavioral activities (locomotion, rearing, grooming, scanning, and immobility) were studied in rats tested during the dark phase ("dark" rats) or the light phase ("light" rats) of the lighting cycle, using a telemetry system for registration of ECGs during the first and fifth tests. Heart rate (HR) was used to monitor sympathetic and parasympathetic activity; the PQ interval was used to monitor parasympathetic activity. Behavior was videotaped simultaneously. In light rats, the first and fifth exposures to the test box resulted in higher increases of active behavior and HR than in dark rats, whereas the duration of the PQ interval of the ECG was increased in light rats only. This indicates that in the light phase novelty induces active behavior associated with an increase in both sympathetic and vagal outflow, whereas in the dark phase behavioral activation is predominantly associated with increased sympathetic activity, without appreciable changes in vagal outflow. In addition, light rats showed less active behavior during the fifth than during the first exposure, indicating behavioral habituation. This behavioral habituation to the test box in the light phase coincided with vagal habituation (a diminution of the PQ interval). The increase of the tachycardiac response during the fifth exposure as compared to the first exposure suggests that it is not likely that sympathetic outflow was part of the habituation process. In dark rats no behavioral or cardiac habituation was found.

Sustained diurnal blood pressure reduction in SHR with ramipril assessed by telemetric monitoring

Subchronic bolus administration of the angiotensin-converting-enzyme inhibitor ramipril caused a dose-dependent (0.1 to 10 mg.kg-1.d-1), yet each in his own way 24-hour sustained, reduction in arterial blood pressure in conscious unrestrained spontaneously hypertensive rats with telemetric recording, and 0.01 mg.kg-1.d-1 was a no-effect dose. This was accompanied by dose-dependent attenuation of heart rate lowering during periods of low locomotor activity. Neither effect persisted for more than a few days after cessation of the medication with full restoration of pretreatment levels. No dosage altered the circadian rhythm of blood pressure, heart rate and locomotor activity of the rats.

The influence of restraint on blood pressure in the rat

We examined the influence of procedures used in blood pressure measurement on blood pressure and the effects of antihypertensive agents. Subjects were spontaneously hypertensive rats (SHR) and their Wistar/Kyoto (WKY) controls. Blood pressure was recorded by telemetry. Twenty-four h baseline pressure were measured, and the effect of minor handling on blood pressure and heart rate was examined. The influence of restraint such as is used for tail-cuff blood pressures was examined. The effects of three different antihypertensive drugs was also examined in the SHR. In the home-cage environment, the SHRs showed higher systolic blood pressures, but had similar hypertensive responses to minor handling as the WKYs. Both strains had elevated heart rate and blood pressure when restrained in the manner used for tail-cuff readings. The antihypertensive effects of captopril and losartan in the SHR were unchanged when the animals were restrained but the hypotensive effect of hydralazine was greater. These results confirm that significant changes in heart rate and blood pressure can occur as a result of the minor procedures frequently used in blood pressure recording in both SHR and WKY rats. This suggests that telemetry may have significant advantages as a method for continuous blood pressure monitoring. The pharmacological profile of antihypertensive drugs may well be different in animals where telemetry is employed and are not subject to the stresses involved in previous methods of monitoring blood pressure.

24-hour recordings of blood pressure, heart rate and behavioural activity in rabbits by radio-telemetry: effects of feeding and hypertension

We used radio-telemetry to measure 24-hour rhythms of systolic, diastolic and mean blood pressure, heart rate and behavioural activity in conscious rabbits, which were maintained under normal day/night rhythms and restricted feeding. Over three consecutive days, all variables showed little change between day-period and night-period, except for a pronounced rise in the afternoon, coinciding with the presentation of pellet food. Mean blood pressure increased during this period from baseline values between 78-82 mm Hg to a peak of 89-91 mm Hg. At the same time heart rate rose from baseline values of 147-161 b/min to a peak of 206-234 b/min and behavioural activity scores rose from 11-31 counts/h to a peak of 52-81 counts/h. Changing the time at which pellet food was presented to the rabbits from the early afternoon to the early morning, caused a complete and immediate shift of the peak of blood pressure and heart rate to the morning period. Chronic intravenous infusion of angiotensin II caused a significant increase in blood pressure (24-hour average: 80 +/- 1 vs. 114 +/- 7 mm Hg) but did not alter basal heart rate or behavioural activity. The increase in heart rate and blood pressure seen with food presentation was attenuated with angiotensin II infusion. These data show that in rabbits diurnal changes in blood pressure, heart rate and activity were determined to a large extent by timed feeding. In addition, in rabbits with angiotensin-induced hypertension the food-induced changes in blood pressure and heart rate were blunted.

Cardiopulmonary interactions following REM sleep deprivation in Sprague-Dawley rats

We characterized the effects of 48 h of rapid-eye-movement (REM) sleep deprivation on cardiovascular and respiratory variables and on sleep-related cardiopulmonary interactions in adult male Sprague-Dawley rats. Rats were instrumented for monitoring EEG, EMG, and aortic blood pressure. Respiratory rate and minute ventilation were measured by unrestrained single-chamber plethysmography. By using radiotelemetry to monitor blood pressure we clearly demonstrated progressive decreases in mean blood pressure with transitions from wakefulness to non-rapid-eye-movement and REM sleep which were unaffected by REM sleep deprivation. Mirror-image state-dependent increases in heart period suggest that baroreflexes were augmented during sleep with respect to wakefulness. REM sleep deprivation was also associated with lower blood pressure and longer heart period over all sleep/wake states, although this achieved statistical significance only during REM sleep and only during the first hour of recovery sleep. These cardiovascular changes coupled with the observed decreases in respiratory rate and minute ventilation suggest a further augmentation of baroreflexes following REM sleep deprivation.

Autonomic control of ultradian and circadian rhythms of blood pressure, heart rate, and baroreflex sensitivity in spontaneously hypertensive rats

OBJECTIVE

To examine the influence of the autonomic nervous system on ultradian and circadian rhythms of blood pressure, heart rate and baroreflex sensitivity of heart rate (BRS) in spontaneously hypertensive rats (SHR).

METHODS

Spontaneous fluctuations in blood pressure, heart rate and BRS in SHR were recorded continuously for 24 h using a computerized system and compared with those in Wistar-Kyoto (WKY) rats. Furthermore, 24 h recordings were performed in SHR during cardiac autonomic blockade by metoprolol and methyl-atropine, vascular autonomic blockade by prazosin, ganglionic blockade by hexamethonium and vagal stimulation by a low dose of scopolamine. The magnitudes of the ultradian fluctuations in blood pressure, heart rate and BRS were assessed by wide-band spectral analysis techniques.

RESULTS

The BRS was lower in SHR than it was in WKY rats throughout the 24 h cycle. In both strains high values were found during the light, resting period, whereas low values were found during the first hours of the dark, active period. The circadian rhythmicity of the blood pressure in SHR was abolished completely during the infusions of prazosin and hexamethonium. In contrast, the circadian rhythmicities of the blood pressure and heart rate were not altered by infusions of metoprolol, methyl-atropine and the low dose of scopolamine. Power spectra of the blood pressure and heart rate lacked predominant peaks at ultradian frequencies and showed 1/f characteristics. In the absence of autonomic tone, the ultradian fluctuations in heart rate, but not in blood pressure, were decreased. The ultradian BRS spectra had no 1/f shape, but showed a major peak at approximately equal to 20 min for 71% of the WKY rats and 42% of the SHR.

CONCLUSIONS

The influence of the autonomic nervous system on the blood pressure and heart rats in SHR is frequency-dependent. The circadian, but not ultradian, blood pressure rhythmicity is controlled by vascular autonomic activity. Conversely, the circadian, but not ultradian, heart rate rhythmicity is independent of autonomic tone. In rats, just as in humans, the trough in baroreflex sensitivity occurred after the sleeping period, when locomotor activity is resumed.

Possible confounding influence of strain, age and gender on cognitive performance in rats

There are substantial differences in the performance of various rat strains in tasks of learning, memory and attention. Strain, age and sex differences are not consistent over procedures: poor performance in one paradigm does not predict poor performance in a different paradigm. Some strain differences are not readily apparent until a direct comparison is made between one or more strains. Moreover, large differences in nominally the same strain but obtained from different suppliers have been observed in behavioural, pharmacological and physiological parameters and can have important consequences for interpretation of drug effects. Longevity, and the effects of ageing can differ dramatically from one strain to another; drug effects can alter radically with increasing age and show strain (and individual) differences in their action. Sex can further complicate interpretation of results. Thus, non-cognitive factors may exert a major effect on results in cognitive testing, and strain-dependent effects may account for many conflicting results in the literature concerning mnemonic performance. Strain differences in particular must be identified and used to help identify fundamental effects on memory, rather than continue to be ignored and allowed to obscure interpretation of drug effects on cognitive processes.