Characterization of the relationship between spontaneous locomotor activity and cardiovascular parameters in conscious freely moving rats

In freely behaving rats, variations in heart rate (HR) and blood pressure (BP) are coupled closely with changes in locomotor activity (Act). We have attempted to characterize this relationship mathematically. In 10- and 16-week-old rats, HR, BP and Act were recorded telemetrically every minute for 2 days under 12h:12h light-dark cycling. After examining data for individual rats, we found that the relationship between Act and HR could be approximated by the negative exponential function HR(Act)=HRmax-(HRmax-HRmin)∗exp(-Act/Acte), where HRmax, HRmin, and Acte are constants. These constants were calculated separately for light and dark periods by non-linear curve fitting. HR corresponding to maximal locomotion was similar during the light and dark phases, while HR at rest during the dark phase was higher than during the light phase. The range of HR variability associated with Act during the dark phase was similar in young and older animals, but minimal HR was significantly lower in older rats. The relationship between Act and BP was approximated with a similar function. We have found no differences between BP at rest and at maximal locomotion between light and dark and between 10-week and 16-week-old rats. Our results indicate that in rats, cardiovascular parameters are coupled to locomotion to a high degree; however both the HR and the BP reach maximal values when locomotor activity is relatively low. We also found that the phase of daily cycle affects HR in conscious rats independent of locomotor activity

Effect of Low Dose of Amphetamine on Thermoregulation System and Performance of Rats Running on Treadmills

poster abstractAmphetamine has been used widely as a performance-enhancing drug among athletes. There are numerous reports showing that low dose of amphetamine increases one’s performance by suppressing sensations of fatigues. However, a little has been known about the mechanism by which such an effect of amphetamine is caused. The goal of this study was to investigate how a low dose of amphetamine changed the duration and the capacity of running in rats by studying thermoregulation system of rats running on treadmills with experimental results and a mathematical model. 12 rats were separated into two groups of 6 and rats in the experimental group were injected with 2mg/kg of amphetamine and ones in the control group were injected with saline. Then each rat in both groups ran on a treadmill at the room temperature (25°) while the speed and the incline of the treadmill were increased stepwise in every 3 minutes. The running time of individual rats were determined by their ability of keeping up with the intensity of running and the core body temperatures and the oxygen consumptions ()of rats were recorded during the experiments. Then a mathematical model was constructed to describe rates of temperature changes in the core and muscles by quantifying the heat dissipations and heat productions using . Modeling revealed that amphetamine increases the heat dissipation in the core body, which slowed down the core temperature increase. Therefore rats injected with amphetamine were kept their core temperatures below approximately 40 °C for longer time, at which both groups were unable to run anymore. Additionally, the fact that the core temperature at the end of run was not significantly different between two groups, while muscle temperature was significantly different, suggests that the indicator of running capacity was the core temperature, rather than the muscle temperature. Finally, the level of overheating in muscles for the amphetamine group was severe enough to cause damages in muscles

Yohimbine is a 5-HT1A agonist in rats in doses exceeding 1 mg/kg.

Yohimbine is a prototypical alpha2-adrenergic receptor antagonist. Due to its relatively high selectivity, yohimbine is often used in experiments whose purpose is to examine the role of these receptors. For example, yohimbine has been employed at doses of 1–5 mg/kg to reinstate drug-seeking behavior after extinction or to antagonize general anesthesia, an effects presumably being a consequence of blocking alpha2-adrenergic receptors. In this report we characterized dose-dependent autonomic and behavioral effects of yohimbine and its interaction with an antagonist of 5-HT1A receptors, WAY 100635. In low doses (0.5 – 2 mg/kg i.p.) yohimbine induced locomotor activation which was accompanied by a tachycardia and mild hypertension. Increasing the dose to 3–4.5 mg/kg reversed the hypertension and locomotor activation and induced profound hypothermia. The hypothermia as well as the suppression of the locomotion and the hypertension could be reversed by the blockade of 5-HT1A receptors with WAY 100635. Our data confirm that yohimbine possesses 5-HT1A properties, and demonstrated that in doses above 1 mg/kg significantly activate these receptors

Inhibition of the dorsomedial hypothalamus, but not the medullary raphe pallidus, decreases hyperthermia and mortality from MDMA given in a warm environment.

The central mechanisms through which MDMA mediates life-threatening hyperthermia when taken in a warm environment are not well described. It is assumed that MDMA alters normal thermoregulatory circuits resulting in increased heat production through interscapular brown adipose tissue (iBAT) and decreased heat dissipation through cutaneous vasoconstriction. We studied the role of the dorsomedial hypothalamus (DMH) and medullary raphe pallidus (mRPa) in mediating iBAT, tail blood flow, and locomotor effects produced by MDMA. Rats were instrumented with guide cannulas targeting either the DMH or the mRPa-brain regions involved in regulating iBAT and cutaneous vascular beds. In all animals, core temperature and locomotion were recorded with surgically implanted telemetric transmitters; and additionally either iBAT temperature (via telemetric transmitter) or tail artery blood flow (via tail artery Doppler cuff) were also recorded. Animals were placed in an environmental chamber at 32°C and microinjected with either control or the GABA agonist muscimol (80pmol) followed by an intravenous injection of saline or MDMA (7.5 mg kg-1). To prevent undue suffering, a core temperature of 41°C was chosen as the surrogate marker of mortality. Inhibition of the DMH, but not the mRPa, prevented mortality and attenuated hyperthermia and locomotion. Inhibition of either the DMH or the mRPa did not affect iBAT temperature increases or tail blood flow decreases. While MDMA increases iBAT thermogenesis and decreases heat dissipation through cutaneous vasoconstriction, thermoregulatory brain regions known to mediate these effects are not involved. Rather, the finding that inhibiting the DMH decreases both locomotion and body temperature suggests that locomotion may be a key central contributor to MDMA-evoked hyperthermia

Treadmill running restores MDMA-mediated hyperthermia prevented by inhibition of the dorsomedial hypothalamus

The contribution of exercise to hyperthermia mediated by MDMA is not known. We recently showed that inhibiting the dorsomedial hypothalamus (DMH) attenuated spontaneous locomotion and hyperthermia and prevented deaths in rats given MDMA in a warm environment. The goal of this study was to confirm that restoring locomotion through a treadmill would reverse these effects thereby confirming that locomotion mediated by the DMH contributes to MDMA-mediated hyperthermia. Rats were randomized to receive bilateral microinjections, into the region of the DMH, of muscimol (80pmol/100nl) or artificial CSF followed by a systemic dose of either MDMA (7.5mg/kg, i.v.) or saline. Immediately after the systemic injection, rats were placed on a motorized treadmill maintained at 32°C. Rats were exercised at a fixed speed (10m/min) until their core temperature reached 41°C. Our results showed that a fixed exercise load abolished the decreases in temperature and mortality, seen previously with inhibition of the DMH in freely moving rats. Therefore, locomotion mediated by neurons in the DMH is critical to the development of hyperthermia from MDMA

Circadian variability of body temperature responses to Methamphetamine (Meth)

poster abstractVital parameters of living organisms exhibit circadian rhythmicity. Despite rats are nocturnal animals, most of drugs of abuse studies in rodents are performed during the day. Virtually no data on circadian variability of responses to amphetamines is currently available. However, the amplitude of circadian variations of body temperature is comparable to the magnitude of temperature responses to Meth. Accordingly, one can expect that the responses may be qualitatively different during the day and at night. Experiments were performed on male Sprague-Dawley rats implanted with telemetric probes reporting body temperature. Rats received i.p. injections of Meth (1 or 5 mg/kg) or saline at 10-11am or at 10-11pm. Each rat received only one injection of Meth to avoid the effects of repeated administration. The responses were recorded for at least 5 h. The baseline body temperature at night was 0.8ºC higher than during the day. The body temperature increased after injections of saline during both day and night but returned to its baseline within 1 h. This response was developing faster, and more pronounced at night. The temperature responses to Meth were different during the day and at night. In both cases the lower dose of Meth (1 mg/kg) induced monophasic hyperthermia. However, the maximal deviation of the temperature from baseline was appr. twice smaller at night than during the day. Injection of the higher dose of Meth (5 mg/kg) at day time caused a delayed hyperthermic response, preceded by a slight increase of the body temperature immediately after injection. In contrast, at night the same dose produced immediate hypothermia, which was not observed during the day. Recently, we created a model which showed that the complex dose-dependence of day-time temperature responses to Meth results from the delicate balance between inhibitory and excitatory drives which have different sensitivity to the drug. To interpret the night time data, we extended this mathematical model by assuming that the excitatory and/or inhibitory components and general metabolism are affected by the circadian input. Our model revealed that during the night the baseline activity of the excitatory node is greater than during the day. Besides, after injection of either dose of Meth the equilibrium body temperature appears significantly lower than the temperature observed before injection. The suppression of the response to the lower dose of Meth is, therefore, explained by a combination of two factors. First, the excitatory drive, which is predominantly responsible for monophasic hyperthermia after low doses of Meth, gets partially saturated. Second, the reduced general metabolism, which underlies the lower equilibrium temperature, leads to gradual cooling thus limiting the hyperthermia. Same mechanisms mediate the observed hypothermia during the night after the higher dose of Meth, as the inhibitory drive starts dominating the excitatory one. The reduction of the equilibrium temperature after Meth injection during the active time period represents a major perturbation of the thermoregulatory system status, and may reflect a Meth-triggered disturbance of circadian rhythmicity

Automatic analysis of treadmill running to estimate times to fatigue and exhaustion in rodents

Introduction: The determination of fatigue and exhaustion in experimental animals is complicated by the subjective nature of the measurement. Typically, it requires an observer to watch exercising animals, e.g. rats running on the treadmill, and to identify the time of the event. In this study, we hypothesized that automatic analysis of the time-averaged position of a rat on a treadmill could be an objective way for estimating times to fatigue and exhaustion. To test this hypothesis, we compared these times measured by a human observer to the results of an automated video tracking system. Methods: Rats, previously familiarized to running on the treadmill, ran at a fixed speed with zero incline, until exhaustion. The experiments were performed at either room temperature (24 °C) or in a hot environment (32 °C). Each experiment was video recorded. A trained observer estimated the times to fatigue and exhaustion. Then, video tracking software was used to determine the position of the animals on the treadmill belt. The times to fatigue and exhaustion were determined, based on the position on the treadmill using predefined criteria. Results: Manual scores and the average position on the treadmill had significant correlation. Both the observer and the automated video tracking determined that exercise in a hot environment, compared with the exercise at room temperature, results in shorter times to exhaustion and fatigue. Also, estimates of times made by the observer and the automated video tracking were not statistically different from each other. Discussion: A similarity between the estimates of times to fatigue and exhaustion made by the observer and the automated technique suggests that video tracking of rodents running on a treadmill can be used to determine both parameters in experimental studies. Video tracking technique allows for a more objective measure and would allow for an increased performance in experimentation. The Supplemental information to this manuscript contains an Excel file, which includes the code in Virtual Basic with freeware license, to process and visualize running data and automatically estimate the times to fatigue and exhaustion. Instructions for the software are also included

Tissue oxidative metabolism can increase the difference between local temperature and arterial blood temperature by up to 1.3oC: Implications for brain, brown adipose tissue, and muscle physiology

Tissue temperature increases, when oxidative metabolism is boosted. The source of nutrients and oxygen for this metabolism is the blood. The blood also cools down the tissue, and this is the only cooling mechanism, when direct dissipation of heat from the tissue to the environment is insignificant, e.g., in the brain. While this concept is relatively simple, it has not been described quantitatively. The purpose of the present work was to answer two questions: 1) to what extent can oxidative metabolism make the organ tissue warmer than the body core, and, 2) how quickly are changes in the local metabolism reflected in the temperature of the tissue? Our theoretical analysis demonstrates that, at equilibrium, given that heat exchange with the organ is provided by the blood, the temperature difference between the organ tissue and the arterial blood is proportional to the arteriovenous difference in oxygen content, does not depend on the blood flow, and cannot exceed 1.3oC. Unlike the equilibrium temperature difference, the rate of change of the local temperature, with respect to time, does depend on the blood flow. In organs with high perfusion rates, such as the brain and muscles, temperature changes occur on a time scale of a few minutes. In organs with low perfusion rates, such changes may have characteristic time constants of tens or hundreds of minutes. Our analysis explains, why arterial blood temperature is the main determinant of the temperature of tissues with limited heat exchange, such as the brain

Amphetamine enhances endurance by increasing heat dissipation

Athletes use amphetamines to improve their performance through largely unknown mechanisms. Considering that body temperature is one of the major determinants of exhaustion during exercise, we investigated the influence of amphetamine on the thermoregulation. To explore this, we measured core body temperature and oxygen consumption of control and amphetamine‐trea ted rats running on a treadmill with an incrementally increasing load (both speed and incline). Experimental results showed that rats treated with amphetamine (2 mg/kg) were able to run significantly longer than control rats. Due to a progressively increasing workload, which was matched by oxygen consumption, the control group exhibited a steady increase in the body temperature. The administration of amphetamine slowed down the temperature rise (thus decreasing core body temperature) in the beginning of the run without affecting oxygen consumption. In contrast, a lower dose of amphetamine (1 mg/kg) had no effect on measured parameters. Using a mathematical model describing temperature dynamics in two compartments (the core and the muscles), we were able to infer what physiological parameters were affected by amphetamine. Modeling revealed that amphetamine administration increases heat dissipation in the core. Furthermore, the model predicted that the muscle temperature at the end of the run in the amphetamine‐treated group was significantly higher than in the control group. Therefore, we conclude that amphetamine may mask or delay fatigue by slowing down exercise‐induced core body temperature growth by increasing heat dissipation. However, this affects the integrity of thermoregulatory system and may result in potentially dangerous overheating of the muscles