Biotelemetry transmitter implantation in rodents: impact on growth and circadian rhythms

Growth failure after recurrent fever in young guinea pigs

Infection causes fever and suppression of appetite, a combination of effects which threatens normal growth in infected children. We have used an animal model to study the effects on growth of recurrent simulated Gram-positive bacterial infection. After weaning, 10 guinea pig pups underwent surgery under general anaesthesia for the implantation of temperature-sensitive radiotelemeters and thereafter were assigned to receive intramuscular injections of either 50 microg/kg muramyl dipeptide (MDP), or sterile saline. During a 30-day period corresponding to their rapid growth phase, the pups were given eight injections. MDP resulted in fevers of about 1.5 degrees C on each occasion, but no significant change in body temperature occurred after saline injections. Food intake was suppressed during each febrile episode such that 24-h intake was significantly lower on days of injections of MDP, compared to days between MDP injections in the same animals, and compared to that of animals injected with saline. The rate of weight gain of the MDP-injected guinea pigs was significantly lower than that of the control group and failed even to achieve a rate similar to the saline-injected group in their more adult-like growth phase. Plasma zinc concentration was significantly lower in MDP-compared to saline-injected animals sampled 8 days after the last injection. Our results show that recurrent fever during the growth phase of young guinea pigs results in irreversible growth failure, and that reduced food intake on days when the animals were febrile was at least partly responsible for this reduced rate of growth.

Intrauterine growth restriction increases blood pressure and central pulse pressure measured with telemetry in aging rats

OBJECTIVES

Intrauterine growth restriction (IUGR) is associated with a higher risk of hypertension in adulthood. In Western countries, IUGR is based on uteroplacental dysfunction. We hypothesize that aging augments the increased baseline blood pressure after IUGR and alters the cardiovascular response to acute stress.

METHODS

To evaluate blood pressure during aging in the rat, we used a model of uteroplacental dysfunction (bilateral uterine artery ligation). Blood pressure was measured in male offspring at the ages of 6, 9, and 12 months using telemetry, allowing for unstressed measurements in conscious animals. At 6 and 12 months of age, cardiovascular data were obtained during acute olfactory stress induced by ammonia and subsequent recovery.

RESULTS

Rats born after IUGR had lower birth weights (4.6 versus 6.5 g, P < 0.001) and did not completely catch up in weight by 12 months of age (519 versus 567 g, P < 0.01). Systolic blood pressure was significantly higher in IUGR animals at all ages. Pulse pressure (PP) was identical in both groups at the age of 6 months. However, PP increased in the IUGR group with increasing age, unlike the control group, and was significantly higher at 9 and 12 months of age. At the age of 12 months, there was a highly significant negative correlation between birth weight and PP (r = -0.82, P < 0.001). IUGR rats reached a higher peak in systolic blood pressure during stress, and showed a longer period for the raised heart rate to recover after stress.

CONCLUSIONS

IUGR is associated with raised baseline blood pressure, an increasing PP with age, and an altered stress response.

Time course of cytokine, corticosterone, and tissue injury responses in mice during heat strain recovery

Elevated circulating cytokines are observed in heatstroke patients, suggesting a role for these substances in the pathophysiological responses of this syndrome. Typically, cytokines are determined at end-stage heatstroke such that changes throughout progression of the syndrome are poorly understood. We hypothesized that the cytokine milieu changes during heatstroke progression, correlating with thermoregulatory, hemodynamic, and tissue injury responses to heat exposure in the mouse. We determined plasma IL-1α, IL-1β, IL-2, IL-4, IL-6, IL-10, IL-12p40, IL-12p70, IFN-γ, macrophage inflammatory protein-1α, TNF-α, corticosterone, glucose, hematocrit, and tissue injury during 24 h of recovery. Mice were exposed to ambient temperature of 39.5 ± 0.2°C, without food and water, until maximum core temperature (Tc,Max) of 42.7°C was attained. During recovery, mice displayed hypothermia (29.3 ± 0.4°C) and a feverlike elevation at 24 h (control = 36.2 ± 0.3°C vs. heat stressed = 37.8 ± 0.3°C). Dehydration (∼10%) and hypoglycemia (∼65–75% reduction) occurred from Tc,Max to hypothermia. IL-1α, IL-2, IL-4, IL-12p70, IFN-γ, TNF-α, and macrophage inflammatory protein-1α were undetectable. IL-12p40 was elevated at Tc,Max, whereas IL-1β, IL-6, and IL-10 inversely correlated with core temperature, showing maximum production at hypothermia. IL-6 was elevated, whereas IL-12p40 levels were decreased below baseline at 24 h. Corticosterone positively correlated with IL-6, increasing from Tc,Max to hypothermia, with recovery to baseline by 24 h. Tissue lesions were observed in duodenum, spleen, and kidney at Tc,Max, hypothermia, and 24 h, respectively. These data suggest that the cytokine milieu changes during heat strain recovery with similarities between findings in mice and those described for human heatstroke, supporting the application of our model to the study of cytokine responses in vivo.

Association of birth weight with cardiovascular parameters in adult rats during baseline and stressed conditions

Low birth weight (LBW) due to intrauterine growth restriction (IUGR) in humans is associated with increased blood pressure (BP) in adulthood. In Western countries, IUGR is based on uteroplacental dysfunction. We used an animal model of uteroplacental dysfunction to evaluate this correlation. We hypothesize that IUGR increases baseline BP and alters the BP response to acute stress, which may explain BP differences in previous studies using stressful methods to obtain BP. IUGR was induced by bilateral uterine artery ligation in pregnant Wistar rats according to a modified method of Wigglesworth. BP was measured in the offspring using telemetry, allowing for unstressed measurements in conscious animals. Cardiovascular data were obtained at the age of 12 wk during baseline and acute olfactory stress induced by an ammonia gauze. Rats born after IUGR had a lower birth weight versus controls and did not completely catch up in weight. At baseline, systolic BP (SBP), mean arterial pressure (MAP), and pulse pressure (PP) were elevated in IUGR rats versus controls, by 8, 6, and 5 mm Hg, respectively. There was a strong negative correlation between birth weight and SBP and between birth weight and PP. During acute stress, there was a tendency to reach a higher peak in SBP and to need a longer period to recover in IUGR animals. We conclude that IUGR is associated with increased baseline BP.

Heat stress induces a biphasic thermoregulatory response in mice

Previous animal models of heat stress have been compromised by methodologies, such as restraint and anesthesia, that have confounded our understanding of the core temperature (Tc) responses elicited by heat stress. Using biotelemetry, we developed a heat stress model to examine Tcresponses in conscious, unrestrained C57BL/6J male mice. Before heat stress, mice were acclimated for >4 wk to an ambient temperature (Ta) of 25°C. Mice were exposed to Taof 39.5 ± 0.2°C, in the absence of food and water, until they reached maximum Tcof 42.4 ( n = 11), 42.7 ( n = 12), or 43.0°C ( n = 11), defined as mild, moderate, and extreme heat stress, respectively. Heat stress induced an ∼13% body weight loss that did not differ by final group Tc; however, survival rate was affected by final Tc(100% at 42.4°C, 92% at 42.7°C, and 46% at 43°C). Hypothermia (Tc< 34.5°C) developed after heat stress, with the depth and duration of hypothermia significantly enhanced in the moderate and extreme compared with the mild group. Regardless of heat stress severity, every mouse that transitioned out of hypothermia (survivors only) developed a virtually identical elevation in Tcthe next day, but not night, compared with nonheated controls. To test the effect of the recovery Ta, a group of mice ( n = 5) were acclimated for >4 wk and recovered at Taof 30°C after moderate heat stress. Recovery at 30°C resulted in 0% survival within ∼2 h after cessation of heat stress. Using biotelemetry to monitor Tcin the unrestrained mouse, we show that recovery from acute heat stress is associated with prolonged hypothermia followed by an elevation in daytime Tcthat is dependent on Ta. These thermoregulatory responses to heat stress are key biomarkers that may provide insight into heat stroke pathophysiology.

Differential effects of infralimbic cortical lesions on temperature and locomotor activity responses to feeding in rats

The time of food availability induces important behavioral and metabolic adaptations. Animals subjected to feeding restricted to a few daytime hours show increased locomotor activity and body temperature in anticipation of mealtime. In addition, animals under ad libitum feeding show a marked postprandial raise in body temperature and in thermogenesis. The areas of the brain commanding these responses to food are partially known. We investigated in the rat the role of the infralimbic area, located in the medial prefrontal cortex, and considered a visceral-autonomic motor area, in the responses to ad libitum or restricted feeding schedule. We performed infralimbic cortex excitotoxic lesions using injections of ibotenic acid, and measured body temperature and locomotor activity by telemetry in rats under ad libitum and restricted feeding conditions. We found that bilateral infralimbic area lesions prevented both the anticipatory and the postprandial increases in core temperature, decreased mean temperature by nearly 0.3 degrees C during both light/dark phases, and increased daily temperature variability. In contrast, the lesion caused a rapid induction of the anticipatory locomotor activity. These results show that behavioral and metabolic responses to the time of food availability are commanded separately and that the infralimbic area is a key structure to adjust the body temperature to an upcoming meal.