Aging alters regulation of visceral sympathetic nerve responses to acute hypothermia

Renal and segmental artery hemodynamics during whole body passive heating and cooling recovery

High environmental temperatures are associated with increased risk of acute kidney injury, which may be related to reductions in renal blood flow. The susceptibility of the kidneys may be increased because of heat stress-induced changes in renal vascular resistance (RVR) to sympathetic activation. We tested the hypotheses that, compared with normothermia, increases in RVR during the cold pressor test (CPT, a sympathoexcitatory maneuver) are attenuated during passive heating and exacerbated after cooling recovery. Twenty-four healthy adults (22 ± 2 yr; 12 women, 12 men) completed CPTs at normothermic baseline, after passive heating to a rise in core temperature of ~1.2°C, and after cooling recovery when core temperature returned to ~0.2°C above normothermic baseline. Blood velocity was measured by Doppler ultrasound in the distal segment of the right renal artery (Renal, n = 24 during thermal stress, n = 12 during CPTs) or the middle portion of a segmental artery (Segmental, n = 12). RVR was calculated as mean arterial pressure divided by renal or segmental blood velocity. RVR increased at the end of CPT during normothermic baseline in both arteries (Renal: by 1.0 ± 1.0 mmHg·cm−1·s, Segmental: by 2.2 ± 1.2 mmHg·cm−1·s, P ≤ 0.03), and these increases were abolished with passive heating ( P ≥ 0.76). At the end of cooling recovery, RVR in both arteries to the CPT was restored to that of normothermic baseline ( P ≤ 0.17). These data show that increases in RVR to sympathetic activation during passive heating are attenuated and return to that of normothermic baseline after cooling recovery.

NEW & NOTEWORTHY Our data indicate that increases in renal vascular resistance to the cold pressor test (i.e., sympathetic activation) are attenuated during passive heating, but at the end of cooling recovery this response returns to that of normothermic baseline. Importantly, hemodynamic responses were assessed in arteries going to (renal artery) and within (segmental artery) the kidney, which has not been previously examined in the same study during thermal and/or sympathetic stressors.

Does acute heat stress differentially-modulate expression of ionotropic neurotransmitter receptors in the RVLM of young and aged F344 rats?

The rostral ventral lateral medulla (RVLM) is a brainstem area that plays a role in regulating numerous physiological systems, especially their responsiveness to acute stress. Aging affects the responsiveness of RVLM neural circuits to acute stress. Based on the relationship between ionotropic neurotransmitter receptors in the RVLM and the physiological functions mediated via activation of these receptors, we hypothesized that in response to acute heat stress the expression of ionotropic neurotransmitter receptors in the RVLM of aged rats would be characterized by upregulation of inhibitory subunits and downregulation of excitatory subunits. The goal of the present study was to determine the effect of acute heating on the gene expression profile of RVLM inhibitory (GABA_A and Glycine) and excitatory (NMDA and AMPA) ionotropic neurotransmitter receptor subunits in young and aged F344 rats. RVLM tissue punches from young and aged F344 rats were analyzed using TaqMan qPCR and immunoblotting. When compared to age-matched controls, heat stress increased the gene expression of RVLM inhibitory receptor subunits in aged (Gabra1, Gabra2, Gabra5, Glra1) and young (Gabra1) F344 rats at mRNA level, with little change in the expression of RVLM excitatory receptor subunits. Significant age x heat interaction effects were observed with increased expression of Gabra2 and Gabrb1 inhibitory receptor subunits and decreased expression of Gria1 and Gria2 excitatory receptor subunits in the RVLM of aged F344 rats, with the most marked change observed with the Gabra2 subunit, which was validated by immunoblotting. These findings demonstrate that in response to acute heat stress there is enhanced expression of inhibitory ionotropic receptor subunits in aged compared to young rats, supporting the idea that advanced age may alter RVLM responsivity by affecting the molecular substrate of ionotropic receptors.

Microarray analysis of aging-associated immune system alterations in the rostral ventrolateral medulla of F344 rats

The rostral ventrolateral medulla (RVLM) is an area of the brain stem that contains diverse neural substrates that are involved in systems critical for physiological function. There is evidence that aging affects some neural substrates within the RVLM, although age-related changes in RVLM molecular mechanisms are not well established. The goal of the present study was to characterize the transcriptomic profile of the aging RVLM and to test the hypothesis that aging is associated with altered gene expression in the RVLM, with an emphasis on immune system associated gene transcripts. RVLM tissue punches from young, middle-aged, and aged F344 rats were analyzed with Agilent's whole rat genome microarray. The RVLM gene expression profile varied with age, and an association between chronological age and specific RVLM gene expression patterns was observed [P < 0.05, false discovery rate (FDR) < 0.3]. Functional analysis of RVLM microarray data via gene ontology profiling and pathway analysis identified upregulation of genes associated with immune- and stress-related responses and downregulation of genes associated with lipid biosynthesis and neurotransmission in aged compared with middle-aged and young rats. Differentially expressed genes associated with the complement system and microglial cells were further validated by quantitative PCR with separate RVLM samples (P < 0.05, FDR < 0.1). The present results have identified age-related changes in the transcriptomic profile of the RVLM, modifications that may provide the molecular backdrop for understanding age-dependent changes in physiological regulation.

Does aging alter the molecular substrate of ionotropic neurotransmitter receptors in the rostral ventral lateral medulla? - A short communication

Aging alters sympathetic nervous system (SNS) regulation, although central mechanisms are not well understood. In young rats the rostral ventral lateral medulla (RVLM) is critically involved in central SNS regulation and RVLM neuronal activity is mediated by a balance of excitatory and inhibitory ionotropic neurotransmitters and receptors, providing the foundation for hypothesizing that with advanced age the molecular substrate of RVLM ionotropic receptors is characterized by upregulated excitatory and downregulated inhibitory receptor subunits. This hypothesis was tested by comparing the relative mRNA expression and protein concentration of RVLM excitatory (NMDA and AMPA) and inhibitory (GABA and glycinergic) ionotropic neurotransmitter receptor subunits in young and aged Fischer (F344) rats. Brains were removed from anesthetized rats and the RVLM-containing area was micropunched and extracted RNA and protein were subsequently used for TaqMan qRT-PCR gene expression and quantitative ELISA analyses. Bilateral chemical inactivation of RVLM neurons and peripheral ganglionic blockade on visceral sympathetic nerve discharge (SND) was determined in additional experiments. The relative gene expression of RVLM NMDA and AMPA glutamate-gated receptor subunits and protein concentration of select receptor subunits did not differ between young and aged rats, and there were no age-related differences in the expression of RVLM ionotropic GABA_A and Gly receptors, or of protein concentration of select GABA_A subunits. RVLM muscimol microinjections significantly reduced visceral SND by 70±2% in aged F344 rats. Collectively these findings from this short communication support a functional role for the RVLM in regulation of sympathetic nerve outflow in aged rats, but provide no evidence for an ionotropic RVLM receptor-centric framework explaining age-associated changes in SNS regulation.

Postanesthesia Care for the Elderly Patient

PURPOSE

As the general population lives longer, the perioperative physician is more likely to encounter disease states that increase in incidence in an aging population. This review focuses on anesthetic considerations for rational drug choices during the perioperative care of elderly patients. The primary aim of the review was to identify intraoperative and postanesthetic considerations for diseases associated with advancing age; it includes highlights of the commonly impaired major organs (eg, cardiovascular, pulmonary, neurologic, renal, hepatic systems). We also outline an approach to frequent issues that arise in the immediate postsurgical period while caring for these patients.

METHODS

A systematic review was performed on aspects of the perioperative and postoperative periods that relate to the elderly. A list of pertinent key words was derived from the authors, and a PubMed database search was performed.

FINDINGS

The anesthesiologist must account for changes in various organ systems that affect perioperative care, including the cardiovascular, pulmonary, renal, hepatic, and central nervous systems. The pharmacokinetic principles frequently differ and are often unpredictable because of anatomic changes and decreased renal and hepatic function. The most important pharmacodynamic consideration is that elderly patients tend to exhibit an exaggerated hypoactivity after anesthesia.

IMPLICATIONS

Before surgery, it is essential to identify those patients at risk for delirium and other commonly encountered postanesthesia scenarios. Failure to manage these conditions appropriately can lead to an escalation of care and prolonged hospitalization.

Prearrest hypothermia improved defibrillation and cardiac function in a rabbit ventricular fibrillation model

BACKGROUND

Hypothermia when cardiopulmonary resuscitation begins may help achieve defibrillation and return of spontaneous circulation (ROSC), but few data are available.

OBJECTIVE

The objective of this study was to determine whether prearrest hypothermia improved defibrillation and cardiac function in a rabbit ventricular fibrillation (VF) model.

RESULTS

Thirty-six New Zealand rabbits were randomized equally to receive normothermia (Norm) (~39°C), post-ROSC hypothermia (~33°C), or prearrest hypothermia (~33°C). Ventricular fibrillation was induced by alternating current. After 4 minutes of VF, rabbits were defibrillated and given cardiopulmonary resuscitation until ROSC or no response (≥30 minutes). Hemodynamics and electrocardiogram were monitored; N-terminal pro-brain natriuretic peptideand troponin I were determined by enzyme-linked immunosorbent assay. Myocardial histology and echocardiographic data were evaluated. First-shock achievement of perfusion rhythm was more frequent in prearrest than normothermic animals (7/12 vs 1/12; P=.027). After ROSC, dp/dtmax was higher in prearrest than normothermic animals (P<.001). Left ventricular end-systolic pressure was higher in prearrest than normothermic animals (P=.001). At 240 minutes after ROSC, troponin I and N-terminal pro-brain natriuretic peptide were lower in prearrest than normothermic animals (15.74±2.26 vs 25.09±1.85 ng/mL and 426±23 vs 284±45 pg/mL, respectively), the left ventricular ejection fraction and cardiac output were lower in the Norm group than other 2 groups (P<.01). Myocardial histology was more disturbed in normothermic than post-ROSC and prearrest animals, but was not different in the latter 2 groups.

CONCLUSIONS

Induction of hypothermia before VF led to improved cardiac function in a rabbit VF model through improving achievement of perfusing rhythm by first-shock defibrillation and facilitating resuscitation.

Dexmedetomidine and regulation of splenic sympathetic nerve discharge in aged F344 rats

Sedatives influence the immune system and centrally-acting alpha2-adrenergic receptor agonists, including Dexmedetomidine (Dex), modulate sympathetic nerve discharge (SND). Because sedatives are used under medical conditions that include elderly patients, and because advancing age attenuates SND responsivity to various interventions, we tested the hypothesis that splenic sympathoinhibitory responses to Dex would be attenuated in aged compared with young Fischer 344 rats. Dex-mediated reductions in splenic SND were similar in aged and young baroreceptor-intact and -denervated rats, indicating that SND changes to Dex administration occur in an age-independent manner. These findings provide new information regarding interactions between alpha2-adrenergic agonists, advanced age, and SND regulation.

Autonomic regulation of cellular immune function

The nervous system and the immune system (IS) are two integrative systems that work together to detect threats and provide host defense, and to maintain/restore homeostasis. Cross-talk between the nervous system and the IS is vital for health and well-being. One of the major neural pathways responsible for regulating host defense against injury and foreign antigens and pathogens is the sympathetic nervous system (SNS). Stimulation of adrenergic receptors (ARs) on immune cells regulates immune cell development, survival, proliferative capacity, circulation, trafficking for immune surveillance and recruitment, and directs the cell surface expression of molecules and cytokine production important for cell-to-cell interactions necessary for a coordinated immune response. Finally, AR stimulation of effector immune cells regulates the activational state of immune cells and modulates their functional capacity. This review focuses on our current understanding of the role of the SNS in regulating host defense and immune homeostasis. SNS regulation of IS functioning is a critical link to the development and exacerbation of chronic immune-mediated diseases. However, there are many mechanisms that need to be further unraveled in order to develop sound treatment strategies that act on neural-immune interaction to resolve or prevent chronic inflammatory diseases, and to improve health and quality of life.

Age-dependent changes in temperature regulation - a mini review

It is now well recognized that the body temperature of older men and women is lower than that of younger people and that their tolerance of thermal extremes is more limited. The regulation of body temperature does not depend on a single organ, but rather involves almost all the systems of the body, i.e. systems not exclusively dedicated to thermoregulatory functions such as the cardiovascular and respiratory systems. Since these deteriorate naturally with advancing age, the decrement in their functions resonates throughout all the bodily processes, including those that control body temperature. To the extent that the age-related changes in some of these, e.g. in the musculoskeletal system, can be slowed, or even prevented, by certain measures, e.g. fitness training, so can the decrements in thermoregulatory functions. Some deficits, however, are unavoidable, e.g. structural skin changes and metabolic alterations. These impact directly on the ability of the elderly to maintain thermal homeostasis, particularly when challenged by ambient thermal extremes. Since the maintenance of a relatively stable, optimal core temperature is one of the body's most important activities, its very survival can be threatened by these disorders. The present article describes the principal, age-associated changes in physiological functions that could affect the ability of seniors to maintain their body temperature when exposed to hot or cold environments.

Influence of age on heart rate variability during therapeutic hypothermia in a rat model

OBJECTIVES

To evaluate the effect of age on heart rate variability (HRV) in a rat model of therapeutic hypothermia.

METHODS

Thirty-six male Sprague-Dawley rats (18 were 2 months old and 18 were 18 months old) were randomized into one of three groups: normothermia (37°C), mild hypothermia (34°C), and moderate hypothermia (31°C). An electrocardiogram (ECG) was recorded at baseline and continuously for 1h once the target core body temperature was reached. Various heart rate variability measurements were calculated.

RESULTS

Significant effects of age were observed in respect to the ratio of standard deviation of all normal to normal R-R [NN] intervals (SDNN)/standard deviation of the differences between adjacent NN intervals (SD of delta NN) (P=0.037), low frequency (LF) power, normalized units (nu, %) (P<0.001), and the ratio of LF and high frequency (HF) (P<0.001). Significant effects of temperature were found in LF power and a significant body-temperature interaction was found in HF power. HF power was significantly lower in the young rats at mild and moderate hypothermic conditions. For the LF/HF, the ratio was significantly lower in the young animals compared to the older animals at normal body temperatures and during mild hypothermia. LF/HF increased significantly at both 34°C and 31°C in the young rats compared to the young rats at 37°C. In contrast, LF/HF was significantly lower in the older group of rats at 34°C and 31°C compared to the older group of rats maintained under normothermic conditions.

CONCLUSIONS

This study noted that autonomic regulation determined via HRV, primarily the ratio of LF to HF, was different between different age groups. Additional studies on this topic are needed to achieve a more detailed understanding of therapeutic hypothermia.

Animal aging and regulation of sympathetic nerve discharge

Studies completed in human subjects have made seminal contributions to understanding the effects of age on sympathetic nervous system (SNS) regulation. Numerous experimental constraints limit the design of studies involving human subjects; therefore, completion of studies in animal models of aging would be expected to provide additional insight regarding mechanisms mediating age-related changes in sympathetic nerve discharge (SND) regulation. The present review assesses the current state of the literature regarding contributions from animal studies on the effects of advancing age on SND regulation, focusing primarily on studies that have used direct recordings of sympathetic nerve outflow. Few studies using direct SND recordings have been completed in animal models of aging, regardless of the fundamental component of SND regulation reviewed (basal levels, acute responsiveness, relationships between the discharges in sympathetic nerves, central neural regulation). SNS responsiveness to various acute stressors is altered in aged compared with young animals; however, mechanisms remain virtually unexplored. There is a marked dearth of studies that have used central neural microinjection techniques in conjunction with SND recordings in aged animals, making it difficult to develop an evidence-based framework regarding potential age-associated effects on central regulation of SND. Determination of age-related changes in mechanisms regulating SND is important for understanding relationships between chronic disease development and changes in SNS function; however, this can only be achieved by substantially extending the current knowledge base regarding the effects of age on SND regulation in animal studies.

The amplitude spectrum area correctly predicts improved resuscitation and facilitated defibrillation with head cooling

OBJECTIVES

When systemic hypothermia was maintained before inducing cardiac arrest, the likelihood of successful defibrillation and meaningful survival was increased. When hypothermia is induced during cardiopulmonary resuscitation, mortality is also improved. With the introduction of the amplitude spectrum area as a predictor of the success of electrical defibrillation, we investigated the effect of preferential head cooling initiated coincident with cardiopulmonary resuscitation on amplitude spectrum area as a predictor. We hypothesized that rapid head cooling initiated coincident with cardiopulmonary resuscitation improves amplitude spectrum area, and therefore is predictive of successful defibrillation.

DESIGN

Prospective randomized controlled study.

SETTING

University-affiliated research institute.

SUBJECTS

Domestic pigs.

INTERVENTIONS

Sixteen pigs, weighing 40.6 +/- 1.4 kg, were randomized to the hypothermia (n = 8), or control (n = 8) group. Ventricular fibrillation was induced and untreated for 10 mins. Cardiopulmonary resuscitation was then initiated for 5 mins followed by attempted defibrillation with a biphasic 150-J electric shock. Coincident with starting cardiopulmonary resuscitation, hypothermia was induced with evaporative intranasal cooling using a perfluorochemical. If spontaneous circulation was not restored after defibrillation, cardiopulmonary resuscitation was resumed for 1 min before the next defibrillation attempt until the animal was either successfully resuscitated or for a total of 15 mins. The target core temperature was 34 degrees C. Control animals were identically treated except for hypothermia.

MEASUREMENTS AND MAIN RESULTS

Five seconds of ventricular fibrillation waveform were recorded immediately preceding delivery of a shock. The ventricular fibrillation waveforms were analyzed using the amplitude spectrum area algorithm. A smaller epinephrine dose (60 +/- 32.1 vs. 30 +/- 0 mg/mL, p = .01) and shorter cardiopulmonary resuscitation duration (365 +/- 42 sec vs. 600 +/- 243 sec, p = .01) were required to achieve return of spontaneous circulation in the hypothermia group, compared with control. Five minutes after starting cardiopulmonary resuscitation, head temperature was reduced from 38 degrees C to 34 degrees C in the hypothermia group (p = .028). Hypothermia improved the success of electrical shocks before return of spontaneous circulation (88 +/- 18% vs. 66 +/- 19%, p = .034). Both the amplitude spectrum area values of initial shock (26.1 +/- 5.3 vs. 21.4 +/- 2.16 mV-Hz, p = .049) and total shocks (26.1 +/- 5.3 vs. 21.4 +/- 2.16 mV-Hz, p = .006) were significantly higher in the hypothermia group than control.

CONCLUSIONS

Amplitude spectrum area served as a useful predictor for improved resuscitation and facilitated defibrillation in the setting of rapid head cooling initiated coincident with cardiopulmonary resuscitation.

Intra-arrest selective brain cooling improves success of resuscitation in a porcine model of prolonged cardiac arrest

AIMS OF STUDY

We have previously demonstrated that early intra-nasal cooling improved post-resuscitation neurological outcomes. The present study utilizing a porcine model of prolonged cardiac arrest investigated the effects of intra-nasal cooling initiated at the start of cardiopulmonary resuscitation (CPR) on resuscitation success. Our hypothesis was that rapid nasal cooling initiated during "low-flow" improves return of spontaneous resuscitation (ROSC).

METHODS

In 16 domestic male pigs weighing 40+/-3 kg, VF was electrically induced and untreated for 15 min. Animals were randomized to either head cooling or control. CPR was initiated and continued for 5 min before defibrillation was attempted. Coincident with starting CPR, the hypothermic group was cooled with a RhinoChill device which produces evaporative cooling in the nasal cavity of pigs. No cooling was administrated to control animals. If ROSC was not achieved after defibrillation, CPR was resumed for 1 min prior to the next defibrillation attempt until either successful resuscitation or for a total of 15 min.

MAIN RESULTS

Seven of eight animals in the hypothermic group (87.5%) and two of eight animals in control group (25%) (p=0.04) were successfully resuscitated. At ROSC, brain temperature was increased from baseline by 0.3 degrees C in the control group, and decreased by 0.1 degrees C in the hypothermic animals. Pulmonary artery temperature was above baseline in both groups.

CONCLUSION

Intra-nasal cooling initiated at the start of CPR significantly improves the success of resuscitation in a porcine model of prolonged cardiac arrest. This may have occurred by preventing brain hyperthermia.

Melatonin and tryptophan affect the activity-rest rhythm, core and peripheral temperatures, and interleukin levels in the ringdove: changes with age

Aging is known to alter the circadian rhythms of melatonin, serotonin, thermoregulatory responses, cytokine production, and sleep/wakefulness which affect sleep quality. We tested the possible palliative effects of a 3-day administration of melatonin (0.25 or 2.5 mg/kg of body weight [b.w.] to young and old ringdoves, respectively) or tryptophan (300 mg/kg of b.w. to old ringdoves) on these rhythms. Doves are a monophasic, diurnal species; these characteristics are similar in humans. Old animals presented lower melatonin and serotonin levels; higher interleukin (IL)-1beta, IL-6, and tumor necrosis factor alpha values; and reductions in the Midline-Estimating Statistic of Rhythm and amplitude of activity-rest rhythm and in the amplitude of the core temperature rhythm. Melatonin raised serum melatonin levels; tryptophan increased both melatonin and serotonin levels. Melatonin and tryptophan lowered nocturnal activity, core temperature, and cytokine levels and increased peripheral temperature in both groups. Melatonin or tryptophan may limit or reverse some of the changes that occur in sleep-wake rhythms and temperature due to age.

Thermoregulation: some concepts have changed. Functional architecture of the thermoregulatory system

While summarizing the current understanding of how body temperature (Tb) is regulated, this review discusses the recent progress in the following areas: central and peripheral thermosensitivity and temperature-activated transient receptor potential (TRP) channels; afferent neuronal pathways from peripheral thermosensors; and efferent thermoeffector pathways. It is proposed that activation of temperature-sensitive TRP channels is a mechanism of peripheral thermosensitivity. Special attention is paid to the functional architecture of the thermoregulatory system. The notion that deep Tb is regulated by a unified system with a single controller is rejected. It is proposed that Tb is regulated by independent thermoeffector loops, each having its own afferent and efferent branches. The activity of each thermoeffector is triggered by a unique combination of shell and core Tbs. Temperature-dependent phase transitions in thermosensory neurons cause sequential activation of all neurons of the corresponding thermoeffector loop and eventually a thermoeffector response. No computation of an integrated Tb or its comparison with an obvious or hidden set point of a unified system is necessary. Coordination between thermoeffectors is achieved through their common controlled variable, Tb. The described model incorporates Kobayashi’s views, but Kobayashi’s proposal to eliminate the term sensor is rejected. A case against the term set point is also made. Because this term is historically associated with a unified control system, it is more misleading than informative. The term balance point is proposed to designate the regulated level of Tb and to attract attention to the multiple feedback, feedforward, and open-loop components that contribute to thermal balance.