Repeated nitrous oxide exposure in rats causes a thermoregulatory sign-reversal with concurrent activation of opposing thermoregulatory effectors

Nitrous oxide consistently attenuates thermogenic and thermoperceptual responses to repetitive cold stress in humans

Divers are at enhanced risk of hypothermia, due to the independent action of the inspired inert gases on thermoregulation. Thus, narcosis induced by acute (≤2 h) exposure to either hyperbaric nitrogen or normobaric nitrous oxide (N2O) impairs shivering thermogenesis and accelerates body core cooling. Animal-based studies, however, have indicated that repeated and sustained N2O administration may prevent N2O-evoked hypometabolism. We, therefore, examined the effects of prolonged intermittent exposure to 30% N2O on human thermoeffector plasticity in response to moderate cold. Fourteen men participated in two ∼12-h sessions, during which they performed sequentially three 120-min cold-water immersions (CWIs) in 20°C water, separated by 120-min rewarming. During CWIs, subjects were breathing either normal air or a normoxic gas mixture containing 30% N2O. Rectal and skin temperatures, metabolic heat production (via indirect calorimetry), finger and forearm cutaneous vascular conductance (CVC; laser-Doppler fluxmetry/mean arterial pressure), and thermal sensation and comfort were monitored. N2O aggravated the drop in rectal temperature (P = 0.01), especially during the first (by ∼0.3°C) and third (by ∼0.4°C) CWIs. N2O invariably blunted the cold-induced elevation of metabolic heat production by ∼22%-25% (P < 0.001). During the initial ∼30 min of the first and second CWIs, N2O attenuated the cold-induced drop in finger (P ≤ 0.001), but not in forearm CVC. N2O alleviated the sensation of coldness and thermal discomfort throughout (P < 0.001). Thus, the present results demonstrate that, regardless of the cumulative duration of gas exposure, a subanesthetic dose of N2O depresses human thermoregulatory functions and precipitates the development of hypothermia.NEW & NOTEWORTHY Human thermoeffector plasticity was evaluated in response to prolonged iterative exposure to 30% N2O and moderate cold stress. Regardless of the duration of gas exposure, N2O-induced narcosis impaired in a persistent manner shivering thermogenesis and thermoperception.

Individual differences in biological regulation: Predicting vulnerability to drug addiction, obesity, and other dysregulatory disorders

Physiological regulation is so fundamental to survival that natural selection has greatly favored the evolution of robust regulatory systems that use both reactive and preemptive responses to mitigate the disruptive impact of biological and environmental challenges on physiological function. In good health, robust regulatory systems provide little insight into the typically hidden complex array of sensor-effector interactions that accomplish successful regulation. Numerous health disorders have been traced to defective regulatory mechanisms, and generations of scientists have worked to discover ways to correct these defects and restore normal physiological function. Despite progress, numerous chronic health disorders remain resistant to treatment, and indeed for some disorders the incidence is increasing. We propose that an individual's susceptibility to acquire certain persistent dysregulatory disorders can be traced to interindividual variation in how that individual's regulatory system responds to challenges. Preexisting reliable individual differences among regulatory systems are typically unrecognized until appropriate regulatory challenges (e.g., exposure to a drug of abuse) lead to dysregulation (e.g., drug addiction). Specific characteristics of an individual's regulatory responsiveness may include etiological factors that participate in the acquisition, escalation and maintenance of health disorders characterized by dysregulation. By appropriately challenging a healthy individual's regulatory systems to identify its underlying characteristics, it is possible to ascertain whether an individual has an elevated risk for acquiring a dysregulated health condition and thereby enable strategies designed to prevent, rather than treat, the condition. This model is applied to drug addiction, and in addition we relate this approach to other dysregulated conditions such as obesity. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Validation of an equation for energy expenditure that does not require the respiratory quotient

Background

Energy expenditure (EE) calculated from respirometric indirect calorimetry is most accurate when based on oxygen consumption (VO₂), carbon dioxide production (VCO₂) and estimated protein metabolism (PM). EE has a substantial dependence of ~7% on the respiratory quotient (RQ, VCO₂/VO₂) and a lesser dependence on PM, yet many studies have instead estimated EE from VO₂ only while PM has often been ignored, thus reducing accuracy. In 1949 Weir proposed a method to accurately calculate EE without using RQ, which also adjusts for estimated PM based on dietary composition. This RQ^- method utilizes the calorimeter airflow rate (FR), the change in fractional O₂ concentration (ΔFO₂) and the dietary protein fraction. The RQ^- method has not previously been empirically validated against the standard RQ⁺ method using both VO₂ and RQ. Our aim was to do that.

Methods

VO₂ and VCO₂ were measured repeatedly in 8 mice fed a high protein diet (HPD) during exposure to different temperatures (n = 168 measurements of 24h gas exchange). The HPD-adjusted RQ⁺ equation was: EE [kcal/time] = VO₂ [L/time]×(3.853+1.081RQ) while the corresponding RQ^- equation was: EE = 4.934×FR×ΔFO₂. Agreement was analyzed using the ratios of the RQ^- to RQ⁺ methods along with regression and Bland-Altman agreement analyses. We also evaluated the standard equation using the dietary food quotient (FQ) of 0.91 as a proxy for RQ (FQ⁺ method).

Results

Ratio analysis revealed that the mean error of the RQ^- method was only 0.11 ± 0.042% while the maximum error was only 0.21%. Error using the FQ⁺ method was 4 -and 10-fold greater, respectively. Bland-Altman analysis demonstrated that the RQ^- method very slightly overestimates EE as RQ decreases. Theoretically, this error can be eliminated completely by imposing an incurrent fractional oxygen concentration at a value only slightly greater than the atmospheric level.

Conclusions

The Weir ‘RQ-free’ method for calculating EE is a highly valid alternative to the ‘gold standard’ method that requires RQ. The RQ^- approach permits reduced cost and complexity in studies focused on EE and provides a way to rescue EE measurement in studies compromised by faulty CO₂ measurements. Practitioners of respirometry should consider adjusting EE calculations for estimated protein metabolism based on dietary composition.

Plasma corticosterone, epinephrine, and norepinephrine levels increase during administration of nitrous oxide in rats

Nitrous oxide (N2O) is a gaseous drug with abuse potential. Despite its common clinical use, little is known about whether N2O administration activates the HPA axis and/or the sympathetic adrenomedullary system. The goal of this study was to determine whether 60% N2O alters plasma concentrations of corticosterone (CORT), epinephrine (EPI), and norepinephrine (NE) in male Long-Evans rats. A gas-tight swivel assembly in the lid of a gas administration chamber allowed the remote collection of blood samples from an indwelling jugular vein catheter at four time-points: baseline and at 30, 60, and 120 min during a two-hour administration of 60% N2O. Relative to baseline, plasma CORT (n = 9) was significantly elevated at all three time-points during N2O inhalation (mixed model analysis, p = .001) and plasma EPI and NE levels were each significantly elevated (n = 8, p ≤ .001) at the 30 min assessment. EPI then declined and did not differ from baseline at the 60 and 120 min assessments (p > .05) whereas NE remained elevated (120 min, p = .001). Administration of 60% N2O increases circulating CORT, EPI, and NE, supporting N2O as a physiological stressor. An N2O-induced increase in CORT is consistent with the observation that addictive drugs typically activate the HPA axis causing increased plasma levels of glucocorticoids. Allostatic models of drug addiction typically involve stress systems and the possible role of stress hormones in N2O-induced allostatic dysregulation is discussed.

Concentration-related metabolic rate and behavioral thermoregulatory adaptations to serial administrations of nitrous oxide in rats

Background

Initial administration of ≥60% nitrous oxide (N₂O) to rats evokes hypothermia, but after repeated administrations the gas instead evokes hyperthermia. This sign reversal is driven mainly by increased heat production. To determine whether rats will behaviorally oppose or assist the development of hyperthermia, we previously performed thermal gradient testing. Inhalation of N₂O at ≥60% causes rats to select cooler ambient temperatures both during initial administrations and during subsequent administrations in which the hyperthermic state exists. Thus, an available behavioral response opposes (but does not completely prevent) the acquired hyperthermia that develops over repeated high-concentration N₂O administrations. However, recreational and clinical uses of N₂O span a wide range of concentrations. Therefore, we sought to determine the thermoregulatory adaptations to chronic N₂O administration over a wide range of concentrations.

Methods

This study had two phases. In the first phase we adapted rats to twelve 3-h N₂O administrations at either 0%, 15%, 30%, 45%, 60% or 75% N₂O (n = 12 per group); outcomes were core temperature (via telemetry) and heat production (via respirometry). In the second phase, we used a thermal gradient (range 8°C—38°C) to assess each adapted group’s thermal preference, core temperature and locomotion on a single occasion during N₂O inhalation at the assigned concentration.

Results

In phase 1, repeated N₂O administrations led to dose related hyperthermic and hypermetabolic states during inhalation of ≥45% N₂O compared to controls (≥ 30% N₂O compared to baseline). In phase 2, rats in these groups selected cooler ambient temperatures during N₂O inhalation but still developed some hyperthermia. However, a concentration-related increase of locomotion was evident in the gradient, and theoretical calculations and regression analyses both suggest that locomotion contributed to the residual hyperthermia.

Conclusions

Acquired N₂O hyperthermia in rats is remarkably robust, and occurs even despite the availability of ambient temperatures that might fully counter the hyperthermia. Increased locomotion in the gradient may contribute to hyperthermia. Our data are consistent with an allostatic dis-coordination of autonomic and behavioral thermoregulatory mechanisms during drug administration. Our results have implications for research on N₂O abuse as well as research on the role of allostasis in drug addiction.

Does environmental exposure to the greenhouse gas, N2O, contribute to etiological factors in neurodevelopmental disorders? A mini-review of the evidence

BACKGROUND

Neurodevelopmental disorders are increasing in prevalence worldwide. Previous work suggests that exposure to the environmental air pollutant and greenhouse gas - nitrous oxide (N₂O) - may be an etiological factor in neurodevelopmental disorders through the targeting of several neural correlates.

METHODOLOGY

While a number of recent systematic reviews have addressed the role of general anesthesia in the surgical setting and neurodevelopmental outcomes, a narrative mini-review was conducted to first define and characterize the relevant variables (i.e., N₂O, attention-deficit hyperactivity disorder [ADHD] and autism spectrum disorders [ASD]) and their potential interactions into a coherent, hypothesis-generating work. The narrative mini-review merges basic principles in environmental science, anesthesiology, and psychiatry to more fully develop the novel hypotheses that neurodevelopmental impairment found in conditions like ADHD and ASD may be due to exposure to the increasing air pollutant, N₂O.

RESULTS

The results of the present mini-review indicate that exposure to N₂O, even at non-toxic doses, may modulate central neurotransmission and target many neural substrates directly implicated in neurodevelopmental disorders, including the glutamatergic, opioidergic, cholinergic, and dopaminergic systems. Epidemiological studies also indicate that early and repeated exposure to general anesthesia, including N₂O, may contribute to later adverse neurodevelopmental outcomes in children.

CONCLUSIONS

The current evidence and subsequent hypotheses suggest that a renewed interest be taken in the toxicological assessment of environmental N₂O exposure using validated biomarkers and psychiatric endpoints. Given the relevance of N₂O as a greenhouse gas, societies may also wish to engage in a more robust monitoring and reporting of N₂O levels in the environment for climactic benefit as well.

Brown adipose tissue thermogenesis does not explain the intra-administration hyperthermic sign-reversal induced by serial administrations of 60% nitrous oxide to rats

Initial administration of ≥60% nitrous oxide (N2O) to rats promotes hypothermia primarily by increasing whole-body heat loss. We hypothesized that the drug promotes heat loss via the tail and might initially inhibit thermogenesis via brown adipose tissue (BAT), major organs of thermoregulation in rodents. Following repeated administrations, N2O inhalation evokes hyperthermia underlain by increased whole-body heat production. We hypothesized that elevated BAT thermogenesis plays a role in this thermoregulatory sign reversal. Using dual probe telemetric temperature implants and infrared (IR) thermography, we assessed the effects of nine repeated 60% N2O administrations compared to control (con) administrations on core temperature, BAT temperature, lumbar back temperature and tail temperature. Telemetric core temperature, telemetric BAT temperature, and IR BAT temperature were reduced significantly during initial 60% N2O inhalation (p≤0.001 compared to con). IR thermography revealed that acute N2O administration unexpectedly reduced tail temperature (p=0.0001) and also inhibited IR lumbar temperature (p<0.0001). In the 9th session, N2O inhalation significantly increased telemetric core temperature (p=0.007) indicative of a hyperthermic sign reversal, yet compared to control administrations, telemetric BAT temperature (p=0.86), IR BAT temperature (p=0.85) and tail temperature (p=0.47) did not differ significantly. Thus, an initial administration of 60% N2O at 21°C may promote hypothermia via reduced BAT thermogenesis accompanied by tail vasoconstriction as a compensatory mechanism to limit body heat loss. Following repeated N2O administrations rats exhibit a hyperthermic core temperature but a normalized BAT temperature, suggesting induction of a hyperthermia-promoting thermogenic adaptation of unknown origin.

Predicting addictive vulnerability: individual differences in initial responding to a drug's pharmacological effects

Considerable data suggest that individuals who appear minimally disrupted during an initial drug administration have elevated risk for abusing the drug later. A better understanding of this association could lead to more effective strategies for preventing and treating drug addiction. To investigate this phenomenon using a rigorous experimental model, we first administered the abused inhalant nitrous oxide (N₂O) to rats in a total calorimetry and temperature system to identify groups that were sensitive or insensitive to the drug’s hypothermic effect. We then enrolled the two groups in a novel N₂O self-administration paradigm. The initially insensitive rats self-administered significantly more N₂O than sensitive rats, an important step in the transition to addiction. Continuous non-invasive measurement of core temperature and its underlying determinants during screening revealed that both groups had similarly increased heat loss during initial N₂O administration, but that insensitive rats generated more heat and thereby remained relatively normothermic. Calorimetry testing conducted after self-administration revealed that whereas N₂O’s effect on heat loss persisted comparably for both groups, initially insensitive rats actually over-responded by generating excess heat and becoming hyperthermic. Thus, rats with the greatest initial heat-producing compensatory response(s) appeared initially insensitive to N₂O-induced hypothermia, subsequently self-administered more N₂O, and developed hyperthermic overcompensation during N₂O inhalation, consistent with increased abuse potential and an allostatic model of addictive vulnerability.

Temperature in the spotlight of drug abuse research

This editorial summarizes Temperature's special issue entitled "Temperature and Toxicology with a Focus on Drugs of Abuse" (2014, volume 1, issue 3), dedicated to the multiple recent discoveries related to the thermoregulatory effects of xenobiotics. Several basic and clinical studies on xenobiotic-induced hyperthermia are reported that propose novel mechanisms and treatments.

Correctly identifying responses is critical for understanding homeostatic and allostatic regulation

Homeostasis stabilizes critical biological variables within appropriate limits via corrective regulatory effector responses that adequately counter disturbing effects. Identifying individual effects and responses, and distinguishing their individual influences on a regulated state, is challenging. Studying effector responses can reveal regulatory phenomena that depart from homeostasis into the realm of allostasis.

A unifying theory for the functional architecture of endothermic thermoregulation

Developing a unifying theory for the functional architecture of endothermic thermoregulation has been proven to be a challenging endeavor. Three papers published in this issue of Temperature take a closer look at this problem and add interesting views to our knowledge about the way that endothermic thermoregulation works.

Drug-induced regulatory overcompensation has motivational consequences: Implications for homeostatic and allostatic models of drug addiction

Initial administration of 60% nitrous oxide (N₂O) at 21°C ambient temperature reduces core temperature (Tc) in rats, but tolerance develops to this hypothermic effect over several administrations. After additional N₂O administrations, a hyperthermic overcompensation (sign-reversal) develops such that Tc exceeds control levels during N₂O inhalation. This study investigated whether rats would employ behavioral thermoregulation to facilitate, or oppose, a previously acquired hyperthermic overcompensation during N₂O administration. To establish a hyperthermic sign-reversal, male Long-Evans rats (N = 12) received 10 3-h administrations of 60% N₂O while housed in a gas-tight, live-in, “inactive” thermal gradient (∼21°C). Following the tenth N₂O exposure, the thermal gradient was activated (range of 10–37°C), and rats received both a control gas session and a 60% N₂O test session in counterbalanced order. Mean Tc during N₂O inhalation in the inactive gradient was reliably hypothermic during the first exposure but was reliably hyperthermic by the tenth exposure. When subsequently exposed to 60% N₂O in the active gradient, rats selected a cooler Ta, which blunted the hyperthermic sign-reversal and lowered Tc throughout the remainder of the N₂O exposure. Thus, autonomic heat production effectors mediating the hyperthermia were opposed by a behavioral effector that promoted increased heat loss via selection of a cooler ambient temperature. These data are compatible with an allostatic model of drug addiction that suggests that dysregulatory overcompensation in the drugged-state may motivate behaviors (e.g., drug taking) that oppose the overcompensation, thereby creating a vicious cycle of escalating drug consumption and recurring dysregulation.

Persistence of a hyperthermic sign-reversal during nitrous oxide inhalation despite cue-exposure treatment with and without a drug-onset cue

We asked whether chronic tolerance and the hyperthermic sign-reversal induced by repeated 60% N2O exposures could be extinguished using a cue-exposure paradigm. Rats received 18 N2O administrations in a total calorimetry system that simultaneously measures core temperature (Tc), metabolic heat production (HP), and body heat loss (HL). Each exposure entailed a 2-h baseline period followed by a 1.5-h N2O exposure. The 18 drug exposures induced a robust intra-administration hyperthermia in which the initial hypothermic effect of N2O inverted to a significant hyperthermic sign-reversal during N2O inhalation due primarily to an acquired robust increase in HP. The rats were then randomized to one of three extinction procedures (n=8/procedure) over a 20-d interval: 1) a N2O-abstinent home-cage group (HC) that received only the usual animal care; 2) a cue-exposure group (CEXP) in which the animals were placed in the calorimeter 8 times but received no N2O; and 3) a drug-onset-cue group (DOC) in which animals received a brief N2O exposure in the calorimeter that mimicked the first 3 min of an actual 60% N2O trial. Following the extinction sessions, all rats received a 60% N2O test trial and Tc, HP and HL were assessed. The hyperthermic sign-reversal remained fully intact during the test trial, with no significant differences observed among groups in any post-baseline change in any thermal outcome. These data suggest that cue exposure may not be an efficacious strategy to reduce sign-reversals that develop with chronic drug use.