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

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.

5-HT2A Receptor Agonist-Induced Hyperthermia Is Induced via Vasoconstriction by Peripheral 5-HT2A Receptors and Brown Adipose Tissue Thermogenesis by Peripheral Serotonin Loss at a High Ambient Temperature

Recreational drugs such as 3,4-methylenedioxymethamphetamine and cocaine induce hyperthermia, which is affected by ambient temperature. 2-(4-Bromo-2,5-dimethoxyphenyl)-N-(2-methoxybenzyl)ethanamine (25B-NBOMe), a selective agonist of 5-HT_2A receptor used as a recreational drug, reportedly induces hyperthermia. This study aimed to verify whether 25B-NBOMe induces ambient temperature-dependent hyperthermia and to clarify its mechanism. Eight-week-old male Sprague-Dawley rats were administered intraperitoneal injection of 25B-NBOMe at an ambient temperature of 23°C or 29°C. 25B-NBOMe administration at 23°C did not change the core body temperature of the rats, whereas administration at 29°C induced significant hyperthermia 30-120 minutes postadministration. Tail surface temperature temporarily decreased 30 minutes postadministration, indicating heat storage by peripheral vasoconstriction despite a high ambient temperature. Because 25B-NBOMe-induced-hyperthermia was suppressed by sarpogrelate, but not by destruction of central noradrenaline or serotonin neurons, peripheral 5-HT_2A receptors were considered contributors to the development of hyperthermia at a high ambient temperature, independently from central neurons. The temperature of brown adipose tissue (BAT) increased 60-120 minutes postadministration of 25B-NBOMe at 29°C, indicating thermogenesis. Previous studies have reported that peripheral serotonin contributes to the inhibition of BAT thermogenesis. Decreased plasma serotonin levels were observed at 29°C, and serotonin administration partially suppressed 25B-NBOMe-induced hyperthermia at a high ambient temperature, suggesting that decreased levels of peripheral serotonin induced BAT thermogenesis. Our findings indicate that 25B-NBOMe induces hyperthermia at a high ambient temperature via vasoconstriction regulated by 5-HT_2A receptors and BAT thermogenesis mediated by decreased levels of plasma serotonin. Thus, peripheral serotonin plays a partial but important role in thermoregulation.

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.