Robust thermoregulatory overcompensation, rather than tolerance, develops with serial administrations of 70% nitrous oxide to rats

Abstract

Changes in typical whole-animal dependent variables following drug administration represent an integral of the drug’s pharmacological effect, the individual’s autonomic and behavioral responses to the resulting disturbance, and many other influences. An archetypical example is core temperature (Tc), long used for quantifying initial drug sensitivity and tolerance acquisition over repeated drug administrations. Our previous work suggested that rats differing in initial sensitivity to nitrous oxide (N2O)-induced hypothermia would exhibit different patterns of tolerance development across N2O administrations. Specifically, we hypothesized that rats with an initially insensitive phenotype would subsequently develop regulatory overcompensation that would mediate an allostatic hyperthermic state, whereas rats with an initially sensitive phenotype would subsequently compensate to a homeostatic normothermic state. To preclude confounding due to handling and invasive procedures, a valid test of this prediction required non-invasive thermal measurements via implanted telemetric temperature sensors, combined direct and indirect calorimetry, and automated drug delivery to enable repeatable steady-state dosing. We screened 237 adult rats for initial sensitivity to 70% N2O-induced hypothermia. Thirty highly sensitive rats that exhibited marked hypothermia when screened and 30 highly insensitive rats that initially exhibited minimal hypothermia were randomized to three groups (n=10 each/group) that received: 1) twelve 90-min exposures to 70% N2O using a classical conditioning procedure, 2) twelve 90-min exposures to 70% N2O using a random control procedure for conditioning, or 3) a no-drug control group that received custom-made air. Metabolic heat production (via indirect calorimetry), body heat loss (via direct calorimetry) and Tc (via telemetry) were simultaneously quantified during N2O and control gas administrations. Initially insensitive rats rapidly acquired (3rd administration) a significant allostatic hyperthermic phenotype during N2O administration whereas initially sensitive rats exhibited classical tolerance (normothermia) during N2O inhalation in the 4th and 5th sessions. However, the sensitive rats subsequently acquired the hyperthermic phenotype and became indistinguishable from initially insensitive rats during the 11th and 12th N2O administrations. The major mechanism for hyperthermia was a brisk increase in metabolic heat production. However, we obtained no evidence for classical conditioning of thermal responses. We conclude that the degree of initial sensitivity to N2O-induced hypothermia predicts the temporal pattern of thermal adaptation over repeated N2O administrations, but that initially insensitive and sensitive animals eventually converge to similar (and substantial) magnitudes of within-administration hyperthermia mediated by hyper-compensatory heat production.

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

“…4) revealed a hyperthermic overcorrection of Tc during N 2 O exposure similar to the hyperthermia that eventually developed during the early period of N 2 O inhalation in the thermal gradient. Consistent with our previous work, 22 the early-period hyperthermic Tc in drug-adapted rats was primarily underlain by an increase of metabolic HP that exceeded the effect of the drug to augment HL. However, the late-period HL was not elevated compared to control values (Fig.…”

“…The evolution of patterns of Tc within and across N 2 O inhalation sessions in the thermal gradient are similar to those we have observed in calorimetry experiments at typical lab temperatures (~21–22°C). 22 Specifically, in Session 1 the Tc of drug-naïve rats initially decreased rapidly from baseline, achieving a nadir by 30–45 min, and subsequently returned toward baseline, consistent with the development of acute tolerance. In subsequent sessions, rats developed chronic tolerance to the hypothermic effect of N 2 O, and this stage of adaptation became fully expressed by sessions 4-6 (i.e., Tc remained commensurate with baseline during N 2 O inhalation); subsequently, Tc exhibited a hyperthermic sign-reversal during N 2 O inhalation (Sessions 7-12).…”

“…19 Telemetric measurement of Tc coupled with simultaneous total calorimetry (combining indirect and direct calorimetry) allows HP and HL to be measured non-invasively and continuously in undisturbed rats. 19–22 …”

“…19,20 Increases in HP can occur during the course of an initial N 2 O administration and result in the development of acute tolerance, 19 while progressive increases in HP over subsequent N 2 O administrations result in the development of chronic tolerance. 21,22 …”

“…The magnitude of the HP response increases progressively over repeated N 2 O administrations, which contributes to chronic tolerance development, and with additional administrations causes rats to eventually exhibit a hyperthermic overcompensation of Tc. 21,22,29 …”