Assessment of heat production, heat loss, and core temperature during nitrous oxide exposure: a new paradigm for studying drug effects and opponent responses

Abstract

Studies using core temperature (Tc) have contributed greatly to theoretical explanations of drug tolerance and its relationship to key features of addiction, including dependence, withdrawal, and relapse. Many theoretical accounts of tolerance propose that a given drug-induced psychobiological disturbance elicits opponent responses that contribute to tolerance development. This proposal and its theoretical extensions (e.g., conditioning as a mechanism of chronic tolerance) have been inferred from dependent variables, such as Tc, which represent the summation of multiple underlying determinants. Direct measurements of determinants could increase the understanding of opponent processes in tolerance, dependence, and withdrawal. The proximal determinants of Tc are metabolic heat production (HP) and heat loss (HL). We developed a novel system for simultaneously quantifying HP (indirect calorimetry), HL (direct gradient layer calorimetry), and Tc (telemetry) during steady-state administrations of nitrous oxide (N2O), an inhalant with abuse potential that has been previously used to study acute and chronic tolerance development to its hypothermia-inducing property. Rats were administered 60% N2O ( n = 18) or placebo gas ( n = 16) for 5 h after a 2-h placebo baseline exposure. On average, N2O rapidly but transiently lowered HP and increased HL, each by ∼16% ( P < 0.001). On average, rats reestablished and maintained thermal equilibrium (HP = HL) at a hypothermic Tc (−1.6°C). However, some rats entered positive heat balance (HP > HL) after becoming hypothermic such that acute tolerance developed, i.e., Tc rose despite continued drug administration. This work is the first to directly quantify the thermal determinants of Tc during administration of a drug of abuse and establishes a new paradigm for studying opponent processes involved in acute and chronic hypothermic tolerance development.