Karl Kaiyala, PhD - US grants

DESCRIPTION (provided by applicant): This is an R21 proposal to investigate the physiological mechanisms of tolerance development to nitrous oxide (N2O)-induced hypothermia. Influential theories of drug tolerance contend that CNS-mediated drug-opposing responses underlie the etiology of drug tolerance and dependence. Evidence for such responses is primarily based on the observation of withdrawal effects when the drug is discontinued (or antagonized) as well as on the elicitation of the responses by conditioned stimuli. This proposal uses a novel approach for measuring these responses directly, while the drug is present, during both an initial drug administration as well as over repeated administrations. This research will also determine bow individual differences in the identity, latency and/or intensity of these responses are related to individual differences in initial sensitivity and acute tolerance to N2O hypothermia. Using a rat model that combines direct and indirect calorimetry with implanted temperature sensors, synchronous measures of core temperature and the dynamic processes of metabolic heat production and heat loss will be made during a steady-state administration of 60 percent N20. In an initial experiment, experimental (N2O) and control (placebo gas) rats will receive a 5-hour gas exposure during which core temperature and the components of heat balance are measured. The same rats will be retested 2 weeks later to determine the reliability of the mechanisms that determine core temperature. In a second experiment, individual rats given an initial N2O exposure will be classed as: a) insensitive to N2O-induced hypothermia, b) sensitive to N2O-induced hypothermia and develop acute tolerance, or c) sensitive to N2O-induced hypothermia with little or no acute tolerance. Rats in each group will be randomly assigned to receive either N2O or placebo during three additional five-hour gas exposures at one-week intervals to determine how the physiological responses controlling heat balance change as chronic tolerance develops. These studies have theoretical importance for understanding the mechanisms of drug tolerance and will begin to suggest the physiological basis of individual differences in vulnerability to addiction and drug abuse.

DESCRIPTION (provided by applicant): This is an R21 proposal to identify brain regions that participate in the neuroadaptive responses underlying tolerance to nitrous oxide (N2O) hypothermia. Addictive drugs perturb variables that are subject to homeostatic control. Drug effects are thereby countered by adaptive biological responses. Many addiction researchers believe that these adaptive changes give rise to central features of the addiction phenotype, including drug tolerance, dependence and withdrawal. During an initial drug administration, reflexive adaptive responses can oppose a drug effect, causing acute (intrasessional) tolerance. With repeated drug exposures, the magnitude and timing of adaptive responses improve such that the drug has little observable effect (i.e., chronic tolerance develops). We propose to use c-fos immunostaining to identify brain areas that participate in neuroadaptive responses to the pharmacologically-active gas, N2O. The proposed experiments exploit the principle that adaptive response mechanisms are activated only if the regulated variable is perturbed by the drug. As with ethanol, N2O has a potent hypothermic effect at typical room temperatures, and tolerance develops to this, both acutely and chronically. However, preventing ethanol-induced hypothermia by elevating ambient temperature prevents development of chronic tolerance despite repeated drug administrations because the adaptive responses are not recruited. Accordingly, we will manipulate ambient temperature to either provide (21 degrees C) or deny (30 degrees C) rats the hypothermic state that typically occurs with 60% nitrous oxide. Two experiments are proposed to investigate the adaptive responses that develop acutely during an initial nitrous oxide exposure, as well as chronically over repeated exposures. Core temperature, and c-fos gene expression, a widely-used measure of neuronal activity, are the primary dependent measures. This work will be the first to identify brain areas activated by a drug-induced hypothermic stimulus and its adaptive consequences. These studies have theoretical importance for understanding and treating drug addiction.