Corticosterone response underlying tolerance to stress induced by a gaseous (nitrous oxide) environment

Conditioned taste aversion (CTA) was demonstrated in rats following a single pairing between ingestion of a novel saccharin solution (CS) and 60 min. inhalation of an 80% N2O2:20% N2O gas mixture (UCS). CTA from a single novel CS-N2O UCS pairing was achieved by methodological improvements including: an inhalation chamber for small animals that achieved complete gas diffusion, an increased UCS duration (60 min. of inhalation) and a decreased CS-UCS interval (1 min.). The increased N2O UCS inhalation probably contributed to CTA, since no duration less than 60 min. of N2O inhalation produced significantly greater plasma corticosterone elevation than 60 min. of N2:O2 inhalation. The hypothesis that CTA is stress-induced was supported, in that plasma corticosterone levels were elevated by N2O inhalation and adrenal catecholamines (CA) were depleted. Further, after 4 N2O pretreatments, which produced partial behavioral tolerance to N2O treatment, plasma corticosterone elevation abated somewhat. However, corticosterone level also was raised by N2:O2 treatment. As a result, abatement of corticosterone elevation cannot account for behavioral tolerance to N2O-induced CTA, though it may be a contributor. Since adrenal CA were depleted by N2O inhalation, this response also might abate and underlie or contribute to behavioral tolerance.