Urethane anesthesia in adult female rats: preliminary observations

Urethane is widely used as a rodent anesthetic in the laboratory setting, and is characterized as producing long‐lasting anesthesia. The purpose of this study was to evaluate the quality of anesthesia provided by a single dose of urethane based on the response to a noxious stimulus. If the quality of anesthesia was insufficient to prevent gross purposeful movement (GPM), isoflurane was also administered until no response to noxious stimulation occurred. Five adult Harlan Sprague Dawley rats (6 months of age, 250–300 g) were given urethane (1.4 g kg^?1 IP) and evaluated for 120 minutes post‐injection. If the rats became laterally recumbent by 20 minutes post‐injection, a large hemostat was positioned around the tail and the response to tail clamping was assessed. If no GPM occurred, an additional 20 minutes was allowed to elapse. If the rats were not laterally recumbent or GPM was present, they were placed in a chamber and isoflurane in oxygen was administered. Inspired isoflurane concentrations (ISO) were measured using a S/5 anesthetic gas analyzer (Datex‐Ohmeda Division, Helsinki, Finland) calibrated before each experiment with a standardized calibration gas mixture (DOT‐34 NRC 300/375 m 1014, Datex‐Ohmeda Division, Helsinki, Finland). A period of 20 minutes was allowed for equilibration to inspired ISO. The tail‐clamp stimulus was then re‐applied and the animal's response recorded. If GPM was absent, ISO was lowered by 10–20% and an additional 20 minute interval elapsed. In contrast, if GPM was present, ISO was increased by 10–20%. This procedure was repeated until the ISO required to prevent GPM was determined in duplicate. The position within the estrus cycle influenced pain thresholds in the rats. As such, a vaginal smear was prepared from each rat and the position in the estrus cycle was determined based on vaginal cytology. Rats were euthanatized at the end of the study period. All values were mean ± SD. Four rats became recumbent after urethane injection (time to recumbency: 45 ± 17 seconds). Of these, two rats (one estrus, one metestrus) did not require isoflurane supplementation for the duration of the study. The three remaining rats (two metestrus, one estrus) required isoflurane supplementation. The mean ISO required to prevent GPM was 0.26 ± 0.16%. Position within the estrus cycle did not appear to affect the animal's response to urethane. These results indicate that urethane anesthesia is not long lasting in all rats and provides variable quality of anesthesia. This is of particular concern in the laboratory setting where muscle relaxants are often administered to rats shortly after urethane injection.