Evaluation of five agents/methods for anesthesia of neonatal rats

Although neonatal altricial rodents are frequently used in experimental projects in which they must undergo survival surgical procedures, there are few published guidelines for anesthetizing them. Many of the drugs and methods that are commonly used to anesthetize newborn rodents provide inadequate anesthesia or are associated with problems, such as excessively high mortality. A two-part study was undertaken with the intention of identifying agents or methods that can be used to provide humane, safe, and effective anesthesia for neonatal rats. In part I, 1- to 3-day-old rat pups were anesthetized with methoxyflurane or one of several injectable agents, or by induction of hypothermia (immersion in ice water). Times to loss of the righting reflex, surgical anesthesia (indicated by loss of the pedal reflex and failure to respond to a skin incision), and recovery of the righting reflex were measured, and heart rate, respiratory rate, and oxygen saturation were monitored. Ketamine (100 mg/kg of body weight, i.p.), pentobarbital (30 to 40 mg/kg, i.p.), and fentanyl-droperidol combination (0.16 mg of fentanyl and 8.0 micrograms of droperidol/g, i.p.) proved unsafe (> 50% mortality) and/or ineffective at inducing short-term surgical anesthesia. In contrast, methoxyflurane and hypothermia were safe and effective. On the basis of subjective observations that they struggled and vocalized less, pups placed in protective latex sleeves prior to immersion in ice water appeared to be less distressed than pups immersed unprotected. In part II of the study, it was possible to safely maintain 1- to 3-day-old pups at a surgical plane of anesthesia for 30 min with either methoxyflurane or hypothermia. Supplementation with pentobarbital (20 mg/kg, i.p.) offered no advantages but significantly prolonged time to recovery. Surviving pups in both studies grew at similar rates during the month after testing. It was concluded that methoxyflurane or hypothermia are good choices for short- or long-term anesthesia of neonatal rats, and that use of a protective sleeve appears to reduce distress associated with induction of profound hypothermia.