Comparative Study of the Aftereffect of CO2 Inhalation or Tiletamine-Zolazepam-Xylazine Anesthesia on Laboratory Outbred Rats and Mice

Abstract

CO₂ inhalation is currently the most common method of euthanasia for laboratory rats and mice, and it is often used for further terminal blood sampling for clinical biochemical assays. Lately, this method has been criticized due to animal welfare issues associated with some processes that develop after CO₂ inhalation. The stress reaction and the value of the clinical laboratory parameters significantly depend on the used anesthetics, method, and the site of blood sampling. Especially in small rodents, an acute terminal state followed by a cascade of metabolic reactions that can affect the studied biochemical profile may develop and cause unnecessary suffering of animals. The aim of this study was to compare the stability of biochemical parameters of outbred Sprague Dawley rats and CD-1 mice serum collected after CO₂ inhalation or the intramuscular injection of tiletamine-zolazepam-xylazine (TZX). The serum content of total protein and albumin, cholesterol, triglycerides, aspartate aminotransferase (AST), alanine aminotr ansferase (ALT), alkaline phosphatase (ALP), total bilirubin, and creatinine was decreased by the injection of TZX in comparison with CO₂ inhalation. In addition, the levels of calcium, phosphates, chlorides and potassium were lowered by TZX vs. CO₂ administration, while the level of sodium increased. Finally, the level of the majority of serum clinical biochemical parameters in rats and mice tend to be overestimated after CO₂ inhalation, which may lead to masking the possible effect of anti-inflammatory drugs in animal tests. Injection anesthesia for small rodents with TZX is a more feasible method for terminal blood sampling, which also reduces the suffering of animals.

Keywords: CD-1 mice; Sprague Dawley rats; animal care and protection; laboratory rodents; serum biochemistry.

Figure 1

Blood glucose levels in males and females of CD-1 mice (A), and Sprague Dawley rats (B), in the awake state and following 5 min after CO₂ inhalation or tiletamine–zolazepam–xylazine (TZX) anesthesia. # p < 0.05 TZX versus awake, * p < 0.05 TZX versus CO₂.