Euthanasia in Endocrinology: The Choices Get More Complex

Abstract

In 1927, the German physicist Werner Heisenberg published a paper that developed the principle of imprecision or uncertainty in measuring the position and momentum of subatomic particles; simply stated, “the more precisely the position is determined, the less precisely the momentum is known, . . . and vice versa.” This principle reflects a fact of life that most scientists learn at an early age:preciseexperimentalmeasurementsmayonlybepossibleunder conditions in which the properties of the system can no longer be completely defined. For endocrinologists, the problems are particularly acute because of the sensitivity of endocrine systems to metabolic, sensory, or pharmacologically induced perturbation. This makes itdifficult tostudythese systems invivo,without introducing the potential for experimentally induced error. The challenge is to develop procedures that induce as little disturbance of normal hormonal interactions as possible. Experiments involving euthanasia present a particular problem because of the need to balance ethical concerns in the treatment of animals with avoidance of procedures that might alter the experimental results. Most countries have now adopted guidelines for research that stress the principle that animals should not be unnecessarily exposed to pain and suffering. In the euthanasia guidelines of the American Veterinary Medical Association, for example, euthanasia isdefinedastheactof inducinghumanedeathinananimal, “with an emphasis on making the death as painless and distress free as possible” (1). Ensuring that death is distress free, however, may be possible only by depressing consciousness, through modulation of neuronal activity in the central nervous system. This is hard to do without also affecting the hypothalamic and limbic brain centers that normally regulate neuroendocrine function. Measures to achieve painless euthanasia may, therefore, themselves alter the endocrine endpoints under study, particularly if those endpoints are sensitive to changes in neuroendocrine hormone secretion. The demonstration in the mid-1970s that neural activity may continue in the brain for 13–14 sec after decapitation (2) resulted in adoption of policies encouraging sedation or complete anesthesia beforephysicalmethodsofeuthanasia,basedonthehypothesis that continued brain activity might reflect a perception of pain. Because many inhalant and noninhalant anesthetic drugs themselves induce stress responses and changes in hormone levels, their usefulness is limited (3,4). Several studies, however, indicated that sedationwith CO2 gas might provide a nonstressful alternative. Exposure to CO2 concentrations as low as 7.5% in air increases pain thresholds, whereas30–100%CO2 canproducerapid lossofconsciousness (5, 6). Importantly, brief exposure to CO2 ( 30 sec) did not seem to increase the levelsofACTH,ahormone that isnotoriously sensitive tostress (7).Forthisreason,CO2sedationhasbecomeawidelyused method to limit pain and distress before euthanasia. In the current issue of Endocrinology, however, Reed et al. (8) report that only 20–25 sec exposure of male rats to CO2 gas, sufficient to render them recumbent and unresponsive but still alive, results in a dramatic elevation in the levels of vasopressin and oxytocin in the circulation. Using methods based on a combination of RIA with liquid capillary chromatography-mass spectrometry to achieve definitive identification of the peptides, Reed et al. (8) show that plasma levels of vasopressin and oxytocin increase at least 10fold after brief CO2 exposure. The authors speculate that these findings may explain previous observations of CO2 induced analgesia, because of the reported ability of iv vasopressin to induce non-opioidergic analgesic effects (9) as well as observations on induction of ACTH release after longer periods (2 min) of CO2 exposure (7). The mechanisms of these effects remain unclear. Reed et al. (8) speculate that it may involve changes in a number of neurotransmitter systems in the brain, perhaps secondary to changes in blood pressure triggered by CO2 exposure, but additional work will be required to define the underlying mechanisms, given that the systems in the brain that control the posterior pituitary may also influence other aspects of endocrine function. The data also have ramifications for studies that may not appear to have a direct neuroendocrine connection, because of the powerful effects of the posteriorpituitaryhormonesaroundthebody. Inadditionto itsactions as an ACTH secretagogue, vasopressin increases blood pressure by increasing peripheral vascular resistance (10), stimulates water resorption in the kidney (11), and influences aggressive and social