Prolonged antinociception following carbon dioxide anesthesia in the laboratory rat

Cardiovascular and Metabolic Responses to Carbon Dioxide Euthanasia in Conscious and Anesthetized Rats

Euthanasia is a necessary component in research and must be conducted humanely. Currently, regulated CO₂ exposure in conscious rats is acceptable, but data are divided on whether CO₂ alone is more distressing than anesthesia prior to CO₂. To evaluate distress in rats, we compared physiologic responses to CO₂ euthanasia with and without isoflurane preanesthesia. Male Sprague-Dawley rats were implanted with telemetry devices to measure mean arterial pressure (MAP), heart rate (HR), and blood glucose. Animals recovered for 2 wk and were then exposed to either 5% isoflurane (n = 6) or 100% CO₂ (n = 7; calculated 30% chamber volume/min displacement) in their home cages to induce loss of consciousness. Euthanasia was then completed with CO₂ in both groups. MAP and HR increased when the gas delivery lids were placed on the home cages of both groups. Both MAP and HR gradually decreased with isoflurane exposure. MAP increased and HR decreased with CO₂ exposure. Glucose levels remained stable throughout the procedure, except for a small drop in conscious animals initially exposed to 100% CO₂. These data suggest that both gases affect the measured parameters in a similar manner, and that environmental factors, such as gas delivery lid placement, also change these measurements.

Evaluation of Low versus High Volume per Minute Displacement CO₂ Methods of Euthanasia in the Induction and Duration of Panic-Associated Behavior and Physiology

Current recommendations for the use of CO ₂ as a euthanasia agent for rats require the use of gradual fill protocols (such as 10% to 30% volume displacement per minute) in order to render the animal insensible prior to exposure to levels of CO ₂ that are associated with pain. However, exposing rats to CO ₂ , concentrations as low as 7% CO ₂ are reported to cause distress and 10%-20% CO ₂ induces panic-associated behavior and physiology, but loss of consciousness does not occur until CO ₂ concentrations are at least 40%. This suggests that the use of the currently recommended low flow volume per minute displacement rates create a situation where rats are exposed to concentrations of CO ₂ that induce anxiety, panic, and distress for prolonged periods of time. This study first characterized the response of male rats exposed to normoxic 20% CO ₂ for a prolonged period of time as compared to room air controls. It demonstrated that rats exposed to this experimental condition displayed clinical signs consistent with significantly increased panic-associated behavior and physiology during CO ₂ exposure. When atmospheric air was then again delivered, there was a robust increase in respiration rate that coincided with rats moving to the air intake. The rats exposed to CO ₂ also displayed behaviors consistent with increased anxiety in the behavioral testing that followed the exposure. Next, this study assessed the behavioral and physiologic responses of rats that were euthanized with 100% CO ₂ infused at 10%, 30%, or 100% volume per minute displacement rates. Analysis of the concentrations of CO ₂ and oxygen in the euthanasia chamber and the behavioral responses of the rats suggest that the use of the very low flow volume per minute displacement rate (10%) may prolong the duration of panicogenic ranges of ambient CO ₂ , while the use of the higher flow volume per minute displacement rate (100%) increases agitation. Therefore, of the volume displacement per minute rates evaluated, this study suggests that 30% minimizes the potential pain and distress experienced by the animal.

The effect of local or general anesthesia on the physiology and behavior of tail docked pigs

Tail docking of pigs is a routine procedure on farms to help control tail-biting behavior; however, docking can cause pain. The objective of this research was to evaluate the effect of local or general anesthesia on the physiology (experiment 1) and behavior (experiment 2) of tail docked pigs. Pigs were allocated to one of six treatment groups: (i) sham docking (CON); (ii) docking using conventional cutting (CUT) with side-cutting pliers; (iii) CUT docking plus local anesthesia injected immediately before docking (LA); (iv) CUT docking plus short-acting local anesthesia applied topically to the tail wound (SHORT); (v) CUT docking plus long-acting anesthesia applied topically to the tail wound (LONG) and (vi) CUT docking while the pig was anesthetized with carbon dioxide gas (CO(2)). In experiment 1, blood samples were collected from pigs (10 pigs per treatment) before and 30, 60 and 120 min after docking to measure leukocyte counts and percentages and cortisol concentrations. In experiment 2, the above treatments were repeated (10 pigs per treatment); the percentage of stress vocalizations were recorded during the administration of the treatments and behavior was recorded for up to 120 min after docking or handling. All pigs were weighed before and 24 h after docking and wound healing was recorded until weaning. The neutrophil/lymphocyte ratio was greater (P < 0.05) in CUT, LA, SHORT and LONG compared with CON pigs. At 30 min, cortisol concentrations were greater (P < 0.05) in CUT, LA, LONG and CO(2) compared with CON pigs. Cortisol concentrations did not differ (P > 0.05) between SHORT and CON pigs 30 min after docking. Cortisol concentrations did not differ (P > 0.05) among pigs given pain relief at the time of docking compared with pigs' docked without pain relief. Body weight change and wound scores did not differ (P > 0.05) among treatments. The percentage of stress vocalizations increased (P < 0.05) in CUT, SHORT and LONG, but not in CON, LA and CO(2) pigs in response to docking or handling. The percentage of time pigs spent lying without contact after docking tended to be greater (P = 0.06) in CUT pigs compared with all other docking treatments and CON pigs. In this study, none of the anesthesia treatments tested were effective at significantly changing the physiological or behavioral response to tail docking in pigs.

The physiological and behavioral response of pigs castrated with and without anesthesia or analgesia

Surgical castration is a common management practice performed on male pigs to prevent the occurrence of boar taint. Surgical castration is known to cause physiological and behavioral changes in pigs indicative of pain-induced distress; however, it is commonly performed without pain relief. The objective of this study was to evaluate the effectiveness of carbon dioxide gas (CO(2)) anesthesia and a non-steroidal anti-inflammatory drug (NSAID) to alleviate the pain caused by castration. At 3 d of age, male pigs were either control handled (CON), castrated without pain relief (CAS), given an NSAID and then immediately castrated (CAS+NSAID), anesthetized with CO(2) and then castrated (CAS+CO2), or anesthetized with CO(2) and given an NSAID at the time of castration (CAS+BOTH). Blood samples were collected before castration, and at 30, 60, 120, and 180 min, 24 h, and 3 d after castration or handling for analysis of cortisol, C-Reactive protein (CRP), and substance-P (SP) concentrations. This study was then repeated using the same treatment groups, and the behavioral response to castration and handling were measured using a 1-min scan sampling procedure. The percentage of stress vocalizations was recorded during the administration of all treatments. Anesthesia and analgesia did not effectively reduce (P > 0.05) the cortisol response to surgical castration. Overall, CRP concentrations were greater (P < 0.05) in CAS+CO2 pigs as compared with CON pigs. Sixty minutes after castration or handling, SP concentrations were greater (P < 0.08) in pigs given CO(2) anesthesia (CO2, CAS+CO2, and CAS+BOTH) than CON, CAS, and CAS+NSAID pigs. Pigs castrated without pain relief spent more (P < 0.001) time lying without contact than all other treatments during the first 30 min after castration, but thereafter CAS+CO2 pigs spent more (P < 0.001) time lying without contact than other treatments. During the first 30 min after the treatments were applied, CAS+CO2 pigs spent more (P < 0.01) time displaying pain-like behaviors than CON, CAS, CAS+NSAID, and CAS+BOTH pigs. The percentage of stress vocalizations was greater (P < 0.05) in CAS and CAS+NSAID pigs than all other treatments. Neither CO(2) anesthesia nor a NSAID, given separately or combined, markedly reduced the pain-induced distress caused by castration in pigs. More research is needed to evaluate practical methods of on-farm pain relief for pigs.

Chronic hypercapnia downregulates arginase expression and activity and increases pulmonary arterial smooth muscle relaxation in the newborn rat

In rats, chronic hypercapnia has been reported to ameliorate hypoxia-induced pulmonary hypertension in newborn and adult and to enhance endothelium-dependent vasorelaxation in adult pulmonary arteries. The underlying mechanisms accounting for chronic hypercapnia-induced improvements in pulmonary vascular function are not understood. Hypothesizing that downregulation of arginase activity may be contributory, we examined relaxation responses and arginase activity and expression in pulmonary arteries from newborn rats that were exposed (from birth to 14 days) to either mild-to-moderate (5.5% inhaled CO(2)) or severe (10% CO(2)) hypercapnia with either normoxia or hypoxia (13% O(2)). Pulmonary arteries from pups exposed to normoxia and chronic hypercapnia (5.5 or 10% CO(2)) contracted less in response to a thromboxane A(2) analog, U-46619, and showed enhanced endothelium-dependent (but not independent) relaxation compared with arteries from normocapnic pups (P < 0.01). Parallel with these changes, arginase activity and arginase I (but not II) expression in lung and pulmonary arterial tissue were significantly decreased (P < 0.05). Exposure to 10% CO(2) significantly increased (P < 0.01) pulmonary arterial tissue nitric oxide (nitrite) generation. In pups chronically exposed to hypoxia (13% O(2)), severe hypercapnia (10% CO(2)) significantly (P < 0.05) enhanced endothelium-dependent relaxation, increased nitric oxide generation, and decreased arginase activity but not expression. We conclude that chronic hypercapnia-induced downregulation of lung arginase expression and/or activity may reduce pulmonary vascular resistance by enhancing nitric oxide generation and thus endothelium-dependent relaxation. This mechanism may explain some of the beneficial effects of chronic hypercapnia on experimental pulmonary hypertension.

Carbon dioxide-induced anesthesia results in a rapid increase in plasma levels of vasopressin

Brief anesthesia, such as after exposure to high levels of carbon dioxide, prior to decapitation is considered a more humane alternative for the euthanasia of rodents, compared with use of decapitation alone. Studies of the levels of certain stress hormones in plasma such as corticosterone and ACTH have supported the use of this method of euthanasia in endocrinological and molecular studies. In the current study, rats were briefly exposed to a chamber filled with carbon dioxide until recumbent (20-25 sec), immediately killed via decapitation, and trunk blood collected; findings were compared with rats killed via decapitation with no exposure to carbon dioxide. RIAs were used to measure arginine vasopressin (AVP) and ACTH immunoreactivity (ir) in plasma. Whereas ACTH-ir levels remained steady after brief exposure to carbon dioxide (in accordance with results of other investigators), AVP-ir levels were increased by more than an order of magnitude. These results were confirmed by quantitative capillary-liquid chromatography-mass spectrometry, indicating this observation of rapid increase in plasma AVP-ir levels is not due to nonspecific recognition by the antibody used in the RIA. Likewise, using capillary-liquid chromatography-mass spectrometry, we observed a rapid increase in plasma oxytocin levels after carbon dioxide exposure. These surprising findings have important implications for the design and interpretation of studies involving brief carbon dioxide exposure prior to decapitation as well as those with euthanasia resulting from carbon dioxide-induced asphyxiation.

Carbon dioxide negatively modulates N-methyl-D-aspartate receptors

BACKGROUND

Carbon dioxide (CO2) dose-dependently decreases minimum alveolar concentration (MAC) of anaesthetics in rats. CO2 also dose-dependently decreases cerebrospinal fluid pH. N-methyl-D-aspartate (NMDA) channels exhibit pH sensitivity and are putative targets for inhaled anaesthetics. We hypothesized that CO2 dose-dependently decreases rat NMDA channel current via an acidifying effect at concentrations relevant to CO2 MAC.

METHODS

To test this hypothesis, we studied rat NR1/NR2A glutamate receptors expressed in voltage-clamped Xenopus oocytes. To measure pH effects, we used perfusates adjusted between 7.3 and 5.3 with HCl. To measure CO2 effects, we used equimolar sodium perfusates containing either 0 or 24 mM NaHCO3 and CO2 between 0% and 87% atm. Solution compositions were measured using a blood gas analyser with values corrected using a calibrated pH meter and gas chromatograph with solutions at 37 degrees C.

RESULTS

We found that decreasing pH decreased NMDA current. Moreover, pH effects produced by adding CO2 to NaHCO3-containing perfusates were identical to those produced by adding HCl to normal perfusates. The pH inhibiting 50% of NMDA current was 6.52. The CO2 concentration inhibiting 50% of rat NMDA current was 63% for solutions with 24 mM NaHCO3. CO2 exhibited a linear dose-dependent NMDA response analogous to that observed for in vivo CO2 anaesthetic potency in rats.

CONCLUSIONS

CO2 and hydrogen ions act via the same mechanism to inhibit NMDA receptors. Moreover, CO2 inhibits rat NMDA receptors in a manner that is consistent with CO2 MAC-sparing effects in rats.

Aversion to carbon dioxide stunning in pigs: effect of carbon dioxide concentration and halothane genotype

Aversion to the dip-lift stunning system and to the inhalation of 70 and 90% carbon dioxide was assessed in 18 halothane-free (NN) and 14 heterozygous halothane (Nn) slaughter weight pigs using aversion learning techniques and behavioural studies in an experimental slaughterhouse. Pigs were subjected to the treatments individually. When the dip lift system contained atmospheric air, the proportion of pigs that entered the crate voluntarily increased on subsequent days, indicating that pigs habituate to the stunning system. Based on the number of attempted retreats, for the first descent into the well with atmospheric air, Nn pigs were more reactive than NN pigs. On repeating the descent, Nn pigs showed greater habituation to the procedure. When the pit contained (either 70 or 90%) carbon dioxide, the time taken to enter the crate and the incidence of pigs that attempted to retreat increased on subsequent days, indicating aversion to the carbon dioxide concentrations. The aversion was higher when the stunning system contained 90 as opposed to 70% carbon dioxide due possibly to increased irritation of the nasal mucosal membranes and more severe hyperventilation. Conversely, a decrease in the concentration of carbon dioxide increased the time to loss of posture and, therefore, lengthened the perception of the aversive stimulus till the animal lost consciousness. These results suggest that stunning with carbon dioxide is not free from pain or distress. The degree of aversion depends on the carbon dioxide concentration. Therefore, if higher concentrations of carbon dioxide are recommended for rapid induction of anaesthesia, it needs to be assumed that this may be more aversive to pigs.

Possibilities for refinement and reduction: future improvements within regulatory testing

Approaches and challenges to refining and reducing animal use in regulatory testing are reviewed. Regulatory testing accounts for the majority of animals reported in the most painful and/or distressful categories in the United States and Canada. Refinements in testing, including the use of humane endpoints, are of increasing concern. Traditional approaches to reduction (e.g., improving experimental design) are being supplemented with complementary approaches, such as the use of tier testing to eliminate some chemicals prior to in vivo testing. Technological advances in telemetry and noninvasive techniques will help decrease either the demand for animals in testing or animal suffering. Further decreases in animal use will stem from international harmonization and coordination of testing programs. Progress in refinement and reduction faces a variety of broad challenges, including limited funding for research. In the specific area of refinement, a key challenge is the issue of distress (as distinct from pain). In the area of reduction, the practice of using unjustifiably high numbers of animals from small species (e.g., rodents) should be challenged. One case study of the use of carbon dioxide as a euthanasia agent illustrates the need for further analysis and research. Notwithstanding the complexities and challenges, the potential for refinement and reduction in regulatory testing is encouraging.

The conditioning of dyspneic suffocation fear. Effects of carbon dioxide concentration on behavioral freezing and analgesia

Previous studies in our laboratory have shown that a single exposure to 100% carbon dioxide (CO2) can serve as an effective unconditioned stimulus (US) in a Pavlovian aversive-context conditioning paradigm in rats. Although the US exposure parameters employed in the initial studies were sufficient for producing a context-specific enhancement of behavioral freezing and analgesia, it had yet to be determined whether variations of these CO2 conditioning procedures would produce other conditioning effects. Thus, the purpose of the following experiment was to investigate the intensity of the US on the conditioned response (CR). The findings confirm that variations in CO2 concentrations produce changes in the CR that are consistent with principles of Pavlovian conditioning. The findings lend additional support to the tenability of a dyspneic suffocation fear theory of panic disorder, a theory that postulates that at least one type of panic attack could be a consequence of Pavlovian conditioning.

Newborn tissue concentrations of bupivacaine following maternal epidural administration during labor in guinea pigs

Newborn plasma concentrations of maternally administered bupivacaine are often measured, but it is unclear how well they reflect tissue concentrations. Bupivacaine (0.25%) was administered epidurally (0. 12 ml/kg) 10 min prior to labor induction in 6 term-pregnant guinea pigs. Plasma, brain, heart and liver samples were obtained for bupivacaine analysis from newborns (n = 22) after spontaneous delivery. Liver bupivacaine concentrations were 2-3 times greater than those in the plasma, brain, and heart. A similar pattern of tissue concentrations was seen in a smaller number of newborns delivered by cesarean section. Liver bupivacaine concentration decreased with drug-delivery interval in littermates, while heart and brain concentrations showed no relationship with drug-delivery interval. Blood gases of newborns reflected acidosis, which may have influenced tissue drug concentrations. Under conditions of the study, bupivacaine concentrations in heart and brain, potential sites of bupivacaine action, were lower than those observed in a peripheral blood sample.