M.M. Puig

Tail immersion test for the evaluation of a nociceptive reaction in mice: Methodological considerations☆☆☆

The effect of two commonly used methods to immobilize the animals, viz. tube restrainer and wrapping in a diaper (chux) on the tail flick latency in immersion test, was evaluated in mice using a stimulus temperature of 50 degrees C. The animals were immobilized either in the tube or chux briefly (25-30 sec) during the tail flick measurements. The basal tail flick latency was 2.8 +/- 0.2 in the tube restrained and 5.5 +/- 0.3 sec in chux restrained groups (p less than 0.001). The analgesic effect of morphine (1, 3, 4, 7, and 10 mg/kg) was significantly higher in the chux-restrained animals as indicated by the dose ratio of 2.16 for the 50% analgesic response in the chux versus tube restrained mice. The tail flick latency, 15 min after naloxone injection (1 and 3 mg/kg), expressed as % of predrug latency was significantly reduced in the chux- but not the tube-restrained group. The hyperalgesic effect of naloxone could not be detected in chux-restrained animals, when the water temperature was increased to 55 degrees C. The results demonstrate that the restraining procedure will influence the analgesic effects of test drugs in tail immersion test. Furthermore, the stimulus temperature appears to be an important variant that could influence the results in this test. The present results demonstrate the hyperalgesic effect of naloxone after systemic administration in the tail immersion test and supports the concept that tail flick response is tonically inhibited by endogenous opioid systems.

The hyperalgesic effect of naloxone is attenuated in streptozotocin-diabetic mice

In mice with streptozotocin-induced hyperglycemia, nociception was tested after naloxone administration in hot plate and tail immersion tests. The choice of these two tests was to include a supra-spinal nociceptive reflex indicative of higher cognitive process (hot-plate test) as well as a reflex which predominantly represents lower spinal motor mechanisms (tail immersion test). Naloxone-induced hyperalgesia was attenuated in both tests in mice with streptozotocin-induced diabetes. In mice with hyperglycemia induced by intraperitoneal dextrose administration, naloxone hyperalgesia was significantly enhanced in the hot plate test. The basal nociceptive threshold in streptozotocintreated animals was decreased in the immersion but not in the hot plate test. These results indicate that hyperglycemia per se does not adequately explain the changes in naloxone hyperalgesia in experimental models of diabetes. They also suggest that acute hyperglycemia may modify the interaction of endogenous opioid peptides with their receptors only at supra-spinal sites. However, chronic hyperglycemia appears to affect endogenous opioid peptides both at spinal and supra-spinal levels and their interaction with the opiate receptors.

Influence of hypo and hyperthermia on disposition of morphine.

Using morphine as a prototype opiate anesthetic, the dispositional changes and cardiovascular effects during hypothermia (30°C) and hyperthermia (40°C) in dogs under isoflurane anesthesia was assessed. Single intravenous bolus injection of 1 mg/kg morphine resulted in a significant and sustained decrease in mean arterial pressure in hypothermic, but not in hyperthermic or normothermic (37°C) conditions. Hypothermic dogs showed significantly higher levels of morphine both in plasma and in cerebrospinal fluid. In contrast, hyperthermia did not affect these levels. Body temperature did not affect the t1/2α, however t1/2β and mean residence time were significantly increased while volume of distribution at steady state and total body clearance were decreased during hypothermia. The results provide evidence that hypothermia is likely to be associated with a sustained increase in opiate levels and might be associated with a enhanced side effects. The results suggests the need for a controlled clinical trial to assess the dose of opiate anesthetics during hypothermia.

Stereospecific inhibition of gastrointestinal transit by κ opioid agonists in mice

Abstract Systemic administration of highly selective κ opiate agonists, U-50488H and U-69593 (3, 10 and 30 mg/kg i.p.) produced significant inhibition of the gastrointestinal transit in mice as assessed by charcoal meal test. In contrast, the (+) stereoisomer of U-50488H, U-53455E did not inhibit the gastrointestinal motility. Furthermore, the κ-selective antagonist, Mr 2266 (3 mg/kg) when administered along with the agonists, reversed the effects of the agonists. These results suggest that stereospecific κ opiate receptors are involved in inhibition of gut motility in mice.

Droperidol enhances fentanyl and sufentanil, but not morphine, analgesia

1. The effect of droperidol pre-treatment on the analgesic potency of morphine, fentanyl and sufentanil was assessed in mice. 2. Acetic acid writhing test and tail immersion test were used to measure the analgesic response. 3. The neuroleptic augments the effects of sufentanil and fentanyl. 4. Thus, the dose-response curves for fentanyl and sufentanil were shifted to the left and the ED50 of the analgesics lowered in droperidol pre-treated animals. 5. However, morphine analgesia was not influenced by droperidol. 6. The results suggest that combination of sufentanil or fentanyl with droperidol may be better than morphine to produce neuroleptanalgesia and anesthesia.

Effects of acute and chronic hyperglycemia on morphine-induced inhibition of gastrointestinal transit in mice.

Using the charcoal meal test, the effects of morphine (3, 5 and 7 mg/kg) on gastrointestinal transit was assessed in normoglycemic as well as in acute and chronic hyperglycemic mice. Acute hyperglycemia was induced by intraperitoneal injection of glucose (5.04 g/kg), while chronic hyperglycemia was induced by streptozotocin injection (200 mg/kg i.p., 7-8 days before). Acute hyperglycemia augmented the inhibitory effect of morphine on gut transit, however, chronic hyperglycemia failed to modify the effects of morphine. The results indicate that hyperglycemia per se may not be the primary mechanism for the altered sensitivity to morphine in experimental models of diabetes.

Bretylium tosylate and electrically induced cardiac arrhythmias during hypothermia in dogs

The effect of bretylium tosylate on plasma catecholamines and on electrically induced arrhythmias was evaluated in anesthetized hypothermic dogs. Bretylium at a dose of 7.5 mg/kg was administered prior to cooling from 37 degrees C to 27 degrees C. During cooling, the ventricular arrhythmia threshold (VAT) in control animals decreased from 10.1 +/- 1.9 to 4.4 +/- 1.3 impulses, while the VAT in bretylium-treated animals increased from 9.8 +/- 2.9 to 23.2 +/- 2.7 impulses. Catecholamine levels increased during cooling in all animals. In control animals, the epinephrine/norepinephrine ratio was unchanged, but in animals treated with bretylium tosylate, the ratio increased more than 10-fold (from 0.48 +/- 0.1 to 5.49 +/- 0.32 at 29.9 degrees C). The demonstrated increase in catecholamine levels during hypothermia suggests that the protection offered by bretylium tosylate against cardiac arrhythmias is not explained by modification of catecholamine levels, and is more likely due to an alteration of the electrophysiologic properties of cardiac tissues.

Effects of Mu receptor agonists and droperidol on motor coordination in mice

The effects of morphine, fentanyl, sufentanil and droperidol on motor coordination in mice were studied. Animals were trained to complete successfully the rotarod test before assessing the effects of drugs. Administration of analgesic doses of the mu agonists morphine, fentanyl and sufentanil did not inhibit motor coordination. Droperidol produced a dose related inhibition of motor coordination. When a subthreshold dose of droperidol was administered followed by an opiate, a significant inhibition of motor coordination was observed. The results indicate that although analgesic doses of mu opioid agonists do not affect motor coordination, their combination with droperidol results in motor incoordination. The mechanisms and/or opioid receptor sub-types involved in this in vivo interaction remain to be established.

High doses of L-naloxone but neither D-naloxone nor β-funaltrexamine prevent hyperthermia-induced seizures in rat pups

Abstract— The effects of the non‐specific opiate antagonist L‐naloxone and the inactive isomer D‐naloxone, as well as the specific mu receptor antagonist β‐funaltrexamine, have been examined on hyperthermia‐induced seizures in unrestrained 15 days old rats. Saline‐injected animals exposed to an ambient temperature of 40°C showed a gradual increase in body temperature reaching a maximum of 42±0.1°C at 50 min exposure. At this time all the pups had seizures and died. Similar results were obtained when the animals were pretreated with different doses of D‐naloxone and β‐funaltrexamine. Rats pretreated with L‐naloxone also showed an increase in rectal temperature; but the temperature was lower than in saline‐injected animals. Only high doses of L‐naloxone prevented seizures and deaths. These data indicate that endogenous opioid peptides may play a role in seizures induced by hyperthermia and that receptors other than mu receptors could be involved in hyperthermia‐induced seizures.

κ-Opiate Agonist-Induced Inhibition of Gastrointestinal Transit in Different Strains of Mice

The effects of kappa-opiate agonists on gastrointestinal motility was assessed in Swiss Webster, C57BL/6, BALB/c and DBA/2 strains of mice. The kappa-agonists, PD 117302, U-69593 and U-50488H (3 mg kg-1), were injected subcutaneously and the distance travelled by a charcoal meal in the gastrointestinal tract was measured. All kappa-agonists induced significant inhibition of charcoal meal transit; however, there were significant strain differences in the antitransit effect. It is concluded that kappa-receptors are involved in the inhibition of gastrointestinal transit and that the negative data reported in the literature may be due to a genotype-dependent effect of kappa-agonists.