K. Ramabadran

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.

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.

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.

Intracerebroventricular administration of κ-agonists induces convulsions in mice

Abstract Intracerebroventricular (ICV) administration of κ-agonists (PD 117302, U-50488H and U-69593) induced convulsions in a dose-related manner in mice. The dose at which 50% of animals convulsed (CD 50 ) was in nmol ranges for all opioids. Among the opioids used, PD 117302 was the most potent convulsant. ICV administration of either vehicle alone or U-53445E, a non-κ-opioid ( + ) enantiomer of U-50488H did not induce convulsions. The convulsive response of κ-agonists was differentially susceptible for antagonism by naloxone and/or MR 2266. Collectively, these findings support the view that convulsions induced by κ-agonists in mice involve stereospecific opioid receptor mechanisms. Furthermore, the convulsant effect of κ-agonists could not be modified by pretreatment with MK-801, ketamine, muscimol or baclofen. It is concluded that κ-opioid but not NMDA or GABA receptor mechanisms are involved in convulsions induced by κ-agonists. These results are the first experimental evidence implicating stereospecific κ-receptor mechanisms in opioid-induced convulsions in mice.

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.

Effects of the benzomorphan κ-opiate, MR 2266 and its (+) enantiomer MR 2267, on thermonociceptive reactions in different strains of mice

The effects of the benzomorphan kappa opiate MR 2266 and its dextro enantiomer MR 2267 were assessed on thermonociception in Swiss Webster, C57BL/6, BALB/c and DBA/2 strains of mice. In the hot plate (60 +/- 0.5 degrees C, cut-off time 120 s and tail immersion tests, MR 2266 (10 mg/kg, s.c., 15 min before) decreased, while MR 2267 (10 mg/kg s.c., 15 min before) increased the reaction latencies. In the hot plate test, the sensitivities for the effects of MR 2266 and MR 2267 on jump latency in different strains of mice were as follows: MR 2266; BALB greater than Swiss greater than C57BL greater than DBA and MR 2267; DBA greater than BALB = Swiss greater than C57BL. In the immersion test, for the hyperalgesic response of MR 2266, the rank order of strains was; BALB greater than C57BL and DBA greater than or equal to Swiss while the rank order for the analgesic effect of MR 2267 was; Swiss greater than DBA and BALB. The results indicate the presence of tonic kappa-receptor-mediated regulation of the spinal and supra-spinal thermonociceptive reactions which is stereospecific and strain dependent.

Effect of Yohimbine on Nociceptive threshold in normoglycemic and streptozotocin-treated hyperglycemic mice ☆

The effects of yohimbine (0.1, 1, 3 and 10 mg/kg SC) on nociceptive threshold were tested in mice using the tail-immersion and hot-plate tests. The tail-flick (withdrawal) latency, a monosynaptic spinal nociceptive response, was significantly lowered by yohimbine. This hyperalgesic response was at its peak 0.5 hr after yohimbine injection. The tail-flick latencies expressed as % of basal latencies were, 95 +/- 8, 100 +/- 10, 62 +/- 10, 33 +/- 7 and 28 +/- 4 in vehicle and 0.1, 1, 3 and 10 mg/kg in yohimbine-treated groups respectively. Yohimbine-induced hyperalgesia was observed when stimulus temperature was either 50 degrees C or 45 degrees C; however, the opiate antagonist naloxone (3 mg/kg SC) induced a hyperalgesic response at 50 degrees C and an analgesic response at 45 degrees C stimulus temperature. Streptozotocin-induced hyperglycemia did not influence the hyperalgesic response of yohimbine. In the hot-plate (60 degrees C) test, which involves higher centers and a polysynaptic nociceptive reflex, yohimbine did not modify the jump latency. The data provide evidence for the presence of a tonic spinal noradrenergic inhibitory control of nociceptive mechanism(s) which does not appear to be readily altered by hyperglycemia.

κ-Opioid receptor-mediated thermonociceptive mechanisms in streptozotocin diabetes

The effects of the benzomorphan kappa-opiate antagonist MR 2266 and its dextro enantiomer MR 2267 were assessed on thermonociception in male Swiss Webster mice. Experimental diabetes was induced by injecting streptozotocin (200 mg/kg IP, 7-8 days before). Animals with dextrose treatment (5 g/kg, IP, at the time of opiate injection) were used as acute hyperglycemic controls. Nociception was assessed by supraspinal nociceptive reflex (licking and jumping in hot plate test) indicative of higher cognitive process as well as a predominantly lower spinal monosynaptic reflex (tail immersion test). In normoglycemic, acute hyperglycemic and diabetic mice, MR 2266 decreased, while MR 2267 increased, the reaction latencies. The results indicate tonic stereospecific kappa-opiate receptor-mediated spinal and supraspinal thermonociceptive reactions are not modulated by experimental diabetes and thus are distinct from those of naloxone-sensitive mu-opiate receptor sites.