Hyperglycaemia as a factor affecting kappa-opiate agonist-induced inhibition of the gastrointestinal transit in mice

mu-Opioid receptor-knockout mice: the role of mu-opioid receptor in gastrointestinal transit

The role of mu-opioid receptor in gastrointestinal transit was investigated using mu-opioid receptor knockout mice (MOR-KO). Our result establishes unequivocally that inhibition of GI transit by morphine is a mu-opioid receptor mediated function. In addition, we show that neither delta nor kappa receptor agonist given supraspinally or peripherally are able to inhibit GI transit in MOR-KO animals. It was interesting to observe that basal GI motility was lower in MOR-KO (-/-) compared to heterozygous (+/-) and wild type (+/+) animals.

The inhibitory effects of alpha(2)-adrenoceptor agonists on gastrointestinal transit during croton oil-induced intestinal inflammation

1. The peripheral effects of alpha(2)-adrenoceptor agonists were investigated in a model of intestinal inflammation induced by intragastric administration of croton oil (CO). Our hypothesis was that inflammation would 'sensitize' adrenoceptors in peripheral and/or central terminals of myenteric and submucous plexus neurones, and enhance systemic effects of alpha(2)-adrenoceptor agonists. 2. Male swiss CD-1 mice, received intragastrically CO (0.05 ml), castor oil (CA, 0.1 ml) or saline (SS) 3 h before the study: gastrointestinal transit (GIT) was evaluated 20 min afterwards with a charcoal meal. The presence of inflammation was assessed by electron microscopy. 3. The intragastric administration of CA or CO caused an increase in GIT and weight loss, but only CO induced an inflammatory response. Both clonidine (imidazoline1/alpha(2)-agonist) and UK-14304 (alpha(2)-agonist) produced dose-related inhibitions of GIT in all groups. During inflammatory diarrhoea (CO), potencies of systemic (s.c.) clonidine and UK-14304 were significantly increased 3.5 and 2.1 times, respectively, while potencies remained unaltered in the presence of diarrhoea without inflammation (CA). The effects were reversed by administration (s.c.) of receptor-specific adrenoceptor antagonists, but not by naloxone. 4. Clonidine was 8.3 (SS) and 2.8 (CO) times more potent when administered intracerebroventricularly (i.c.v.), than when administered s.c. Inflammation of the gut did not alter the potency of i.c.v. clonidine, demonstrating that enhanced effects of s.c. clonidine are mediated by peripheral receptors. During inflammation, i.c.v. efaroxan did not antagonize low doses of s.c. clonidine (ED20 and ED50S), but partially reversed ED80S, further supporting the peripheral effects of the agonists in CO treated animals. 5. The results demonstrate that inflammation of the gut enhances the potency of alpha(2)-adrenoceptor agonists by a peripheral mechanism. The results also suggest that the inflammatory response induces an up-regulation or sensitization of alpha(2)-adrenoceptors and/or imidazoline receptors.

Influence of blood glucose levels on rat liquid gastric emptying

The glycemic influence on liquid gastric emptying in rats was studied. Diabetic hyperglycemia was induced by streptozotocin-treated rats further received a daily insulin injection ( 2.5 or 10 IU/kg). Immediate hyperglycemia was induced in a separate group of rats by continuous intravenous glucose infusion (44 or 88 mg/kg/min) 10 min before the experiment. Rats were killed 15 min after radiochromium feeding; then the radioactivity of stomach and small intestine were counted to obtain the gastric emptying value. Emptying in diabetic rats was delayed compared with controls (mean +/- SE: 40.9 +/- 2.6% vs. 54.2 +/- 2.8%, P < 0.01). Low-dose insulin treatment reversed the impairment, while high-dose treatment even enhanced emptying. Immediate hyperglycemia induced with two glucose infusions also inhibited gastric emptying. Present results indicate that hyperglycemia elicited with any hyperglycemic model is at least one of the important mechanisms to delay liquid gastric emptying.

Hyperglycaemia is responsible for the inhibited gastrointestinal transit in the early diabetic rat

The effect of plasma glucose levels on the gastrointestinal motility of the rat was studied. Chronic hyperglycaemia was induced by i.v. injection of streptozotocin 1 week before the motility experiment. Some rats received additional daily insulin therapy (1.25, 2.5 or 10 IU kg-1) after induction of diabetes mellitus. Acute hyperglycaemia was induced by the continuous i.v. infusion of glucose solution (11, 22, 44 or 88 mg kg-1 min-1) 10 min before the motility experiments. The rats were killed 15 min after successful orogastric feeding of a charcoal-contained suspension. Gastrointestinal transit was calculated as the percentage of the overall length of the small intestine to which the charcoal moved during this time period. The diabetic rats were found to have delayed transit compared with controls (mean +/- SEM: 32.2 +/- 2.1% vs. 42.9 +/- 4.2%, P < 0.05). Correction of hyperglycaemia with moderate doses of insulin therapy failed to inhibit transit, whereas hypoglycaemia induced by high-dose insulin treatment enhanced transit. High doses of glucose elicited acute hyperglycaemia and delayed transit when compared with saline infused non-diabetic rats. In early diabetes, hyperglycaemia probably mediates the inhibited gastrointestinal transit, since correction of hyperglycaemia usually restores the delayed transit.

Pertussis and cholera toxins modulate kappa-opioid receptor agonists-induced hypothermia and gut inhibition

In mice pretreated intracerebroventricularly (i.c.v.) with either saline (10 microliters/mouse), pertussis (1 microgram/mouse) or cholera (2.5 micrograms/mouse) toxins, effect of kappa-opioid receptor agonists on the colonic temperature and charcoal meal transit time were assessed. The kappa-opioid receptor agonist, trans-(+)-3,4-dichloro-N-methyl-[2-(1- pyrrolidinyl)cyclohexyl]-benzeneacetamide methane sulfonate hydrate (U-50488H, 50, 100 and 200 micrograms/mouse, i.c.v.) produced dose dependent hypothermia. Pertussis toxin pretreatment (72 and/or 144 h before) antagonized (P < 0.05) the hypothermic effect of U-50488H (100 micrograms/mouse) and (+)-trans-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl[benz[b]-thio-phene-4-acetamide (PD 117302, 30 micrograms/mouse). In contrast, cholera toxin pretreatment (48 and/or 96 h before) did not antagonize the hypothermic effect of the kappa-opioid receptor agonists. Moreover, both i.c.v. and intrathecal (i.t.) administration of kappa-opioid receptor agonists, U-50488H, }[5R-(5 alpha,7 alpha,8 beta)]-(+/-)-N-methyl-N-[7-(1- pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]-benzeneacetamide¿ (U-69593) and PD 117302, produced dose dependent inhibition of the charcoal meal transit. Cholera toxin pretreatment (48 and 96 h before) augmented (P < 0.05) the antitransit effect of i.c.v. administered U-50488H (100 micrograms/mouse), U-69593 (100 micrograms/mouse) and PD 117302 (50 micrograms/mouse). However, pertussis toxin pretreatment did not affect the gastrointestinal inhibitory effect of the kappa-opioid receptor agonists. The present results extend our previous results on the effect of kappa-selective agonists on gastrointestinal motility and indicate, like the prototype opiate agonist morphine, kappa-opioid receptor agonists are effective in inhibiting the gastrointestinal motility when administered either by intrathecal or intracerebroventricular routes.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracerebroventricular administration of kappa-agonists induces convulsions in mice

Intracerebroventricular (ICV) administration of kappa-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 (CD50) 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-kappa-opioid (+) enantiomer of U-50488H did not induce convulsions. The convulsive response of kappa-agonists was differentially susceptible for antagonism by naloxone and/or MR 2266. Collectively, these findings support the view that convulsions induced by kappa-agonists in mice involve stereospecific opioid receptor mechanisms. Furthermore, the convulsant effect of kappa-agonists could not be modified by pretreatment with MK-801, ketamine, muscimol or baclofen. It is concluded that kappa-opioid but not NMDA or GABA receptor mechanisms are involved in convulsions induced by kappa-agonists. These results are the first experimental evidence implicating stereospecific kappa-receptor mechanisms in opioid-induced convulsions in mice.

Attenuation of central alpha 2 adrenergic action in diabetic rats

Molecular components in transmembrane signaling may be dysfunctional in insulin-deficient states. To investigate whether the alpha 2 adrenergic receptor-effector mechanism is functionally altered by insulin deficiency, we determined the hypnotic response to dexmedetomidine [(+)4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole, a highly-selective alpha 2 agonist, in streptozotocin-induced diabetic rats. The duration of the loss of righting reflex (sleep time) in response to dexmedetomidine, 0.25 mg.kg-1 IP, was measured in rats pretreated with streptozotocin, 50 mg.kg-1 IP. Dexmedetomidine sleep time was significantly shortened when tested 10 days (-25%), 3 (-29%), 6 (-35%) and 8 (-47%) weeks into the diabetic state. Supplementation of the diabetic rats with insulin normalized alpha 2 responsiveness. Acute hyperglycemia did not reduce dexmedetomidine-induced sleep time. Sleep time was also reduced when dexmedetomidine was administered via the intracerebroventricular (ICV) route at 4 (-21%) and 8 (-29%) weeks after streptozotocin. Thus the central nervous system response to the alpha 2 adrenergic agonist has become attenuated. The mechanism may involve a perturbation of an insulin-sensitive molecular component of the signal transduction pathway responsible for alpha 2 adrenoceptor-mediated anesthetic action.

Kappa-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.

Streptozotocin-diabetes attenuates alpha 2-adrenoceptor agonist-induced delay in small intestinal transit in mice

1. The effect of alpha 2-adrenoceptor agonists on gastrointestinal motility was assessed in normoglycaemic and streptozotocin-diabetic mice. 2. The alpha 2-adrenoceptor agonists used were: clonidine (0.1, 0.3 and 1 mg kg-1, azepexole (10, 20 and 40 mg kg-1), tizanidine (1, 3 and 10 mg kg-1) and ST-91 (10, 20 and 30 mg kg-1). 3. Acute hyperglycaemia was induced by D-(+)-glucose (5 g kg-1) and chronic hyperglycaemia by streptozotocin (200 mg kg-1) injection. 4. The gut motility was quantitated using the charcoal meal test. 5. The results indicate that in normoglycaemic and acutely hyperglycaemic mice, all of the alpha 2-adrenoceptor agonists used produced significant inhibition of meal transit. 6. However, in streptozotocin-diabetic mice, the anti-transit effect of alpha 2-adrenoceptor agonists was attenuated. 7. Since streptozotocin-induced diabetes but not acute hyperglycaemia was associated with the attenuation of anti-transit effect, elevated blood sugar is not the mechanism for the observed effect. 8. As with groups treated with clonidine, azepexole or tizanidine, the anti-transit effect of a peripherally acting alpha 2-adrenoceptor agonist, ST-91, was attenuated in streptozotocin-diabetic mice. This suggests the involvement of peripheral mechanism(s) in attenuating the anti-transit effect of alpha 2-adrenoceptor agonists. 9. These results identify the need for critical evaluation of the role and efficacy of alpha 2-adrenoceptor agonists in the therapeutic management of diabetic diarrhoea.

Effects of kappa- and mu-selective opiate agonists on colonic temperature in normoglycaemic and streptozotocin-treated hyperglycaemic mice

The thermic responses of highly selective k-(U-69593 and U-50488H) and mu-(sufentanil and fentanyl) agonists were assessed in normoglycaemic and hyperglycaemic mice. Hyperglycaemia was induced by streptozotocin injection. The opiates were injected subcutaneously and colonic temperatures monitored for 90 min. after injection. In normoglycaemic animals the k-agonists induced a significant hypothermic response and hyperglycaemia did not modify this effect. In normoglycaemic mice, low doses of mu-selective agonists predominantly produced hypothermia, while in high doses resulted in hyperthermia. In chronic hyperglycaemic mice, hyperthermia was the predominant effect of mu-opiates. The present results characterize the time course and dose response of some of the currently available highly selective kappa- and mu-opiate agonists for the first time in mice and suggest that hyperglycaemia differentially affects the opiate receptor subtypes that mediate thermoregulatory events.