A comparison of the receptor constants of morphine and ethylketocyclazocine for analgesia and inhibition of gastrointestinal transit in the rat

Reversal by kappa-agonists of peritoneal irritation-induced ileus and visceral pain in rats

Peritoneal irritation in rats induced by i.p. administration of acetic acid produces abdominal contractions reflecting visceral pain, and gastrointestinal ileus characterized by inhibition of gastric emptying and small intestine transit. In this study, gastric emptying (GE) and intestinal transit, calculated by the geometric center (GC) method, were estimated using a test meal labeled with 51Cr-EDTA. Visceral pain was assessed by counting abdominal contractions. Acetic acid produced abdominal contractions (80.8 +/- 3.3) and inhibition of GE (-54%) and GC (-63%) during the test-period. The kappa-opioid receptor agonists, CI-977 (+/-)-U-50,488H, (+/-)-bremazocine, PD-117,302, (-)-cyclazocine, and U-69,583, reversed abdominal contractions and inhibitions of gastrointestinal transit in a dose-related manner. The mu-opioid receptor agonists and potent analgesics, morphine and fentanyl did not restore normal gastric emptying and intestinal transit. These data suggest that selective kappa-opioid receptor agonists might be used to treat abdominal pain associated with motility and transit impairment during postoperative ileus.

Hypothalamus involvement in the reticulo-rumen motor and behavioural disturbances induced by morphine in sheep

Morphine (20 and 40 micrograms/kg) administered into the cerebral ventricle of conscious sheep caused significant inhibition of the mean frequency and the average amplitude of primary ruminal contractions by 45 min after injection. Between 90 and 120 min, morphine (40 micrograms) provoked a significant increase in the amplitude (p < 0.01). At both doses it caused strong psychomotor excitability that lasted for more than 140 min. Isolation of the hypothalamus prevented both the inhibitory effects of morphine on rumen motility and the drug-induced psychomotor excitability. Histopathological analysis of slices of the hypothalamus, pons and medulla indicated descending degenerative changes in the nervous pathways connecting the hypothalamus with lower structures in the brain. These results suggest either that hypothalamic isolation caused degeneration of inhibitory descending pathways that connect the hypothalamus with the gastric centres or that structures of importance for forestomach motility are not located within the gastric centres but elsewhere in the brain, for example in the hypothalamus.

Action of opiates on gastrointestinal function

Opioid peptides and opioid receptors are distributed along the gastrointestinal (GI) tract, indicating endogenous opiates released peripherally may modulate GI motor and secretory functions. Animal studies have revealed that the effects of opiates on gut motility depend on the nature of the subclasses of receptor involved, the species and the part of bowel. Most opiates that have a selective or predominant mu agonist activity inhibit gastric motility and delay gastric emptying by acting centrally; delta and kappa agonist are inactive when injected systemically. The effect of opiates in delaying intestinal transit observed in man, rat and other species is related to an inhibition (rat) or a stimulation (dog and man) of intestinal contractions as premature phase III-like sequences. The constipating effects of morphine probably result mainly from its action on colonic motility. Morphine stimulates colonic motility in humans by action on both central and peripheral sites. This increase in colonic motility and the delay in colonic transit is associated with a reinforcement of tonic contractions and reduced propulsive waves. Opioid peptides have been shown to participate in the colonic motor response to eating in man and animals. Both delta and mu receptors are involved in the stimulatory effects of opiates on colonic motility, while kappa receptors inhibit colonic contractions, mainly by acting centrally. The effects of opiates on gastric acid secretion are still controversial but it has been well demonstrated that opiates act centrally to reduce pancreatic secretion in rats. Opiates also inhibit intestinal secretions via an action on the enteric nervous system as well as in the CNS. All these results reinforce the hypothesis that opioid peptides have a major physiological role in the control of gut motility and secretions, and these actions explain most of the pharmacological effects of opiate substances on the digestive tract.

Effect of beta-funaltrexamine on opioid side-effects produced by morphine and U-50, 488H

Pretreatment of rats with the irreversible mu-opioid receptor antagonist, beta-funaltrexamine (beta-FNA), 20-40 mg kg-1 s.c., produced a dose-related antagonism of the reduction in respiratory rate, gastrointestinal (GI) propulsion, rotarod reaction latencies and body temperature produced by morphine administration 24 h later, suggesting that these effects are mediated via mu-opioid receptors. The kappa-receptor agonist, U-50,488H, was without effect on respiratory rate at the doses tested, but produced hypothermia, sedation and low maximum inhibition of GI propulsion. These effects of U-50,488H were not blocked by beta-FNA suggesting that they are mediated via kappa-receptors.

Bremazocine induces antinociception, but prevents opioid-induced constipation and catatonia in rats and precipitates withdrawal in morphine-dependent rats

Some in vivo agonist and antagonist properties of the putative k-compound bremazocine were characterized in rats. Bremazocine, at doses from 0.015-32 mg/kg i.p., delayed nociceptive reaction on a 55 degrees C hot-plate with a dose-response curve not readily fitting a single straight line; this effect was antagonized by high doses of naloxone. In the same rats bremazocine did not delay the intestinal transit of a charcoal meal fed 5 min earlier and prevented morphine-induced constipation. This antagonism appeared to be opioid-specific and competitive, with apparent pA2 value 8.56. Catatonia induced by etorphine (0.004 mg/kg s.c.) and constipation induced by etorphine (0.004 mg/kg s.c.) and D-Ala2-D-Leu5-enkephalin (0.1 mg/kg i.p.) were completely antagonized by bremazocine (0.03-8 mg/kg i.p.). Antinociception induced by morphine (10 mg/kg i.v.) and etorphine (0.004 mg/kg s.c.) was only partly prevented. Naloxone (1 mg/kg) and bremazocine (0.015-1 mg/kg i.p.) precipitated a withdrawal syndrome, evaluated as jumping frequency, in rats rendered dependent to morphine. These data suggest the involvement of more than one opioid receptor population in bremazocine action in vivo.

The opioid kappa-selective compound U-50,488H does not inhibit intestinal propulsion in rats

The selective kappa-compound U-50,488H, at doses producing strong central pharmacological effects (0.5-32 mg kg-1 i.p.) did not delay the intestinal transit of a charcoal meal and had little or no antagonist action against morphine-induced constipation. It appears that kappa-opioid receptors are probably not involved in the mechanisms responsible for opioid-induced constipation.

A species difference in the slowing effect of intrathecal morphine on gastrointestinal transit

Intrathecal administration of morphine, levorphanol, bremazocine, ethylketocyclazocine or [D-Ser2,Leu5,Thr6]enkephalin to rats, at doses 10-50 times greater than that necessary to elicit analgesia in the tail flick test, had no marked effect on gastrointestinal transit as determined by the charcoal meal test. In contrast, intrathecal administration of various doses of morphine to mice significantly antagonized transit (A50 (that dose which inhibited transit to 50% of controls) = 14.7 (0.71-2.89) micrograms/mouse). These results suggest (1) a lack of involvement of opioid sensitive spinal structures in the control of gastrointestinal transit in rats, and (2) a species difference in the slowing effect of intrathecal morphine on gastrointestinal transit.