On the criteria for classifying opiate agonists in rats

Antidiarrheal and colonic antipropulsive effects of spinal and supraspinal administration of the natural delta opioid receptor agonist, [D-Ala2]deltorphin II, in the rat

The ability of the natural selective delta opioid receptor agonist, [D-Ala2]deltorphin II (DADELT II), to inhibit the diarrhea induced by castor oil and colonic glass bead expulsion, was studied in rats after supraspinal and spinal administration. When injected intracerebroventricularly, DADELT II (0.2, 1, 10 micrograms/rat) inhibited diarrhea and colonic bead expulsion in a dose-related fashion but did not affect the rate of small intestine transit. Similar results were obtained when the same dose of DADELT II was administered spinally. The antidiarrheal and colonic antipropulsive effects of supraspinally and spinally DADELT II were partially or completely antagonized by subcutaneous pretreatment with 1 and 10 mg/kg, respectively, of naltrindole, a selective delta opioid receptor antagonist. These findings indicate that, in the rat, supraspinal and spinal delta opioid receptors play a role in modulating diarrhea and colonic propulsion and that DADELT II is a useful tool for investigating the role of the delta opioid system in gastrointestinal function.

Discriminative and analgesic effects of mu and kappa opioids: in vivo pA2 analysis

This paper illustrates the use of antagonists to study receptor mediation of the discriminative stimulus effects of opioids and to determine if their analgesic effects are mediated in a similar manner. Analysis by pA2 was used to quantify interactions between the opioid antagonist quadazocine and several opioid agonists. Interactions were examined under a drug discrimination procedure and under a tail withdrawal analgesia procedure. Values of pA2 for quadazocine in combination with the kappa agonists bremazocine, ethylketazocine, and U50,488 ranged between 6.1 and 6.4 under the drug discrimination and tail withdrawal procedures. In contrast, the pA2 values for the mu agonists etorphine, fentanyl, and morphine under these procedures ranged between 7.6 and 8.2. The difference between these pA2 values indicates that the affinity of quadazocine for the receptors mediating the effects of the kappa agonists is over one log unit lower than its affinity for the receptors mediating the effects of the mu agonists. This suggests that the discriminative stimulus and analgesic effects of kappa opioid agonists are mediated at the same opioid receptor type, which is different from the type of opioid receptor at which mu agonists produce their discriminative stimulus and analgesic effects.

The occurrence of cross-tolerance between morphine and ethyl-ketocyclazocine using the tail-flick test: lack of effect of diazepam, phenobarbital, or amphetamine

Experiments were designed to test for short-term tolerance to morphine and ethyl-ketocyclazocine (EKC), mu and kappa agonists, respectively, and cross-tolerance between the two drugs. Mice were primed with one of the drugs, using doses that did not affect the tail-flick response when tested at a time 1 or 3 hours later, when the same or alternate test drug was administered. All animals were injected with the priming drug IP. In one series of experiments, the test drugs were given SC, and in the other, the test drugs were injected ICV under brief halothane anesthesia. Priming with morphine (30 or 100 mg/kg) significantly raised the ED50 for ICV morphine. Priming with EKC (2 or 6 mg/kg) similarly elevated the ED50's for SC and ICV EKC. Symmetrical cross--tolerance was produced in experiments where the test drugs were administered SC when tested at 3 hrs. The effects of priming with EKC on morphine analgesia was evident when the interval between priming and test drugs was 1 hour. When the test drugs were given ICV, cross-tolerance was also symmetrical: priming with EKC significantly raised the ED50 for morphine and priming with morphine raised the ED50 for EKC when tested at 3 hrs. These data suggest that both agonists act on a common site to produce analgesia as similar pA2 values for naloxone antagonism were determined. The occurrence of short-term tolerance and cross-tolerance to the opiates was unaltered by chronic pretreatment with diazepam, phenobarbital, or amphetamine.

Cross-tolerance between morphine- and bombesin-induced inhibition of intestinal transit in rats

Intracerebroventricular (i.c.v.) injection of either morphine or bombesin to rats inhibits intestinal transit of an intraduodenally administered radiochromium marker. In this work, we show that tolerance develops to this effect of bombesin after i.c.v. infusion of the peptide (0.5 micrograms/h for 4 days via an s.c. implanted Alzet 2001 osmotic minipump). Tolerance also develops to the inhibition of intestinal transit associated with i.c.v. morphine after s.c. injections of morphine. Bombesin-induced delay of transit is not attenuated by naltrexone (10 mg/kg, s.c.), a standard narcotic antagonist. Nevertheless, two-way cross-tolerance develops between bombesin and morphine in this system. This is a surprising result since both bombesin and morphine are believed to act on different receptors and cause opposite effects on intestinal motility in rats.

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.

Ketazocines and morphine: effects on gastrointestinal transit after central and peripheral administration

The mu agonist, morphine, and the prototype kappa agonists, ketocyclazocine and ethylketocyclazocine (EK), were studied for their effects on gastrointestinal transit. Following s.c. administration, both morphine (0.3-3 mg/kg) and ketocyclazocine (0.3-10 mg/kg) antagonized transit of an opaque marker through the small intestines of mice. Morphine (0.1-1 microgram) was also effective after intracerebroventricular (icv) administration in mice whereas ketocyclazocine (0.3-30 micrograms) was not. Similarly, while both morphine (0.3-5 mg/kg) and EK (0.6-10 mg/kg) slowed transit after s.c. injection to rats, only morphine (1-10 micrograms), but not EK (0.3-300 micrograms), was active following icv administration. Icv infusion of the mu benzomorphan, phenazocine (10-100 micrograms), slowed transit in a dose-related manner. These results indicate that there may be an anatomically distinct distribution of receptors for benzomorphan kappa agonists in both the mouse and rat, with these opiate receptors not being located near the lateral cerebral ventricles. The difference in efficacy between morphine and ketazocines in slowing gastrointestinal transit after icv administration to rodents suggests that (a) inactivity in this endpoint is a characteristic of benzomorphan kappa compounds and (b) the model may serve as a useful screen when establishing in vivo profiles of kappa agonists in mice and rats.

Agonist and antagonist properties of some benzomorphans on nociceptive reaction and intestinal transit in rats

The agonist and antagonist properties of bremazocine, ethylketazocine, Mr 2034 and Mr 2092, benzomorphans with the same pharmacological actions in vitro, were studied in vivo in rats. Overnight fasted animals were tested for nociceptive reaction on a 55 degrees C hot-plate and for the intestinal transit of a charcoal meal fed 5 min earlier. The antagonist activity was assessed by the drugs' ability to prevent morphine-induced constipation. Bremazocine was an agonist on the nociceptive reaction and a pure antagonist on intestinal propulsion; ethylketazocine and Mr 2034 were pure agonists in both tests and Mr 2092 was a mixed agonist-antagonist in the intestinal transit test. Thus the in vivo agonist-antagonist properties of opiates cannot always be predicted from their in vitro characteristics.

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

The efficacies and dissociation constants of proposed mu and kappa receptor agonists (morphine and ethylketocyclazocine, respectively) were compared using the method of partial irreversible blockade (with buprenorphine) and Stephenson's theory of drug action. While there was good agreement between the dissociation constant (KA) of morphine in analgesia (3.3 x 10(-5) M) and in inhibition of gastrointestinal transit (1.1 x 10(-5) M), the KA of ethylketocyclazocine differed by an order of magnitude in these endpoints (3.2 x 10(-6) M and 6.7 x 10(-5) M, respectively). The efficacies of morphine were found to be similar for the two effects studied (4.23 and 5.26), while those for ethylketocyclazocine differed markedly (2.06 and 10.39). The fraction of receptors remaining unblocked after buprenorphine was consistent for the test but not for the agonist, indicating a different distribution of receptors for the two endpoints. Our results strongly suggest that morphine induces analgesia, and slows transit in the small intestine, through the same type of receptor. The same conclusion cannot be drawn for ethylketocyclazocine.