Kappa opioids in rhesus monkeys. I. Diuresis, sedation, analgesia and discriminative stimulus effects.

The effects of the kappa agonists bremazocine, ethylketazocine, tifluadom and U-50,488 were examined in rhesus monkeys with four experimental procedures: urinary output was measured in normally hydrated monkeys; muscle relaxation and stupor were observed; and the time it took monkeys to withdraw their tails from warm water was evaluated. Lastly, the kappa agonists were examined in monkeys trained to respond differentially in the presence or absence of ethylketazocine. For comparison, morphine and another kappa agonist, Mr 2033, were examined under some of the procedures. Kappa agonists produced dose-dependent increases in urine output whereas morphine did not. Morphine, as well as each of the kappa agonists examined, produced dose-dependent increases in tail-withdrawal latencies from both 50 and 55 degrees C water; analgesic doses of kappa agonists produced marked stupor and muscle relaxation. Kappa agonists produced ethylketazocine-appropriate responding in the drug discrimination procedure. Doses of kappa agonists that produced ethylketazocine-appropriate responding were similar to doses that increased urine output and smaller than doses that increased tail-withdrawal latencies. The opioid antagonist quadazocine (WIN 44,441) antagonized the effects of bremazocine, ethylketazocine, tifluadom and U-50,488.

Kappa opioids in rhesus monkeys. II. Analysis of the antagonistic actions of quadazocine and beta-funaltrexamine.

In rhesus monkeys, kappa opioid agonists have been shown to increase urinary output, increase tail-withdrawal latencies from warm water and produce distinct discriminative stimulus effects. In order to explore further the relation between these effects and activity at the kappa opioid receptor type, the antagonist activity of quadazocine against several kappa opioid agonists was examined with the tail-withdrawal and drug-discrimination procedures. Quadazocine dose dependently antagonized the increases in tail-withdrawal latency produced by the kappa agonists bremazocine, ethylketazocine and U-50, 488, as well as the discriminative stimulus effects of these drugs. The dose-ratio analysis of Schild revealed apparent pA2 values for quadazocine in combination with bremazocine, ethylketazocine and U-50, 488 of 6.1, 6.4 and 6.4, respectively, with the tail-withdrawal procedure and 6.3, 6.4 and 6.1, respectively, with the drug-discrimination procedure. Quadazocine also antagonized the effects of a mu agonist (morphine) in the tail-withdrawal procedure, and the apparent pA2 value for these data was 8.2. The activity of the mu-selective alkylating agent, beta-funaltrexamine (beta-FNA), was examined alone and in combination with the kappa agonist ethylketazocine in the urinary-output, tail-withdrawal and drug-discrimination procedures. At about 30 to 60 min postinjection, beta-FNA alone produced ethylketazocine-appropriate responding under the drug-discrimination procedure and increased urine output but did not increase tail-withdrawal latencies. At 24 to 48 hr postinjection, beta-FNA did not antagonize effects of ethylketazocine in any of the three procedures.(ABSTRACT TRUNCATED AT 250 WORDS)

Kappa opioids in rhesus monkeys. III. Dependence associated with chronic administration.

The kappa opioid agonists, Mr 2033 and U-50, 488, or the mu opioid agonist, morphine, were administered chronically to three separate groups of rhesus monkeys. Tolerance developed to the overt signs of intoxication produced by each compound. Monkeys receiving morphine were not cross-tolerant to Mr 2033 or to U-50, 488, and monkeys receiving U-50, 488 were not cross-tolerant to morphine. Monkeys given Mr 2033 chronically were, however, cross-tolerant to morphine. When administration of U-50,488 was interrupted, or the monkeys receiving this compound were given an opioid antagonist, withdrawal behaviors were displayed that were qualitatively different from deprivation or antagonist-induced morphine withdrawal. These signs were suppressed by kappa agonists but not by morphine. Deprivation-induced withdrawal from Mr 2033 resulted in signs similar to those shown by U-50,488-dependent monkeys and some signs were observed in withdrawn morphine-dependent monkeys. Several antagonists, including the mu-selective antagonist beta-funaltrexamine, precipitated signs of withdrawal normally associated with morphine dependence in Mr 2033-dependent monkeys. Withdrawal from Mr 2033 was suppressed by kappa agonists in a stereoselective manner, and by morphine. The asymmetrical cross-tolerance and cross-dependence between Mr 2033 and morphine, and the appearance of morphine-like signs during precipitated withdrawal, suggest that Mr 2033 is kappa receptor selective but not specific. Dependence to U-50,488, however, was qualitatively and pharmacologically distinct from morphine-dependence and is apparently a consequence of specific activity at kappa receptors.

Kappa receptor mediated opioid dependence in rhesus monkeys.

The kappa receptor-selective agonist U-50, 488 was administered chronically to rhesus monkeys. Tolerance developed to the overt behavioral effects of U-50,488 without cross-tolerance to morphine. Withdrawal behaviors produced by deprivation, naloxone or quadazocine administration in U-50, 488-dependent monkeys consisted of hyperactivity, excessive grooming, and yawning. The syndrome was suppressed in a dose-related manner by a kappa agonist, ethylketazocine, but not by doses of morphine that suppressed its own withdrawal. The mu-selective antagonist, beta-funaltrexamine, at doses which are active in morphine-dependent monkeys, did not precipitate withdrawal in U50, 488-dependent monkeys. Dependence, which is the result of activity at the kappa receptor, was distinct from morphine dependence.

Intrathecal bombesin-induced inhibition of gastrointestinal transit: requirement for an intact pituitary-adrenal axis.

The role of the pituitary-adrenal axis in the inhibition of gastrointestinal transit caused by intrathecal administration of bombesin was examined. Bombesin (0.3-10 micrograms) slowed transit by this route in a dose-related manner. Either hypophysectomy or adrenalectomy prevented the inhibition of gastrointestinal transit associated with bombesin (10 micrograms, i.th.). The inhibitory gut effects of this peptide were not prevented in sham-operated rats. Intrathecal bombesin-induced inhibition of gastrointestinal transit is thus dependent upon an intact pituitary-adrenal axis.

Independent central and peripheral mediation of morphine-induced inhibition of gastrointestinal transit in rats.

The individual contributions of central (brain) and peripheral (enteric) sites in the mediation of the systemic actions of opioids are not well established. In this study, we made use of naltrexone methobromide, a quaternary analog of naltrexone, to separate the central and peripheral components of the slowing action of morphine on gastrointestinal transit in rats. It was established that i.c.v., but not s.c., administration of quaternary naltrexone antagonized morphine-induced analgesia in the radiant-heat tail-flick assay in rats. Thus, quaternary naltrexone probably does not enter the central nervous system in significant amounts after systemic administration. Systemic quaternary naltrexone antagonized, in a dose-related manner, the delaying effects of morphine on the movement of a charcoal meal along the gastrointestinal tract. Quaternary naltrexone was 30 or 100 times less potent than naltrexone when administered s.c. or i.c.v., respectively. Unlike naltrexone, quaternary naltrexone antagonized morphine-induced slowing of gastrointestinal transit only when administered by the same route (i.e., both s.c. or both i.c.v.). The apparent pA2 for s.c. quaternary naltrexone against s.c. morphine was not significantly different from the apparent pA2 for i.c.v. quaternary naltrexone against i.c.v. morphine. Distinct and independent central and peripheral systems appear to mediate morphine-induced inhibition of gastrointestinal transit in rats. However, the receptors are probably of the same type. Peripherally selective antagonists such as quaternary naltrexone may be useful in reversing morphine-induced inhibition of gastrointestinal transit without affecting analgesia.

Intrathecal morphine slows gastrointestinal transit in rats.

Intrathecal (i.th.) (by direct lumbar puncture) and intraperitoneal (i.p.) administration of morphine (30-100 micrograms/rat) caused a dose-related inhibition of gastrointestinal transit in the rat. Pretreatment with i.th. naloxone (5 micrograms at -5 min) reversed the effects of i.th., but not i.p., morphine. These results suggest that the spinal cord appears to be a target site for the inhibitory effects of morphine on gastrointestinal transit in the rat.

Effects of beta-funaltrexamine in normal and morphine-dependent rhesus monkeys: observational studies.

The behavioral effects of the opioid receptor alkylating agent beta-funaltrexamine (beta-FNA) were assessed in normal (drug-naive) and morphine-dependent rhesus monkeys. In normal monkeys, beta-FNA (10 mg/kg s.c.) produced muscle relaxation and stupor, which could be reversed by the opioid antagonist Win 44,441. Given as a 48-hr pretreatment, beta-FNA antagonized the behavioral effects of acute morphine, but not those of two kappa agonists, ethylketazocine and Mr 2033 (UM 1072). In morphine-dependent monkeys, beta-FNA (10 mg/kg, s.c. and 0.003 mg i.c.v.) precipitated severe abstinence which lasted for 3 days. beta-FNA was more than 13,000 times more potent in precipitating withdrawal after i.c.v. than s.c. administration, whereas naltrexone and Win 44,441 were equipotent by these routes. Deprivation-induced abstinence (14 hr) and withdrawal of similar severity precipitated by naltrexone, Win 44,441 or naloxonazine were suppressed completely by 17.5 mg/kg of morphine. In contrast, 320 mg/kg of morphine failed to suppress completely a withdrawal syndrome of the same severity elicited by s.c. or i.c.v. beta-FNA. These data are consistent with the view that beta-FNA has reversible opioid agonist and insurmountable mu selective antagonist activity in the rhesus monkey.

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.

Effects of bombesin on behavior.

This report describes the influence of bombesin on the gross behavior of goldfish, frogs, mice, rats, guinea pigs, rabbits, chicks, pigeons and monkeys. Goldfish, frogs, chicks and pigeons were overtly unaffected by bombesin given centrally and/or peripherally. Mice, rats, guinea pigs, rabbits and monkeys responded quickly to intracerebroventricular (i.c.v.) and/or intrathecal (i.th.) administration of bombesin by displaying a range of behaviors suggestive of altered skin sensation. In mice, bombesin was essentially equipotent as a scratch inducer by i.c.v. and i.th. routes (A50 = 0.010-0.019 microgram) but 6800 times less potent i.p. In rats, bombesin-induced grooming and scratching behaviors were shown to be qualitatively different from those associated with ACTH-(1-24) and thyrotropin releasing hormone. Spantide and [D-Arg1, D-Pro2, D-Trp7,9, Leu11]substance P (both at 0.20, 0.50 and 0.80 microgram i.c.v.), two proposed bombesin receptor antagonists, did not markedly influence bombesin-induced scratching or hypothermia in rats.

Substitution and primary dependence studies in animals.

The mixed agonist-antagonist analgesics buprenorphine, butorphanol, nalbuphine, pentazocine and picenadol were compared to the prototype mu and kappa agonists morphine and Mr 2033, respectively, in the following tests in rhesus monkeys: overt behavioral effects upon acute administration in drug-naive animals; discriminative stimulus properties in monkeys trained to respond to either etorphine or ethylketazocine; self-administration of the test agent relative to codeine; single dose suppression and precipitation in withdrawn and non-withdrawn morphine-dependent monkeys, respectively; and primary addiction studies in drug-naive animals. Whereas both buprenorphine and nalbuphine precipitate withdrawal in morphine-dependent monkeys, withdrawal following chronic administration of buprenorphine resulted in no observable signs of abstinence, while nalbuphine withdrawal was similar to that seen in morphine-dependent monkeys. Butorphanol, pentazocine and picenadol all produced mild dependence of the kappa-type; that is, natural withdrawal behavior similar to that seen following chronic Mr 2033 administration.

The suppression of deprivation and antagonist-induced withdrawal in morphine-dependent rhesus monkeys.

The capacity of morphine to suppress natural and precipitated withdrawal was compared in morphine-dependent rhesus monkeys. A similar severity of withdrawal was induced by 14-hr deprivation or precipitated by naloxone, naltrexone, cyclazocine, Win 44,441 or MR 2266. Regardless of the procedure used to induce withdrawal, behavioral signs were completely suppressed by a cumulative dose of 17.5 mg/kg morphine. Thus, an equivalent level of withdrawal induced by reversible antagonists is as sensitive to subsequent morphine administration as is deprivation-induced abstinence. This is in accordance with the theory that vacancy of opiate receptors normally occupied by morphine is related to the level of abstinence observed. In contrast to Win 44,441, however, an equivalent level of withdrawal precipitated by buprenorphine required 175 mg/kg morphine for complete suppression. This is more informative than comparing duration of action; when given as 24-h pretreatments, a high dose of Win 44,441 (10 mg/kg) was only slightly less effective than buprenorphine (3.2 mg/kg) in antagonizing morphine-induced stupor in normal monkeys. Comparison of the ability of morphine to suppress precipitated withdrawal provides evidence of the relative reversibility of antagonists in vivo and demonstrates the extraordinarily stable nature by which buprenorphine acts at opiate receptors.

Pituitary-adrenal mediation of bombesin-induced inhibition of gastrointestinal transit in rats.

Centrally administered bombesin (0.1-3.5 micrograms, i.c.v.) inhibits gastrointestinal transit of a charcoal meal in a dose-related manner in rats. The roles of pituitary and adrenal glands in the mediation of this effect were assessed. The inhibition of gastrointestinal transit associated with bombesin (0.5 microgram, i.c.v.) was prevented by either hypophysectomy or adrenalectomy. Bombesin-induced inhibition of gastrointestinal transit is therefore mediated through the pituitary-adrenal axis. This is in contrast to bombesin-induced scratching and inhibition of gastric acid secretion which are not markedly influenced by either hypophysectomy or adrenalectomy.

In vivo evidence for benzomorphan-selective receptors in rats.

The scratching caused by a standard, submaximal dose of bombesin (0.10 microgram i.c.v.) in rats is antagonized in a stereospecific and dose-related manner by systemically (but not centrally) administered benzomorphan analgesics; other commonly used opioids and opioid peptides are ineffective at behaviorally nondepressent doses. Naloxone attenuates the antibombesin effect of ethylketocyclazocine in a stereospecific, potent and dose-related manner. Tolerance develops to the inhibitory action of ethylketocyclazocine (and phenazocine). Multiple injections of morphine do not influence the ability of ethylketocyclazocine or of phenazocine to antagonize bombesin. Furthermore, when mu opiate receptors are occluded by buprenorphine, ethylketocyclazocine and phenazocine can still antagonize bombesin-induced scratching. Benzomorphan-selective binding sites have previously been postulated; we suggest that this test provides evidence of such sites in vivo. The model affords a simple, yet novel, behavioral endpoint that can be used when defining the pharmacological profile of new benzomorphans and their antagonists.