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.

Rat cold water tail-flick: a novel analgesic test that distinguishes opioid agonists from mixed agonist-antagonists

The models currently used to assess antinociceptive efficacy in animals are far from ideal. Those procedures that detect both opioid agonists and mixed agonist-antagonists fail to differentiate between them unless the noxious stimulus is adjusted. Furthermore, changes in the sensitivity of the test often result in positive responses being elicited from agents that are either not analgesics or only weak ones, at best. The technique described in this report uses cold water as the noxious stimulus in rats. It is simple, requires no complicated instrumentation or training, correlates well with clinical efficacy in man, and allows separation of opioid agonists from mixed agonist-antagonists without detecting non-opioid agents.

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.

Mu and delta, but not kappa, opioid agonists induce contractions of the canine small intestine in vivo

Extraluminal strain gage transducers were sutured along the transverse axis of the duodenum in order to monitor circular muscle contractile activity in the pentobarbital anesthetized dog. Administration by intravenous bolus of a variety of mu- and delta-directed opioid ligands resulted in a dose-dependent increase in duodenal contractile activity. In contrast, all kappa-directed ligands were devoid of stimulatory activity. Naloxone reversed the effects of normorphine and [Met5]enkephalin but was 20 times more effective against normorphine than [Met5]enkephalin. Based on the inactivity of all kappa ligands examined and the differential potency of naloxone against [Met5]enkephalin and normorphine, we suggest that this model may be useful in the classification of opioid ligands as to their receptor selectivity in vivo. Further, these data indicate that the stimulation of duodenal contractile activity is not mediated by enteric kappa receptors.

Studies in vivo with ICI 174864 and [D-Pen2, D-Pen5]enkephalin

We studied the in vivo pharmacology of a selective agonist (DPDPE) and a selective antagonist (ICI 174864) at delta opioid receptors. ICI 174864 (10 micrograms icv) caused postural abnormalities, barrel rotation and hypothermia in rats. DPDPE induced behavioural arousal (at 75 micrograms icv) and barrel rotation (at 125 micrograms) in rats. ICI 174864 (10 micrograms icv) attenuated acetic acid induced writhing in mice. This action was antagonized by naloxone (10 but not 2 mg/kg s.c.). A lower, non-agonist dose of ICI 174864 (5 micrograms) antagonized DPDPE (3 micrograms icv) in this test without affecting DAGO (0.0006 micrograms icv), a selective agonist at mu receptors. In the mouse tail flick test, ICI 174864 (10-50 micrograms icv) did not significantly antagonize the agonist actions of DPDPE (40 micrograms icv) or DAGO (0.3 micrograms icv). At 10-50 micrograms icv, ICI 174864 had no marked effect on gastrointestinal transit in mice. ICI 174864 (25 micrograms icv or 20 mg/kg s.c.) did not interact with mu opioid receptors in mice rendered physically dependent on morphine.

Xorphanol

Xorphanol is a new mixed agonist-antagonist from the morphinan class of analgesics. On the basis of animal experiments, the physical dependence liability of xorphanol is predicted to be of a low order in man. Conceptually, xorphanol is of interest since in vitro experiments have revealed anti-naloxone properties and resistance to antagonism by opioid antagonists. At the practical level, xorphanol is a well tolerated, orally active analgesic that provides effective pain relief clinically.

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.

Studies on the excitatory and inhibitory influence of intracerebroventricularly injected opioids on seizure thresholds in rats

The influence of centrally administered meperidine, normeperidine and pentazocine on the excitability of brain was studied by measuring the threshold for flurothyl-induced convulsions in rats. All three opioids are reported to lower seizure thresholds when given subcutaneously to rats in this test. Dose-and time-dependent changes in the seizure threshold occurred after intracerebroventricular injection of pentazocine (10-160 micrograms), meperidine (25-150 micrograms) and normeperidine (50-150 micrograms). Rapid increases in the seizure threshold were associated with pentazocine and meperidine, whereas a slowly developing decrease in the threshold was caused by normeperidine. Naloxone (10 mg/kg, s.c.) antagonized the anticonvulsant effect of meperidine (but not that of pentazocine) and enhanced the proconvulsant effect of normeperidine. Thebaine (25-150 micrograms), which had no marked influence on the seizure threshold when given intracerebroventricularly, lowered the threshold after subcutaneous injection of 12.5 and 25 mg/kg. This effect was not altered by injection of naloxone. These results show that centrally administered opioids can act on excitatory or inhibitory systems that regulate seizure mechanisms in the rat brain. Furthermore both naloxone-sensitive and naloxone-insensitive components are involved. Meperidine, pentazocine and thebaine have different actions on the seizure threshold after intracerebroventricular, as opposed to subcutaneous, administration. This work has, therefore, identified the route of administration as a critical variable in the effect of opioids on the seizure threshold in rats.

ICI 154,129, a delta-opioid receptor antagonist raises seizure threshold in rats

Acute i.c.v. administration of ICI 154,129 (100-600 micrograms), a delta-opioid receptor antagonist, raised the seizure threshold in a dose-related manner in rats exposed to flurothyl, a volatile convulsant. Pretreatment with naloxone or beta-funaltrexamine (beta-FNA) antagonized this effect. Lower doses of ICI 154,129 (12.5-50 micrograms), which did not influence seizure threshold, selectively antagonized the anticonvulsant action of [D-Ala2,D-Leu5]enkephalin (DADLE) in the same procedure. Consequently, it may be inferred that ICI 154,129 at high doses has mu-agonist and at low doses delta-antagonist properties in the rat flurothyl test.

Pituitary opioid involvement in ECS-postictal electrogenesis and behavioral depression in rats

The role of pituitary opioids in electroconvulsive shock (ECS)-induced postictal electrogenesis and behavioral depression was investigated in sham-hypophysectomized and hypophysectomized rats. These animals were divided into two subgroups and injected SC with either saline or naloxone (3 mg/kg) 10 min prior to transauricular ECS. Sham-hypophysectomized rats given saline responded to a single ECS with a 65 +/- 18% (s.e.) increase in postictal electrogenesis and a behavioral depression lasting 3840 +/- 530 sec. Naloxone significantly antagonized both the postictal increase in EEG voltage output and behavioral depression. Hypophysectomy by itself was without effect on EEG patterns and only partially attenuated the ECS-induced electrogenesis and postictal depression (31.9 +/- 9% and 2360 +/- 511 sec, respectively). However, in hypophysectomized rats, naloxone did not further antagonize these effects of ECS. Thus, it appears that pituitary opioids may, at least in part, mediate postictal electrogenesis and behavioral depression. Alternatively, since hypophysectomy only partially attenuates these phenomena, central or nonpituitary opioid peptide systems may be involved. In view of the observed decrease in responsiveness to naloxone in hypophysectomized rats, nonopioid systems cannot be ruled out as contributors to the opioid-like effects of ECS in these animals.

Studies on bombesin-induced grooming in rats

The gross behavior induced by centrally administered bombesin in rats was compared to that elicited by ACTH-(1-24) and the somatostatin analog, des AA1,2,4,5,12,13[D-Trp8]-somatostatin (ODT8-SS). Bombesin (0.001-1 microgram, ICV) caused dose-related excessive scratching which was qualitatively different from that associated with the other two groom-inducing agents. Bombesin-induced grooming was not markedly affected by behaviorally nondepressant doses of haloperidol, morphine, naloxone or neurotensin. Bombesin was active in genetically hypotrichotic (essentially furless) rats; and, again in such animals, even after numbing the area caudal to the shoulders with lidocaine. Tolerance and cross-tolerance studies with bombesin and ODT8-SS indicated that they produce scratching through different mechanisms. Bombesin caused scratching when injected directly into the periaqueductal gray, but not when administered intravenously. Neither hypophysectomy nor adrenalectomy markedly affected bombesin-induced grooming. This behavior appears to be initiated in the central nervous system and is produced independently of the pituitary-adrenal axis.

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.

Intrathecal bombesin in rats: effects on behaviour and gastrointestinal transit

When bombesin is given intracerebroventricularly to rats, it is known to cause excessive scratching and inhibit gastrointestinal transit. We have administered bombesin via a permanent indwelling cannula into the subarachnoid space of the lumbar spinal cord of rats. By this route, bombesin elicited immediate excessive scratching and rapidly inhibited passage of a charcoal meal along the gastrointestinal tract. The A50 values for these effects were 0.004 (0.001-0.018) micrograms/rat and 0.34 (0.22-0.55) micrograms/rat, respectively. Bombesin-induced scratching and inhibition of transit are therefore mediated at spinal, as well as supraspinal, levels.

An animal model for preclinical screening of systemic antipruritic agents

Reliable antipruritic agents that can be given systemically are not available at present. This may be due to the lack of animal models for screening such compounds. Bombesin, a tetradecapeptide originally isolated from frog skin, induces dose-related excessive scratching when administered intracerebroventricularly (i.c.v.) to rats. With the help of a microcomputer, we monitored the scratching elicited by a standard, submaximal dose of bombesin (0.10 microgram, i.c.v.). This system provides 1) a sensitive and novel way of assessing drug-induced behavioral depression, and 2) a means of quantifying interactions between bombesin and possible antagonists. Thus, bombesin-induced grooming is antagonized by behaviorally nondepressant doses of methdilazine, trimeprazine, and chlorpromazine but not by morphine, haloperidol, diphenhydramine, hydroxyzine, mepyramine, cimetidine, or cyproheptadine. Methdilazine and trimeprazine are used clinically as antipruritic agents. The model therefore offers a means of evaluating new, systemic antipruritic agents, particularly those which may be active in treating histamine-independent pruritus.

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.

ACTH-(1-24) and RX 336-M induce excessive grooming in rats through different mechanisms

ACTH-(1-24) (0.03-6 micrograms i.c.v.) and RX 336-M (7,8-dihydro-5',6'-dimethylcyclohex-5'-eno-1',2',8',14 codeinone) (1.5-6 mg/kg i.p.) induce dose-related excessive grooming and 'wet-dog' shaking in rats. In the present study, the grooming associated with these compounds was compared and analyzed pharmacologically. Grooming caused by RX 336-M and by ACTH-(1-24) was antagonized when rats were pretreated with comparable doses of morphine (0.5-4 mg/kg s.c.), however, only ACTH-(1-24)-induced grooming was attenuated by naloxone (1 and 10 mg/kg s.c.). ICI 154,129 (N,N-bisallyl-Tyr-Gly-Gly-psi-(CH2S)-Phe-Leu-OH) (30 mg/kg s.c.), a selective delta-opiate receptor antagonist, was ineffective against both ACTH-(1-24) and RX 336-M. Although haloperidol is known to antagonize grooming elicited by ACTH-(1-24) (e.g., Wiegant et al., 1977, European J. Pharmacol. 41, 343), even a high dose of this neuroleptic agent (5 mg/kg s.c.) only partially attenuated grooming caused by RX 336-M. Tolerance developed to the grooming elicited by RX 336-M, and by ACTH-(1-24), but there was no cross-tolerance. Both agents were active in genetically hypotrichotic rats; and, again in such animals, even after numbing the area caudal to the shoulders with lidocaine. Given the divergent results with naloxone, and, possibly, with haloperidol, and the lack of cross-tolerance, we conclude that the excessive grooming induced in rats by ACTH-(1-24) and by RX 336-M is mediated by different mechanisms.

A selective role for delta-receptors in the regulation of opioid-induced changes in seizure threshold

In an effort to identify delta-receptor-specific properties for opioid modulation of seizure activity, studies were conducted with ICI 154,129, a putative delta-receptor antagonist, in the rat flurothyl test. Rats were pretreated i.c.v. with ICI 154,129 (50 micrograms) which, at this dose, does not alter normal seizure thresholds. Mean seizure thresholds for control groups (i.c.v. saline) ranged between 323-349 sec. In this test, D-Ala2-D-Leu5 enkephalin (20 micrograms, i.c.v.), metkephamid (40 mg/kg, s.c.), and etorphine (20 micrograms/kg, s.c.) raised seizure thresholds by 117, 128, and 140% of control, respectively. Meperidine (25 mg/kg, s.c.) lowered seizure thresholds by 14% less than control. Pretreatment with ICI 154,129 failed to antagonize the proconvulsant action of meperidine or the anticonvulsant and behavioral depressant actions of etorphine. The increases in seizure threshold produced by DADL and metkephamid (two delta-directed ligands) were significantly attenuated by ICI 154,129. However, the DADL-induced wet-shakes, rigid immobility, and behavioral depression were insensitive to ICI 154,129. These data indicate that ICI 154,129 possesses delta-receptor antagonistic properties in this in vivo model of seizure activity. Furthermore, since only the changes in seizure threshold were antagonized, it may be inferred that opioid-induced behavioral depression and DADLE wet-shakes are not a function of delta-receptor activity.

Studies on opioid peptides as endogenous anticonvulsants

In both normal and hypophysectomized rats, electroconvulsive shock (ECS) produced a significant postictal rise in seizure threshold (S.T.) to flurothyl, a volatile convulsant. This ECS-induced increase in S.T. was markedly attenuated by naloxone (10 mg/kg s.c.), which itself did not alter basal S.T. A dose of bicuculline (0.1 mg/kg i.p.) which was slightly proconvulsant in the flurothyl test did not significantly alter the postictal rise in S.T. produced by ECS. With hypophysectomized rats, there was a 24% increase in basal S.T. to flurothyl challenge. Hypophysectomy had no influence on the postictal rise in S.T. produced by ECS, nor on the attenuation that occurs with naloxone. We propose that the postictal rise in S.T. reflects a change occurring centrally which prevents a static convulsive state, possibly through brain opioid peptides acting as endogenous anticonvulsants.