Agonist and antagonist actions of morphine-like drugs on the guinea-pig isolated ileum

Concurrent Assessment of the Antinociceptive and Behaviorally Disruptive Effects of Opioids in Squirrel Monkeys

Although the clinical application of opioids for pain management is often hindered by undesired behavioral impairment, preclinical assays of antinociception typically do not provide information regarding the behaviorally disruptive effects of opioids that may accompany their antinociceptive effects. To address this, we modified a warm water tail withdrawal procedure to determine concurrently the effects of opioids on tail withdrawal latency (antinociception) and indices of food-maintained operant behavior (rates of responding and reinforcement density) in squirrel monkeys. Six opioid agonists were tested, and all produced dose-dependent antinociception and impairment of operant behavior. The ratio of median effective dose (ED₅₀) values for both measures (behavioral impairment:antinociception) was used as a quantitative measure of therapeutic index. Nalbuphine had the highest ED₅₀ ratio (4.88), reflecting antinociception with minimal behavioral disruption. Oxycodone, heroin, buprenorphine, and methadone all produced similar ED₅₀ ratios (.82-1.14), whereas butorphanol yielded a significantly lower ED₅₀ ratio (.17) reflecting behavioral disruption at doses producing only minimal antinociception. The antinociceptive and behaviorally disruptive effects of oxycodone and buprenorphine were further characterized using Schild analysis to calculate apparent pA₂ values for antagonism of the 2 drugs by naltrexone. These analyses suggest that µ-receptor mechanisms likely mediate the antinociceptive as well as behaviorally disruptive effects of oxycodone (pA₂ values: 8.13 and 8.57) and buprenorphine (pA₂ values: 8.6 and 7.9).

PERSPECTIVE

This article presents an assay that allows for the concurrent assessment of the antinociceptive and behaviorally disruptive effects of opioids. Our results show that the tail withdrawal assay in squirrel monkeys can provide a useful index of the behavioral selectivity with which opioids produce antinociception.

Biphasic regulation of the acute μ-withdrawal and CCk-8 contracture responses by the ORL-1 system in guinea pig ileum

The cloning of the opioid-receptor-like receptor (ORL-1) and the identification of the orphaninFQ/nociceptin (OFQ/N) as its endogenous agonist has revealed a new G-protein-coupled receptor signalling system. The structural and functional homology of ORL-1 to the opioid receptor systems has posed a number of challenges in the understanding the often competing physiological responses elicited by these G-protein-coupled receptors. We had previously shown that in guinea pig ileum (GPI), the acute μ-withdrawal response is under the inhibitory control of several systems. Specifically, we found that the exposure to a μ-opioid receptor agonist activates indirectly the κ-opioid, the A(1)-adenosine and the cannabinoid CB(1) systems, that in turn inhibit the withdrawal response. The indirect activation of these systems is prevented by the peptide cholecystokinin-8 (CCk-8). In the present study, we have investigated whether the ORL-1 system is also involved in the regulation of the acute μ-withdrawal response. Interestingly, we found that in GPI preparation, the ORL-1 system is not indirectly activated by the μ-opioid receptor stimulation, but instead the system is able by itself to directly regulate the acute μ-withdrawal response. Moreover, we have demonstrated that the ORL-1 system behaves both as anti-opioid or opioid-like system based on the level of activation. The same behaviour has also been observed in presence of CCk-8. Furthermore, in GPI, the existence of an endogenous tone of the ORL-1 system has been demonstrated. We concluded that the ORL-1 system acts as a neuromodulatory system, whose action is strictly related to the modulation of excitatory neurotrasmitters released in GPI enteric nervous system.

Chronic morphine induces the concomitant phosphorylation and altered association of multiple signaling proteins: a novel mechanism for modulating cell signaling

Traditional mechanisms thought to underlie opioid tolerance include receptor phosphorylation/down-regulation, G-protein uncoupling, and adenylyl cyclase superactivation. A parallel line of investigation also indicates that opioid tolerance development results from a switch from predominantly opioid receptor G(i alpha) inhibitory to G(beta gamma) stimulatory signaling. As described previously, this results, in part, from the increased relative abundance of G(beta gamma)-stimulated adenylyl cyclase isoforms as well as from a profound increase in their phosphorylation [Chakrabarti, S., Rivera, M., Yan, S.-Z., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 655-662; Chakrabarti, S., Wang, L., Tang, W.-J. & Gintzler, A. R. (1998) Mol. Pharmacol. 54, 949--953]. The present study demonstrates that chronic morphine administration results in the concomitant phosphorylation of three key signaling proteins, G protein receptor kinase (GRK) 2/3, beta-arrestin, and G(beta), in the guinea pig longitudinal muscle myenteric plexus tissue. Augmented phosphorylation of all three proteins is evident in immunoprecipitate obtained by using either anti-GRK2/3 or G(beta) antibodies, but the phosphorylation increment is greater in immunoprecipitate obtained with G(beta) antibodies. Analyses of coimmunoprecipitated proteins indicate that phosphorylation of GRK2/3, beta-arrestin, and G(beta) has varying consequences on their ability to associate. As a result, increased availability of and signaling via G(beta gamma) could occur without compromising the membrane content (and presumably activity) of GRK2/3. Induction of the concomitant phosphorylation of multiple proteins in a multimolecular complex with attendant modulation of their association represents a novel mechanism for increasing G(beta gamma) signaling and opioid tolerance formation.

Isolation and partial characterization of an opioid-like 88 kDa hibernation-related protein

Previous studies show that infusion of hibernating woodchuck albumin (HWA) induces hibernation in summer-active ground squirrels and results in profound behavioral and physiological depression in primates. These effects are reversed by the administration of opiate antagonists, suggesting that the putative hibernation induction trigger (HIT) may act through opioid receptors. We have demonstrated that both HIT-containing plasma and the synthetic alpha opioid D-Ala2-D-Leu5-enkephalin (DADLE), which mimics the activity of HIT in hibernators, extend tissue survival time of a multi-organ autoperfusion system by 3-fold. In this study we present the first data showing biological activity with a much more highly purified plasma fraction from hibernating woodchucks, identified as the hibernation-related factor (HRF). Both the HRF and DADLE show opiate-like contractile inhibition in the mouse vas deferens (Mvd) bioassay. We also have preliminary evidence in an isolated rabbit heart preparation indicating that the HRF and DADLE act similarly to restore left ventricular function following global myocardial ischemia. Furthermore, we have partially sequenced an alpha 1-glycoprotein-like 88 kDa hibernation-related protein (p88 HRP) present in this fraction, which may prove to be the blood-borne HIT molecule.

Tyr-MIF-1 and hemorphin can act as opiate agonists as well as antagonists in the guinea pig ileum

The brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was tested for its effects on electrically stimulated contractions in the guinea pig ileum assay. Tyr-MIF-1 acted as an opiate agonist in reducing these contractions. Its IC50 was about 9 microM, and its effects were reversed by naloxone and CTOP. The ability of Tyr-MIF-1 also to antagonize the inhibitory effects of opiates on electrically stimulated contractions was more evident in the ileum removed from a guinea pig tolerant to morphine or after partial inactivation of opiate receptors with beta-CNA. Similar results were observed with hemorphin. The endogenous peptide Tyr-MIF-1 and the blood-derived peptide hemorphin, therefore, can act as agonists as well as antagonists in the guinea pig ileum. The effects as antagonists are best observed in preparations of ileum with reduced receptor reserve (tolerant or beta-CNA treated) and are consistent with the idea that properties of endogenous peptides as opiate antagonists are enhanced in the tolerant state.

Dermorphin-related peptides from the skin of Phyllomedusa bicolor and their amidated analogs activate two mu opioid receptor subtypes that modulate antinociception and catalepsy in the rat

Three naturally occurring dermorphin-like peptides from the skin of the frog Phyllomedusa bicolor, the related carboxyl-terminal amides, and some substituted analogs were synthesized, their binding profiles to opioid receptors were determined, and their biological activities were studied in isolated organ preparations and intact animals. The opioid binding profile revealed a very high selectivity of these peptides for mu sites and suggested the existence of two receptor subtypes, of high and low affinity. The peptides tested acted as potent mu opioid agonists on isolated organ preparations. They were several times more active in inhibiting electrically evoked contractions in guinea pig ileum than in mouse vas deferens. When injected into the lateral brain ventricle or peritoneum of rats, the high-affinity-site-preferring ligand, [Lys7-NH2]dermorphin, behaved as a potent analgesic agent. By contrast, the low-affinity-site-preferring ligand, [Trp4,Asn7-NH2]dermorphin, produced a weak antinociception but an intense catalepsy.

Suppression of guinea pig ileum induced contractility by plasma albumin of hibernators

Previous studies suggest that hibernation may be regulated by internal opioids and that the putative "hibernation induction trigger" (HIT) may itself be an opioid. This study examined the effect of plasma albumin (known to bind HIT) on induced contractility of the guinea pig ileum muscle strip. Morphine (400 nM) depressed contractility and 100 nM naloxone restored it. Ten milligrams of lyophilized plasma albumin fractions from hibernating ground squirrels, woodchucks, black bears, and polar bears produced similar inhibition, with partial reversal by naloxone. Five hundredths mg of D-Ala2-D-Leu5-enkephalin (DADLE) also inhibited contractility and naloxone reversed it. Conclusions are that hibernating individuals of these species contain an HIT substance that is opioid in nature and summer animals do not; an endogenous opioid similar to leu-enkephalin may be the HIT compound or give rise to it.

Contractile response of guinea pig ileum by repetitive application of morphine

1. The repetitive application of morphine gradually induced a contracture in the isolated guinea pig ileum. 2. The optimum conditions for induction of the contracture were as follows: the concentration, incubation time and washout time of morphine were 1 or 10 microM, 2 and 3 min, respectively. 3. Preincubation with naloxone or TTX blocked this morphine-induced contracture. 4. Among twitch-inhibiting drugs, only clonidine induced a contracture similar to that induced by morphine, while tetrodotoxin (TTX) and adenosine did not. 5. The contracture was also observed in the longitudinal muscle-myenteric plexus preparations. 6. These findings indicate that morphine has a dual inhibitory and excitatory action on the guinea pig ileum and that its repetitive application preferentially diminish the inhibitory one.

A methodological basis for improving the reliability of measurements of opiate abstinence responses in the guinea-pig ileum made dependent in vitro

The purpose of this study was to find a neurogenic response of the isolated guinea-pig ileum that could serve as an internal standard to normalize abstinence responses, which are also neurogenic, during opiate dependence produced in vitro. The internal standard is required because of baseline variability in these responses of the ileum and a time-dependent decay in neuroeffector responsiveness with prolonged incubation. Systematic studies were made of the variability in the responses to neurogenic stimulation by 1) electrical field stimulation, 2) nicotinic stimulation, and 3) precipitation of opiate abstinence with opiate antagonists as well as studies of the time-dependent decay in responsiveness with prolonged incubation. The three neurogenic responses show covariation but the best correlation is found between the nicotinic and the abstinence responses. The nicotinic system presents a pharmacological sensitivity to specific acute opiate action and also shows an improving correlation with abstinence which develops with the progression of dependence. This correlation tends to a direct linear relation with a slope approaching 1.0. The nicotinic response of the ileum seems to be a valid internal control to normalize its abstinence responses after incubation with opiates for different intervals of time.

Structure-activity relationships of the delta-opioid-selective agonists, deltorphins

Deltorphins are naturally occurring peptides with high affinity and selectivity for delta-opioid receptors. They share with dermorphin, another mu-selective opioid agonist, the same N-terminal tripeptide Tyr-D-Xaa-Phe, where D-Xaa is a D-Ala or a D-Met residue. This common sequence appears to be essential for the best fitting of the peptides to both mu- or delta-opioid sites. We studied the changes in receptor affinity and selectivity and in biological potency of deltorphins due to shortening of the sequence, C-terminal deamidation or single amino acid substitutions. The results support the view that a code addressing the molecule towards delta-opioid sites is expressed in the C-terminal region of these peptides. This addressing domain confers high delta-selectivity to the ligand in the following two ways: (i) increased affinity for delta-sites; (ii) decreased affinity for mu-sites. The sequence of the C-terminal tripeptide appears to be responsible for the high delta-affinity of the molecules. Negatively charged side chains inhibit mu-binding and enhance delta-selectivity.

Is the polar bear (Ursus maritimus) a hibernator? Continued studies on opioids and hibernation

Polar bear behavior and biochemistry suggest they may have the ability to hibernate year-round, even though this species is not considered to be a true hibernator. This observation, plus the discovery of a hibernation-induction trigger (HIT) in the blood of black bears, prompted the examination of polar bear blood collected throughout the year for evidence of HIT, and to determine if it displayed opioid activity, as black bear blood does. A bioassay was conducted by injecting summer 13-lined ground squirrels with serum collected from polar bears at different seasons. One group of squirrels was previously implanted with osmotic pumps containing naloxone. The rest had pumps containing saline. Squirrels with saline pumps all hibernated significantly more than those with naloxone, except the group receiving blood from a November polar bear, observed to be highly active and hyperphagic. An in vitro study, using guinea pig ileum, showed that 400 nM morphine inhibited induced contractions and 100 nM naloxone reversed the inhibition. Ten mg of winter polar bear serum albumin fraction (to which HIT binds in ground squirrels and woodchucks) had a similar inhibiting effect, but naloxone, even at 4,000 nM, didn't reverse it. It is concluded that polar bear blood contains HIT, that it has an inhibiting effect, but naloxone, even at 4,000 nM, didn't reverse it. It is concluded that polar bear blood contains HIT, that it has an opioid effect, but may not itself be an opioid.

Antinociceptive effects of [D-Ala2]deltorphin II, a highly selective delta agonist in vivo

The present study has characterized the antinociceptive actions of [D-Ala2]deltorphin II following intracerebroventricular (i.c.v.) administration in the mouse tail-flick test. [D-Ala2]deltorphin II produced dose- and time-related antinociception, with maximal effects at +10 min and significant antinociception which lasted for 40-60 min. [D-Ala2]deltorphin II was 13-fold more potent than i.c.v. [D-Pen2, D-Pen5]enkephalin (DPDPE), a second highly selective delta agonist, and approximately equipotent with i.c.v. morphine in producing antinociception. The antinociceptive effects of i.c.v. [D-Ala2]deltorphin II and DPDPE, but not those of morphine, were antagonized by the selective delta antagonist, ICI 174,864. In contrast, pretreatment with the non-equilibrium mu antagonist, beta-funaltrexamine blocked morphine antinociception, but failed to antagonize [D-Ala2]deltorphin II and DPDPE antinociception. These data indicate that [D-Ala2]deltorphin II produced its antinociceptive effects at a supraspinal delta receptor. [D-Ala2]deltorphin II appears to be the most appropriate delta opioid agonist currently available for studies in vivo and support the involvement of delta receptors in supraspinal antinociception.

Synthesis and biological activity of substance P analogues

The synthesis of the hexapeptide [Glu6]SP6-11 and its glycosylated analogue at the Glu6 gamma-carboxyl position by solution procedures according to several strategies is discussed. The biological activity of SP, [Glu6]SP6-11 (VI) and [Glu(beta-D-Glcp)6]SP6-11 (VIII) have been determined and compared to SP by the GPI and RVD assays. The introduction of a beta-D-glucopyranosyl moiety at the sixth position of the [Glu6]SP6-11 did not affect to a great extent the in vitro activity pattern of the parent hexapeptide.

Agonists, antagonists and modulators of excitatory amino acid receptors in the guinea-pig myenteric plexus

1. The receptors for glutamic acid (L-Glu) present in the guinea-pig myenteric plexus-ileal longitudinal muscle preparation have been studied by measuring the muscle contraction induced by numerous putative endogenous agonists acting at these receptors. Furthermore, the actions of different concentrations of antagonists, glycine, Mg2+ and Ca2+ on the ileal contractions induced by L-Glu have been evaluated. 2. The EC50 values of the most common putative endogenous agonists of these receptors were: L-Glu 1.9 X 10(-5) M; L-aspartate 8 X 10(-5) M; quinolinate 5 X 10(-4) M; L-homocysteate 1.4 X 10(-4) M; the dipeptide aspartyl-glutamate 8 X 10(-5) M, while N-acetyl-aspartyl-glutamate was inactive. Among the molecules used to classify excitatory amino acid receptors, N-methyl-D-aspartate (NMDA) was the most potent (EC50 5 X 10(-4) M). Kainic and quisqualic acids were almost completely inactive. 3. The responses to L-Glu were competitively antagonized by 2-amino-5-phosphonovaleric acid. They were, also, prevented by hyoscine (10(-7) M) and by tetrodotoxin (3 X 10(-7) M), suggesting that the L-Glu-induced ileal contraction was in some way dependent upon an action on the myenteric cholinergic neurones. Kynurenic acid was a non-competitive antagonist, gamma-D-glutamyl-taurine (10(-4) M) and aminophosphonobutyric acid (10(-4) M) did not modify the L-Glu-induced contractions. 4. Glycine (10(-5) M) significantly potentiated the effects of glutamate especially when the ionic composition of the superfusion medium contained concentrations of Ca2+ in the range of 0.6-1.2 mM. Strychnine 3 X 10(-5) M did not modify the actions of glycine. 5. The data presented here confirm the presence of NMDA receptors in the guinea-pig myenteric plexus, and show that these receptors, similar to those present in primary neuronal cultures may be modulated by glycine.

N,N-diallyl-tyrosyl substitution confers antagonist properties on the kappa-selective opioid peptide [D-Pro10]dynorphin A(1-11)

1. In the search for kappa-opioid antagonists, we have designed two N,N-diallyl substituted analogues of the kappa-selective peptide [D-Pro10]dynorphin A (1-11)(DPDYN). In this study, we have examined (i) the binding properties of N,N-diallyl-DPDYN (analogue 1) and N,N-diallyl-[Aib2,3]DPDYN (analogue 2) at the three main types (mu, delta, kappa) of opioid binding sites, (ii) their binding sensitivity to Na+ ions (120 mM NaCl) and guanine nucleotide (50 microM Gpp(NH)p) at mu- and kappa-binding sites and (iii) their biological activity in two pharmacological bioassays specific for mu- and kappa-(guinea-pig ileum) and kappa-(rabbit vas deferens) opioid receptors. 2. Steric hindrance resulting from incorporation of two bulky allyl groups at the tyrosal nitrogen atom greatly altered the binding properties of DPDYN. A dramatic fall in apparent affinity for the three types (mu, delta, kappa) of site as well as selectivity for kappa-sites was observed for the two N,N-diallyl-substituted peptide analogues. 3. At kappa-sites of guinea-pig cerebellum and mu-sites of rabbit cerebellum, N,N-diallyl-substitution led to a complete loss of binding sensitivity to the inhibitory effect of 120 mM NaCl + 50 microM Gpp(NH)p compared to the high sensitivity of DPDYN. This may therefore suggest that the N,N-diallyl-DPDYN analogues are endowed with opioid antagonist properties. 4. No agonist activity of the analogues was observed in guinea-pig myenteric plexus and rabbit vas deferens organ preparations. In contrast, both of the diallyl-substituted peptides displayed similar antagonist properties against the kappa-agonist DPDYN in both preparations. In the guinea-pig ileum, the affinities of the antagonist peptides against the mu-agonist Tyr-D-Ala-Gly-MePhe- NH(CH2)20H(DAGOL) were approximately half that observed against DPDYN. 5. These results show that N,N-diallyl-tyrosyl substitution leads to analogues of DPDYN which act in vitro as pure opioid antagonists and exhibit a reasonable affinity at, but a weak selectivity for, the K-opioid receptors.

Effects of a novel opioid peptide antagonist on rat bladder motility in vivo

The agonist, and opioid antagonist, effects of intracerebroventricularly (ICV) given D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (CTP), a cyclic analogue of somatostatin octapeptide, were evaluated using the micturition reflex of the anesthetized rat as the endpoint. Antagonist effects were evaluated against equieffective doses of selective mu [D-Ala2,NMPhe4,Gly-ol]enkephalin (DAGO) and delta [D-Pen2,D-Pen5] enkephalin (DPDPE) opioid agonists. At low ICV doses, CTP preferentially antagonized DPDPE rather than DAGO; increasing the dose of CTP further effectively antagonized both mu and delta agonists, while even higher doses showed an agonist effect alone which was not blocked by adrenergic, cholinergic or opioid antagonists. Selective opioid antagonist doses of CTP failed to block the inhibition of the micturition reflex produced by pentobarbital. Possible residual somatostatin like properties of CTP were tested by using somatostatin as a possible antagonist of equieffective doses of DPDPE and DAGO; somatostatin did not antagonize these agonists. Repeated exposure to CTP resulted in the development of acute tolerance to the agonist effect, and also prevented the inhibition of the reflex by high doses of somatostatin, with the converse experiment showing a similar pattern; thus, repeated somatostatin resulted in tolerance and subsequent cross-tolerance to the agonist effects of CTP. In animals tolerant to somatostatin, CTP nevertheless behaved as an opioid antagonist. The present results indicate that CTP possesses opioid antagonist properties in vivo which are pharmacological in nature but nevertheless retains residual somatostatin-like activity at higher doses.

Structure-activity relationships of enkephalins containing serially replaced thiomethylene amide bond surrogates

An isomeric series of four leucine-enkephalin analogs containing the thiomethylene ether unit as an amide bond replacement in all positions have been prepared by solid phase methods. The resulting pseudopeptides divulged widely differing retentive behaviors on reversed phase high performance liquid chromatography (HPLC). An analog containing the Phe psi[CH2S]Leu dipeptide replacement at the 4-5 position exhibited binding close to the parent, leucine enkephalin; its guinea pig ileum (GPI) activity was the highest of the analogs tested. Another compound, Tyr psi[CH2S]Gly1-2]-Leu-enkephalin, also displaced 3H-etorphine well in the binding assay, but caused increased contractions in the GPI assay at low concentrations. The Phe psi[CH2S]Leu results are not compatible with the necessity of a beta-turn structure for agonist activity in the GPI assay.

Agonist-antagonist properties of fluorescent opioid probes in the guinea-pig myenteric plexus-longitudinal muscle preparation

The opioid activity of a series of fluorescent derivatives of oxymorphone, naloxone and naltrexone, were characterized on the electrically stimulated guinea-pig myenteric plexus-longitudinal muscle preparation. In all compounds the fluorescent moiety, fluorescein or tetramethylrhodamine B, is attached to the C-6 carbon of the morphinan. Mu-receptor affinity was well retained and there was a good correlation between capacity to displace [3H]-dihydromorphine from binding sites in rat brain membranes and apparent receptor affinity in the myenteric plexus measured as antagonism of normorphine effects. Fluorescein conjugated oxymorphone showed mainly agonist activity. Certain derivatives of the antagonists naloxone and naltrexone showed partial agonism. One compound, 1-(N)-fluoresceinyl naloxone thiosemicarbazone, was an antagonist.

A potent factor in extracts of the skin of the Australian frog, Pseudophryne coriacea. Apparent facilitation of transmitter release in isolated smooth muscle preparations

Extracts of the skin of the Australian frog Pseudophryne coriacea (PsC) displayed potent stimulant effects on isolated smooth muscle preparations of intestine and similar effects on electrically-stimulated vas deferens preparations. These effects must be ascribed to an alkaloid, related in structure to the pumiliotoxins, a class of alkaloid compounds occurring in the skin of neotropical poison frogs. On the basis of results obtained with antagonists and blocking agents, it is suggested that the extract has a pre-synaptic, neurogenic point of attack and that it acts to facilitate the release of transmitters from nerve endings. Acetylcholine is the most important agent involved in the response to the extract by the intestinal muscle and noradrenaline in the response by vas deferens preparations. However, release of other aminergic or peptidergic transmitters may participate, positively or negatively, in the response.

A novel opioid peptide, leumorphin, acts as an agonist at the kappa opiate receptor

The primary structure of the common precursor of porcine beta-neo-endorphin and dynorphin (preproenkephalin B) has shown the existence of a third leucine-enkephalin (leu-enkephalin) sequence with a C-terminal extension of 24 amino acids. This nonacosapeptide, named leumorphin, was approximately 70 times more potent than leu-enkephalin in inhibiting the contraction of the myenteric plexus-longitudinal muscle preparation of the guinea pig ileum. This action of leumorphin, like those of beta-neo-endorphin and dynorphin, was antagonized less effectively by naloxone than that of leu-enkephalin, but more effectively by Mr2266, an antagonist relatively specific for the kappa type opiate receptor. The inhibitory action of leumorphin or beta-neo-endorphin on the contraction of the guinea pig ileum muscle strip was reduced in a dose-dependent manner by pretreatment with dynorphin and vice versa. Leumorphin as well as beta-neo-endorphin and dynorphin inhibits the contraction of the rabbit vas deferens which is known to have only the kappa type opiate receptor. This action was also effectively antagonized by Mr2266. It is concluded that leumorphin has potent opioid activity and acts at the kappa receptor, like other opioid peptides derived from preproenkephalin B.

Synthesis and preliminary biological investigations of O-sulphated dermorphin

Synthesis by a classical solution method of a sulphated analogue of dermorphin is reported. The product has proved to be devoid of any peripheral and central opiate- or CCK-like activity in the tests assayed.

Pharmacological characterization of the opioid receptor in the submucous plexus of the guinea-pig oesophagus

1 The cholinergically mediated electrically-induced contractions of the submucous plexus-longitudinal muscularis mucosae preparation of the guinea-pig oesophagus were used to study the actions of opioid peptides and morphine. 2 The twitch contractions of the tissue (0.1 Hz, 0.5 ms, supramaximal voltage) were inhibited by all the opioid peptides and morphine in a concentration-dependent manner. The order of potency was dynorphin-(1-13) greater than alpha-neo-endorphin greater than beta-endorphin greater than [D-Ala2]-methionine-enkephalin much greater than alpha-endorphin, methionine-enkephalin, leucine-enkephalin and morphine. 3 The inhibitory actions of dynorphin-(1-13) (20 nM), alpha-neo-endorphin (100 nM) and beta-endorphin (3 microM) were completely reversed either by naloxone (1 microM) or by morphine (100 microM). The Ke values of naloxone against dynorphin-(1-13) and alpha-neo-endorphin were 30 and 25 nM, respectively. 4 Increasing the concentration of calcium from 1.8 to 3.6 mM in Tyrode solution decreased the sensitivity of the tissue to dynorphin-(1-13) 7.4 times and to alpha-neo-endorphin 462 times. 5 The inhibitory actions of dynorphin-(1-13) (100 nM) and alpha-neo-endorphin (300 nM) were inversely related to stimulus frequency, being most active at low frequencies (0.1-1 Hz), and least active at high (30 Hz). 6 Exogenously applied acetylcholine produced concentration-dependent contractions of the isolated muscularis mucosae, with an EC50 of 72.6 +/- 4.5 nM. The contractile response of the oesophagus to acetylcholine was not affected by the pretreatment of the tissue with dynorphin-(1-13) (100 nM), alpha-neo-endorphin (300 nM) or beta-endorphin (3 microM). 7 It is concluded that the submucous plexus-longitudinal muscularis mucosae of the guinea-pig oesophagus is inhibited by opioid peptides acting at prejunctional opioid receptors, probably of the kappa-subtype.

Interactions of morphine with PGE1, isoproterenol, dopamine and aminophylline in rat mast cells; their effect on IgE-mediated 14C-serotonin release

The formaldehyde method was used to examine the interactions of morphine with PGE1, isoproterenol, dopamine and aminophylline in rat mast cells by their effects on IgE-mediated 14C-serotonin release. PGE1 (2 x 10(-8) -2 x 10(-5) M), isoproterenol (10(-10) -10(-8) M), dopamine (4 x 10(-8) -4 x 10(-6) M) and aminophylline (6 x 10(-6) -6 x 10(-4) M) caused dose-related inhibition of the mediator release 1 min after an antigen challenge, and propranolol (10(-7) M) blocked the inhibition by isoproterenol (10(-8) M) but not that by dopamine (4 x 10(-6) M), while haloperidol (4 x 10(-6) M) blocked that by dopamine (4 x 10(-6) M) but not that by isoproterenol (10(-8) M). Morphine (3 x 10(-7) -3 x 10(-5) M) reversed the inhibitory effects of PGE1 (2 x 10(-6) M), isoproterenol (10(-8) M) and dopamine (4 x 10(-6) M) dose-dependently and stereospecifically; naloxone (2 x 10(-4) M) antagonized these reversing actions of morphine (3 x 10(-5) M). Morphine (10(-6) -10(-4) M) did not reverse the inhibitory action of aminophylline (6 x 10(-4) M). These results suggest that the inhibitory responses of mast cells to PGE1, isoproterenol and dopamine but not to aminophylline in immunological mediator release were reversed by morphine through opioid receptors, and that the inhibition of adenylate cyclase in mast cells is one of the biochemical actions of morphine.

Improved assays for the assessment of kappa- and delta-properties of opioid ligands

The highly selective non-equilibrium mu-antagonist beta-funaltrexamine (beta-FNA) produced a maximal 20-fold shift in the IC50 for the mu-agonist morphine on the guinea-pig ileum preparation, whilst producing no significant change in the IC50 for the kappa-agonist ethylketazocine. On preparations pretreated with beta-FNA, the pA2 values for the interaction of morphine and ethylketazocine with naloxone were similar. These values were similar to the pA2 value for the interaction of ethylketazocine and naloxone determined on control tissues, but significantly different from the pA2 value for morphine-naloxone on control tissues, indicating that the agonist actions of morphine on preparations pretreated with high concentrations of beta-FNA are mediated by kappa-, rather than mu-receptor interaction. On the mouse vas deferens preparation, co-incubation with the highly selective delta-agonist Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET) and the non-selective non-equilibrium opiate antagonist beta-chlornaltrexamine (beta-CNA) resulted in marked inhibition of the agonist actions of morphine but had no effect upon the agonist actions of the delta-agonist leucine-enkephalin. The pA2 values for the interactions of naloxone with leucine-enkephalin and etorphine were unaltered by pretreatment with beta-CNA and DSLET. In similarly pretreated tissues, the agonist actions of ethylketazocine were markedly inhibited. It is concluded that manipulation of the guinea-pig ileum and mouse vas deferens preparations in the described manner results in assay systems that possess a largely homogeneous receptor population, and as such are valuable tools with which to evaluate opioid activity.

Assessment of the potential agonistic and antagonistic properties of ketamine at opiate receptors in the guinea-pig ileum

The anesthetic agent ketamine was studied for its ability to interact with opiate receptors in the longitudinal smooth muscle-myenteric plexus preparation of the guinea-pig ileum. The drug was found to possess agonistic, but not antagonistic, activity on opiate receptors. Naloxone antagonized the effect of ketamine although to a lesser degree than the antagonism exhibited toward morphine. In addition to the opiate action, ketamine also produced a depression of the contractile responses of the ileal smooth muscle to acetylcholine and histamine. The concentrations of ketamine that produced this non-specific depression were generally higher than those needed to demonstrate the opiate effect. However, some overlap in the concentration ranges for the two actions were observed. Although the nature of the opiate action of ketamine suggested that it interacts with opiate receptors or sub-types of these differently than does morphine, the non-specific action of the drug on the ileal smooth muscle precluded a definitive analysis of differences in the opiate receptor preferences of the drug.

Quaternary narcotic antagonists' relative ability to prevent antinociception and gastrointestinal transit inhibition in morphine-treated rats as an index of peripheral selectivity

Single doses of naloxone (0.025 to 0.5 mg/kg) or of one of four quaternary narcotic antagonists (i.e. nalorphine allobromide, nalorphine methobromide, naloxone methobromide or naltrexone methobromide, 1 to 60 mg/kg) were given s.c. to rats before morphine, 5 mg/kg i.v. In the absence of antagonists morphine reduced G.I. transit of a charcoal meal to about 15% of drug-free controls and consistently delayed nociceptive reactions (55 degrees C hot plate) in all animals. Doses of antagonists slightly reducing morphine antinociception (centrally effective = A) and restoring G.I. transit to about 50% of drug-free rats (peripherally effective = B) were estimated. The A:B ratio, indicating peripheral selectivity, was at least 8 for any of the quaternary antagonists given 10 min before morphine, but prolonging this interval may have resulted in a lower figure (i.e. less peripheral selectivity) because of reduced A and increased B. This was definitely so for naltrexone methobromide (A:B, greater than 60 at 10 min, about 1 at 80 min) and was not apparent for nalorphrine methobromide according to available data, which for nalorphrine allobromide and to a lesser extent for naloxone methobromide showed only an increase in B at intervals longer than 10 min. Both morphine-induced antinociception and inhibition of G.I. transit were reduced by naloxone at the lower doses tested and were fully prevented at the higher. These findings indicated that, unlike naloxone, the investigated quaternary narcotic antagonists are interesting prototype drugs for selective blockade of opiate receptors outside the CNS, although certain critical aspects, possibly biological N-dealkylation to the corresponding tertiary antagonists, condition peripheral selectivity.

Model of opiate dependence in the guinea-pig isolated ileum

1 Segments of ileum, incubated for 2-24 h at 22 degrees C with normorphine (0.01 - 1.0 muM), in the presence of hexamethonium, contracted when challenged with naloxone (0.03 muM). No response to this dose of naloxone was induced either by incubation in control solution without opiate for 2-24 h or by exposure of the preparation to opiate for 30 min at 37 degrees C.2 When segments were incubated for 24 h, the size of the response to naloxone was directly related both to the normorphine concentration in the incubation fluid (0.01 to 0.1 muM), and to the concentration of naloxone applied (0.03 to 0.1 muM).3 A spontaneous withdrawal contracture was elicited in ilea that had been incubated with normorphine (1.0 muM), when the normorphine-containing bathing fluid was exchanged for one without opiate.4 Normorphine restored to resting level the tension of the withdrawal contracture, whether it had been elicited spontaneously or by naloxone challenge.5 Addition of naloxone (1.0 muM) to normorphine (1.0 muM) in the incubation fluid abolished the withdrawal contracture to subsequent challenge with naloxone.6 Naloxone elicited a contracture from segments incubated for 24 h at 22 degrees C with levorphanol (0.1 muM) but not from those incubated with dextrorphan.7 Application of (+)-naloxone (0.03 muM) to segments previously incubated with normorphine (0.1 muM) did not elicit a contracture.8 The contracture elicited by naloxone in preparations incubated with morphine (10 muM) was associated with a reduction in sensitivity to the acute inhibitory effect of morphine on the electrically-evoked response.9 Addition of hyoscine (0.5 muM) immediately after challenge with naloxone restored the tension of the withdrawal contracture to resting level.10 Tetrodotoxin (3.0 muM) given before challenge, prevented naloxone from eliciting a withdrawal contracture.11 The inclusion of 5-hydroxytryptamine (10 muM) with morphine (10 muM) inhibited the induction of tolerance to morphine.12 These experiments, together with those described earlier, indicate that incubation with opiate induces a dependence in the final cholinergic motor neurones of the myenteric plexus, manifested as a contracture of the longitudinal muscle on removal of opiate or administration of an antagonist. This dependence is associated with tolerance, expressed as a decrease in sensitivity to inhibition by morphine of the electrically-evoked contracture. Tolerance and dependence are induced and withdrawal precipitated through specific and stereospecific opiate receptors.

Pharmacological characterization of opiate physical dependence in the isolated ileum of the guinea-pig

1 Physical dependence was produced in ilea from naive guinea-pigs by exposure of the tissue to different opiates for logarithmically-spaced periods of time (20-320 min). The responsiveness of the tissue to naloxone, as indicated by a strong contracture of the ileum, was enhanced in contrast to that found in intestines not exposed to opiates. 2 The dose-response curves to naloxone obtained in tissues individually exposed to different opiates showed that their relative potency in increasing sensitivity to naloxone was as follows: levorphan greater than morphine greater than Met-enkephalin greater than nalorphine greater than pentazocine. 3 The naloxone-induced response was dose-dependent and was directly related to the opiate concentration and length of exposure. 4 Dextrorphan the inactive isomer of levorphan, did not increase the responsiveness of the tissues to the narcotic antagonist, indicating that the phenomenon is stereospecific. 5 The naloxone-induced contraction in ilea exposed for 320 min to morphine (1 x 10(-6)M) was not prevented or suppressed by the administration of a large dose of morphine (1 x 10(-5)M) before or immediately after the naloxone challenge. 6 The evidence presented here shows that a phenomenon resembling in vivo opiate physical dependence can be acutely produced in vitro with pharmacological characteristics similar to other naloxone-induced abstinence effects.

Pharmacological data on dermorphins, a new class of potent opioid peptides from amphibian skin

1 Dermorphin and Hyp6-dermorphin are the first representatives of a new class of potent opioid peptides occurring in amphibian skin. They present the unique feature of having a D-Ala residue incorporated in the peptide molecule. 2 Dermorphin displayed a potent depressive action on electrically stimulated contractions of the guinea-pig ileum and mouse vas deferens preparations. Dermorphin was respectively 57,294, 18 and 39 times more potent than Met-enkephalin, Leu-enkephalin, beta-endorphin, and morphine on the guinea-pig ileum opiate receptors. On the vas deferens receptors, dermorphin was about as potent as the enkephalins and 40 times more potent than morphine. Naloxone was a powerful antagonist to dermorphin in both preparations. 3 Dermorphin produced potent and long-lasting analgesia in mice by intravenous injection, and in rats by intracerebroventricular injection, the ED50 being here of the order of 13-23 pmol/rat. Morphine was 752 and 2170 times less potent, depending on the analgesia test used. At high intracerebroventricular doses analgesia was accompanied by catalepsy. 4 Intracerebroventricular infusion of dermorphin induced development of tolerance and precipitation of withdrawal symptoms upon administration of naloxone. Both tolerance and physical dependence was consistently less marked with dermorphin than with morphine. 5 The minimum sequence requirement for full dermorphin activity was represented by the N-terminal tetrapeptide. The presence of the D-Ala2-residue was of crucial importance.

Clonidine dependence in the guinea-pig isolated ileum

1 Compared with the response of preparations incubated in solutions without clonidine, a three to four fold increase in the magnitude of the contracture of the longitudinal muscle to challenge with phentolamine (1.0 mum) was induced by incubating the guinea-pig isolated ileum at 22 degrees C for 24 h with clonidine (1.0 mum) in Krebs solution containing hexamethonium (70 mum). Incubation of the ileum with clondine (1.0 mum) for 0.5 h at 37 degrees C did not increase responsiveness to phentolamine.2 The increase in responsiveness to phentolamine was directly related to the clonidine concentration in the incubation fluid over the range 0.01 to 1.0 mum.3 The magnitude of the contracture to phentolamine of ilea incubated with clonidine (1.0 mum) (withdrawal contracture) was directly related to the challenge dose of phentolamine over the range 0.3 to 1.0 mum.4 Yohimbine (1.0 mum) or piperoxane (1.0 mum) elicited a response comparable to that elicited by phentolamine but propranolol (1.0 mum) was inactive.5 Addition of phentolamine (1.0 mum) to clonidine (1.0 mum) in the incubation fluid abolished the increased response of the preparation to subsequent challenge with phentolamine.6 Addition of hyoscine (0.5 mum) immediately after challenge with phentolamine restored the tension of the withdrawal contracture to its resting level.7 Tetrodotoxin (3.0 mum) given before challenge, prevented phentolamine from eliciting a withdrawal contracture.8 Ileal segments incubated with clonidine (1.0 mum) were unresponsive to challenge with naloxone (100 nm); and segments incubated with normorphine (1.0 mum) were unresponsive to phentolamine (1.0 mum), although responsive to naloxone.9 Normorphine (1.0 mum) restored to resting level the tension of the clonidine withdrawal contracture; and clonidine (0.1 mum) restored to resting level the tension of the contracture to naloxone in ileal segments incubated with normorphine.10 These experiments indicate that incubation with clonidine induces, in the final cholinergic motor neurones of the myenteric plexus of the isolated ileum, a dependence the withdrawal from which is expressed as a contracture in response to alpha-adrenoceptor antagonists.11 Although opiate receptors are not involved in clonidine dependence nor alpha-adrenoceptors in opiate dependence, the findings that normorphine suppresses the clonidine withdrawal-contracture and that clonidine suppresses the contracture of opiate-dependent ileum to naloxone, suggest that the withdrawal effect studied in both clonidine and normorphine dependence in this preparation is mediated by release of acetylcholine from the final motor neurone.

Action of morphine on the neuro-effector transmission in the guinea-pig ileum and in the mouse vas deferens

Effects of morphine on the neuro-effector junction of the guinea-pig ileum or mouse vas deferens were investigated by the micro-electrode and double sucrose gap methods. 1. Morphine (10(-8)-10(-5) M) did not change the membrane potential, membrane resistance and electrical threshold required to produce the action potential of smooth muscle cells of guinea-pig ileum or mouse vas deferens. 2. Morphine (10(-8)-10(-7) M) markedly suppressed the amplitude of excitatory junction potential (e.j.p.) of ileum or that of vas deferens. However the same concentration of morphine did not suppress the inhibitory junction potential recorded from the guinea-pig ileum or the facilitation phenomena observed with repetitive stimulation in the mouse vas deferens. In addition, this opiate (3.5 x 10(-5) M) did not alter the amplitude or the frequency of miniature excitatory junction potential recorded from the mouse vas deferens. 3. Naloxone (3.5 x 10(-7) M), itself, exerted no effect on the membrane potential and amplitude of the e.j.p. After pretreatment with naloxone, however, the inhibitory action of morphine on the e.j.p. was suppressed. 4. The extracellularly recorded action potential from the small nerve bundle innervating the mouse vas deferens was not affected by morphine (3.5 x 10(-5) M). 5. The amplitude of the e.j.p. of the guinea-pig ileum was dependent on the concentration of [Ca]o. When [Ca]o and the relative amplitude of e.j.p. were plotted on a double logarithmic scale, the above relation yielded a straight line with a slope of 1.7. Application of morphine resulted in a reduction in the slope of the straight line to 1.0. 6. These results indicate that morphine probably suppresses the influx of Ca during spike electrogenesis in the nerve terminal, however, there is no modification of the action of Ca in nerve terminals.

Effects of morphine on canine intestinal absorption and blood flow

1 Intestinal absorption and blood flow were determined in anaesthetized fed or fasted dogs following rapid intravenous injections of morphine (0.01, 0.1, 1 mg/kg). 2 3H2O and 22Na were used to determine the unidirectional fluxes of Na+ and H2O from saline perfused through the ileal lumen and the clearances of 3H2O were used to determine total and absorptive site blood flow. 3 Net Na+ and H2O absorption were increased at each dose of morphine in fed but not in fasted dogs, due primarily to increased absorptive fluxes. 4 Arterial pressure was decreased by morphine but mesenteric vein pressure was little affected. Absorptive site blood flow was increased by morphine due to decreased blood flow resistance but total blood flow resistance was little affected by morphine. 5 The absorptive fluxes of Na+ and H2O were correlated with absorpitve site blood flow in both fed and fasted animals. The secretory fluxes of Na+ and H2O were correlated with estimated capiliary pressure in fasted dogs but morphine decreased the the secretory fluxes at a given capillary pressure in dogs which had been fed. 6 Naloxone (0.12 mg, i.v.) reversed the effects of morphine. The effects of morphine on the gut were reversed more slowly than on systemic blood pressure. 7 It was concluded that morphine can increase net absorption in fed dogs by a selective increase in intestinal absorptive site blood flow and thus increase absorptive fluxes by a washout effect but that there is also an epithelial effect, sensitized by feeding, which reduces the secretory fluxes of Na+ and H2O.

The effect of morphine on human neuromuscular transmission

By utilizing high frequency nerve stimulation, we observed the effects of morphine sulphate, 0.5 mg.kg-1 on human neuromuscular transmission. Tetanic fade at 50, 100 and 200 hz did not change during the one hour period after infusion of morphine. Post-tetanic depression (PTD) of single twitch response increased progressively with time over the one-hour study period. This was most marked with the 100 and 200 hz tetanic frequencies. In contrast, control subjects displayed either no change, or a slight increase (post-tetanic facilitation; PTF), over a similar one-hour test period. We believe these results are best explained by a presynaptic action of morphine, whereby intraterminal mobilization of acetylcholine is impaired by the opiate narcotic drug.

Chloroxymorphamine, and opioid receptor site-directed alkylating agent having narcotic agonist activity

Chloroxymorphamine, the 6beta-N,N-bis(2-chloroethyl) derivative of oxymorphone, is a potent nonequilibrium narcotic agonist in the longitudinal muscle preparation of guinea pig ileum. The corresponding naltrexone analog,chlornaltrexamine, is a potent nonequilibrium antagonist of morphine. These receptor sitedirected alkylating agents possess considerable potenial as pharmacologic and biochemical probes of apoid receptors.

Animal and molecular pharmacology of mixed agonist-antagonist analgesic drugs

1 The pharmacology of analgesic drugs with mixed agonist-antagonist action is reviewed in the light of the theory of drug action at single and multiple receptors. 2 Evidence for a heterogeneous population of opiate receptors in mammalian brain and in isolated tissue preparations is discussed. 3 The unusual pharmacological and pharmacokinetic profiles of the oripavine derivative buprenorphine are discussed in relation to the action of the drug at a molecular level.

Antagonistic effect of compound 48/80 on the inhibitory actions of morphine and methionine-enkephalin on electrically-induced contractions of the guinea-pig ileum

1 The effect of compound 48/80 was studied on the twitch-like contractions of the longitudinal muscle of guinea-pig ileum induced by electrical stimulation of intramural cholinergic nerves. 2 Compound 48/80 alone, at concentrations up to 30 microgram/ml, had no effect on the twitch contractions. The contraction to exogenously applied acetylcholine was slightly depressed by the compound. 3 At 100 microgram/ml, compound 48/80 caused a weak but long-lasting increase in tone and irregular contractile activity in the ileum, part of which was reduced but not completely abolished by pretreatment with chlorpheniramine (1 muM) or by repeated applications of compound 48/80. 4 The inhibitory effects of morphine and methionine-enkephalin on the twitches were antagonized by the presence of compound 48/80 (3 to 30 microgram/ml), possibly in a competitive manner. The antagonism was not affected by pretreatment with the antihistaminics, chlorpheniramine and/or metiamide. 5 The inhibitory effects of noradrenaline and adrenaline on the twitches were slightly but significantly increased by the presence of compound 48/80 (10 or 30 microgram/ml), whereas that of ATP was not modified. 6 Thesese results indicate that compound 48/80 acts as a selective and competitive antagonist at opiate receptors located in the intramural cholinergic nerves of guinea-pig ileum.

The mechanism of action of narcotic analgesics in the guinea-pig ileum

1. Intracellular recordings were made from neurones in the myenteric plexus of the guinea-pig ileum. Single myenteric ganglia were maintained in vitro and drugs were applied by adding them to the perfusing solution. 2. Narcotic analgesics hyperpolarized the membrane of a proportion of neurones in the myenteric plexus. 3. The membrane hyperpolarization was sometimes associated with a decrease in input resistance. These effects reduced the excitability of myenteric neurones. 4. The effects of narcotics occurred at low concentrations (10 nM to 1 micrometer), were stereospecific and were reversed by naloxone. 5. It is proposed that the morphine-sensitive neurones may be the cholinergic efferents to the muscle layers. By hyperpolarizing these neurones, morphine may prevent their excitation by electric field stimulation. This may explain why narcotic analgesics reduce the output of acetylcholine and the contractile response of this preparation when it is excited by field stimulation.

Opiate binding and effect in ileum preparations from normal and morphine pretreated guinea-pigs

1 Dose-response curves for normorphine in the absence and presence of naloxone have been obtained from myenteric plexus-longitudinal muscle strip preparations from normal and morphine pretreated guinea-pigs. In addition, the high affinity stereospecific binding of [3H]-etorphine has been measured in homogenates of the same tissue. 2 Higher concentrations of normorphine were required to produce 50% inhibition of the electrically stimulated contractions of strip preparations from morphine pretreated animals. There was also an increase in the slope of linearized dose-response curves in opiate-tolerant preparations. Maximum opiate effect was unchanged, and responses to exogenous acetylcholine were not affected by the pretreatment. 3 There was a slight increase in the apparent equilibrium constant for naloxone after morphine pretreatment. 4 Tolerance to opiate effect was not accompanied by a change in the affinity or number of stereospecific binding sites for [3H]-etorphine. Hill plots of [3H]-etorphine binding in both control and morphine pretreated preparations gave slopes close to unity. 5 Most of these results can be explained by the assumption that in tolerant preparations, a certain fractional opiate receptor occupation threshold must be exceeded before opiate effects become apparent. It is suggested that the tissue adapts toward a threshold equivalent to the mean receptor occupancy attained during the period of opiate drug pretreatment.

Effects of pentazocine on the intestine and biliary tract of the rabbit in vitro

Pentazocine impairs peristaltic activity and relaxes longitudinal muscle in the colon and in the ileum. The circular coat is excited in the colon, while in the ileum pentazocine exhibits both excitatory and inhibitory effects depending on the concentration employed. Pentazocine does not exert a spasmogenic effect in the smooth muscle of terminal bile duct but instead reduces the electrically-induced contraction. The effect of pentazocine does not seem to involve endogenous acetylcholine or catecholamine release.

Dopamine and the depressant action of morphine on stimulated guinea-pig ileum

1 Morphine reduces the amplitude of the contractions induced by electrical stimulation, in the myenteric plexus-longitudinal muscle preparation of guinea-pig ileum. Dopamine and apomorphine have the same effect but at much higher concentrations. 2 Dopamine, at concentrations lower than those which would normally be inhibitory, partially reverses the depressant effect of morphine. 3 Pre-treatment of guinea-pigs with 6-hydroxydopamine results in a slight supersensitivity of innervated longitudinal muscle preparations to dopamine and has no effect on morphine activity. 4 Naloxone antagonizes the depressant effect of morphine but not that of dopamine or apomorphine. 5 The response of theileum preparation to morphine is not affected by phentolamine or propranolol; the effect of dopamine, however, is abolished by alpha-adrenoceptor blockade.