The effect of kappa agonist U50-488H on [3H]nimodipine receptor binding in rat brain regions

Role of κ- and δ-opioid receptors in the antinociceptive effect of oxytocin in formalin-induced pain response in mice

Oxytocin has been implicated in the modulation of somatosensory transmission such as nociception and pain. The present study investigates the effect of oxytocin on formalin-induced pain response, a model of tonic continuous pain. The animals were injected with 0.1 ml of 1% formalin in the right hindpaw and the left hindpaw was injected with an equal volume of normal saline. The time spent by the animals licking or biting the injected paw during 0-5 min (early phase) and 20-25 min (late phase) was recorded separately. Oxytocin (25, 50, 100 microg/kg, i.p.) dose dependently decreased the licking/biting response, both in the early as well as the late phases. The antinociceptive effect of oxytocin (100 microg/kg, i.p.) was significantly attenuated in both the phases by a higher dose of the non-selective opioid receptor antagonist naloxone (5 mg/kg, i.p.), MR 2266 (0.1 mg/kg, i.p.), a selective kappa-opioid receptor antagonist and naltrindole (0.5 mg/kg, i.p.), a selective delta-opioid receptor antagonist but not by a lower dose of naloxone (1 mg/kg, i.p.) or beta-funaltrexamine (2.5 microg/mouse, i.c.v.), a selective mu-opioid receptor antagonist. Nimodipine, a calcium channel blocker (1 and 5 mg/kg, i.p.) produced a dose-dependent analgesic effect. The antinociceptive effect of oxytocin was significantly enhanced by the lower dose of nimodipine (1 mg/kg, i.p.) in both the phases. Chronic treatment with oxytocin (100 microg/kg/day, i.p. daily for 7 days) did not produce tolerance in both the phases of formalin-induced pain response. The results thus indicate that oxytocin displays an important analgesic response in formalin test; both kappa- and delta-opioid receptors as well as voltage-gated calcium channels seem to be involved in the oxytocin-induced antinociception.

Role of L-type Ca(2+) channels in pertussis toxin induced antagonism of U50,488H analgesia and hypothermia

Previous studies have shown that the kappa-opioid effects are sensitive to pertussis toxin (PTX) and affected by Ca(2+) fluxes. However, the possible involvement of Ca(2+) channels in PTX-induced inhibition of kappa-opioid effects has not been reported. The effect of intracerebroventricular (i.c.v.) treatment of pertussis toxin (1 microg/rat, PTX) or saline on the kappa-opioid agonist, U-50,488H (U5H) induced tail-flick analgesia and hypothermia in rats was determined. The effect of nimodipine (NIM), a dihydropyridine (DHP)-sensitive Ca(2+) channel blocker (CCB), on PTX-induced modulation of U5H effects was examined. The DHP ligand, [3H]PN200-110 binding was also determined in both PTX and saline treated rats to study the possible involvement of L-type Ca(2+) channels in PTX modulation of kappa-opioid agonist effects. The analgesia and change in colonic temperature were determined using tail-flick analgesiometer and telethermometer, respectively. U5H (40 mg/kg, i.p.) produced significant analgesic and hypothermic responses. PTX treatment significantly (P<0.01) antagonized the analgesic and hypothermic effects of U5H. Acute pretreatment of NIM (1 mg/kg, i.p.) 15 min prior significantly (P<0.01) reversed the PTX-induced antagonism of U5H effects. In the binding study, PTX treatment (72 h before) resulted in a significant (P<0.005) upregulation (+45% vs. saline control) of DHP binding (B(max)) with no change in affinity (K(d)). The results showed significant upregulation of DHP binding in accordance with PTX-induced antagonism of U5H effects and this blockade was reversed by NIM. Thus, present results suggest that U5H-induced analgesia and hypothermia may be mediated through PTX-sensitive transducer G-proteins (G(i/o)) coupled to L-type Ca(2+) channels.

Role of Ca2+ channels on the hypothermic response produced by activation of kappa-opioid receptors

The effect of nimodipine (NIM) and lercanidipine (LER) 1,4-dihydropyridine (DHP) calcium channel blockers (CCBs) on the hypothermic response of selective kappa-opioid receptor agonists U50,488H (U50), PD117,302 (PD) and U69,593 (U69) was determined in rats by recording colonic temperature using digital telethermometer. Intraperitoneal (i.p.) injections of U50 (7.5, 15, 22.5 and 40 mg/kg), PD (7.5, 15 and 22.5 mg/kg) and U69 (5 and 20 mg/kg) produced a dose-dependent hypothermic response. However, higher doses of U50 (60 and 80 mg/kg) produced hypothermia, which is less when compared to that produced by 22.5-mg/kg dose of U50. NIM (1 mg/kg i.p.; 15 min prior) and LER (0.3 mg/kg i.p.; 15 min prior) did not produce any change in basal colonic temperature. Treatment of NIM and LER potentiated the U50 (7.5, 15, 22.5 and 40 mg/kg)-induced hypothermic effect. NIM did not potentiate hypothermia produced by U50 (60 mg/kg). On the other hand, PD (7.5, 15 and 22.5 mg/kg)- and U69 (5 and 20 mg/kg)-induced hypothermia was unaffected by the pretreatment of either NIM or LER. This differential modulation of kappa-opioid agonist-induced hypothermia by CCBs suggest that there may be two mechanisms, Ca(2+)-sensitive and Ca(2+)-insensitive, involved in kappa-opioid agonist-induced hypothermic response.

L-type Ca2+ channel modulation by dihydropyridines potentiates kappa-opioid receptor agonist induced acute analgesia and inhibits development of tolerance in rats

The effect of 1,4-dihydropyridine (DHP) calcium channel blockers (CCBs), nimodipine (NIM) and lercanidipine (LDP) on the analgesic response of selective kappa-opioid receptor agonists, U50,488H, PD117,302 and U69,593 was determined in male Sprague-Dawley rats using the tail-flick test. The effect of NIM on development of tolerance to U50,488H-induced analgesia and the status of brain DHP-sensitive Ca(2+) channel (L-type) binding sites in both U50,488H-naive and tolerant rats was determined using the highly selective DHP radioligand, [(3)H]PN200-110. Tolerance was induced by injecting U50,488H (25 mg/kg, i.p.) twice daily for 4 days. Intraperitoneal (i.p.) injection of kappa-opioid receptor agonists produced a dose-dependent acute analgesic response. NIM (1 mg/kg; i.p.) and LDP (0.3 mg/kg; i.p.) used in the study produced no tail-flick analgesia. Administration of NIM and LDP (15 min prior) significantly potentiated the analgesia produced by three kappa-opioid receptor agonists. Tolerance developed completely to the analgesic effect induced by U50,488H (25 mg/kg, i.p.) administered on the 5th day. NIM (1 mg/kg, i.p.) twice daily for 4 days not only completely inhibited the development of tolerance to analgesic response but also significantly potentiated it (supersensitivity). There was a significant up-regulation of DHP binding sites (B(max): +41%) in whole brain membranes of tolerant rats when compared to vehicle treated naive rats, implicating increased influx of Ca(2+) through L-type channels in kappa-opioid tolerance. U50,488H (25 mg/kg, i.p.) and NIM (1 mg/kg, i.p.) twice daily for 4 days also resulted in an equivalent up-regulation of DHP binding sites (+36%) as that of U50,488H alone. These results strongly suggest a functional role of L-type Ca(2+) channels in the regulation of pain sensitivity, mechanism of kappa-opioid analgesia and expression of tolerance.

Effects of kappa opioid receptor-selective agonists on responses of pelvic nerve afferents to noxious colorectal distension

The aim of this study was to examine the effects of kappa-opioid receptor selective agonists on responses of mechanosensitive afferent fibers in the pelvic nerve. Single-fiber recordings were made from pelvic nerve afferents in the decentralized S1 dorsal root of the rat. A total of 572 afferent fibers in the S1 dorsal root were identified by electrical stimulation of the pelvic nerve; 252 (44%) responded to noxious colorectal distension (CRD; 80 mmHg). Of these 252 fibers that responded to CRD, 100 were studied further. All 100 fibers gave monotonic increases in firing to increasing pressures of CRD. Eighty-eight fibers had low thresholds for response (mean: 3 mmHg) and 12 fibers had high-thresholds for response (mean: 28 mmHg). Responses of 17 fibers also were tested after instillation of 5% mustard oil (MO) into the colon. The resting activity of 16/17 fibers significantly increased after MO instillation; 13 (77%) also exhibited sensitization of responses to graded CRD when tested 30 min after intracolonic MO instillation. The effects of kappa1-opioid receptor preferring agonists (U50,488H, U69,593 and U62,066), the kappa2-opioid receptor preferring agonist bremazocine, and the kappa3-opioid receptor preferring agonist naloxone benzoylhydrazone (nalBzoH) were tested on responses of 64 mechanosensitive afferent fibers to noxious CRD. All five agonists dose-dependently inhibited afferent fiber responses to noxious CRD. Doses producing inhibition to 50% of the control response to CRD did not differ among the five agonists, ranging from approximately 4 to 15 mg/kg. The effects of kappa1, kappa2, and kappa3 receptor agonists were attenuated by naloxone; two kappa-opioid receptor-selective antagonists were ineffective. There were no differences in the dose-response relationships of these drugs for fibers recorded from untreated and irritant-treated colons. Conduction velocities of the fibers remained unaffected after high doses of all tested agonists. In an in vitro study, U50,488 (10(-4) M) did not produce any significant change in the tension of colonic smooth muscle. These results document that responses of mechanosensitive pelvic nerve afferent fibers innervating the colon are inhibited by kappa-opioid receptor agonists having varying affinities for putative kappa-opioid receptor subtypes. The inhibitory effects of these drugs likely are mediated by an action at receptors associated with the afferent fibers. The receptor at which these effects are produced is kappa-opioid-like but clearly different from the kappa-opioid receptor characterized in the CNS and is perhaps an orphan receptor.

Inhibition of [3H]-U69593 binding and the cardiac effects of U50, 488H by calcium channel blockers in the rat heart

1. The calcium channel blockers (CCBs), nifedipine, nicardipine, diltiazem and verapamil, were used to displace the binding of [3H]-U69593 ((5a, 7a,8b)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4,5] dec-8-yl)-benzeneacetamide), a specific kappa-opioid agonist, in the rat cardiac sarcolemma. The CCBs competed with the binding of [3H]-U69593 (4 nM) in a dose-dependent manner. The displacing potency of verapamil was 55 times greater than that of nifedipine. 2. The effects of two CCBs, verapamil and nifedipine, on the arrhythmogenic action of kappa-receptor stimulation by a specific kappa-receptor agonist, U50,488H (trans-(+/-(-3),4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeacetamide methanesulphonate), were also studied in the rat isolated perfused heart. U50,488H 80-800 nmol dose-dependently induced arrhythmias, which were completely abolished by a selective kappa-receptor antagonist, nor-BNI (nor-binaltorphimine, 17,17'-(dicyclopropylmethyl)-6,6',7,7'-6,6'-imino-7,7'-binorphinan -3,4',14, 14'-tetrol), at 100 nmol. The arrhythmogenic effect was also attenuated by both verapamil and nifedipine in a dose-dependent manner. The ED50 values for verapamil and nifedipine were 2.75 and 63.7 nmol, respectively. The antiarrhythmic potencies of these two CCBs were correlated to their displacing potencies and inversely related to their well known potencies in inhibiting transmembrane Ca2+ influx in the cardiac muscle. 3. Measurement of [Ca2+]i in the absence of free extracellular Ca2+ by a spectrofluorometric method, with fura-2 as Ca2+ indicator, showed that U50,488H 5 x 10(-5) M slowly increased [Ca2+]i in single ventricular myocytes and this effect was abolished by pretreatment with nor-BNI (5 microM), or ryanodine (5 microM). Verapamil 1 and 10 microM abolished the effect of U50,488H in 37.5% (3 out of 8) and 100% (12 out of 12) of the cells studied, respectively. On the other hand, nifedipine 10 and 100 microM had no effect at all. Neither verapamil nor nifedipine exerted any significant effect on the caffeine-induced Ca2+ transient. 4. The observations suggest that CCBs may inhibit the actions of kappa-receptor stimulation at the level of the kappa-receptor.

Kappa-opioid receptor mediated antinociception in rats is dependent on the functional state of dihydropyridine-sensitive calcium channels

The modulatory effect of the dihydropyridine Ca2+ channel antagonist nimodipine on the analgesic action of the kappa-opioid receptor agonist U-69,593 was analyzed using the tail-flick test in rats. The antinociceptive effect of U-69,593 (0.25-4 mg/kg) was antagonized by L-type Ca2+ channel blockade with nimodipine (200 microgram/kg, i.p.), the ED50 being increased from 1.4 to 7.3 mg/kg. On the contrary, when an increase in the density of these channels was induced by means of chronic and simultaneous treatment with nimodipine (1 microgram/h, 7 days) and sufentanil (2 micrograms/h, 8 days), the analgesic effect of U-69,593 was potentiated by 5-fold. Our results suggest a functional coupling between kappa-opioid receptors and L-type Ca2+ channels in nociception.

Interaction of U-50,488H with calcium channel agonists and antagonists in different cardiac tissues

1. The present study examined the interaction of U-50,488H (specific k-agonist) with diltiazem (calcium channel antagonist) or Bay K 8644 (calcium channel agonist) on isolated left and right atria of the rat. 2. The inhibitory effects of U-50,488H on left and right atria were unaffected by the k-receptor antagonist MR-2266 (10(-7) and 5 x 10(-7) M) suggesting that they were not mediated via opioid receptors. 3. The inhibitory cardiac effects induced by U-50,488H were antagonized in presence of Bay K 8644. The negative inotropic response to the maximum concentration of U-50,488H used in presence of two concentrations of Bay K 8644 (10(-9), 3 x 10(-9) M) were 19 +/- 0.9% and 9 +/- 0.1% reductions in contractility respectively. These values were significantly (P < 0.001) lower than that obtained with the k-agonist alone (76 +/- 3.6%). Similar results were obtained for the negative chronotropism of right atria. 4. The inhibitory effect of U-50,488H was potentiating in the presence of diltiazem (5 x 10(-8) or 10(-7) M). The IC50 values for U-50,488H obtained in the left (22 +/- 2 x 10(-6) M) and right atria (610 +/- 40 x 10(-6) M) were significantly (P < 0.001) decreased in the presence of diltiazem. 5. These data demonstrate that transmembrane calcium influx may play an important role in the inhibitory cardiac effects of U-50,488H, which may be independent of k-receptor stimulation.

Opioid receptor-mediated inhibition of 3H-dopamine and 14C-acetylcholine release from rat nucleus accumbens slices. A study on the possible involvement of K+ channels and adenylate cyclase

The release of 14C-ACh from rat nucleus accumbens slices, induced by 15 mM [K+], was inhibited by the mu- and delta-opioid agonists DAMGO and DPDPE, respectively, whereas only the kappa agonist U50,488 reduced the release of 3H-DA. The opioid receptors involved appear to be localized on nerve terminals, since blockade of action potential propagation by 1 microM TTX did not diminish the inhibitory effects of DAMGO, DPDPE or U50,488. Enhancement of the potassium concentration in the superfusion medium to 56 mM with simultaneous reduction of the Ca2+ concentration from 1.2 mM to 0.12 mM induced a release similar to that caused by 15 mM K+ and 1.2 mM Ca+. Under this conditions, the inhibitory effects of both DAMGO and DPDPE on stimulated 14C-ACh release were reduced, whereas the inhibition of evoked 3H-DA release caused by U50,488 was not affected. Activation of mu- as well as delta-opioid receptors by DAMGO and DPDPE, respectively, inhibited forskolin-stimulated adenylate cyclase activity. However, increasing the intracellular cAMP levels with 0.3 mM 8-bromo-cAMP affected neither the depolarization-induced release of 14C-ACh or 3H-DA from accumbens slices nor the inhibitory effects of opioid receptor activation thereon. The results indicate that the mechanism by which functional mu and delta receptors presynaptically inhibit the depolarization-induced 14C-ACh release from nucleus accumbens slices is likely to involve an increase of potassium channel conductance. In contrast, activation of kappa-opioid receptors, which inhibits depolarization-evoked 3H-DA release, apparently does not result in a hyperpolarization of (dopaminergic) nerve terminals. In none of these inhibitory effects presynaptic adenylate cyclase appears to be involved.

Differential effects of opioid receptor agonists on nociception and cAMP level in the spinal cord of monoarthritic rats

Changes in functional responsiveness of spinal opioid receptors in monoarthritic rats were investigated at the behavioral and the molecular level. After intrathecal administration of morphine, D-Ala2-D-Leu5-enkephalin (DADLE), D-Pen2-D-Pen5-enkephalin (DPDPE) and dynorphin monoarthritic rats showed an enhanced antinociceptive response as measured by a tail-flick latency. No such changes were observed following administration of the selective kappa agonists U50,488H and U69,593. The opioid mu and delta receptor agonists (0.1-1.0 microM) inhibited the basal, as well as the forskolin-stimulated cAMP formation in spinal cord slices obtained from monoarthritic rats, whereas no significant changes were found in control animals. Higher concentrations of the mu and delta opioid receptor agonists were required to attenuate the cAMP level in spinal cord of control animals. The selective kappa agonists U50,488H and U69,593 did not influence the cAMP formation in monoarthritic or control animals. Additionally, we found that the GppNHp-stimulated level of cAMP was higher in the spinal cord slices of monoarthritic rats, which points to an enhanced responsiveness of the adenylate cyclase effector system to the action of this GTP analog. Our data suggest that the enhanced antinociceptive response to intrathecally administered opioids in monoarthritic rats may be connected with the increased sensitivity of adenylate cyclase to the inhibitory effects of mu and delta agonists.

Competitive interactions at [3H]1,3-DI(2-tolyl) guanidine (DTG)-defined σ recognition sites in guinea pig brain

In saturation binding experiments, (+)pentazocine, (+)3-(3-hydroxyphenyl)-N-propylpiperidine (3-PPP), haloperidol and rimcazole did not inhibit the binding of [3H]DTG in a purely competitive fashion. Although Scatchard analysis indicated that [3H]DTG bound to a single site, the inhibition curves of some, but not all, reference compounds exhibited Hill coefficients of less than 0.8. The Scatchard data were consistent with a model of hyperbolic competitive inhibition of binding to the [3H]DTG-defined sigma site, although other possibilities such as negative cooperativity or binding to two sites cannot be definitively excluded. Compounds from numerous pharmacological and structural classes inhibited the binding of [3H]DTG, suggesting that interactions of [3H]DTG with other receptors may have confounded the Scatchard analysis of the binding of [3H]DTG to sigma recognition sites.

Smithkline Beecham Prize for Young Psychopharmacologists: A review of the relationship between calcium channels and psychiatric disorders

The symptoms and etiology of most major psychiatric disorders probably represent an underlying disturbance of neurotransmitter function. Understanding the mechanisms which control neurotransmitter function, and in particular neurotransmitter release, is therefore of considerable importance in determining the appropriate pharmacological treatment for these disorders. Calcium entry into neurons triggers the release of a wide range of neurotransmitters and recently our understanding of the mechanisms which control neuronal calcium entry has increased considerably. Neuronal calcium entry occurs through either voltage-sensitive or receptor-operated calcium channels. This article reviews the different subtypes of calcium channel, with particular reference to their structure; drugs which act upon them; and the possible function of the subtypes identified to date. In addition, it reviews the potential role of calcium channel antagonists in the treatment of a wide range of psychiatric disorders, and concludes that these drugs may have an increasing therapeutic role particularly in the treatment of drug dependence, mood disorders and Alzheimer's disease.

Ca2+ channel modulation by dihydropyridines modifies sufentanil-induced respiratory depression in cats

We analyzed the interaction between sufentanil, a selective mu agonist, and two dihydropyridines, the Ca2+ antagonist, nimodipine, and the Ca2+ agonist, Bay K 8644, on the respiratory actions induced in the brainstem of cats. Drugs were applied topically to the ventral medullary surface. Sufentanil (0.26-26 nmol) consistently induced an immediate and dose-dependent reduction in tidal volume. Respiratory frequency was only depressed by the higher doses of the opiate. Pretreatment with nimodipine (0.19 and 0.38 mumol) potentiated the respiratory depression induced by sufentanil (0.26 nmol). The potentiation included both frequency and tidal volume. On the other hand, under the influence of Bay K 8644 (0.28 nmol), the respiratory effect of the opiate (7.8 nmol) was partially antagonized. Our results indicate that modulation of the L-type Ca2+ channels by dihydropyridines modifies sufentanil-induced respiratory depression at the controlling medullary mechanisms of breathing.

Effect of the kappa-receptor agonist, U-50,488H, on cerebral ischemia-induced impairment of working memory assessed in rats by a three-panel runway task

The effect of U-50,488H, a selective kappa-receptor agonist, on memory functions in an animal model of cerebral ischemia was investigated by use of a three-panel runway task. A 5-min period of ischemia caused a significant increase in the number of errors (pushes made on the two incorrect panels of the three panel-gates at four choice points) in a working memory task but it did not impair a reference memory task. U-50,488H at 10 and 32 mg/kg, administered i.p. immediately after blood flow restoration significantly reduced the increase in errors expected to occur in a working memory task assessed 24 h after 5 min of ischemia. This protective effect of U-50,488H on amnesia in the ischemic rat was antagonized by the kappa-receptor antagonist, MR-2266. We conclude that U-50,488H prevents the impairment of working memory following transient forebrain ischemia, an event mediated by the activation of the kappa-opioid receptor.

The role of neuronal calcium channels in dependence on ethanol and other sedatives/hypnotics

This review discusses the importance of neuronal calcium currents in dependence on ethanol, barbiturates, benzodiazepines and opiates. The main sections describe the actions of ethanol on control of intracellular calcium and on calcium and calcium-dependent conductance mechanisms. In particular, the effects of both acute and chronic ethanol treatment on dihydropyridine-sensitive, voltage-dependent, calcium channels are described. The later sections cover the effects of barbiturates, benzodiazepines and opiates on these systems. The conclusions suggest that dihydropyridine calcium channel antagonists may offer a new therapeutic approach to the treatment of ethanol and opiate dependence.