Serotonergic involvement in the antinociceptive action of and the development of tolerance to the kappa-opioid receptor agonist, U-50, 488H

The antinociceptive action of U-50,488H [( trans-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)- cyclohexyl]benzeneacetamide]), a selective kappa-opioid receptor agonist, was antagonized by serotonergic (5-HT) but not by noradrenergic receptor antagonists. U-50,488H (i.c.v.) was antagonized by more than 2-fold by i.c.v. administered pindolol, methysergide, mianserin, ketanserin or pirenperone. A similar degree of antagonism of U-50,488H (i.c.v.) was found after intrathecal (i.t.) treatments with pindolol or methysergide but not with mianserin, ketanserin or pirenperone. When U-50,488H was given i.t., its antinociceptive action was antagonized by pindolol or methysergide administered i.t. but potentiated by mianserin, ketanserin or pirenperone administered i.t. Tolerance to the antinociceptive action of U-50,488H was induced in mice using slow release preparations of U-50,488H. A smaller degree of cross-tolerance to the antinociceptive action of 5-HT also developed. 5-HT receptor antagonists (pindolol or ketanserin) were coadministered with U-50,488H to test for their effect on the development of tolerance to U-50,488H. A greater degree of tolerance was observed after ketanserin administration whereas the development of tolerance was partially blocked by the coadministration of pindolol. Changes in the properties of the kappa-opioid, 5-HT1 and 5-HT2 receptor binding sites in the cortical, striatal and spinal tissues of control and U-50,488H-tolerant mice were investigated. There were no significant changes in the maximum binding values of [3H]ethylketazocine, [3H]-5-HT or [3H]ketanserin. The Kd values of all ligands increased approximately 2-fold in most of the regions.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential antagonism of delta opioid agonists by naltrindole and its benzofuran analog (NTB) in mice: evidence for delta opioid receptor subtypes.

In this study naltrindole (NTI) and its benzofuran derivative (NTB) were studied for their antagonist activity against various delta opioid receptor agonists in the tail-flick antinociceptive assay in mice. The antinociceptive ED50 of i.c.v. administered DSLET [(D-Ser2, Leu5, Thr6)enkephalin] was shifted about 4-fold by either s.c. NTB or i.c.v. NTI injection. On the other hand, the antinociceptive ED50 of i.c.v. administered DPDPE [(D-Pen2,D-Pen5)enkephalin] was shifted 1.4- and 1.8-fold with s.c. NTB and i.c.v. NTI administration, respectively, which were significantly lower than the shifts observed with DSLET. NTB did not alter the antinociceptive action of i.c.v. administered [(D-Ala2,D-Leu5)enkephalin], morphine sulfate, [(D-Ala2,MePhe4,Gly-ol5)enkephalin] or U-50,488H (trans(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]benzeneacetamide). At spinal sites, the antinociceptive ED50 of intrathecal (i.t.) administered DSLET was increased by 12.5-fold by s.c. NTB injection, whereas that of DPDPE was unaffected. NTB injection at this site also did not alter the antinociceptive action of i.t. administered [D-Ala2, D-Leu5]enkephalin, [(D-Ala2,MePhe4,Gly-ol5)enkephalin] or morphine sulfate. Pretreatment of animals with beta-funaltrexamine caused a large increase in the capacity of NTB to antagonize the antinociceptive activity of i.t. administered DSLET with little change in that of i.t. administered DPDPE. When cross-tolerance between DSLET and DPDPE was studied by i.c.v. injection of a single large dose of either DSLET or DPDPE 24 hr before the antinociceptive assay, there was no development of cross-tolerance between the two peptides. Based on these results, it was concluded that the antinociceptive action of DSLET and DPDPE may be mediated by different receptors, possibly delta opioid subtypes.     

Differential contribution of descending serotonergic and noradrenergic systems to central Tyr-D-Ala2-Gly-NMePhe4-Gly-ol5 (DAMGO) and morphine-induced antinociception in mice

Differences in antinociceptive (inhibition of tail-flick response) action of morphine and Tyr-D-Ala2-Gly-NMePhe4-ol5 (DAMGO) were demonstrated by intracerebroventricular (i.c.v.) administration of these agonists along with intrathecal (i.t.) administration of a variety of antagonists: yohimbine, methysergide, naloxone and nor-binaltorphimine. Intracerebroventricular morphine analgesia was antagonized by either i.t. yohimbine or methysergide, whereas i.c.v. DAMGO analgesia was only antagonized by i.t. methysergide. Thus, for i.c.v. morphine-induced analgesia, descending spinal noradrenergic and serotonergic systems were involved, whereas for DAMGO analgesia, only the serotonergic system was involved. The dose-response curve for i.c.v. morphine reached a plateau at high doses, whereas i.c.v. DAMGO analgesia peaked at 10 ng and then decreased thereafter, producing a bell-shaped dose-response curve. This decrement in analgesic response could be reversed by low doses of i.t. methysergide and i.t. pindolol. It was concluded that activation of serotonin-1 (5-HT1) receptors plays a role in the decrease in analgesia from high doses of DAMGO. Combinations of i.t. morphine with i.t. 5-HT or i.t. clonidine produced additive or greater analgesic responses. Combinations of i.t. DAMGO with i.t. 5-HT or i.t. clonidine produced less than additive interactions. Part of the latter responses appeared to be due to activation of 5-HT1 receptors; blockade of these receptors by pindolol enhanced i.t. DAMGO-induced analgesia. Morphine and DAMGO differ further because i.c.v. morphine activated a descending antianalgesic pathway mediated by spinal dynorphin A(1-17), whereas i.c.v. DAMGO at a high dose did not. Thus, morphine and DAMGO differ in their modes of antinociceptive action as measured by the tail-flick response.     

Kappa opioid analgesia is dependent on serotonergic mechanisms

The serotonergic dependence of mu and kappa opioid analgesia was compared in the mouse tail-flick and hot-plate assays using morphine and the selective kappa agonist, U-50, 488H, respectively. Depletion of serotonin with p-chlorophenylalanine resulted in a marked antagonism of U-50,488H analgesic potency in both assays, as did reserpine pretreatment. Both of these effects were dose-related and the latter was reversed by treatment with the serotonin precursor, 5-hydroxytryptophan. Several reputed serotonin antagonists (cyproheptadine, ketanserin and pirenperone) also antagonized U-50,488H analgesia. In contrast, the analgesic potency of morphine was only decreased slightly by p-chlorophenylalanine and reserpine, and not at all by the serotonin antagonists. Thus, kappa, but not mu, analgesia is strongly dependent upon serotonergic mechanisms in these assays. However 5-hydroxytryptophan did not enhance U-50,488H analgesia in nonpretreated mice or in mice made tolerant to U-50,488H. Likewise, it did not alter the development of U-50,488H tolerance. On this basis it appears that kappa opioid tolerance is not due to serotonergic hypofunction.     

Nor-binaltorphimine, a highly selective kappa-opioid antagonist in analgesic and receptor binding assays

Previously, we reported on an opioid antagonist, nor-binaltorphimine (nor-BNI), that had high selectivity for kappa opioid receptors in smooth muscle preparations. In this study, nor-BNI administered either s.c. or i.c.v. was shown to antagonize significantly the antinociceptive effects of the kappa opioid agonists, ethylketazocine and U-50,488H at doses that had no effect on the antinociceptive effect of mu agonists, morphine and [D-Ala3, MePhe4, Gly-ol5]enkephalin and the delta agonist, [D-Pen3, D-Pen5]enkephalin. Nor-BNI and U-50,488H were used to demonstrate that kappa opioid receptors in the spinal cord were more important than those located supraspinally for kappa-mediated analgesia. Nor-BNI also possessed high affinity and high selectivity for kappa opioid receptors in the receptor binding assay. However, the comparatively low selectivity of BNI in receptor binding studies did not correlate with the high pharmacologic selectivity for kappa receptors.     

Central and systemic morphine-induced antinociception in mice: contribution of descending serotonergic and noradrenergic pathways

The relative importance of descending serotonergic and noradrenergic pathways to the antinociceptive action of supraspinally and systemically administered morphine sulfate, and to the antinociceptive synergism observed between spinally and supraspinally administered morphine was examined in mice. Morphine administered i.c.v., intrathecally (i.t.), s.c. or i.t. + i.c.v. produced a dose-dependent antinociceptive response, as measured by the tail-flick test. The dose-response curve for morphine (i.c.v.) was shifted significantly to the right by equal i.t. doses of methysergide or phentolamine. However, the most pronounced shift to the right was caused by i.t. administration of the selective alpha-2 antagonist yohimbine. Administration of a subantinociceptive dose of morphine (i.t.) along with morphine (i.c.v.) shifted the dose-response curve for morphine (i.c.v.) 6-fold to the left. The resulting dose-response curve for morphine (i.c.v.) was shifted to the right more by phentolamine (i.t.) than by methysergide (i.t.). Similarly, the dose-response curve for morphine (s.c.) was shifted more to the right by phentolamine (i.t.) than by methysergide (i.t.). These results suggest that the descending noradrenergic system, more than the serotonergic system, may be an important component in spinal/supraspinal interactive mechanisms that may mediate the antinociceptive action of systemically administered morphine.     

Chronic l-alpha-acetylmethadol (LAAM) in rhesus monkeys: tolerance and cross-tolerance to the antinociceptive, ventilatory, and rate-decreasing effects of opioids

Although l-alpha-acetylmethadol (LAAM) is a maintenance treatment for opioid dependence, few studies have systematically assessed the behavioral effects of LAAM and other drugs in LAAM-treated subjects. In the current study, we assessed the ventilatory, antinociceptive, and rate-decreasing effects of drugs (s.c. except dynorphin, which was administered i.v.) in rhesus monkeys (n = 3 or 4) before and during chronic treatment with 1.0 mg/kg/12 h LAAM (s.c.). Minute volume (V(E)) was reduced to 62% of baseline during LAAM treatment and remained depressed after more than 10 months of LAAM treatment. A cumulative dose of 10.0 mg/kg morphine decreased V(E) to similar values under baseline (53%) and LAAM-treated (52%) conditions; however, larger doses of morphine (up to 56.0 mg/kg) could be administered safely to LAAM-treated monkeys. LAAM treatment produced dependence as evidenced by a 220% increase in V(E) after a dose of naltrexone (0.032 mg/kg) that did not modify ventilation under baseline conditions. Compared with baseline, LAAM treatment increased the ED(50) values for the rate-decreasing effects of nalbuphine, morphine, and alfentanil by 7-, 7-, and 2-fold, respectively, in monkeys responding under a fixed ratio 10 schedule of food presentation. Similarly, LAAM treatment increased ED(50) values for the antinociceptive effects of morphine and alfentanil by 5- and 3-fold, respectively. LAAM treatment also increased the ED(50) values for the antinociceptive effects of the kappa-agonist enadoline by 5-fold and not those of U-50,488. That tolerance developed differentially to the ventilatory, rate, and antinociceptive effects of mu-agonists in LAAM-treated monkeys suggests that cross-tolerance might not be a safe therapeutic approach for the treatment of some opioid abusers.     

The antinociceptive role of a bulbospinal serotonergic pathway in the rat brain

The antinociceptive role of spinal serotonin (5-HT) neurons descending from 5-HT cells near the ventrolateral surface of the medulla oblongata was investigated by stimulating these cells in normal rats, in rats with generalized or selective chemical ablation of 5-HT nerves, and in rats with postsynaptic blockade of 5-HT receptors. Electrical stimulation of the lateral medulla elicited analgesia in normal rats; the increase in pain threshold was proportional to the intensity and to the frequency of stimulation. In addition, microinjection of kainic acid or L-glutamate at the same sites also produced analgesia. However, generalized destruction of CNS 5-HT nerves produced by intraventricular injection of 5,7-dihydroxytryptamine (5,7-DHT) or selective destruction of spinal 5-HT nerves produced by intraspinal injection of 5,7-DHT reduced the magnitude of the antinociceptive responses to electrical stimulation. Postsynaptic blockade of CNS 5-HT receptors produced by intraventricular injection of cyproheptadine also reduced the stimulation-produced analgesia. The specificity of the lesions for 5-HT nerves is demonstrated by the lack of effect on the levels of noradrenaline in the same brain regions. The results indicate that the activity of 5-HT nerve cells adjacent to the ventrolateral surface of the medulla oblongata and projecting to the spinal cord serves to elevate pain threshold.     

Interaction of morphine with intrathecally administered calcium and calcium antagonists: evidence for supraspinal endogenous opioid mediation of intrathecal calcium-induced antinociception in mice

The interaction between morphine [i.p. and intrathecal (i.t.)] and calcium and its antagonists (i.t. and i.c.v.) was studied in the mouse tail-flick test for antinociception. Calcium (0.66 mumol i.t.) produced antinociception comparable to that of morphine (0.5 microgram i.t.) but had a significantly longer duration. A lower dose of calcium (0.16 mumol i.t.) significantly potentiated morphine (0.2 and 0.5 micrograms i.t.). The antinociceptive effect of i.p. morphine was also potentiated by i.t. calcium, but was antagonized by the i.t. administration of ethylene glycol bis(beta-aminoethyl ether)-N,N'-tetraacetic acid (3.7-7.5 nmol), verapamil (15 micrograms), magnesium (9.4 nmol) and barium (1-2 nmol). In contrast, i.t. calcium and i.p. morphine were significantly potentiated by the i.c.v. administration of verapamil (15 micrograms) and antagonized by i.c.v. calcium (0.33 mumol). The antinociceptive effect of i.t. calcium was antagonized by naloxone administered s.c. (1 mg/kg) or i.c.v. (0.5 microgram), but not i.t. (0.5 and 10 microgram). It is concluded that the antinociceptive effect of i.t. calcium is mediated, at least partly, by a reflex supraspinal release of endogenous opioids, and that the administration of calcium and its antagonists modify the antinociceptive effect of morphine in opposite directions, depending upon whether they are administered by the i.t. or i.c.v. routes. Calcium may serve as a useful adjunct for opioid-induced analgesia via the i.t. route.     

Kappa-opioid receptor-mediated antinociception in the rat. II. Supraspinal in addition to spinal sites of action

This study examines whether there is a supraspinal, in addition to spinal, component to the antinociceptive actions against heat and pressure stimuli of kappa-opioid receptor agonists (U-69,593, U50,488H, bremazocine and tifluadom) as compared to mu-opioid receptor agonists (Tyr-D-Ala-Gly-NMe-Gly-ol, fentanyl and morphine) in the rat. The antinociception induced by kappa- and mu-opioids (applied s.c.) was unaffected by systemic quaternary naltrexone (50 mg/kg) revealing that it is mediated in the central nervous system. All kappa- and mu-opioids produced dose-dependent antinociception upon intrathecal application, in each case reversible by naloxone (5 mg/kg s.c.). However, intrathecal application of naloxone could only partially (by ca. 50%) antagonize the antinociception evoked by systemically applied U50,488H and morphine: this suggests sites of action in brain in addition to spinal cord for both mu- and kappa-opioids. Intraventricular application of mu-agonists produced maximal, dose-dependent antinociception. All kappa-agonists were also active in producing dose-dependent antinociception although curves were shallow and maximal antinociception could not be attained. The action of tifluadom was shown to be stereospecific. Naltrexone was 10-fold more potent in blocking morphine as compared to U50,488H whereas nor-binaltorphimine, a preferential kappa-antagonist, was 6-fold more potent against U50,488H than morphine. Indeed, whereas a dose of 0.2 mg/kg of naltrexone reversed mu-agonist actions, this dose was inactive against all kappa-agonists: the actions of these could be antagonized only by 2.0 mg/kg. These data indicate that in addition to kappa-receptors in the spinal cord, kappa-receptors in the brain can mediate antinociception against noxious heat and pressure.     

(+/-)-3,4-Methylenedioxymethamphetamine administration to rats does not decrease levels of the serotonin transporter protein or alter its distribution between endosomes and the plasma membrane

We showed that the serotonin (5-HT) neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) reduces brain tissue 5-HT, decreases expression of 5-HT transporter (SERT) protein, and increases expression of glial fibrillary acidic protein (GFAP). In contrast, doses of (+/-)-3,4-methylenedioxymethamphetamine (MDMA) that decrease brain tissue 5-HT fail to alter expression of SERT or GFAP. Using a new and highly sensitive anti-SERT antibody, we determined whether MDMA alters the subcellular distribution of SERT protein by measuring SERT expression in endosomes and plasma membranes 2 weeks after MDMA administration. Rat brain tissues (caudate, cortex, and hippocampus) were collected 3 days and 2 weeks after MDMA (7.5 mg/kg i.p., every 2 h x 3 doses) or 5,7-DHT (150 microg/rat i.c.v.) administration. Representative results from cortex are as follows. At both 3 days and 2 weeks postinjection, MDMA decreased tissue 5-HT (65%) and had no effect on GFAP expression. MDMA increased heat shock protein 32 (HSP32; a marker for microglial activation) expression (30%) at 3 days, but not 2 weeks. MDMA did not alter SERT expression at either time point and did not alter SERT levels in either endosomes or plasma membranes (2 weeks). 5,7-DHT decreased tissue 5-HT (80%), increased HSP32 expression at both time points (about 50%), and increased GFAP expression at 2 weeks (40%). 5,7-DHT decreased SERT expression (33%) at 2 weeks, but not at 3 days. These findings indicate that a dosing regimen of MDMA that depletes brain 5-HT does not alter SERT protein expression or the distribution of SERT between endosomes and the plasma membrane and does not produce detectable evidence for neurotoxicity.     

Role of spinal 5-HT(1A) receptors in morphine analgesia and tolerance in rats.

We here studied the involvement of spinally located 5-HT(1A) and opioid receptors, in the paradoxical effects that their activation can produce on nociception. Intrathecal (i.t.) injection of the 5-HT(1A) receptor agonist 8-hydroxy-2-[di-n-propylamino] tetralin (8-OH-DPAT) (1-10 microg) induced analgesic effects in the formalin model of tonic pain whereas in the paw pressure test, it decreased the vocalization threshold. In this latter test, i.t. 8-OH-DPAT also markedly reduced the analgesic effect of systemic morphine (5-10 mg/kg, s.c.). At 10 microg, 8-OH-DPAT totally abolished the effect of 5 mg/kg of morphine; this inhibitory effect was antagonized by pre-treatment with 0.63 mg/kg of the 5-HT(1A) antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)-cyclohexanecarboxamide-trihydrochloride). In contrast, the i.t. injection of WAY-100635 (1-10 microg) dose-dependently potentiated the antinociceptive activity of a dose of morphine (2.5 mg/kg, s.c.). Furthermore, WAY-100635 (10 microg, i.t.) potentiated morphine analgesia in morphine-tolerant rats. These findings demonstrate that 5-HT(1A) receptor agonists can act in the spinal cord to produce both hyper- and hypo-algesic effects and play a major role in the opioid analgesia and tolerance.     

Mechanisms of effects of intrathecal serotonin on nociception and blood pressure in rats

The effects of intrathecal (i.t.) serotonin (5-HT) and a number of serotonergic receptor agonists on nociception and blood pressure were examined in rats. Intrathecal 5-HT produced dose-dependent inhibition of the nociceptive tail-flick reflex (ED50 = 100.0 micrograms) and dose-dependent depressor effects. The 5-HT1A agonist 8-hydroxy-N,N-dipropyl-2-aminotetralin and the 5HT1B agonist 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU-24969) also produced depressor effects but, in contrast to 5-HT, facilitated the tail-flick reflex, whereas the 5-HT2 agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, 6-chloro-2-(1-piperazinyl)-pyrazine (MK-212) and quipazine produced dose-dependent antinociception and had little or no effect on blood pressure. These results suggest that the antinociceptive and depressor effects of i.t. 5-HT are mediated by spinal 5-HT2 and 5-HT1 receptors, respectively. In other experiments, rats chronically treated with i.t. 5-HT developed tolerance to its antinociceptive effects, whereas chronic i.t. morphine or clonidine did not produce cross-tolerance to i.t. 5-HT. These results suggest that serotonergic spinal antinociceptive mechanisms are distinct from the mechanisms by which opioid receptor and alpha-2 adrenoceptor agonists produce antinociception in the spinal cord.     

Analgesia produced by E-2078, a systemically active dynorphin analog, in mice

E-2078 is a very stable dynorphin analog that has the same affinity and selectivity for opioid receptors as dynorphin-A by in vitro bioassay. In the present study, we have characterized the receptor selectivity of E-2078 in mu-, delta- and kappa-representative binding assays, and have evaluated the analgesic effect of systemically administered E-2078 by the tail pinch, tail flick and formalin tests in mice. E-2078 possessed a higher affinity for kappa-receptors than for mu- or delta-receptors in the receptor-binding assay. Dose-related, long-lasting analgesia was produced by s.c. injection of E-2078, and its peak effect was observed 2 hr after s.c. administration in all the analgesic tests. This analgesic effect was produced at doses that did not affect rotarod latency. Post-treatment with naloxone dose-dependently reversed the analgesic effects of both E-2078 and morphine, but E-2078-induced analgesia was relatively resistant to naloxone antagonism. Pre-treatment with the kappa-antagonist, nor-binaltorphimine, antagonized the analgesic effect of E-2078 but had little effect on the analgesic action of morphine. These data indicate that E-2078 is a systemically active analgesic and suggest that the activation of kappa-opioid receptors contributes to analgesia.     

Roles of central and peripheral mu, delta and kappa opioid receptors in the mediation of gastric acid secretory effects in the rat

The opioid receptors involved in the mediation of gastric acid secretory effects were studied in the pylorus-ligated rat. The effects of i.c.v. and i.v. administration of morphine and mu ([D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePheD-Pro-NH2)-, delta ([D-Pen2,D-Pen5]enkephalin)- and kappa-selective [trans-3,4-dichloro-N-methyl-N-[2-91-pyrrolidinyl)-cyclohexyl]- benzeneacetamide methanesulfonate (U-50,488H), dynorphin-(1-9), dynorphin-(1-17), nalorphine, alpha-neoendorphin and ethyl-ketocyclazocine) opioid receptor agonists on gastric volume and acid output were examined. Morphine, [D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePhe-D-Pro-NH2 decreased gastric acid secretion more potently after i.c.v. than after i.v. administration. The inhibitory effect of i.v. administered morphine on gastric acid secretion was not blocked by the quaternary opioid antagonist naltrexone methylbromide when given s.c. However, when naltrexone methylbromide was administered i.c.v., it blocked completely the effects of i.c.v. morphine and partially antagonized the effects of i.v. morphine, indicating a central site of action for morphine. The delta-selective agonist [D-Pen2,D-Pen5]enkephalin did not alter gastric acid secretion after i.c.v. or i.v. administration. The kappa-selective opioid agonist U-50,488H produced a dose-dependent increase in gastric acid secretion after i.v. but not i.c.v. administration. The other kappa-selective agonists tested did not produce a significant increase in gastric acid secretion after i.c.v. or i.v. administration. The increase in gastric acid secretion produced by U-40,488H was blocked by pretreatment with the opioid receptor antagonist naloxone, the nonselective muscarinic receptor antagonist atropine and the M1 selective muscarinic receptor antagonist pirenzepine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Activation of kappa-opioid receptors inhibits pruritus evoked by subcutaneous or intrathecal administration of morphine in monkeys

Pruritus (itch sensation) is the most common side effect associated with spinal administration of morphine given to humans for analgesia. A variety of agents have been proposed as antipruritics with poorly understood mechanisms and they are effective with variable success. kappa-Opioid agonists possess several actions that are opposite to micro -opioid agonists. We proposed to investigate the role of kappa-opioid receptors (KORs) in morphine-induced scratching and antinociception in monkeys. Scratching responses were counted by observers blinded to treatment. Antinociception was measured by a warm water (50 degrees C) tail-withdrawal assay. Pretreatment with low doses of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide (U-50488H) (0.032-0.18 mg/kg s.c.), a selective KOR agonist, dose dependently suppressed the s.c. morphine dose-effect curve for scratching and potentiated s.c. morphine-induced antinociception. In addition, s.c. U-50488H attenuated i.t. morphine (10 and 32 micro g)-induced scratching while maintaining or enhancing i.t. morphine-induced antinociception. The combination of s.c. or i.t. morphine with low doses of U-50488H did not cause sedation. More importantly, pretreatment with 3.2 mg/kg nor-binaltorphimine, a selective KOR antagonist, blocked the effects of s.c. U-50488H on both s.c. and i.t. morphine-induced scratching. These results indicate that activation of KOR attenuates morphine-induced scratching without interfering with antinociception in monkeys. This mechanism-based finding provides functional evidence in support of the clinical potential of KOR agonists as antipruritics in the presence of MOR agonist-induced pruritus.     

D-Pro10]-dynorphin(1-11) is a kappa-selective opioid analgesic in mice

The synthetic opioid agonist [D-Pro10]-dynorphin(1-11) (DPDYN) binds kappa receptors with both high affinity and selectivity in vitro. We have examined the in vivo characteristics (i.e., analgesic properties) of the peptide in mice. The analgesic effects of i.c.v. administered DPDYN were determined in two nociceptive tests, involving thermal cutaneous (tail-flick) and chemical visceral (AcOH-induced writhing) stimuli, in which mu and kappa receptors are known to be activated differentially. The antinociceptive action of DPDYN was compared with that of 1) morphine and U-50,488H, which are, respectively, the prototypical mu and kappa agonists, 2) dynorphin A which is the endogenous parent peptide and 3) Leu-enkephalin, which represents the N-terminal sequence of DPDYN. DPDYN did not show any activity against thermal stimulus but, in contrast, produced a dose-related effect against chemical pain. In the AcOH-writhing test, there was no significant cross-tolerance between morphine and DPDYN in mice made tolerant to morphine. Pretreatment with low doses of s.c. naloxone (less than 1 mg/kg) antagonized completely the antinociceptive action of morphine but only partially reversed the analgesic action of DPDYN. In gastrointestinal studies, DPDYN as well as U-50,488H were ineffective (maximum effect lower than 25%) in inhibiting intestinal transit in mice, in contrast to morphine which produced a dose-related antitransit effect reaching 100%. These data indicate that the in vivo properties, and particularly analgesia, of i.c.v. administered DPDYN are mediated by a "non-mu" (presumably kappa) opioid receptor at the supraspinal level of the opioid system of the mouse.     

Multiplicative interaction between intrathecally and intracerebroventricularly administered mu opioid agonists but limited interactions between delta and kappa agonists for antinociception in mice

Simultaneous action of morphine on supraspinal and spinal sites produces a multiplicative interaction for antinociception which may be important for the analgesia produced by systemically administered morphine. The purpose of this study was to see whether other agonists with more receptor selective opioid actions than morphine would also produce this multiplicative interaction. DAMPGO (Tyr-D-Ala2-Gly-NMePhe4-Gly-ol5), DPDPE (D-Pen2, D-Pen5, enkephalin) and U50-488H, opioid agonists highly selective for mu, delta and kappa receptors, respectively, were administered alone i.c.v. or intrathecally (i.t.) or in combination (i.c.v. plus i.t.) to determine ED50 values for the tail-flick response in mice. These ED50 values were examined isobolographically in relation to the theoretical additive ED50 values by the potency ratio method. First, DAMPGO given i.cv and i.t. was similar to morphine, indicating that simultaneous supraspinal and spinal mu agonist administration produce the multiplicative interaction. Second, concurrent administration of DPDPE or U50,488H, i.c.v. and i.t., as well as cross-over combinations of DPDPE at one and U50,488H at the other site, produced additive interactions only. The multiplicative interaction was a property characteristic of mu but not delta and kappa agonists. Based on the similarity between morphine and DAMPGO, it was postulated that both mu agonists act on redundant descending pain inhibitory pathways to produce multiplication. A second mechanism for multiplicative interaction was based on the difference between DAMPGO and morphine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cocaine-induced elevation of plasma adrenocorticotropin hormone and corticosterone is mediated by serotonergic neurons.

We investigated the hypothesis that cocaine-induced elevations of plasma adrenocorticotropin hormone (ACTH) and corticosterone are mediated by brain serotonin (5-HT) neurons. Adult male rats were pretreated with the 5-HT depleting agent p-chlorophenylalanine, the 5-HT neurotoxin 5,7-dihydroxytryptamine, the partial 5-HT1A agonist 8-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-8- azaspirol[4,5]-decane-7,9-dione (BMY 7378) or the 5-HT1C/2 antagonist ritanserin. The effects of cocaine (2-15 mg/kg, i.p.) on plasma ACTH and corticosterone were then examined. Cocaine dose-dependently increased ACTH and corticosterone concentration. This increase was prevented by 5-HT depletion with PCPA and by destruction of 5-HT neurons with i.c.v. injections of 5,7-dihydroxytryptamine. The cocaine-induced elevation of ACTH and corticosterone was not significantly modified by administration of the partial 5-HT1A agonist BMY 7378, suggesting that 5-HT1A receptors probably do not mediate ACTH and corticosterone secretion. However, pretreatment with the 5-HT2/5-HT1C antagonist ritanserin virtually eliminated the cocaine-induced elevation of corticosterone. To determine whether these effects of cocaine are centrally mediated, conscious rats received cocaine injections into the cerebral ventricle through chronically implanted cannulas. Plasma ACTH concentrations were dose-dependently increased, whereas low doses (50 micrograms/kg, i.c.v.) produced a maximal increase in corticosterone concentration. These data indicate that the cocaine-induced stimulation of ACTH and corticosterone secretion is mediated by 5-HT neurons in brain, and furthermore, that 5-HT2 or 5-HT1C receptors are responsible for this effect.     

Spinal adenosine modulates descending antinociceptive pathways stimulated by morphine

Adenosine-mediated analgesia and interactions between opioids and adenosine in the brain have been observed by several researchers. Our investigations were designed to examine opioid-adenosine interactions at spinal sites and possible modulation of opioid-stimulated descending antinociceptive pathways by adenosine in the spinal cord. Methylxanthines administered intrathecally (i.t.) were used as adenosine receptor antagonists to determine possible interactions between opioids and endogenous adenosine. Theophylline administered i.t. dose-dependently antagonized analgesia induced by morphine administered i.t. or i.c.v. as measured by tail-flick and hot-plate assays. Analgesia induced by i.t. injections of 2-chloroadenosine, an adenosine agonist, was also antagonized by theophylline. However, doses of naloxone (i.t.) that antagonized analgesia induced by i.t. injections of morphine had no effect on 2-chloroadenosine (i.t.)-induced analgesia. These data support morphine-stimulated release of adenosine in the spinal cord. Antagonism of morphine (i.c.v.)-induced analgesia by theophylline was mimicked by caffeine and isobutylmethylxanthine. The results could not be explained as a consequence of effects on phosphodiesterease enzymes, drug-induced hyperalgesia or spinal redistribution of i.c.v. administered morphine. Therefore, our studies suggest that endogenous adenosine in the spinal cord is involved in analgesia mediated by opioid-stimulated descending antinociceptive pathways.