Possible mechanisms of salt-induced hypertension in Dahl salt-sensitive rats

The effects of acute and chronic administration of cocaine on the antinociception and tolerance to the antinociceptive actions of mu-(morphine), kappa-(U-50,488H), and delta-([D-Pen2,D-Pen5]enkephalin; DPDPE), opioid receptor agonists were determined in male Swiss-Webster mice. Intraperitoneal injection of 40 mg/kg of cocaine by itself produced weak antinociceptive response as measured by the tail-fick test but the lower doses were ineffective. Administration of morphine (10 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (10 microg/mouse, ICV) produced antinociception in mice. Cocaine (20 mg/kg) potentiated the antinociceptive action of morphine and DPDPE but had no effect on U-50,488H-induced antinociception. Administration of morphine (20 mg/kg, SC), U-50,488H (25 mg/kg, IP) or DPDPE (20 microg/mouse, ICV) twice a day for 4 days resulted in the development of tolerance to their antinociceptive actions. Tolerance to the antinociceptive actions of morphine and U-50,488H was inhibited by concurrent treatment with 20 or 40 mg/kg doses of cocaine; however, tolerance to the antinociceptive action of DPDPE was not modified by cocaine. It is concluded that cocaine selectively potentiates the antinociceptive action of mu- and delta- but not of the kappa-opioid receptor agonist. On the other hand, cocaine inhibits the development of tolerance to the antinociceptive actions of mu- and kappa- but not of delta-opioid receptor agonists in mice.     

Characterization of the membrane-associating domain of the sperm adhesive protein, bindin

Male Swiss-Webster mice were rendered tolerant to morphine by subcutaneous implantation of a morphine pellet, each containing 75 mg morphine base, for 3 days. Mice implanted with placebo pellets served as controls. A high degree of tolerance to the analgesic effect of morphine developed as evidenced by decreased analgesic response to various doses of morphine. A selective kappa-opiate agonist, U-50,488H (8, 16 and 32 mg/kg, i.p.) produced dose-dependent analgesic and hypothermic effects in mice implanted with placebo pellets. A significant decrease in the analgesic and hypothermic effects of U-50,488H was observed in morphine tolerant mice as compared to placebo-treated mice. Mice were rendered tolerant to U-50,488H by injecting the drug (25 mg/kg, i.p.) twice daily for 4 days. Vehicle injected mice served as controls. Tolerance to the analgesic and hypothermic effects of U-50,488H in mice injected chronically with the drug was evidenced by the decreases in the intensity of these responses when compared to those observed in vehicle injected controls. Morphine produced a dose-dependent analgesic and hypothermic effects in mice injected chronically with vehicle but the intensity of these effects was significantly lower in mice injected chronically with U-50,488H. These results indicate that a substantial tolerance to analgesic and hypothermic effects of U-50,488H develops in morphine tolerant mice. The effect of chronic injections of U-50,488H on the binding of [3H]ethylketocyclazocine (EKC) and [3H]D-Ala2,MePhe4,Gly-ol5-enkephalin (DAMGO) to whole brain and spinal cord kappa- and mu-opiate receptors was determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

The competitive alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor antagonist LY293558 attenuates and reverses analgesic tolerance to morphine but not to delta or kappa opioids

Antagonists of the NMDA type of excitatory amino acid (EAA) receptor attenuate or reverse the development of tolerance to the analgesic effects of the mu opioid agonist morphine, the delta-1 opioid agonist DPDPE but not the kappa-1 agonist U50,488H or the kappa-3 agonist naloxone benzoylhydrazone. The role of the AMPA subtype of EAA receptor in analgesic tolerance was examined using LY293558, a selective competitive antagonist that is active after systemic administration. Administration of morphine, DPDPE, or U50,488H three times daily for 3 days according to an escalating dosing schedule resulted in analgesic tolerance as indicated by an increase in analgesic ED50 values using the tail-flick test in mice. Analgesic tolerance was attenuated when mice received a continuous subcutaneous infusion of LY293558 at doses of 30, 45 or 60 mg/kg/24 hr via an osmotic pump concurrent with the morphine treatment. Continuous subcutaneous infusion of LY293558 (45 mg/kg/24 hr) also reversed established morphine tolerance. In contrast, continuous subcutaneous infusion of the highest dose of LY293558 (60 mg/kg/24 hr) was ineffective in preventing the development of analgesic tolerance to DPDPE or U50,488H. Continuous subcutaneous infusion of LY293558 (60 mg/kg/24 hr) for 3 days protected mice from generalized convulsions produced by the selective AMPA agonist ATPA, indicating that the dosage of LY293558 that attenuated morphine tolerance was effective as an antagonist at AMPA receptors. These results demonstrate that AMPA receptors may play a role in the development and maintenance of morphine, but not DPDPE or U50,488H, analgesic tolerance.     

Cross-tolerance to acute administration of mu and kappa opioid agonists at the spinal cord level in the rat

Development of tolerance and cross-tolerance after acute administration of the mu agonist morphine and the kappa agonist U-50,488H was assessed in rats, through recording of a C-fiber-evoked spinal nociceptive reflex. Rats rendered tolerant to morphine (a single dose of 1 mg/kg i.p.) showed, after a 5-hour period, tolerance to morphine and cross-tolerance to the kappa-opioid receptor agonist U-50,488H, as revealed by depressed C-reflex responsiveness. In contrast, pretreatment with U-50,488H (a single dose of 1 mg/kg i.p.) rendered tolerant the rats to U-50,488H, but the animals did not develop cross-tolerance to morphine. Results indicate that acute administration of mu and kappa ligands leads to development of unidirectional cross-tolerance in rat spinal cord. This points to limitations in using alternated mu and kappa opioid agonists to bypass the problem of development of opioid tolerance in chronic pain complaints.     

Binding of [3H][D-Ala2, MePhe4, Gly-ol5] enkephalin, [3H][D-Pen2, D-Pen5]enkephalin, and [3H]U-69,593 to airway and pulmonary tissues of normal and sensitized rats

The role of endogenous opioid peptides in the regulation of bronchomotor tone, as well as in the pathophysiology of asthma is uncertain. We have studied the binding of highly selective [3H]labeled ligands of mu-([D-Ala2, MePhe4, Gly-ol5]enkephalin; DAMGO), delta ([D-Pen2, D-Pen5]enkephalin; DPDPE), and kappa-(U-69,593) opioid receptors to membranes of trachea, main bronchus, lung parenchyma and pulmonary artery obtained from normal (unsensitized) and actively IgE-sensitized rats acutely challenged with the specific antigen. [3H]DAMGO, [3H]DPDPE and [3H]U-69,593 bound to membranes of normal and sensitized tissues at a saturable, single high-affinity site. The rank order of receptor densities in normal tissues was delta- > or = kappa- > or = mu-, with lung parenchyma exhibiting the greatest binding capacity for delta- and mu- receptors compared to the other regions examined. The Kd values showed small differences between ligands and regions tested. The mu- and delta-opioid receptor densities were decreased in sensitized main bronchus and lung parenchyma, respectively, compared to normal tissues. By contrast, kappa-opioid receptor density was augmented in sensitized lung parenchyma but an increase in Kd values was also observed. These differential changes in the density and affinity of opioid receptor types may be related to alterations in endogenous opioid peptides during the process of sensitization.     

Differential antagonism by MK-801 against antinociception induced by opioid receptor agonists administered supraspinally in mice

Various doses of MK-801 ((+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate), a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.001-1 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. MK-801 (0.001-1 microgram i.c.v.) dose dependently attenuated the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered morphine (1 microgram), [D-Pen2, D-Pen5]enkephalin (DPDPE; 10 micrograms), and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeoce tamide ) 60 micrograms). However, the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered beta-endorphin (1 microgram) was not changed by i.c.v. administered MK-801. Our results indicate that, at the supraspinal level, NMDA receptors are involved in the production of antinociception induced by supraspinally administered morphine, DPDPE, and U50,488H but not beta-endorphin.     

Alteration of calcitonin gene related peptide and its receptor binding sites during the development of tolerance to mu and delta opioids

Calcitonin gene related peptide (CGRP), one of the most abundant peptides in the spinal cord, is localized in primary afferents and released following nociceptive stimuli. Its colocalization and corelease with substance P, a well-known nociceptive neuropeptide, support the importance of CGRP in pain mechanisms. However, its distinctive function in that regard remains to be fully established. Recently, we reported that increases in CGRP-like immunostaining and decrements in specific 125I-labelled human CGRP alpha ([125I]hCGRP alpha) binding sites in the spinal cord were correlated with the development of tolerance to the spinal antinociceptive action of a mu opioid agonist, morphine. The goal of the present study was to investigate whether the development of tolerance to other classes of opioids, namely, delta and kappa agonists, can also alter CGRP-like immunostaining and receptors in the rat spinal cord. The antinociceptive effects of all opioids were monitored by the tail-immersion test. Tolerance to their antinociceptive properties was induced by the infusion for 7 days of mu (morphine sulfate, 7.5 micrograms/h), delta D([D-Pen2,D-Pen5]enkephalin (DPDPE), 2.0 micrograms/h), and kappa (U-50488H, 10.0 micrograms/h) related agonists at the spinal level (L4), using osmotic minipumps. We confirmed that rats chronically treated with morphine showed significant decreases in [125I]CGRP alpha binding in laminae I, II, and III of the L4 spinal cord, while CGRP-like immunostaining was increased in these same laminae. Similar effects were observed following a treatment with the delta agonist, DPDPE, while the kappa agonist, U-50488H, apparently only slightly decreased [125I]CGRP alpha] binding in lamina II. Binding in other laminae and CGRP-like immunostaining were not affected. These results suggest a specific interaction between spinal CGRP systems and the development of tolerance to the spinal antinociceptive effects of mu- and delta-related agonists.     

Effects of naturally acquired decompensated mitral valve regurgitation on the renin-angiotensin-aldosterone system and atrial natriuretic peptide concentration in dogs

We examined whether opioids, especially morphine, would centrally elicit scratching in mice and determined some characteristics of the scratch-inducing action of opioids. When intracisternally (i.c.) injected, morphine (0.1-3 nmol/mouse) produced a dose-dependent increase in scratching of the face, but not of the ears, head and body trunk. When injected intradermally into the rostral part of the back, morphine (at most potent i.c. dose of 3 nmol/mouse or higher) did not increase the scratching of the injected site. Facial scratching of the mouse induced by i.c. injection of morphine (0.3 nmol/mouse) was almost abolished by distraction and by naloxone (1 mg/kg, s.c.). [D-Ala2, N-Me-Phe4, Gly5-ol]Enkephalin (DAMGO) (0.03-2 nmol), but not [D-Pen2,5]enkephalin (DPDPE) and U-50,488, dose-dependently elicited facial scratching by i.c. injection. These results suggest that morphine and DAMGO increased facial scratching, probably mediated by central opioid mu-receptors in mice, and such scratching was due to a sensation, probably itching. The present animal model may be useful for analyzing opioid-mediated central itching.     

Majonoside-R2, a major constituent of Vietnamese ginseng, attenuates opioid-induced antinociception

The effects of majonoside-R2 on antinociceptive responses caused by the mu-opioid receptor agonist morphine and the selective kappa-opioid receptor agonist U-50, 488H were examined by the tail-pinch test in mice. Intraperitoneal (IP) or intracerebroventricular (ICV) injection of majonoside-R2 (3.1-6.2 mg/kg, IP or 5-10 micrograms/mouse, ICV) and diazepam (0.1-0.5 mg/kg, IP or 0.5-1.0 microgram/mouse, ICV), as well as an opioid receptor antagonist naloxone (2 mg/kg, IP or 5 micrograms/mouse, ICV), dose-dependently attenuated the antinociception caused by subcutaneously administered morphine and U-50,488H. Moreover, when co-administered ICV or intrathecally (IT) with morphine (4 micrograms/mouse) or U-50,488H (60 micrograms/mouse), majonoside-R2 (5-20 micrograms/mouse) also exhibited antagonism against the antinociceptive action of these opioid receptor agonists in the tail-pinch test. The inhibitory effects of majonoside-R2 (10 micrograms/mouse, ICV) and diazepam (1 microgram/mouse, ICV) were reversed by flumazenil (2.5 micrograms/mouse, ICV), a selective benzodiazepine receptor antagonist, and picrotoxin (0.25 microgram/mouse, ICV), a GABA-gated chloride channel blocker. These results suggest that majonoside-R2 attenuates the opioid-induced antinociception by acting at the spinal and supraspinal levels, and that the GABAA receptor complex at the supraspinal level is involved in the effect of ICV administered majonoside-R2.     

Effects of mu and delta opioid agonists and antagonists on affective vocal and reflexive pain responses during social stress in rats

The present experiments evaluated the influence of intraventricular mu and delta opioid receptors on affective vocal and reflexive responses to aversive stimuli in socially inexperienced, as well as defensive and submissive responses in defeated, adult male Long-Evans rats. Defeat stress consisted of: (1) an aggressive confrontation in which the experimental intruder rat exhibited escape, defensive and submissive behaviors [i.e., upright, supine postures and ultrasonic vocalizations (USV)], and subsequently, (2) protection from the resident stimulus rat with a wire mesh screen for 10-20 min. Defeat stress was immediately followed by an experimental session with tactile startle (20 psi). The mu opioid receptor agonists morphine (0.1-0.6 microg i.c.v.) and [D-Ala2-N-Me-Phe4-Gly5-ol]-enkephalin (DAMGO; 0.01-0.3 microg i.c.v.), and the delta opioid receptor agonist [D-Pen2,5]-enkephalin (DPDPE; 10-100 microg i.c.v.) dose-dependently decreased startle-induced USV and increased tail-flick latencies in socially inexperienced and defeated rats. Of greater interest, morphine, DAMGO and DPDPE increased the occurrence of the submissive crouch posture, and defeated rats were more sensitive than socially inexperienced rats to the startle-induced USV-suppressive and antinociceptive effects of morphine and DPDPE. The antinociceptive effects of DAMGO were likewise obtained at lower doses in defeated rats. Finally, the USV-suppressive effects of morphine and DAMGO were reversed with the mu receptor antagonist naltrexone (0.1 mg/kg i.p.), but the USV-suppressive effects produced by DPDPE were not reversed with the delta receptor antagonist naltrindole (1 mg/kg i.p.). These results confirm mu, but not delta opioid receptor activation as significant in affective vocal, passive-submissive behavior, as well as reflexive antinociception. Furthermore, similar to previous studies with restraint and electric shock stress, the facilitation of mu opioid effects on vocal responses and antinociception is consistent with the proposal that defeat stress activated endogenous opioid mechanisms.     

Heroin antinociception changed from mu to delta receptor in streptozotocin-treated mice

CD-1 mice were treated intravenously with streptozotocin, 200 mg/kg, and tested 2 weeks later or treated with 60 mg/kg and tested 3 days later. Both treatments changed the tail flick response of heroin and 6-monoacetylmorphine (6 MAM) given intracerebroventricularly from a mu- to delta-opioid receptor-mediated action as determined by differential effects of opioid receptor antagonists. The response to morphine remained mu. Heroin and 6 MAM responses involved delta1 (inhibited by 7-benzylidenenaltrexone) and delta2 (inhibited by naltriben) receptors, respectively. These delta-agonist actions did not synergize with the mu-agonist action of morphine in the diabetic mice. The expected synergism between the delta agonist, [D-Pen2-D-Pen5]enkephalin (DPDPE), and morphine was not obtained in diabetic mice. Thus, diabetes disrupted the purported mu/delta-coupled response. In nondiabetic CD-1 mice, heroin and 6 MAM produced a different mu-receptor response (not inhibited by naloxonazine) from that of morphine (inhibited by naloxonazine). Also, these mu actions, unlike that of morphine, did not synergize with DPDPE. The unique receptor actions and changes produced by streptozotocin suggest that extrinsic in addition to genetic factors influence the opioid receptor selectivity of heroin and 6 MAM.     

ACEA-1328, an NMDA receptor antagonist, increases the potency of morphine and U50,488H in the tail flick test in mice

The effect of 5-nitro-6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione (ACEA-1328), a competitive and systemically bioavailable NMDA receptor/glycine site antagonist, was examined on opioid-induced antinociception in the tail flick test. Swiss Webster mice were injected with ACEA-1328 either alone or in combination with morphine or (+/-)-trans-U-50488 methanesulfonate (U50,488H), a mu- and a kappa-opioid receptor agonist, respectively, and tested for antinociception. Systemic administration of ACEA-1328 alone increased the tail flick latencies with an ED50 of approximately 45 mg kg-1. Concurrent administration of ACEA-1328 with morphine, or U50,488H, at doses that did not affect tail flick latencies, potentiated the antinociceptive effect of the opioid analgesics and vice versa. Naloxone, an opioid receptor antagonist, while not modifying the effect of ACEA-1328, did block the augmentation, suggesting that opioid receptors might be involved in the latter effect. 5-Aza-7-chloro-4-hydroxy-3-(m-phenoxyphenyl)quinoline-2(1H)-one (ACEA-0762), a selective NMDA receptor/glycine site antagonist, also showed enhancement of the antinociceptive effect of morphine and U50,488H. However, concurrent administration of 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol[f]quinoxaline (NBQX), a selective non-NMDA receptor antagonist, with morphine did not alter the antinociceptive potency of the opioid analgesic. Overall, the data suggest that ACEA-1328 may increase the potency of the opioid analgesics by antagonising the glycine site associated with the NMDA receptor.     

Establishing morphine and U-50,488H as discriminative stimuli in a three-choice assay with pigeons.

Pigeons were trained in a 3-choice assay to discriminate among injections of 5.6 mg/kg U-50,488H, 5.6 mg/kg morphine, and vehicle solution. In dose-response tests, subjects rarely responded on the U-50,488H-appropriate key when morphine was administered or on the morphine-appropriate key when they received U-50,488H. A high dose of naltrexone (1.0 mg/kg) completely blocked the morphine cue but failed to block completely the U-50,488H cue. In generalization tests, d-amphetamine primarily engendered saline-appropriate responding. Ethylketazocine produced mixed results, in that moderate doses produced responding on both the morphine- and U-50,488H-appropriate keys, but 3.2 mg/kg engendered primarily morphine-appropriate responding. These results demonstrate the feasibility, but not necessarily the value, of 3-choice discrimination procedures involving mu and kappa agonists and vehicle. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Antisense oligodeoxynucleotides to the kappa-1 receptor block the antinociceptive effects of delta 9-THC in the spinal cord

Intrathecal pretreatment of mice with an antisense oligodeoxynucleotide directed against the kappa-1 receptor significantly reduced the antinociceptive effects of the kappa receptor agonist U50,488 as well as delta 9-THC, the major psychoactive ingredient found in cannabis. A mismatched oligodeoxynucleotide which contained four switched bases did not block the antinociception produced by U50,488 or delta 9-THC. Furthermore, kappa-1 antisense did not alter the antinociceptive effects of either the mu receptor-selective opioid DAMGO, or the delta receptor-selective opioid DPDPE. By using kappa-1 antisense, we were able to demonstrate that an interaction occurs between the cannabinoids and opioids in the spinal cord.     

Up-regulation of brain N-methyl-D-aspartate receptors following multiple intracerebroventricular injections of [D-Pen2, D-Pen5] enkephalin and [D-Ala2, Glu4]deltorphin II in mice

The effects of chronic administration of [D-Pen2, D-Pen5]enkephalin and [D-Ala2, Glu4]deltorphin II, the selective agonists of the delta 1- and delta 2-opioid receptors, on the binding of [3H]MK-801, a noncompetitive antagonist of the N-methyl-D-aspartate receptor, were determined in several brain regions of the mouse. Male Swiss-Webster mice were injected intracerebroventricularly (i.c.v.) with [D-Pen2, D-Pen5]enkephalin or [D-Ala2, Glu4]deltorphin II (20 micrograms/mouse) twice a day for 4 days. Vehicle injected mice served as controls. Previously we have shown that the above treatment results in the development of tolerance to their analgesic activity. The binding of [3H]MK-801 was determined in brain regions (cortex, midbrain, pons and medulla, hippocampus, striatum, hypothalamus and amygdala). At 5 nM-concentration, the binding of [3H]MK-801 was increased in cerebral cortex, hippocampus, and pons and medulla of [D-Pen2, D-Pen5]enkephalin treated mice. In [D-Ala2, Glu4]deltorphin II treated mice, the binding of [3H]MK-801 was increased in cerebral cortex and hippocampus. The changes in the binding were due to increases in the Bmax value of [3H]MK-801. It is concluded that tolerance to delta 1- and delta 2-opioid receptor agonists is associated with up-regulation of brain N-methyl-D-aspartate receptors, however, some brain areas affected differ with the two treatments. The results are consistent with the recent observation from this laboratory that N-methyl-D-aspartate receptors antagonists block tolerance to the analgesic action of delta 1- and delta 2-opioid receptor agonists.     

Logistic regression analysis of twin data: estimation of parameters of the multifactorial liability-threshold model

This study with the rat evaluated the contribution of omega-conotoxin GVIA-(omega-CgTx) and verapamil-sensitive Ca2+ channels in behavioural, antinociceptive and thermoregulatory responses to intracerebroventricular (i.c.v.) injection of [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAMGO), [D-Pen2,D-Pen5]enkephalin (DPDPE) and dynorphin A-(1-17), which are selective agonists for putative mu, delta and kappa-opioid receptors, respectively. The rats treated with omega-CgTx (8-32 pmol i.c.v.) showed transient, dose-dependent shaking behaviour, hyperalgesia and hypothermia which gradually disappeared within 4 h. The behaviour of the rats was normal by 24 h. Histological examination of brain sections showed morphological alterations of neurons in the hippocampus, medial-basal hypothalamus and pyriform cortex. antinociception, catalepsy and thermoregulatory responses elicited by DAMGO (0.4 and 2.0 nmol) were significantly prolonged and potentiated by verapamil (20 pmol i.c.v. 15 min before) or omega-CgTx (8 pmol 24 h before). Antinociception and hypothermia induced by DPDPE were antagonized by verapamil and omega-CgTx, whereas only omega-CgTx prevented the behavioural arousal observed after DPDPE. Similarly, hypothermia induced by dynorphin A-(1-17) (5.0 nmol) and by the kappa-opioid receptor agonist U50,488H (215 nmol) was antagonized by the two Ca2+ channel blockers but only omega-CgTx prevented the barrel rolling and bizarre postures caused by the opioid peptide.     

Streptozotocin-induced diabetes selectively enhances antinociception mediated by delta 1- but not delta 2-opioid receptors

We assessed the effect of diabetes on antinociception produced by intracerebroventricular injection of delta-opioid receptor agonists [D-Pen2,5]enkephalin (DPDPE) and [D-Ala2]deltorphin II. The antinociceptive effect of DPDPE (10 nmol), administered i.c.v., was significantly greater in diabetic mice than in non-diabetic mice. The antinociceptive effect of i.c.v. DPDPE was significantly reduced in both diabetic and non-diabetic mice following pretreatment with 7-benzylidenenaltrexone (BNTX), a selective delta 1-opioid receptor antagonist, but not with naltriben (NTB), a selective delta 2-opioid receptor antagonist. There were no significant differences in the antinociceptive effect of [D-Ala2]deltorphin II (3 nmol, i.c.v.) in diabetic and non-diabetic mice. Furthermore, the antinociceptive effect of i.c.v. [D-Ala2]deltorphin II was significantly reduced in both diabetic and non-diabetic mice following pretreatment with NTB, but not with BNTX. In conclusion, mice with diabetes are selectively hyper-responsive to supraspinal delta 1-opioid receptor-mediated antinociception, but are normally responsive to activation of delta 2-opioid receptors.     

Opioid antagonist profile of SC nor-binaltorphimine in the formalin paw assay

The antinociceptive effects of mu and kappa agonists were examined after the systemic administration of the opioid antagonists nor-binaltorphimine (nor-BNI) and naloxone in the late response or tonic nociceptive phase of the mouse formalin assay. Initially, SC morphine (ED50, 0.97 mg/kg), racemic U-50488H (ED50, 0.79 mg/kg), (-)U-50488 (ED50, 0.41 mg/kg), and another agonist PD 117,302 (ED50, 0.28 mg/kg) were found to produce graded increases in the level of antinociception as measured by this procedure; naloxone, administered immediately before morphine and U-50488H, antagonized their antinociceptive actions. The effects of morphine and U-50488H then were evaluated 10 min to 96 h after the administration of nor-BNI. Subcutaneous nor-BNI at 30.0 mg/kg, but not at 3.0 or 10.0 mg/kg, attenuated the antinociceptive effects of morphine and U-50488H when the interval separating nor-BNI and the agonists was kept constant at 1 h. Time-course analysis of the effects of combinations of nor-BNI with morphine led to irregular findings: 10.0 mg/kg of nor-BNI lessened the effects of morphine (2.0 mg/kg) if the dosing interval was 10 min, whereas 30.0 mg/kg of nor-BNI attenuated the effects of morphine (2.0 mg/kg) if the dosing interval was 1 or 4 h; 10.0 mg/kg of nor-BNI also diminished the antinociceptive effects of U-50488H (1.7 mg/kg) only if the interval spacing the two drugs was 24 h. In comparison, a threefold higher dose of nor-BNI (30.0 mg/kg) reduced the effects of U-50488H (1.7 mg/kg) if the interval was 1 h or more. In these latter experiments, the antagonist effects of SC nor-BNI (30.0 mg/kg) were evident up to 96 h posttreatment. These results show that the mu opioid antagonist activity of nor-BNI is variable and that the kappa opioid antagonist selectivity of nor-BNI is a function of dose and treatment interval and is long-lasting even after systemic administration.     

Attenuation of cocaine kindling by 7-nitroindazole, an inhibitor of brain nitric oxide synthase

Recent studies suggest the involvement of the N-methyl-D-aspartate (NMDA) type of glutamate receptors and nitric oxide synthase (NOS) in the process of increased sensitivity to the convulsive effect of cocaine ("cocaine kindling"). The present study was undertaken to analyze the various behavioral stages in the development of cocaine kindling and to investigate the effect of 7-nitroindazole (7-NI), a relatively selective inhibitor of the neuronal NOS isoform, on the induction and expression of sensitization to the convulsive effect of cocaine. Also, the effect of 7-NI on responses produced by acute systemic administration of cocaine or N-methyl-D,L-aspartate (NMDLA) was investigated. Cocaine kindling was assessed on a five-stage scale following the administration of a sub-convulsant dose of the drug (35 mg/kg/day; i.p.) to Swiss Webster mice for 10 days. Stage 5 seizures developed following the 9th day of cocaine administration. Pre-treatment with 7-NI (25 mg/kg/day; i.p.) 15 min before cocaine for 10 days completely prevented the appearance of stage 4 and 5 seizures, and it significantly attenuated stage 3 behavior in response to a challenge cocaine dose (35 mg/kg) given either 24 hr or 10 days after 7-NI/cocaine administration was stopped. A single injection of 7-NI (25 mg/kg; i.p.) completely prevented the expression of cocaine kindled seizures. Whereas 7-NI had no effect on the responses elicited by acute cocaine administration (60 mg/kg; i.p.), this agent partially attenuated the effects induced by systemic administration of the NMDA receptor agonist NMDLA (250 mg/kg; i.p.). The present study indicates that 7-NI attenuates both the induction and expression of sensitization to the convulsive effect of cocaine. The findings that 7-NI attenuated cocaine kindling and partially blocked the effects produced by activation of the NMDA receptor, but not the effects induced by acute cocaine administration, support the role of the NMDA receptor and brain NOS in the development of cocaine kindling rather than in the acute effects of the drug.     

Study of the antinociceptive effect of tetrahydro-papaveroline derivatives. Interaction with opioids

The antinociceptive effect of racemic tetrahydropapaveroline (THP), of its two R(+)- and S(-) enantiomers, of 1-2-dehydro-THP and of 1-carboxy-THP was assessed using different pain tests in mice. None of these drugs possessed a significant activity in the hot-plate and tail-flick tests. However, after i.p. injection, they reduced the number of abdominal writhes induced by phenylbenzoquinone, with ED50 values of 51 +/- 7, 73 +/- 9 and 79 +/- 7 mg/kg for the most potent compounds: 1,2-dehydro-THP, +/- THP and -THP, respectively. This activity was not antagonized by naloxone (1 mg/kg, s.c.). However combination of inactive doses of these three compounds (32 mg/kg, i.p.) and of morphine (0.5 mg/kg, s.c.) led to a significant antinociceptive effect (83 to 85% of reduction of the number of writhes). This synergistic potentiation confirmed with the combination of +/- THP (16 mg/kg, i.p.) and morphine (0.5 mg/kg, s.c.) was totally inhibited by naloxone (1 mg/kg, s.c.). These results, although excluding a direct agonistic effect of THP derivatives on opiate receptors, suggest an indirect interaction of these drugs with the endogenous opioid system.