Blockade of bradykinin-induced nociception in the rat as a test for analgesic drugs with particular reference to morphine antagonists

Optimization of a Nucleophilic Two-Step Radiosynthesis of 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) for PET Imaging of Brain Opioid Receptors

We have established a method for nucleophilic one-pot, two-step radiosynthesis of the popular opioid receptor radioligand 6-O-(2-[18F]fluoroethyl)-6-O-desmethyl-diprenorphine ([18F]FE-DPN) from the novel precursor 6-O-(2-tosyloxyethyl)-6-O-desmethyl- 3-O-trityl-diprenorphine (TE-TDDPN), which we designate as the Henriksen precursor. We undertook an optimization of the synthesis conditions, aiming to enhance the accessibility of [18F]FE-DPN for positron emission tomography (PET) studies of μ-opioid receptors. Herein, we report an optimized direct nucleophilic 18F-fluorination and the deprotection conditions for a fully automated radiosynthesis of [18F]FE-DPN on a modified GE Tracerlab FX FE synthesis panel. Starting from 1-1.5 GBq of [18F]fluoride and applying an Oasis Max 1cc cartridge for fluorine-18 trapping with a reduced amount of K2CO3 (5.06 μmol K+ ion), [18F]FE-DPN ([18F]11) was produced with 44.5 ± 10.6 RCY (decay-corrected), high radiochemical purity (>99%), and a molar activity of 32.2 ± 11.8 GBq/μmol in 60-65 min.

Antinociceptive and Anti-Inflammatory Activities of the Ethanolic Extract from Synadenium umbellatum Pax. (Euphorbiaceae) Leaves and Its Fractions

Synadenium umbellatum Pax., popularly known in Brazil as “cola-nota,” “avelós,” “cancerola,” and “milagrosa”, is a plant species used in folk medicine for the treatment of inflammation, pain, and several diseases. This study aimed to investigate the antinociceptive and anti-inflammatory activities of the ethanolic extract from Synadenium umbellatum Pax. leaves (EES) and its hexane (HF), chloroform (CF), and methanol/water (MF) fractions using the acetic acid-induced abdominal writhing test, formalin-induced paw licking test, tail flick test, croton oil-induced ear edema test, and carrageenan-induced peritonitis test. EES and MF reduced the number of acetic acid-induced abdominal writhes, while CF and HF did not. EES effect on acetic acid-induced abdominal writhing was reversed with a pretreatment with naloxone. EES reduced licking time in both phases of the formalin-induced paw licking test, but did not prolong the latency in the tail flick test. These results show that EES presented antinociceptive activity, probably involving the opioid system, anti-inflammatory activity in the croton oil-induced ear edema test, and leukocyte migration into the intraperitoneal cavity. MF also presented anti-inflammatory activity in the croton oil-induced ear edema test. In conclusion, EES and MF have antinociceptive activity involving the opioid system and anti-inflammatory activity.

An automated method to analyze vocalization of unrestrained rats submitted to noxious electrical stimuli

Unrestrained rats were subjected to electrical stimuli applied to their paws via an electrified cage floor. Intensity, duration, and order of stimulation were chosen after preliminary tests. Vocalization threshold and vocalization as a behavioral response were studied. The vocalization was recorded and the signal analyzed by a simple computerized method that calculated five parameters: delay, maximal amplitude, duration, area, and maximal derivative with respect to time. The last four parameters increased with increasing intensity of stimulation and remained stable when the same stimulation was given repeatedly. Sensitivity to morphine (2.5 and 5 mg/kg s.c.) was tested. Morphine raised the threshold and lowered vocalization parameters, and it was antagonized by naloxone, thus validating the method. The sensitivity of the test and its capacity to separate sensory and affective components of pain are discussed.

The role of stimulus intensity and stress in opioid-mediated analgesia

Rats exposed to a Pavlovian conditioning paradigm developed naloxone-reversible analgesia only when the intensity of a noxious unconditioned stimulus was suprathreshold and the level of stress was augmented. The time course of the onset of this conditioned analgesia was reproduced by systemic administration of morphine. These findings suggest that both a minimal level of stimulus intensity and stress are necessary for the activation of endogenous opioid-mediated analgesia.

Pain-induced vocalization in the rat and its modification by pharmacological agents

Vocalization was induced in rats by electrical stimulation of the tail (pain-induced vocalization), and its components were characterized in terms of latency, duration, frequency spectrum and energy. Noxious stimuli at threshold elicit a single vocalization component (V1). Increases in stimulus intensity produce additional discrete vocalization components (V2-Vn) with successively longer latencies, termed the vocalization afterdischarge (AD). The AD components are acoustically similar to each other but differ significantly from the V1 component. The duration, the specific acoustic measures and the sound energy of both V1 and AD components are positively correlated with intensity of the stimulus. The dependence of the V1 and AD components on the affective state of the rat was evaluated by comparing the acoustic characteristics of both components to those of stress-induced vocalizations, and by studying the effects of the anxiolytic drug diazepam and physical restraint on the threshold of V1 and AD. The AD components were markedly more dependent on the affective state of the rat then was the V1 component. A moderately low dose of morphine (3.0 mg/kg) also preferentially affected the AD component, suggesting that a significant portion of the action of morphine on pain-induced vocalization is mediated through its action on the affective state of the rat.

[3H]diprenorphine receptor binding in vivo and in vitro

In order to investigate opiate receptor binding in vivo, [3H]diprenorphine was given s.c. to rats, and the tracer specifically bound to membraneous high affinity sites was determined with a rapid filtration technique after brain homogenization. Bound [3H]diprenorphine accounted for 70% of the total brain activity after tracer doses. The in vivo binding sites were saturable at 25-30 pmol/g brain. Fifty percent occupancy of the [3H]diprenorphine binding sites in vivo occurred at a dose (10-15 micrograms/kg) that is similar to the antagonistic ED50 of diprenorphine for reversing morphine analgesia. The in vitro binding capacity for [3H]diprenorphine was also approximately 30 pmol/g brain in fresh untreated Tris buffer brain homogenate; however, extensive homogenate dilution or standard membrane washing procedures resulted in a reduction of the [3H]diprenorphine binding site population to 13-22 pmol/g. These results indicate that the opiate receptor system is modified in vitro. Previous studies have shown that the [3H]diprenorphine tracer is retained at cerebral binding sites over several hours in vivo. A diffusion boundary model was proposed to account for the dose dependent tracer retention. In order to investigate the mechanism of the in vivo binding kinetics, [3H]diprenorphine dissociation was measured in brain homogenates after in vivo labeling, immediately following sacrifice of the animals to minimize in vitro artefacts. No differences were found in the dissociation curves at 'infinite' homogenate dilution in the presence or absence of saturating diprenorphine concentrations under various ionic incubation conditions. This result argues against cooperative binding. It is consistent with the hypothesis that the [3H]diprenorphine tracer is retained in vivo because of a diffusion boundary next to the binding sites (receptor micro-compartment) that is destroyed during brain homogenization.

Aspirin clearly depresses responses of ventrobasal thalamus neurons to joint stimuli in arthritic rats

This study dealt with the effect of aspirin upon activities of 17 ventrobasal thalamic neurons recorded in 17 rats rendered arthritic by injection of Freund's adjuvant into the tail. These neurons presented reproducible responses to mobilization and/or mild lateral pressure on a joint and were recorded for at least 40 min after aspirin administration. After intravenous injection of aspirin at the dose of 50 mg/kg (13 neurons tested), there was a progressive decrease in the number of spikes in the discharges. The maximum effect occurred at 30 min where the mean value of the response expressed as a percentage of the control was m = 34.62 +/- 7.5% (n = 13, p less than or equal to 0.001). Recovery was progressive and could be considered as complete at 60 min. By contrast, no significant modification of the spontaneous firing has been observed. With lower doses of aspirin (12.5 or 25 mg/kg tested with 4 neurons) there was respectively no clear depressive effect or only a transient decrease of the response.

Morphine analgesia and the bulbospinal serotonergic system: increase in concentration of 5-hydroxyindoleacetic acid in the rat spinal cord with analgesics

Administration of an analgesic dose of morphine increased the concentration of the metabolite of 5-hydroxytryptamine (5HT), 5-hydroxyindoleacetic acid (5HIAA), in the spinal cord of rats. The time course of the change in 5HIAA concentration in the dorsal half of the spinal cord corresponded to that of the morphine analgesia, while such was not the case in the ventral half. When the spinal cord was transected at C1, the effect of morphine on 5HIAA concentration disappeared. After transection at the inter-collicular level, this effect remained. The possible relationship between morphine analgesia and descending serotonergic neurons in the spinal cord is discussed.

Depression of taste responses by local or intravascular administration of salicylates in the rat

The effects of aspirin and sodium salicylate on the taste response in the chorda tympani proper nerve have been studied during stimulation of the taste receptors in rats with salty, sweet, sour and bitter-tasting solutions. Three methods of administering the drugs were used: locally on the tongue, intravenously into one femoral vein and intraarterially close to the branching of the lingual artery. It was observed that salicylates, given in any of these three ways, depressed the response to taste stimuli. This is discussed in relation to present views on the action of salicylates.

Morphine analgesia and the bulbospinal noradrenergic system: increase in the concentration of normetanephrine in the spinal cord of the rat caused by analgesics

1 Administration of an analgesic dose (10 mg/kg, s.c.) of morphine increased the concentration of a noradrenaline metabolite, normetanephrine (NM) in the spinal cord of normal rats. The time course of the change in the NM concentration corresponded approximately to that of the morphine analgesia. The concentration of noradrenaline was not affected.2 A similar effect on the NM concentration was also observed after the administration of pentazocine (30 mg/kg, s.c.) and nalorphine (20 mg/kg, s.c.).3 The NM increasing effect of morphine, pentazocine and nalorphine was found in the dorsal half of the spinal cord but not in the ventral half.4 The increase in the concentration of NM induced by morphine, pentazocine or nalorphine was completely suppressed by naloxone (1 mg/kg, s.c.) given 5 min before the administration of these drugs.5 When the spinal cord was transected at C1, the NM increasing effect of morphine disappeared, yet when the brain stem was transected at the inter-collicular level, the effect remained.6 In morphine-tolerant rats, the concentration of NM in the spinal cord was almost the same as that observed in normal rats, but the increase in the concentration of NM in the spinal cord after the acute administration of morphine did not take place.7 The NM concentration in the spinal cord of normal rats was not modified by aminopyrine (75 mg/kg, s.c.), chlorpromazine (10 mg/kg, s.c.), mephenesin (100 mg/kg, i.p.) or naloxone (25 mg/kg, s.c.).8 The possible relation between morphine analgesia and the descending noradrenergic neurones in the spinal cord of rats is discussed.

A study of compounds which initiate and block nerve impulses in the perfused rabbit liver

1. A technique for the perfusion of the rabbit liver with Krebs solution is described: the hepatic nerves were monitored for centripetal action potentials. Spontaneous afferent activity, and a good response to stimulating agents persisted for 7 h or longer.2. Action potentials were elicited by injections or infusions of acetylcholine, 5-hydroxytryptamine, bradykinin, phenyldiguanide, adrenaline and various other compounds. From comparison with experiments carried out in vivo by other authors, it would appear that most of the nerves stimulated were afferent.3. Inhibition of action potentials, either those occurring spontaneously or those elicited by injections, was produced by aspirin, paracetamol, mepyramine, chlorpheniramine, procaine and lignocaine but not by morphine or atropine. Pentolinium produced some inhibition, as did hexamethonium in concentrations of about 7.5 mug/ml.4. Three methods for using the preparation for the assessment of local anaesthetic action are described.5. Lobeline, potassium cyanide and 2,4-dinitrophenol stimulated nerve bundles which were not stimulated by anoxia or hypoxia.

Studies on the receptors involved in the action of the various agents in the phenylbenzoquinone analgesic assay in mice

1. Tolerance to the activity of several narcotic analgesics (morphine, levorphanol, and methadone) and several narcotic-antagonist analgesics (pentazocine, cyclazocine, and nalorphine) was studied in the mouse phenylbenzoquinone stretching test. Virtually complete tolerance was induced by chronic treatment with each of the narcotic agents, while no apparent tolerance was induced by the narcotic antagonists.2. In morphine-tolerant mice there was a high degree of cross-tolerance to the effects of not only the other narcotic drugs but also to those of the narcotic antagonists, acetylsalicylic acid, and physostigmine.3. The effects of morphine and pentazocine were antagonized by naloxone but not by atropine, while the effects of physostigmine were antagonized by atropine but not by naloxone. Neither atropine nor naloxone antagonized the effect of acetylsalicylic acid.4. The results of the tolerance study suggest that there is a fundamental difference in the consequences of receptor interaction for the narcotic and the narcotic-antagonist analgesics. Morphine-tolerant mice exhibit cross-tolerance non-specifically. The selectivity of naloxone and atropine differentiates the narcotic and narcotic-analgesics from the other two agents used in this analgesic test.

Interactions of narcotic antagonists and antagonist-analgesics

In the rat the potent narcotic antagonist N-cyclopropylmethyl-6,14-endoethano-7α- (1-hydroxy-1-methylethyl)-tetrahydronororipavine (M5050, Reckitt), which itself lacks analgesic activity, resembled naloxone in its capacity to reverse the antinociceptive effects of morphine antagonist-analgesics. The nociceptive stimulus employed was bradykinin administered by an intra-arterial route. ED50 values were established for the reversal by M5050 of the analgesic effect of nalorphine, levallorphan, pentazocine and some newer compounds. When antagonist-analgesics were given concomitantly with morphine the response varied from antagonism of the analgesic effect of the morphine to synergism, depending on the dose combination.