The antinociceptive activity of meptazinol depends on both opiate and cholinergic mechanisms

(3S,4S)-3-Ethyl-4-hydr-oxy-3-(3-methoxy-phen-yl)-1-methyl-azepan-1-ium d-tartrate dihydrate

In the title compound, C(16)H(26)NO(2) (+)·C(4)H(5)O(6) (-)·2H(2)O, a meptaz-inol derivative, three C atoms of the azepane ring are disordered over two positions, with site-occupancy factors of 0.80 and 0.20; the major disorder component adopts a twist-chair conformation, while the minor component has a chair conformation. The benzene ring is axially substituted on the heterocyclic ring, resulting in a folded conformation of the cation. The absolute configuration was determined with reference to d-tartaric acid. The crystal structure is stabilized by an extensive network of intra- and inter-molecular O-H⋯O hydrogen bonds.

Reflex activity caused by laryngoscopy and intubation is obtunded differently by meptazinol, nalbuphine and fentanyl

BACKGROUND AND OBJECTIVE

To evaluate the different potencies of several opioids in obtunding reflex mechanisms of laryngoscopy and intubation.

METHODS

Three groups of patients (each n = 25, ASA 1-2) undergoing elective plastic surgery were randomly given meptazinol (2.5 mg kg-1), nalbuphine (0.3 mg kg-1) or fentanyl (5 microg kg-1) in a blinded fashion prior to laryngoscopy and intubation. This was followed by a standardized bolus induction of a barbiturate and a muscle relaxant. The response to laryngoscopy and intubation was studied, using blood pressure, heart rate and bispectral index.

RESULTS

With fentanyl, there was an increase of heart rate by 17%, and systolic blood pressure by 7% when compared to control. Bispectral index dropped an additional 8% when compared to 1 min after barbiturate induction. In the nalbuphine group there was a 16% increase in systolic blood pressure, and a 16% increase in heart rate when compared to control. Also, bispectral index increased by 18% when compared to 1 min after barbiturate injection. The group receiving meptazinol demonstrated no cardiovascular changes although bispectral index dropped by an additional 19% when compared to 1 min after barbiturate injection.

CONCLUSION

Meptazinol, appears to depress cardiovascular stimulatory effects and electroencephalogram arousal induced by laryngoscopy and intubation better than nalbuphine or fentanyl.

Investigation of the binding mode of (−)-meptazinol and bis-meptazinol derivatives on acetylcholinesterase using a molecular docking method

Molecular docking has been performed to investigate the binding mode of (-)-meptazinol (MEP) with acetylcholinesterase (AChE) and to screen bis-meptazinol (bis-MEP) derivatives for preferable synthetic candidates virtually. A reliable and practical docking method for investigation of AChE ligands was established by the comparison of two widely used docking programs, FlexX and GOLD. In our hands, we had more luck using GOLD than FlexX in reproducing the experimental poses of known ligands (RMSD<1.5 A). GOLD fitness values of known ligands were also in good agreement with their activities. In the present GOLD docking protocol, (-)-MEP seemed to bind with the enzyme catalytic site in an open-gate conformation through strong hydrophobic interactions and a hydrogen bond. Virtual screening of a potential candidate compound library suggested that the most promising 15 bis-MEP derivatives on the list were mainly derived from (-)-MEP with conformations of (S,S) and (SR,RS) and with a 2- to 7-carbon linkage. Although there are still no biological results to confirm the predictive power of this method, the current study could provide an alternate tool for structural optimization of (-)-MEP as new AChE inhibitors. [Figure: see text].

Inhibitory effect of intrathecal meptazinol on carrageenan-induced thermal hyperalgesia in rats

The effect of meptazinol in the spinal cord on carrageenan-induced hyperalgesia was investigated. The latency of paw withdrawal (PWL) to a thermal stimulus was used as an index of inflammatory hyperalgesia in awake rats. Intrathecal (i.t.) injection of 10 and 100 microg meptazinol markedly increased the PWL of the carrageenan-injected paw (P<0.01). The PWL of the non-injected paw was not detectably affected by the administration of meptazinol at the doses tested. I.t. injection of naloxone (5 microg) or atropine (1 microg) alone exhibited no effect on the PWLs of either the carrageenan-injected or non-injected paw. Pretreatment with naloxone, but not atropine, completely blocked the meptazinol-induced anti-hyperalgesia. These observations suggested that mu opioid receptor rather than muscarinic acetylcholine receptor may be involved in the anti-hyperalgesia of meptazinol in the spinal cord.

A comparison of subjective, psychomotor and physiological effects of a novel muscarinic analgesic, LY297802 tartrate, and oral morphine in occasional drug users

This study compared the subjective, physiological and psychomotor effects of a novel muscarinic analgesic (LY297802) and oral morphine in healthy volunteers. Nine, non-dependent, occasional drug users participated in nine experimental sessions in which they received the following conditions: placebo, 0.1, 0.3, 0.56 and 1 mg of oral LY297802 and 10, 30, 56 and 100 mg of oral morphine. Subjective drug effects were assessed using the Visual Analog Scale (VAS), the Addiction Research Center Inventory (ARCI) and subjective and objective agonist and antagonist scales of the Adjective Rating Scale (ARS). These measures were collected 30 min before and every 30 min post drug administration for a 4-h period. Psychomotor performance was evaluated using the Digit Symbol Substitution Test (DSST) at these same time intervals. Physiological measures were collected continuously throughout the sessions. Oral morphine produced significant increases in some subjective effects scales, including elevations on the VAS, ARCI and ARS. In contrast, LY297802 did not engender changes different from placebo on any of these indices. Morphine produced significant dose-dependent effects in DSST performance, heart rate, blood oxygen saturation and pupil diameter. LY297802 significantly and dose dependently increased heart rate, mean arterial pressure and diastolic blood pressure. These results suggest that LY297802 does not induce subjective effects similar to morphine, but that it has some significant physiological effects.

Opioid agonist-antagonist drugs in acute and chronic pain states

The agonist-antagonist opioid analgesics are a heterogeneous group of drugs with moderate to strong analgesic activity comparable to that of the pure agonist opioids such as codeine and morphine but with a limited effective dose range. The group includes drugs which act as an agonist or partial agonist at one receptor and an antagonist at another (pentazocine, butorphanol, nalbuphine, dezocine) and drugs acting as a partial agonist at a single receptor (buprenorphine). These drugs can be classified as nalorphine-like or morphine-like. Meptazinol does not fit into either classification and occupies a separate category. Pentazocine, butorphanol and nalbuphine are weak mu-antagonists and kappa-partial-agonists. All three drugs are strong analgesics when given by injection: pentazocine is one-sixth to one-third as potent as morphine, nalbuphine is slightly less potent than morphine, and butorphanol is 3.5 to 7 times as potent. The duration of analgesia is similar to that of morphine (3 to 4 hours). Oral pentazocine is closer in analgesic efficacy to aspirin and paracetamol (acetaminophen) than the weak opioid analgesics such as codeine. Neither nalbuphine nor butorphanol is available as an oral formulation. At usual therapeutic doses nalbuphine and butorphanol have respiratory depressant effects equivalent to that of morphine (though the duration of such effects with butorphanol may be longer). Unlike morphine there appears to be a ceiling to both the respiratory depression and the analgesic action. All of these 3 drugs have a lower abuse potential than the pure agonist opioid analgesics such as morphine. However, all have been subject to abuse and misuse, and pentazocine (but not the others) is subject to Controlled Drug restrictions. Buprenorphine is a potent partial agonist at the mu-receptor, and by intramuscular injection is 30 times as potent as morphine. A ceiling to the analgesic effect of buprenorphine has been demonstrated in animals and it is also claimed in humans. However, there are no reliable data available to define the maximal dose of buprenorphine in humans. A practical ceiling exists for sublingual use in that the only available formulation is a 2 micrograms tablet and few patients will accept more than 3 or 4 of these in a single dose. The duration of analgesia is longer than that of morphine, at 6 to 9 hours. There have been suggestions that buprenorphine causes less respiratory depression than morphine, but viewed overall it appears that in equianalgesic doses the 2 drugs have similar respiratory depressant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

On the mechanism involved in the ability of meptazinol to potentiate the effects of sympathetic nerve stimulation

Mouse isolated vas deferens responded to field stimulation (0.1 Hz) with twitch responses which were abolished by alpha beta-methyleneadenosine 5'-triphosphate (0.5 microM) and were potentiated 2 to 3 fold by meptazinol (5-300 microM). Exogenous adenosine 5'-triphosphate (4-30 microM) also caused a twitch response but was unaffected by meptazinol (30 microM) as was the response to phenylephrine. The effect of meptazinol on the electrically-induced twitch was reproducible, fast in onset, easily reversed by washing and was still seen in the presence of prazosin (1 microM), yohimbine (1 microM), propranolol (0.1 microM), atropine (0.1 microM), physostigmine (1 microM), cocaine (1 microM) or desmethylimipramine (0.3 microM) indicating that the mechanism involved does not depend on adrenoceptors, cholinergic mechanisms or blockade of uptake. Mouse isolated atria responded to stimulation (1, 2 or 5 Hz) of their sympathetic nerves via transmural electrodes with chronotropic responses which were abolished by atenolol (5 and 50 microM) but were unaffected by alpha beta-methyleneadenosine 5'-triphosphate (0.5 microM). Meptazinol (100 microM) failed to potentiate the responses. It is suggested that meptazinol potentiates the effects of the non-adrenergic non-cholinergic transmitter thought to be involved in the response of the mouse vas deferens to electrical stimulation.

Protection against diisopropylfluorophosphate intoxication by meptazinol

The protective action of meptazinol against diisopropylfluorophosphate (DFP) was evaluated in mice which were not receiving any other therapy and in preparations of electric eel AChE and horse serum BuChE. Meptazinol injected subcutaneously in mice produced a dose-dependent reduction in the mortality resulting from a LD99.1 (8 mg/kg sc) of DFP administered later. The effectiveness of protection was inversely correlated to the time between meptazinol and DFP administrations. Under these conditions, the ED50s (95% confidence limits) of meptazinol given 15, 30, and 60 min before poisoning were 7.2 (6.4-8.1), 15.8 (13.7-18.2), and 28 (23.5-33.3) mg/kg, respectively, while full protection (100% of survivors) was obtained with 15, 30, and 60 mg/kg drug doses, respectively. Meptazinol was completely ineffective against DFP-induced lethality when administered 3 min after the poison. The protective ratio of 30 mg/kg meptazinol injected 15 min before DFP was 5.0. Pretreatment of mice with 15, 30, and 60 mg/kg meptazinol 15 min before DFP (8 mg/kg) increased brain AChE activity in DFP-treated mice from 5 +/- 0.5% to 16.2 +/- 2.5%, 42.5 +/- 4%, and 81.2 +/- 4% of control values, respectively, while it failed to increase plasma BuChE activity. Finally, concentrations of meptazinol ranging between 0.1 and 10 microM were found to afford complete protection of eel AChE against irreversible inhibition by 40 microM DFP. By contrast, horse serum BuChE was not protected against the same inhibitor by concentrations of meptazinol up to 1 mM. It is concluded that protection against DFP intoxication by meptazinol is most probably due to its protective action toward AChE.

Meptazinol and pentazocine: plasma catecholamines and other effects in healthy volunteers

1. This double-blind, random-order study was designed to compare the clinical effects and the plasma catecholamine responses after i.v. administration of meptazinol at doses 0.7 and 1.4 mg kg-1, pentazocine at doses 0.3 and 0.6 mg kg-1 and saline placebo to six healthy volunteers. 2. Mean arterial pressure was not affected by either drug. Heart rate showed slight drug-related changes. Respiratory rate fell slightly with both drugs, but independently of dose. 3. The critical flicker fusion threshold-test and Maddox wing readings could both clearly differentiate active drugs from placebo. Meptazinol caused more nausea and dysphoria as expressed with visual analogue scales. Both analgesics caused short-lived feelings of euphoria. 4. After pentazocine plasma noradrenaline increased almost two-fold in 10-20 min. The effect of meptazinol was slightly smaller, whereas meptazinol caused a pronounced increase in plasma adrenaline concentrations in two of six subjects. Pentazocine had a smaller, but significant effect on plasma adrenaline. 5. We conclude that the effects of meptazinol in healthy volunteers do not differ markedly from those of pentazocine, although it may cause more nausea and dysphoria. The pronounced increase in plasma adrenaline concentrations in two of six subjects calls for caution in its use in patients with cardiac diseases.

Prevention of physostigmine-induced lethality by the opioid analgesic meptazinol in the mouse

The prophylactic action of meptazinol against physostigmine- and neostigmine-induced lethality was evaluated in mice. Meptazinol proved to be effective against physostigmine (1 mg kg-1 i.p.), but not against neostigmine (0.5 mg kg-1 i.p.). The antagonism by meptazinol of physostigmine-induced poisoning was maximal when the drug was administered 15 min before physostigmine. Under these conditions the ED50 (95% confidence limits) of meptazinol was 24 (22.0-26.1) mg kg-1 s.c. A 30 mg kg-1 dose of the drug prevented lethality in 89% of the animals. The action of meptazinol was not antagonized by naloxone hydrochloride (2 mg kg-1 i.p.), injected 10 min before meptazinol. Pretreatment of mice with 30 mg kg-1 meptazinol 15 min before physostigmine (1 mg kg-1) poisoning increased brain acetylcholinesterase (AChE) activity on average, from 8 to 31% of control values. The protection of cholinesterases against physostigmine- and neostigmine-induced inactivation was demonstrated in vitro directly on purified preparations of the enzymes using a dilution method. The ED50 values (95% confidence limits) for the protective effect of meptazinol of electric eel AChE against 1 and 3 microM physostigmine and 1 microM neostigmine were 2.6 (1.4-4.9), 9.5 (5-18) and 3 (1.6-5.7) microM, respectively, while for protection of horse serum butyrylcholinesterase (BuChE) against the same inhibitors, the ED50 values were 12 (5.4-26.4), 42 (27-65.1) and 8 (3.6-17.6) microM, respectively. It is suggested that prevention of physostigmine-induced lethality by meptazinol is a consequence of its protective action on AChE in the central nervous system.

The effect of meptazinol on the guinea-pig sphincter of Oddi in-vitro

Meptazinol causes a dose-dependent contraction of the guinea-pig sphincter of Oddi in-vitro. This was antagonized by atropine in concentrations which blocked the contractile response to acetylcholine but not that to KCl. Naloxone was unable to block the response of the tissue to meptazinol, and other opioid drugs had inconsistent effects. Although meptazinol has significant anticholinesterase activity on this preparation, comparison with neostigmine suggests that this is irrelevant to its contractile action.

An investigation of the mechanism involved in the cholinergic action of meptazinol

In concentrations above 20 microM, (+/-)-meptazinol produced a contraction of the guinea-pig isolated ileum and this effect was antagonized by atropine (0.01 to 0.3 microM) in a manner which was not competitive. Cooling the preparation to 15 degrees C blocked the contractile action of meptazinol and of dimethylphenylpiperazinium (DMPP) but did not affect the action of carbachol. Twitch responses of the rat phrenic nerve-diaphragm preparation induced by indirect electrical stimulation in the presence of naloxone (20 nM) were potentiated by meptazinol (1 to 40 microM) which also reversed a partial blockade of the twitch induced by tubocurarine. Neither of these effects was seen in tissues which had been pretreated with the cholinesterase inhibitor BW284C51 (0.2 microM) though tetraethylammonium iodide (40 microM) was still able to enhance the responses to stimulation. In the presence of naloxone (20 nM) electrically induced responses of the rat isolated rectum were abolished by cinchocaine (10 microM), partially blocked by atropine (0.1 to 0.4 microM) and potentiated by meptazinol (1 to 30 microM). The latter action was not seen when meptazinol was administered in the presence of BW284C51. It is concluded that the cholinergic action of meptazinol in these tissues is due to an indirect effect, probably involving inhibition of cholinesterase and that no evidence was seen of any ability to increase the release of acetylcholine itself.

Studies on performance with aspirin and paracetamol and with the centrally acting analgesics meptazinol and pentazocine

Effects of aspirin (325 and 650 mg) and paracetamol (500 and 1000 mg), and of the centrally acting analgesics meptazinol (100 and 200 mg) and pentazocine (25 and 50 mg) on visuo-motor coordination and dynamic visual acuity, together with critical flicker fusion, digit symbol substitution, complex reaction time and subjective assessments of mood, were studied from 0.75-2.0 h after ingestion by seven healthy female adults. The study was double-blind and placebo controlled, and triprolidine (10 mg) was used as the active control. No effects of meptazinol and paracetamol on performance were observed. Pentazocine (25 mg) impaired performance on digit symbol substitution (p less than 0.05) and aspirin (650 mg) appeared to have shortened complex reaction time (p less than 0.05). Meptazinol (100 mg) increased the component of mood assessments related to wakefulness (p less than 0.05). Impaired performance with pentazocine may involve opioid receptor activity, while the apparent alerting effect of meptazinol may relate to its cholinergic activity. The possible effect of aspirin on reaction time needs to be confirmed.

The effects of alfentanil and selected narcotic analgesics on the rate of action potential discharge of medullary respiratory neurones in anaesthetized rats

The effects of intravenous injections of alfentanil, fentanyl, phenoperidine or morphine on respiratory and peak inspiratory air flow rate, the diaphragm electromyogram (EMG), the activity recorded extracellularly from respiratory neurones located in the ventral respiratory group and the cardiovascular system were examined in anaesthetized rats. Alfentanil produced dose-dependent changes in peripheral and central respiratory parameters, which were prevented by naloxone pretreatment. Minimal effects were produced on the cardiovascular system. The bradypnoea was principally due to a prolongation of the inspiratory phase and was accompanied by a comparable decrease in the peak inspiratory air flow rate. Alfentanil prolonged the discharge duration of inspiratory neurones such that it still maintained a strict phase correlation with the diaphragm EMG, but changes in firing frequency were inconsistent and negligible. The action on expiratory neuronal discharge was analogous to that on inspiratory neuronal discharge but delayed in onset. Hypotension produced by morphine limited the dose used but the respiratory responses to morphine and other selected narcotic analgesics were otherwise similar to that of alfentanil, differing mainly in time-course and magnitude. From the respiratory parameters assessed, the order of duration of effect was morphine greater than phenoperidine greater than fentanyl greater than alfentanil and the relative potencies were 0.1, 0.5, 2.5 and 1 respectively. The selective prolongation of inspiration and the immediate action on inspiratory neurones suggests that systemically administered narcotic analgesics may alter the mechanisms within the central respiratory rhythm generator which determine the cessation of inspiration.

Characterization of the effects of (+/-)-meptazinol, its individual enantiomers and N-methyl meptazinol on food consumption in the rat

Both (+/-)-meptazinol (2 mg kg-1) and levorphanol (1 mg kg-1) produced hyperphagia over a 4 h period after intraperitoneal injection in free feeding rats during the daylight phase. The individual (+)- and (-)-enantiomers of meptazinol (2 mg kg-1 i.p.) induced comparable increases in cumulative food intake. N-methyl meptazinol (2-10 mg kg-1 i.p.), the quaternary analogue of meptazinol, produced no modification of food intake though it increased food consumption when injected intracerebroventricularly (10-100 micrograms per animal). Meptazinol and levorphanol hyperphagia was abolished by 1 mg kg-1 doses (i.p.) of the opioid antagonists naltrexone, naloxonazine and (-)-Mr 1452 but not by its (+)-enantiomer Mr 1453 which is not effective as an opioid antagonist. Intracerebroventricular administration of the delta-opioid antagonist ICI 154,129 (10 micrograms per animal) suppressed meptazinol but not levorphanol hyperphagia. It was concluded that meptazinol produces centrally mediated stereospecifically reversible hyperphagia through a mu-opioid receptor mechanism common to levorphanol, and also through delta-opioid receptor mechanism(s).

Can the effects of meptazinol on the guinea-pig isolated ileum be explained by inhibition of acetylcholinesterase?

It has previously been shown that there is a cholinergic component in the antinociceptive action of the opioid analgesic drug meptazinol. In the present study meptazinol was shown to be an inhibitor of acetylcholinesterase in-vitro with a potency one hundredth that of physostigmine. This activity was found to reside only in the (-)-enantiomer of meptazinol. The anticholinesterase activity of meptazinol may explain the increase in the size of the electrically-evoked contraction of the guinea-pig isolated ileum preparation since by using a long pulse width (5 ms) it was found that the (-)-enantiomer of meptazinol modified only the component of the response due to neuronally released acetylcholine and had no direct effect on the smooth muscle. This property of meptazinol may also be responsible for the cholinergic effects of the drug in-vivo.

Meptazinol. A review of its pharmacodynamic and pharmacokinetic properties and therapeutic efficacy

Meptazinol is a new opioid-type analgesic with mixed agonist/antagonist properties. It may be given orally, intravenously or intramuscularly. In studies in patients with moderate to severe pain of various aetiologies, usually following surgery or in obstetrics, the characteristics of analgesia with meptazinol were comparable to those seen with equianalgesic doses of pentazocine, pethidine or a combination of dextropropoxyphene and paracetamol. Preoperative use and use as a component of anaesthesia require further investigation before conclusions may be drawn on its effectiveness in these areas. Onset of action, recorded in a few studies, was faster than that with the other analgesics but duration was shorter than that of morphine, buprenorphine and pentazocine. Only a small number of patients with chronic pain have received long term therapy with meptazinol; in such patients there was no need for increased doses as treatment progressed. Respiratory depression has only been observed in patients receiving meptazinol as a premedication or while undergoing anaesthesia. Similarly any haemodynamic changes have been limited to preoperative patients or patients undergoing anaesthesia. Like other agonist/antagonist analgesic drugs, the abuse potential of meptazinol seems relatively low, but only wider clinical use for longer periods can establish this with certainty. The most commonly reported side effects have been gastrointestinal in nature, and although the incidence of central nervous system side effects has been relatively low, drowsiness and dizziness have caused occasional problems. Thus, meptazinol is a relatively potent but safe addition to the analgesics available for treatment of the patient with moderate to severe pain.

The effects of a new opioid analgesic, meptazinol, on the respiration of the conscious rat

In the conscious rat arterial PCO2 was measured as an index of respiratory status. The opioid analgesic meptazinol (7.5 - 30 mg kg-1) evoked small but significant increases in arterial PCO2 which were attenuated by naloxone. Meptazinol significantly reduced the increase in arterial PCO2 evoked by morphine. The respiratory depression induced by meptazinol, but not that induced by morphine, was enhanced by pretreatment with atropine. The (+)-enantiomer, but not the (-)-enantiomer of meptazinol increased arterial PCO2. In contrast, only the (-)-enantiomer reduced the respiratory depressant effect of morphine. It is proposed that the degree of respiratory depression induced by meptazinol is limited by its opioid antagonist and cholinomimetic properties.

Similarity between mu-opioid receptors in mouse vas deferens and guinea-pig ileum

The effects of the opioid receptor agonist RX783006 and of the opioid receptor partial agonist (+)-meptazinol have been examined on electrically-induced twitch responses of the guinea-pig isolated ileum and of the mouse isolated vas deferens. Log10 concentration-tissue state curves were determined for (+)-meptazinol and for RX783006, alone, in combination and, when appropriate, in the presence of naloxone (30 nM). Analysis of these log10 concentration-tissue state curves using the null equations derived and verified in the previous paper allows quantitation of the characteristics of the interaction of (+)-meptazinol with the opioid receptors in these tissues. The results indicate that the apparent differences in the actions of (+)-meptazinol on isolated electrically-stimulated guinea-pig ileum and mouse vas deferens can be accounted for without the need to postulate differences between mu-opioid receptors in these two tissues.

A comparison of analgesia and suppression of oxytocin release by opiates

The potency of opiates for suppressing oxytocin release relative to their potency as analgesics was tested in lactating rats. Oxytocin release was evoked by the sucking of the young in urethane-anaesthetized and unanaesthetized rats, and was detected by the characteristic behaviour of the young and milk yield respectively. The tail-flick test, using noxious radiant heat, was used to assess analgesia. Intraperitoneal injection of morphine (1 mg kg-1 and 5 mg kg-1) significantly reduced milk yield in unanaesthetized rats. Urethane-anaesthetized rats displayed a pattern of reflex milk-ejection responses similar to that found in conscious rats. This reflex was significantly inhibited in a dose-related, naloxone-reversible manner by buprenorphine (ED50 0.18 mg kg-1), meptazinol (ED50: 14.0 mg kg-1), morphine (ED50: 0.67 mg kg-1), pentazocine (ED50: 15.0 mg kg-1) and pethidine (ED50: 7.9 mg kg-1). Although intraperitoneal injection of morphine (5 mg kg-1) abolished the increase in intramammary pressure occurring at reflex milk-ejection, that evoked by intravenous oxytocin (0.5-1 mu) was unaffected. Each opiate also caused significant, dose-related, naloxone-reversible increases in tail-flick latency. The ED50 doses were buprenorphine (ED50: 0.14 mg kg-1), meptazinol (ED50: 12.5 mg kg-1), morphine (ED50: 5.0 mg kg-1), pentazocine (ED50: 12.5 mg kg-1) and pethidine (ED50: 6.1 mg kg-1). The order of potency for analgesia and for suppression of oxytocin release were identical, namely: buprenorphine greater than morphine greater than pethidine greater than meptazinol greater than pentazocine. The results obtained with lactating rats suggest that secretion of the hormone oxytocin is substantially reduced during opiate-induced analgesia.

Meptazinol cross-tolerance studies between morphine or oxotremorine

The time course of the development of auto-tolerance to meptazinol as determined by combined heat and pressure nociceptive tests has been examined using two dose levels of meptazinol (10 mg and 30 mgkg-1s.c.) corresponding to the partial agonist and cholinergic components respectively. In mice treated twice daily with meptazinol for eight days, there was cross tolerance to morphine in both tests at each dose level of meptazinol. In chronic morphine treated mice challenged with meptazinol (30 mg kg-1 s.c.), there was no cross tolerance with morphine since meptazinol still retained its antinociceptive effects in both tests. Mice treated chronically with meptazinol (30 mgkg-1 s.c.) did not respond to the cholinomimetic agent oxotremorine, implying that there was cross tolerance between these two analgesics. These data suggest the existence of a one-way tolerance between morphine and meptazinol whilst at higher doses of meptazinol a full tolerance occurs with oxotremorine.

A comparison of the effects of meptazinol and morphine on the release of acetylcholine from slices of mouse cerebral cortex

Morphine and the new centrally-active analgesic agent, meptazinol, both increased the K+-evoked release of tritium from slices of mouse cortex preloaded with [3H]choline. The effect of both compounds was antagonised by naloxone whereas the response to meptazinol, but not that to morphine, was reduced in the presence of either scopolamine or tetrodotoxin. Oxotremorine produced a concentration-related inhibition of tritium release, presumably via an action on presynaptic muscarinic receptors, which was also blocked by scopolamine. These results suggest that there may be an indirect component in the action of meptazinol which may be related to a previous finding in which the antinociceptive response to meptazinol in the mouse was antagonised by both scopolamine and naloxone.

Differential effects of di-isopropylfluorophosphate poisoning and its treatment on opioid antinociception in the mouse

Compounds which enhance cholinergic activity have been reported to interact with opioid drugs. We have shown, using the hot-plate test in mice that di-isopropylfluorophosphate potentiates the antinociceptive activity of alfentanil but has no effect on the activity of morphine or fentanyl. Administration of atropine and pralidoxime as a treatment for DFP poisoning does not reverse this effect, and itself potentiates morphine antinociception. The results suggest that a cholinergic/opioid interaction is dependent on the opioid studied, and may have clinical importance when opioid drugs are required in patients poisoned by irreversible anticholinesterases.