A comparison of the effects of imipramine, trimipramine, and some other drugs in rabbits treated with a monoamine oxidase inhibitor

The serotonin syndrome. Implicated drugs, pathophysiology and management

The serotonin syndrome has increasingly been recognised in patients who have received combined serotonergic drugs. This syndrome is characterised by a constellation of symptoms (confusion, fever, shivering, diaphoresis, ataxia, hyperelflexia, myoclonus or diarrhoea) in the setting of the recent addition of a serotonergic agent. The most common drug combinations causing the serotonin syndrome are monoamine oxidase inhibitors (MAOIs) and serotonin selective reuptake inhibitors (SSRIs), MAOIs and tricyclic antidepressants, MAOIs and tryptophan, and MAOIs and pethidine (meperidine). This syndrome is caused by excess serotonin (5-hydroxytryptamine; 5-HT) availability in the CNS at the 5-HT1A-receptor. There may also be some interaction with dopamine and 5-HT2-receptors. This syndrome probably has a low incidence, even among patients taking these drug combinations, and there is likely to be some other as yet unidentified inciting factor causing some patients to develop a full serotonin syndrome. Because fatalities and severe complications have accompanied the serotonin syndrome, the previously described drug combinations should be used cautiously or not at all. The serotonin syndrome is usually mild and, if managed with drug withdrawal and supportive therapy, generally improves within hours. Patients who develop hyperthermia should be treated aggressively with external cooling and paralysis. Methysergide and cyproheptadine appear to be useful adjuncts in treating the serotonin syndrome.

Clinical originality and new biology of trimipramine

Trimipramine differs from other antidepressant drugs in a number of ways. Although trimipramine shares equivalent efficacy with doxepin, imipramine, maprotiline and amitriptyline, as evidenced by double-blind studies, it possesses a different side-effect profile. Trimipramine is considered to be less cardiotoxic, and data presented in this paper support its minimal effect on orthostatic hypotension, as compared with clomipramine. In addition, trimipramine has less epileptogenic potential than other antidepressants such as imipramine, amitriptyline and maprotiline. Besides this different side-effect profile, trimipramine exerts differing effects on neurotransmitter functions and their reuptake. For example, trimipramine does not inhibit reuptake of noradrenaline and serotonin, and does not down-regulate beta 1-adrenoceptors. Furthermore, in common with lithium, trimipramine produces enhancement of antidepressant action in treatment-resistant depressed patients. There is evidence that trimipramine enhances the sensitivity of cortical neurons to noradrenaline after prolonged administration and may also increase the activity of serotonin neurons.

Effect of nonselective and selective inhibitors of monoamine oxidases A and B on pethidine toxicity in mice

The LD50 of pethidine was determined in mice pretreated (4 h) either with the nonselective monoamine oxidase (MAO) inhibitor, phenelzine or with clorgyline, a selective inhibitor of MAO A or deprenyl, a selective inhibitor of MAO B. Phenelzine or combined clorgyline plus deprenyl pretreatments decreased pethidine LD50. Clorgyline or deprenyl alone did not affect pethidine toxicity. Whole brain 5-hydroxytryptamine (5-HT) concentrations were measured in the pretreated mice. 5-HT levels were approximately doubled (P less than 0.001) after phenelzine or clorgyline plus deprenyl treatment, but not after clorgyline or deprenyl given alone. These results indicate that both MAO A and MAO B need to be inhibited to increase pethidine toxicity and brain 5-HT levels. They support the involvement of 5-HT in the toxic interaction between pethidine and MAO inhibitors.

Effects of LM 5008, a selective inhibitor of 5-hydroxytryptamine uptake, on blood pressure and responses to sympathomimetic amines

LM 5008 (4-[2-(3-indolyl)ethyl]piperidine) (10, 20 and 50 mg kg-1) had no significant effect on pressor responses to noradrenaline or tyramine in rats anaesthetized with urethane. Desmethylimipramine (1 mg kg-1) blocked the response to tyramine but chlorimipramine (5 mg kg-1) had no significant effect on responses to noradrenaline or tyramine. In the rabbit, anaesthetized with chloralose, LM 5008 (5 mg kg-1) had no effect on pressor responses to noradrenaline, tyramine or angiotensin II, while desmethylimipramine (0.25 mg kg-1) inhibited responses to tyramine and potentiated those to noradrenaline. LM 5008 (10 mg kg-1) had no effect on resting blood pressure of conscious normotensive or DOCA-saline hypertensive rats. Tranylcypromine (5 mg kg-1) produced a fall in blood pressure in conscious normotensive and in DOCA hypertensive rats. Treatment with a combination of LM 5008 (10 mg kg-1) and tranylcypromine (5 mg kg-1) resulted in the appearance of a behavioural hyperactivity syndrome, but blood pressure was not different from that of animals treated with tranylcypromine alone. These results further demonstrate the selectivity of LM 5008 for 5-hydroxytryptamine as opposed to catecholamine uptake.

Toxic interaction between narcotic analgesics and inhibitors of catechol-O-methyltransferase

A lethal synergism between morphine and tropolone, an inhibitor of catechol-O-methyltransferase, was previously noted in adult male Holtzman rats. The present research demonstrates that this phenomenon generalizes across factors of sex, age, strain (Sprague--Dawley, Wistar) and species (Swiss albino mice). Acute toxicity was also significantly increased (1.5--1.9 times) in the case of codeine, methadone, meperidine and levorphanol, but to a lesser extent than for morphine (4.0 times) in the S-D strain. Another COMT inhibitor, 3,5-dihydroxy-4-methoxybenzoic acid, interacted with morphine in S-D rats to an equal degree as did tropolone. Post-treatment with 1 mg/kg of naloxone in rats or naltrexone in mice reduced the high lethality associated with morphine plus tropolone. There was a pronounced lowering of whole brain norepinephrine (NE) level after morphine plus tropolone in Wistar rats with doses of each component that alone caused no change in NE. Brain dopamine (DA) was elevated by tropolone and by its combination with morphine. Each drug alone caused slight lowering of brain serotonin. Enhancement by tropolone of the toxicity of (+)-amphetamine in mice and rats was of similar magnitude as for morphine. The possible role of brain NE and/or DA in the sensitivity to acute toxic effects of opioids in rodents is suggested by these data, as well as a parallel in this regard with amphetamine-type stimulants.

Attenuating the rate-decreasing effects of phenylpiperidine analgesics by pentobarbital

The ability of pentobarbital, diazepam, and chlorpromazine to attenuate the rate-decreasing effects of a high dose (10 or 30 mg/kg) of meperidine was tested in pigeons responding under a multiple fixed-ratio, fixed-interval schedule of food presentation. Pentobarbital (10 mg/kg) attenuated the meperidine-induced rate decreases, whereas diazepam (0.3--3 mg/kg) or chlorpromazine (3--30 mg/kg) did not reliably attenuate the response rate decreases. The combination of 10 mg/kg of pentobarbital and meperidine resulted in a marked disruption of the pattern of responding in the fixed-interval component of the multiple schedule. Pentobarbital (1, 3, 10, and 17.5 mg/kg) was also tested in combination with rate-decreasing doses of normeperidine (17.5 mg/kg), anileridine (10 mg/kg), alphaprodine (10 mg/kg), and fentanyl (0.3 mg/kg). Pentobarbital reliably attenuated the rate-decreasing effects of normeperidine, anileridine, and alphaprodine, but not the rate decreases induced by fentanyl.

Dextromethorphan inhibits 5-hydroxytryptamine uptake by human blood platelets and decreases 5-hydroxyindoleacetic acid content in rat brain

The effect of dextromethorphan on the uptake and metabolism of 5-hydroxytryptamine (5-HT) was studied in human blood platelets and in rat brain. In the concentration of 120 nM dextromethorphan inhibited the uptake of 5-HT (1 mu-M) into platelets by 50%. The corresponding concentrations of imipramine and methadone under similar conditions were 22 and 590 nM, respectively. Dextromethorphan (20 to 40 mg kg-1) decreased the concentration of brain 5-hydroxyindoleacetic acid (5-HIAA) and the probenecid-induced accumulation of 5-HIAA time- and dose-dependently. However, dextromethorphan did not alter the pargyline-induced changes in brain 5-HT metabolism. Dextromethorphan-induced changes in brain 5-HT metabolism could arise from the inhibition of the re-uptake of 5-HT into neurons.

Dextromethorphan-monoamine oxidase inhibitor interaction in rabbits

Severe toxic reactions may occur clinically when imipramine, pethidine or dextromethorphan is administered to a patient being treated with a monoamine oxidase inhibitor (MAOI). Previous reports indicate that imipramine or pethidine produces symptoms characterized by motor restlessness, tremor, extreme hyperpyrexia and death when administered to phenelzine-pretreated rabbits. The present study shows that dextromethorphan (5 mg kg−1) produces identical symptoms in rabbits pretreated with phenelzine sulphate (30 mg kg−1) or nialamide HCl (50 mg kg−1) 42 and 18 h before temperature recording. The dextromethorphan-MAOI interaction appears to be due to a 5-hydroxytryptamine potentiation. In the unanaesthetized cat nictitating membrane preparation, dextromethorphan (5 mg kg−1) markedly enhanced the response of noradrenaline and 5-HT but antagonized the effects of tyramine. This suggests that dextromethorphan blocks the uptake of these amines in the adrenergic nerve endings.

Role of brain monoamines in the fatal hyperthermia induced by pethidine or imipramine in rabbits pretreated with a monoamine oxidase inhibitor

1. The intravenous infusion of pethidine or imipramine, in doses of 5 mg/kg, caused fatal hyperpyrexia in rabbits premedicated with pargyline.2. The drug interaction was not antagonized when either reserpine or alpha-methyl-p-tyrosine were administered with pargyline. Neither reserpine nor alpha-methyl-p-tyrosine prevented the rise in brain stem 5-hydroxytryptamine content following monoamine oxidase inhibition, although the increase in catecholamines normally produced by pargyline was prevented.3. The development of fatal hyperthermia was completely prevented when rabbits were treated with p-chlorophenylalanine prior to pargyline premedication. In these animals, the concentration of brain stem catecholamines, but not 5-hydroxytryptamine, was increased.4. The results indicate that the hyperthermia evoked by pethidine or imipramine in combination with monoamine oxidase inhibitors can take place only in the presence of raised concentrations of 5-hydroxytryptamine in the brain stem.

The effect of narcotic analgesics on the uptake of 5-hydroxytryptamine and (-)-metaraminol by blood platelets

1. The effects of narcotic analgesic and related drugs were studied on the uptake of 5-hydroxytryptamine (5-HT) and (-)-metaraminol by blood platelets.2. The most potent drug in inhibiting the uptake of 5-HT (10 muM) by human platelets was methadone, followed by pentazocine>piminodine approximately pethidine approximately anileridine approximately cyclazocine approximately thebaine > dextropropoxyphene. Alphaprodine, papaverine, apomorphine, nalorphine, codeine, and morphine were almost without effect. Methadone was slightly less active than desipramine, and had 10% of the activity of imipramine under similar conditions. Naloxone did not antagonize the effect of methadone on 5-HT uptake.3. The most potent inhibitor of metaraminol (3 muM) uptake by human platelets was piminodine, followed by pentazocine>/=anileridine>cyclazocine=methadone > dextropropoxyphene approximately thebaine >/= papaverine approximately alphaprodine >pethidine>morphine. The activity of morphine was 1% of that of piminodine. Piminodine was more potent than desipramine and protriptyline under similar conditions. The order of potency of drugs studied in inhibiting the uptake of metaraminol by rabbit platelets was similar to that obtained with human platelets.4. The effects of the analgesics studied on inhibiting uptake of monoamines did not correlate with their pain-relieving properties.

Analysis of the inhibition of pethidine N-demethylation by monoamine oxidase inhibitors and some other drugs with special reference to drug interactions in man

1. N-Demethylation of pethidine was studied in microsomal suspensions from unstarved male rat liver and the N-demethylase identified as belonging to the class of hepatic microsomal mixed function oxidases.2. A study of the structure/action relationships of compounds inhibiting pethidine N-demethylase revealed that hydrazine derivatives including phenylhydrazine, methylphenylhydrazine and mebanazine were all potent competitive inhibitors.3. Pethidine N-demethylase was only slightly inhibited by histamine and amphetamine but not by adrenaline and ephedrine nor by several miscellaneous compounds including piperidine, N-ethylpiperidine, N-methylpiperidine, N-methylammonium, hydrallazine or pethidinic acid.4. Several psychotropic drugs were all found to be potent competitive inhibitors of pethidine N-demethylase. These included monoaminoxidase inhibitors (the most active being nialamide and phenoxypropazine [K(i)=0.01 mM]; the least active iproniazid [K(i)=1.05 mM]); the tranquillizers promazine, propiomazine and chlorpromazine and tricyclic antidepressants (opipramol [K(i)=0.01 mM], imipramine [K(i)=0.03 mM], desipramine [K(i)=0.03 mM] and amitryptyline [K(i)=0.03 mM]). Hydrocortisone [K(i)=0.3 mM], prednisolone [2.8 mM] and nalorphine [0.07 mM] were also inhibitors, whilst SKF 525A was the most active of all [K(i)=0.002 mM].5. These results are discussed in relation to the clinically observed drug interactions which may occur between monoamineoxidase inhibitors and pethidine. It is concluded that since many different groups of drugs, including monoamineoxidase inhibitors, tranquillizers, tricyclic antidepressants, steroids, nalorphine, SKF 525A and barbiturates compete for cytochrome P(450) reductase, it is possible that this mechanism may account, at least in part, for the observed interactions of these various drugs in man.

Comparison of the effects of morphine, pethidine and pentazocine in rabbits pretreated with a monoamine oxidase inhibitor

1. Rabbits were premedicated with pargyline and the changes in rectal temperature measured after the intravenous infusion of morphine, pentazocine and pethidine.2. Pethidine produced pronounced rises in rectal temperature which were dose dependent. One out of the four rabbits given 1 mg/kg died in hyperthermia. Four out of the four rabbits given 5 mg/kg died in hyperthermia. Doses of 10 mg/kg of pethidine caused no significant change in the rectal temperature of rabbits not pretreated with pargyline.3. Morphine and pentazocine in doses of 1 mg and 10 mg/kg did not significantly alter the rectal temperature of rabbits pretreated with pargyline except for one rabbit which developed a delayed hyperthermia following the injection of morphine 1 mg/kg.

The interaction between monoamine oxidase inhibitors and narcotic analgesics in mice

1. The administration of either iproniazid or tranylcypromine to mice potentiates the acute toxicity of pethidine, morphine, pentazocine and phenazocine.2. Blood levels of pentazocine in mice pretreated with tranylcypromine do not differ from the levels in animals not receiving the monoamine oxidase (MAO) inhibitor.3. There is no correlation between changes in brain and liver MAO activity and the increased pethidine toxicity.4. A comparison is made between the change in pethidine toxicity and the changes in the concentration of cerebral noradrenaline, dopamine and 5-hydroxytryptamine following the injection of tranylcypromine.5. It is concluded that the increased toxicity of potent analgesics in combination with MAO inhibitors is not due to a decelerated metabolism of the analgesic drug, but is related to an increased concentration of cerebral 5-hydroxytryptamine. A critical level of this monoamine, in the brain, may be necessary before the drug interaction will take place.