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Phosphodiesterase-4 (PDE4) is one of the main enzymes that specifically terminate the action of cAMP, thereby contributing to intracellular signaling following stimulation of various G protein-coupled receptors. PDE4 expression and activity are modulated by agents affecting cAMP levels. The selective PDE4 inhibitor (R)-rolipram labeled with C-11 was tested in vivo in rats to analyze changes in PDE4 levels following drug treatments that increase synaptic noradrenaline (NA), serotonin (5HT), histamine (HA) and dopamine (DA) levels. We hypothesized that increasing synaptic neurotransmitter levels and subsequent cAMP-mediated signaling would significantly enhance (R)-[(11)C]rolipram retention and specific binding to PDE4 in vivo. Pre-treatments were performed 3 h prior to tracer injection, and rats were sacrificed 45 min later. Biodistribution studies revealed a dose-dependent increase in (R)-[(11)C]rolipram uptake following administration of the monoamine oxidase (MAO) inhibitor tranylcypromine, NA and 5HT reuptake inhibitors (desipramine [DMI], maprotiline; and fluoxetine, sertraline, respectively), and the HA H(3) receptor antagonist (thioperamide), but not with DA transporter blockers GBR 12909, cocaine or DA D(1) agonist SKF81297. Significant increases in rat brain and heart reflect changes in PDE4 specific binding (total-non-specific binding [coinjection with saturating dose of (R)-rolipram]). These results demonstrate that acute treatments elevating synaptic NA, 5HT and HA, but not DA levels, significantly enhance (R)-[(11)C]rolipram binding. Use of (R)-[(11)C]rolipram and positron emission tomography as an index of PDE4 activity could provide insight into understanding disease states with altered NA, 5HT and HA concentrations.

Using selective monoamine uptake blockers and appropriate selective monoamine receptor antagonists, we have previously shown that cocaine enhances the frequency of 5-HT2A receptor-mediated 5-hydroxytryptophan (5-HTP)-induced head-twitch response (HTR) in mice via inhibition of serotonin uptake. Concomitantly, cocaine prevented the maximal producible HTR frequency via simultaneous indirect stimulation of the inhibitory presynaptic 5-HT1A and postsynaptic alpha 2 receptors. In the present study, we have investigated the effects of cocaine and the selective 5-HT (sertraline), norepinephrine (nisoxetine) and dopamine (GBR 12935) uptake inhibitors on the L-tryptophan-induced HTR in the presence of a nonselective monoamine oxidase inhibitor, tranylcypromine. We utilized two experimental protocols where cocaine or sertraline were administered either after (protocol 1) or prior to (protocol 2) L-tryptophan injection. Cocaine potentiated the ability of L-tryptophan to induce HTR to a greater extent in protocol 1, whereas sertraline induced a greater effect in protocol 2. However, in our earlier study cocaine (and also sertraline) up to 10 mg/kg produced a similar degree of potentiation in both experimental protocols on the 5-HTP-induced HTR. Furthermore, as in the latter study on the 5-HTP-induced HTR, in the present investigation nisoxetine potently attenuated whereas GBR 12935 did not modulate the induced HTR. The results show that the respective serotonergic and noradrenergic effects of cocaine also operate on the L-tryptophan-induced HTR. The differential effects of cocaine and sertraline in experimental protocols 1 and 2 on the L-tryptophan- versus 5-HTP-induced HTRs suggest that cocaine has additional effects on the conversion of L-tryptophan to 5-HT.

Modification in gamma-aminobutyric acid-B (GABA(B)) receptors may contribute to the symptoms of some neurological and psychiatric disorders and to the clinical response to psychotherapeutics. The present study was undertaken to determine whether chronic administration of tranylcypromine (TCP), an antidepressant, and chronic stress influence GABA(B) receptor function in rat brain. The results indicate that TCP treatment, but not stress, increases GABA(B) receptor activity in the cerebral cortex, as measured by baclofen-stimulated GTPgammaS binding. In addition, chronic administration of TCP enhances significantly the locomotor response to a single dose of amphetamine, an effect that is abolished by restraint stress. These results indicate that although TCP administration modifies brain GABA(B) receptor activity, which may contribute to the antidepressant response to this agent, this effect is unrelated to the interaction of stress and TCP treatment on the locomotor response to amphetamine.

Chronic administration of antidepressants has been shown to reduce the number of escape failures in the rat learned helplessness test (LH). In the present study we investigated the role of D1, D2 and D3 receptors in mediating this effect. In our first series of experiments, we demonstrated that SKF38393, D1 receptor agonist, in a dose of 2.5 mg/kg (i.p.) and quinpirole, D2 receptor agonist in a dose of 0.5 mg/kg (i.p.), significantly decreased the number of escape failures in LH, and these were reversed by SCH23390 (0.015 mg/kg), D1 receptor antagonist, and by sulpiride (25 mg/kg), D2 receptor antagonist, respectively. In contrast, 7-OH-DPAT, a D3 receptor agonist, in a dose of 10 mg/kg (i.p.) did not affect the number of escape failures in LH. In a second series of experiments, we showed that eight days of repeated treatment with imipramine (10 mg/kg, p.o.), fluvoxamine (1.25 mg/kg, p.o.) and tranylcypromine (1.25 mg/kg, p.o.) significantly decreased the number of escape failures in LH. The decrease in escape failures seen with use of imipramine and tranylcypromine was reversed by sulpiride in LH, but not by SCH23390. On the other hand, the effect of fluvoxamine was reversed by both SCH23390 and sulpiride. These findings indicate that stimulation of D1 and D2 receptors decreased the number of escape failures in LH, respectively. Thus, D2 and/or D1 receptors are probably involved in the decreased number of escape failures in case of repeated treatment with antidepressants in LH.

Recently, a new imidazoline (I 2 ) ligand, [ 3 H]2-(2-benzofuranyl)-2-imidazoline (BFI) was shown to be more selective for I 2 vs α 2 binding in rodent brain. We characterized [ 3 H]BFI binding in human brain cortex and lateral reticular nucleus (NRL). Membranes were incubated with [ 3 H]BFI at 22 °C in 50 mM Tris, 1.5 mM EDTA at pH 7.5. Saturation experiments with [ 3 H]BFI (0.5–80 nM) were analyzed using non-linear curve fitting. The NRL had 4X more binding sites than cortex with similar affinity (B max = 2085 ± 732 and 471 ± 41 fmol mg protein; K D = 9.3 ± 3.5 and 11.9 ± 2.7 nM, respectively). In competition studies, cortical [ 3 H]BFI binding was displaced in order of decreasing potency by clorgyline > BFI ≥ cirazoline > idazoxan ≥ guanabenz > clonidine > RX821002. The monoamine oxidase (MAO) inhibitor tranylcypromine (TCP) (1 nM-10 μM), markedly enhanced [ 3 H]BFI binding in both NRL and cortex. Enhanced binding was maximal at 300 nM (12 X control) and returned to baseline at 30 μM. Potentiation was not seen with pargyline or clorgyline. TCP did not effect [ 3 H]BFI binding in rat cortex, or [ 3 H]idazoxan binding in human cortex and NRL. In human cortex, inhibition of MAO by preincubation with pargyline (10 μM) abolished the TCP effect. Upon preincubation with TCP, the stimulation of [ 3 H]BFI binding was dose-dependently related to a simultaneous inhibition of MAO. Thus, [ 3 H]BFI labels a site in human NRL and cortex that appears similar to the previously described I 2 site labeled by [ 3 H]idazoxan. However, [ 3 H]BFI binding is dramatically stimulated by TCP in human brain via a mechanism dependent on endogenous MAO activity.

The brain of Alzheimer's disease (AD) patients contains deposits of amyloid beta-peptide (A beta). Recent studies have shown that A beta is a substrate for tissue transglutaminase (TGase), which induces the formation of cross-linked dimers and polymers, and that tacrine, indomethacin and deferoxamine, which have widely different chemical structures, attenuate the progression of symptoms of AD. This report evaluated the potential of a total of ten different pharmacological agents to inhibit TGase-induced cross-linking of A beta, including known TGase inhibitors (dansylcadaverine, spermine), non-steroidal anti-inflammatory drugs (indomethacin, meclofenamic acid, diflunisal, salicylic acid), monoamine oxidase inhibitors (tranylcypromine, phenelzine), an acetylcholinesterase inhibitor (tacrine), and an iron chelating agent (deferoxamine). All but one (salicylic acid) of these ten agents had an inhibitory effect on TGase-induced A beta cross-linking. These results suggest that inhibition of TGase-induced cross-linking of A beta is a potential pharmacologic target for the treatment of AD. A method is also presented for the determination of percent inhibition of TGase-induced A beta cross-linking based on the separated monomer, dimer and polymer bands on SDS-PAGE gels.

Withdrawal from chronic cocaine exposure potentiates the ability of direct 5-HT2A agonists to induce the head-twitch response (HTR) in rodents. This supersensitivity is assumed to be a consequence of cocaine-induced deficits in presynaptic serotonin neurochemistry. The present study utilized the HTR produced by L-tryptophan (TP) to investigate the dose- and time-response effects of cocaine-induced 5-HT deficit. Thus, different groups of mice were injected with cocaine twice daily (0, 0.1, 0.5, 2.5, 5 or 10 mg kg , i.p.) for 7 or 13 days. During HTR testing procedure, at 24 h after last chronic injection, treated-mice received either: 1) no cocaine; 2) their corresponding daily dose; or 3) a 10 mg kg dose. In paradigm 1, the frequency of TP-induced HTR was attenuated in a dose-dependent manner in both chronic cocaine regimens. In paradigm 2, small challenge doses (0.1–2.5 mg kg ) of cocaine in their respective pretreatment groups failed to alter HTR, but larger challenge doses (5 and 10 mg kg ) potentiated the behavior. In paradigm 3, the 10 mg kg challenge dose potentiated the HTR to a similar degree in both chronically exposed vehicle and various cocaine-treated groups in both treatment regimens. Extended withdrawal studies from cocaine exposure (0, 0.5 and 5 mg kg twice daily for 7 or 13 days) indicated attenuations in HTR persisted up to 96 h postcocaine abstinence in paradigm 1, whereas paradigm 2 revealed significant attenuations between 48–72 h for 0.5 mg kg dose; and potentiations for the 5 mg kg dose persisted throughout the 96 h abstinence. In paradigm 3, no significant effect was observed at 96 h abstinence, but the 10 mg kg challenge dose significantly potentiated HTR in chronically exposed 10 mg kg cocaine group 10 days following cocaine abstinence in both exposure regimens. Overall, these results support the notion that chronic cocaine exposure produces prolonged deficits in presynaptic serotonin neurochemistry. Furthermore, serotonergic mechanisms appear to be exquisitely sensitive to chronic administration of both low and high doses of cocaine.

The effects of brofaromine, clorgyline (reversible and irreversible type A MAO inhibitors, respectively) and tranylcypromine (non-selective MAO inhibitor) on rat striatal levels of phenylethylamine, tryptamine, m-tyramine and p-tyramine were determined. Brofaromine and clorgyline increased m- and p-tyramine levels, but not phenylethylamine levels. Brofaromine given at a dose of 100 mg/kg did increase tryptamine levels. Tranylcypromine increased the levels of all four amines greatly. The effects of chronic treatment with brofaromine on amine levels were not different from those following acute treatment. By contrast, chronic treatment with clorgyline caused greater increases in striatal m- and p-tyramine levels than did acute clorgyline. These data show that changes in the rat striatal levels of m-tyramine and p-tyramine may be used as in vivo indicators of the selectivity and reversiblity of inhibition of type A MAO, while tryptamine levels reflect non-selective inhibition of both types of MAO.

The hallucinogen analog trans -2-(2,5-dimethoxy-4-methylphenyl)-cyclopropylamine (DMCPA) was resolved into its two optical isomers. Examination of selected behavioral profiles in mice and cats clearly showed that the levorotatory isomer of DMCPA possesses stereoselective activity when compared with the dextro isomer. The results parallel those obtained using the isomers of the known hallucinogen, DOM (STP) in the same animal models. Comparison of the optical rotatory dispersion (ORD) curves for the N-(5-bromosalicylidene) derivatives of DMCPA and trans -2-phenylcyclopropylamine (tranylcypromine) of known absolute configuration established the configuration of DMCPA to be (-)-1R,2S. This stereoselective activity and proof of absolute configuration lend strong support to a new model of the hallucinogen receptor. The proposed model suggests possible structural similarities between LSD and phenethylamine hallucinogens.

Injection of norepinephrine (NE) into the paraventricular nucleus of the hypothalamus has been shown to elicit eating in satiated rats. In the present study, the monoamine oxidase inhibitor tranylcypromine (TCP), which is known to release endogenous NE, was found to produce a similar effect in rats maintained and tested on a palatable milk-mash combination diet. The effect of this antidepressant drug was positively correlated in magnitude with the NE effect in the same animals. It was selectively antagonized by central injection of drugs which block α-adrenergic receptors. Local pretreatment with NE synthesis inhibitors similarly blocked the TCP eating response but had no effect on eating elicited by exogenous NE. It is suggested from these results that TCP is stimulating eating through the release of endogenous NE from adrenergic neurons innervating the region of the paraventricular nucleus.

Dopamine and related compounds were administered directly into the caudate nucleus ipsilateral to a unilateral 6-OHDA induced lesion of the substantia nigra in rats. Dopamine, norepinephrine, epinine, epinephrine, apomorphine and ADTN produced vigorous long-lasting, dose-related contraversive circling. Dopamine was the most potent substance tested, its threshold for significant activity was 0.06 μg and the dose producing the maximal rate of rotation was 0.40 μg. Serotonin produced modest rotation which was not dose-related. Piribedil, although more effective than its metabolite S584, produced only a short period of activity at very high doses. Isoproterenol and clonidine were inactive. The effects of dopamine were compared in lesioned and non-lesioned rats. The supersensitivity of the lesioned rats was manifested as an increase in the potency of dopamine and as an increase in the maximal response, defined as rate of rotation. Rotation was produced in non-lesioned rats only after pretreatment with tranylcypromine, which had no significant effect in lesioned rats.

Effects of intravenous administration of the serotonin precursor tryptophan (TRP) on serum prolactin, neuromotor function, subjective mood, and blood pressure and pulse were determined in nine depressed patients before and during placebo-controlled treatment with the monoamine oxidase inhibitor (MAOI) tranylcypromine. Tranylcypromine significantly increased the prolactin response to TRP. Four patients developed a distinctive neuromotor syndrome following TRP during tranylcypromine, but not placebo, treatment. Symptoms included hyperreflexia, ankle clonus, nystagmus, incoordination, tremor, myoclonic jerks, and nausea. There were no differences in peak prolactin, mood, or autonomic responses between patients with and without the syndrome, but those with the syndrome had received active tranylcypromine for a significantly shorter duration. Tranylcypromine had little effect on TRP-induced changes in mood or autonomic function, except for a modest enhancement of the TRP-induced rise in diastolic blood pressure. These results suggest that tranylcypromine treatment may enhance serotonin function in depression.

Morphine hyperglycemia in the rabbit was blocked by hydergine but not by propranolol. Two monoamine oxidase inhibitors -- iproniazid and nialamide -- were also shown to prevent the hyperglycemia, but this effect could not be demonstrated with tranylcypromine.

Recent evidence suggests that dopamine can undergo axoplasmic transport in nigro-neostriatal neurons by binding to amine storage granules. In the present experiments it was demonstrated that reserpine pretreatment (10 mg/kg) 24 hours before stereotaxic injections of 3H-DOPA or 3H-dopamine into the substantia nigra increases the amount of 3H-dopamine transported to the neostriatum by about 300 percent. The activity recovered from the substantia nigra was significantly reduced by reserpine pretreatment however. Stereotaxic injection of 14C-leucine into the substantia nigra indicated that neither fast nor slow axoplasmic transport of protein was influenced by reserpine pretreatment in these same neurons. The increased transport of dopamine appears therefore to be due to a relatively selective action of reserpine. The results suggest that reserpine either (i) increases the binding of dopamine to newly synthesized amine storage granules, (ii) increases the number of newly synthesized amine storage granules, or (iii) accelerates the rate of transport of amine storage granules. In addition, the results support the view that reserpine can increase the membrane permeability of adrenergic neurons to the outward movement of catecholamines.

The turnover rates of monoamine oxidases can be determined by measuring the rates of recovery of the irreversibly inhibited enzyme. The rates obtained reflect de novo enzyme synthesis and follow first-order kinetics. The rates of recovery of irreversibly inhibited MAO are shown to be specific for monoamine oxidases in different tissues and independent of the irreversible inhibitor used. Similar rates are obtained when protein turnover is directly measured.- In the submandibular glands of rats, isoproterenol enhances both synthesis and turnover of MAO, whereas in the heart no effect of isoproterenol on MAO can be seen.

The effects of cysteine with and without tranylcypromine, a monoamine oxidase inhibitor, were examined in 4 schizophrenic patients in order to determine whether or not alterations in metabolic pathways of tryptophan would be produced. During the administration of L-cysteine alone, urinary tryptamine rose significantly. With the combined treatment of L-cysteine and tranylcypromine, the rise in tryptamine excretion was greater. 1-methylnicotinamide showed no consistent trend with either L-cysteine alone or combined with tranylcypromine. In general the results of the combined treatment of cysteine and tranylcypromine may be compared to those of methionine and tranylcypromine in demonstrating correlations between behavioral worsening and increased excretions of tryptamine.

Tryptophan injected intracerebrally in rats increases the level of brain serotonin. This increase occurs also in animals pretreated with reserpine or with a monoamineoxidase inhibitor. Other studies show that tryptophan, tryptamine or 6-hydroxytryptamine are not interfering in the determination of brain serotonin.

Rat muricidal activity has been shown to be selectively inhibited by some antidepressants, antihistaminics and stimulants. Several agents from each class, including representative structural prototypes, were tested for anti-muricidal activity. Selective inhibition was considered only when it occurred at doses which were significantly lower than those impairing rotarod performance. The order of potency for agents found selectively active in this study was: tranylcypromine > methamphetamine > d-amphetamine > desipramine > imipramine > chlorpheniramine > pargyline. Structurally each of these drugs has a sidechain substitution which is an alkylamine. However, not all drugs with alkylamine sidechains were selectively effective. These studies seem to indicate those agents structurally related to d-amphetamine, i.e., methamphetamine and tranylcypromine, are the most potent selective inhibitors of mouse killing behavior in rats.

The benzo[b]thiophene analogs of harmine and harmaline have been tested as MAOI in vitro; harmine and S-harmine are similar in potency, while S-harmaline is 50 times more potent than harmaline. S-Harmaline has a higher lipid solubility, a shorter biological half-life, and less tissue binding than the nitrogen analog. When administered to mice, the S-harmaline produces a significant elevation in brain 5HT.

Rats were trained to press a lever for moderately rewarding electrical stimulation of the lateral hypothalamus. Following training the animals were treated with drugs which inhibit brain monoamine oxidase. Pargyline, tranylcypromine, and iproniazid produced marked excitatory effects on self-stimulation which appeared after a conspicuous delay. These effects appeared to be associated with the elevation of brain norepinephrine levels following MAO blockade. Etryptamine, on the other hand, produced a more-or-less immediate excitatory effect in accordance with its known direct amphetamine-like action. The results were presented within the context of a theory that some types of clinical depression are associated with deficient norepinephrine levels in the reward system of the brain.

Intradermal injection of 5 to 10 μg norepinephrine or coli endotoxin was followed 1 to 5 hours later by neutrophilia and lymphopenia in the rat. Administration of a monoamine oxidase inhibitor (tranylcypromine) or of desmethylimipramine, which interferes with the catecholamine-inactivating process, potentiated the neutrophilia producing effect of norepinephrine. Histamine did not produce any changes in the differential leucocyte count when given alone, but did inhibit the neutrophilia normally resulting from norepinephrine treatment. The role of norepinephrine in the delayed phase of the acute inflammatory process is discussed.

The effect of a number of ingibitors of thyroid monoamine oxidase on the incorporation of radioactive iodide into the protein of bovine thyroid slices has been studied. Tranylcypromine inhibited the incorporation of iodide into thyroid protein to approximately the same extent as it inhibited the monoamine oxidase activity of the combined mitochondria-microsome fraction isolated from the thyroid slices. All of the substances tested which inhibit thyroid monoamine oxidase activity in vitro also inhibited the incorporation of iodide into thyroid slice protein. Tyramine and phenylethylamine, substrates for thyroid monoamine oxidase, stimulated the incorporation of iodide into protein. This amine-stimulated incorporation of iodide was inhibited by tranyl cypromine. These data are consistent with the hypothesis that thyroid monoamine oxidase plays an important role in the generation of the H2O2 required for thyroxine biosynthesis.

Rats working for stimulation of the hypothalamus were injected with Parnate and alpha -methyl- meta -tyrosine. A sustained high-level of free norepinephrine was thereby made available to central reactive sites. Self-stimulation behavior was greatly facilitated. Norepinephrine therefore seems to exert excitatory effects on the reward system. Other experiments with tetrabenazine supported this conclusion.

Publisher Summary This chapter discusses the technical strategies for the study of catecholamines in man. MAO has been studied in human platelets and plasma. The platelet enzyme is located in mitochondria, and has many substrates, and inhibitor characteristics in common with mitochondrial MAOs of the B type found in other tissues. The purified platelet enzyme is electrophoretically homogenous, with a molecular weight of 235,000. The soluble plasma enzyme is a pyridoxal-containing enzyme which has different substrate specificities compared to platelet, and other tissue MAOs, and is inhibited by carbonyl reagents but is relatively insensitive to tissue MAO inhibitors such as pargyline, tranylcypromine, and iproniazid. Treatment with MAO inhibiting drugs under ordinary clinical conditions leads to marked inhibition of the human platelet, and brain enzymes, with MAO activity reduced 70–90% after several days of treatment. Studies comparing MAO activity in normal, and psychiatrically-ill monozygotic, and dizygotic twins, sibling pairs, and random unrelated pairs suggest a high order of genetic influence on platelet MAO activity.

After a single i.v. injection, 6-OHDOPA is capable of causing destruction and/or functional impairment of noradrenergic nerve terminals in both the central and the peripheral nervous system. By analogy with the effects of 6-hydroxydopamine and histochemical observations of preterminal accumulation of neurotransmitters, the conclusion that 6-OHDOPA causes degenerative changes in the terminal plexus is derived. There is no question that the final determination of neuronal degeneration should come from electron microscopic studies. The accumulation of histochemically observable catecholamine fluorescence in nonterminal axons, the time course of depletion and recovery of NE within various peripheral organs and impairment of uptake of exogenously administered NE support the conclusion that 6-OHDOPA has the potential to cause selective destruction of noradrenergic terminals. 6-OHDOPA is a valuable tool for the investigation of central pathways and functional organization of the noradrenergic neurons in the brain. Injection of 6-OHDOPA into the cerebral ventricles can produce a highly specific reduction in the NE content of the central nervous system while leaving dopaminergic neurons unaffected. This factor will be useful for the exploration of the behavioral contributions of central noradrenergic neurons.

The inhibition of type A and B monoamine oxidase (MAO A and B) in rat brain, liver and heart by MD780515, 3-[4-(3 cyanophenylmethoxy) phenyl]-5-(methoxymethyl)-2-oxazolidinone, has been investigated ex vivo with 5-hydroxytryptamine (5-HT) and β-phenylethylamine (PEA) as substrates. MAO A was strongly inhibited for four hours after oral administration of 10 mg/kg MD780515 (maximum inhibition : 72%, 86% and 83% in brain, liver and heart respectively. In contrast, in heart where PEA is deaminated by type A MAO, the predominant form of MAO in that tissue, the inhibition was 68% 30 minutes after administration of the compound. In all cases, MAO activities reached control values 24 hours after drug administration (10 mg/kg), whereas some inhibitory activity was still present 24 hours after oral administration of higher doses. The strong MAO A inhibition (68 to 83%) remaining in the three tissues 24 hours after oral administration of clorgyline (5 mg/kg) was completely removed by pretreatment with MD780515 (10 mg/kg). In the same conditions, MD780515 protected against the inhibition (53%) by clorgyline of PEA deamination in heart. Oral pretreatment with increasing doses of MD780515 (2.6 to 84 mg/kg) gradually removed brain MAO A inhibition caused by clorgyline (92%, 28.2 mg/kg) or tranylcypromine (88%, 4.8 mg/kg), the complete removal being observed at the dose of 21 mg/kg of MD780515 for clorgyline, and at 42 mg/kg for tranylcypromine. Inhibition of brain MAO B by tranylcypromine (96%) was not modified by pretreatment with the same range of oral doses of MD780515. The results are consistent with a specific and reversible inhibition of MAO A activity by MD780515 which can protect against long acting MAO A inhibitory effects of clorgyline and tranylcypromine. MD780515 enhances the selectivity of tranylcypromine.

Milacemide is a glycine prodrug with reported antiepileptic antimyoclonic properties. In this study, milacemide increased “wet dog shakes” in rats pretreated with 5-Hydroxytryptophan (5-HTP) and carbidopa. Moreover, it worsened the serotonin behavior syndrome precipitated by 5-HTP and the monoamine oxidase inhibitor tranylcypromine. The serotonin syndrome was also elicited by the combination of milacemide and 5-HTP without tranylcypromine. In vitro, milacemide inhibited both monoamine oxidase A and B from the frontal cortex of rats, to a greater extent for MAO B. This drug is currently under investigation in humans as an anti-epileptic agent and precautions for the consequences of monoamine oxidase inhibition should be considered when the drug is used in high doses.

Ten male mongrel dogs were treated in Phase I with tranylcypromine, 6 mg/kg IM b.i.d., for 21 days. Dogs were anesthetized at weekly intervals before, during, and up to four weeks after drug treatment with a combination of amylbarbital, 25 mg/kg, 1% enflurane in 70:30 N2O:O2 and fentanyl, 500 mcg. Bolus IV injections of norepinephrine, 0.1-0.6 mcg/kg, and ephedrine, 0.03-0.12 mg/kg were given while continuously recording arterial blood pressure, lead II of the ECG, heart rate, and rectal temperature. Awakening times were noted. In Phase II, the dogs were given imipramine, 25 mg/kg IM b.i.d., for 21 days. During Phase III, 14 days of tranylcypromine, 7 days of tranylcypromine plus imipramine, and 7 days of imipramine were administered. Anesthetic techniques were repeated in phases II and III. The fourth phase consisted of tranylcypromine injections, 6 mg IM b.i.d., and anesthesia with amylbarbital, 25 mg/kg, 2.5% enflurane in 70:30 N2O:O2. Vasopressor challenges were repeated during each phase of the study. Following induction of anesthesia and prior to fentanyl challenge, baseline blood pressures and heart rates did not differ from control in Phase I, II, and III of this study. Responses to norepinephrine during all of the tranylcypromine phases were not significantly different from control but ephedrine responses were prolonged, peaking by the second week of treatment. During Phase II, dysrhythmias occurred following norepinephrine and ephedrine with one lethality following norepinephrine, 0.2 mcg/kg. Responses to norepinephrine and awakening times were significantly greater during Phase II compared to Phase I. In Phase III, during the first week of combined therapy the responses to norepinephrine were significantly greater than any other week of this phase. During Phase IV, resting blood pressure and the ephedrine responses were significantly increased during tranylcypromine when the anesthesia regimen did not include fentanyl. These results suggest that during initial treatment with tranylcypromine or imipramine, cardiovascular responses to vasopressor challenges were predicted by the pharmacology of the antidepressant. During tranylcypromine phases, we did not observe exaggerated cardiovascular effects during anesthesia and vasopressor challenges as had been previously reported.

One of the ways of determining whether a particular compound has a role in synaptic transmission is to study the way in which isolated nerve endings accumulate its radiolabelled congener and subsequently release it under resting or stimulated conditions. In the past, this strategy has been used to examine the uptake and release of radiolabeled m-tyramine (mTA), p-tyramine (pTA), and tryptamine (T) (Dyck, 1984; Baker and Dyck, 1986). Most previous studies of the uptake and release of labelled PE and T, in general, have found that these two amines are poorly taken up, poorly retained and that they are released mainly by diffusion rather than by a depolarizing stimulus. In contrast, the more polar amines, mTA and pTA, are actively accumulated, stored and released by depolarizing stimuli in a calcium-dependent manner; i.e., labelled mTA and pTA behave in a manner consistent with the appellation, neurotransmitter. Because radiolabeled congeners do not necessarily give an accurate reflection of the release of endogenous neurochemicals, recent studies attempt to measure basal or stimulated release of endogenous compounds.

We describe a sensitive specific and simple high pressure liquid chromatographic procedure for determining 3,4-dihydroxyphenyl-ethylene glycol (DHPG) in both plasma and tissue. DHPG is extracted from plasma or tissue extracts by adsorption onto alumina. DHPG in the alumina eluate is detected electrochemically following chromatography on a C18 reverse phase column. The method is sensitive enough to detect approximately 20 pg/ml of plasma DHPG. Both clonidine (100 micrograms/kg) and desmethylimipramine (2.5 mg/kg) when administered to rabbits for 3 days induced significant falls in both cardiac and plasma DHPG concentrations. These experiments indicate that both tissue and plasma DHPG concentrations may be of value in assessing both the release and re-uptake of norepinephrine at peripheral sympathetic nerve endings in vivo.

Challenge with tranylcypromine (Tcp) during barbital (B) withdrawal induces dose-related clonic-tonic convulsion (C-TC), which is also related to the severity of withdrawal signs and their changes with the passage of time. The effects of neuropharmacological agents on the Tcp-induced convulsions were observed. dl-Propranolol, phentolamine, phenoxybenzamine and methysergide had been administered intraperitoneally 20 approximately 30 minutes before Tcp challenge. B-withdrawn rats had been pretreated with alpha-methyl-p-tyrosine, 5-hydroxytryptophan, p-chlorophenylalanine or reserpine, or with the combination of iproniazid and reserpine (5 hrs after iproniazid administration) before Tcp challenge. alpha-MT and dl-propranolol inhibited B withdrawal convulsion markedly, though high doses of dl-propranolol rather tended to show a less effect on the convulsion. alpha-Adrenoceptor blockers scarcely inhibited the convulsion. Methysergide or 5-HTP failed to inhibit, but PCPA intensified the convulsion. Reserpine, when administered alone, aggravated the convulsion, but when administered after iproniazid, inhibited it significantly. These findings suggested that the balance between the activities of noradrenergic and serotonergic neurons might be of importance in the manifestation of B withdrawal convulsions, the former probably being excitatory and the latter, inhibitory.

Brain levels of 5-HT, 5-HIAA and L-trytophan were measured in reserpinized rats given the enantiomers of tranylcypromine (TCP). (+)-TCP (1.5–30 mg/kg) increased levels of 5-HT, whereas (−)-TCP failed to affect 5-HT levels. (+)-TCP at high dosage (30 mg/kg) decreased brain levels of 5-HIAA, whereas (−)-TCP failed to affect 5-HIAA levels. Neither (+)-TCP nor (−)-TCP influenced brain levels of L-tryptophan. The effects of (+)-TCP on behavior resembled those of drugs which influence 5-HT processes, whereas (−)-TCP failed to produce 5-HT-dependent behaviors. The findings support the notion that TCP enantiomers have stereoselective effects on serotonergic mechanisms.

Both monoamine oxidase inhibitors, harmine and tranylcypromine, potentiated the pressor responses to tryptamine in the reserpine-pretreated dog and had little effect on the pressor responses to norepinephrine. Whereas harmine also potentiated the pressor responses to tyramine, tranylcypromine was found to be antagonistic to tyŕamine. It is suggested that the block of tyramine by tranylcypromine in the reserpine-pretreated dog is the result of the amphetamine-like activity of tranylcypromine.

In the brain stem and hemispheres the concentration of norepinephrine and serotonin increased gradually and adult levels were not attained until sometime between the 30th and 50th day of life. In contrast the EEG developed mere rapidly showing alerting patterns by 6–8 days and rather mature characteristics at 9–14 days. Reserpine and tranylcypromine were found to have their characteristic actions on serotonin and norepinephrine in 10-day old rabbits: tranylcypromine to raise their concentrations in the brain and reserpine to lower them. Chlorpromasine also characteristically blocks pain-induced KEG alerting beginning at 6 days of age, i.e. the earliest appearance of alerting in the newborn rabbit. Rabbits 9–14 days of age required higher doses of chlorpromazine to prevent EEG alerting in response to pain than either younger rabbits or adult ones.

Changes in the severity of the audiogenic seizure in mice are induced by drugs acting on serotonin and norepinephrine metabolisms. A hypothesis was stated suggesting that the amount of biogenic amines released at the receptor site in a physiologically active form is more important, in this phenomenon, than their total brain level. To support this hypothesis the effect of several drugs was studied: d. amphetamine and 4-chloro-N-methylamphetamine were studied alone, monoamineoxidase inhibitors and thymoleptics were studied alone or associated with reserpine or a catechol-o-methyl transferase inhibitor (pyrogallol). The data obtained are discussed according to Axelrod's theory on the role of norepinephrine in the nervous system.