Neuropharmacological and neurochemical properties of N-(2-cyanoethyl)-2-phenylethylamine, a prodrug of 2-phenylethylamine

Subchronic glucocorticoids, glutathione depletion and a postpartum model elevate monoamine oxidase a activity in the prefrontal cortex of rats

Recent human brain imaging studies implicate dysregulation of monoamine oxidase-A (MAO-A), in particular in the prefrontal cortex (PFC) and anterior cingulate cortex (ACC), in the development of major depressive disorder (MDD). This study investigates the influence of four alterations underlying important pathologies of MDD, namely, chronic elevation of glucocorticoid levels, glutathione depletion, changes in female gonadal sex hormones and serotonin concentration fluctuation, on MAO-A and MAO-B activities in rats. Young adult rats exposed chronically to the synthetic glucocorticoid dexamethasone at 0, 0.05, 0.5, and 2.0mg/kg/day (osmotic minipumps) for eight days showed significant dose-dependent increases in activities of MAO-A in PFC (+17%, p<0.001) and ACC (+9%, p<0.01) and MAO-B in PFC (+14%, p<0.001) and increased serotonin turnover in the PFC (+31%, p<0.01), not accounted for by dexamethasone-induced changes in serotonin levels, since neither serotonin depletion nor supplementation affected MAO-A activity. Sub-acute depletion of the major antioxidant glutathione by diethyl maleate (5mmol/kg, i.p.) for three days, which resulted in a 36% loss of glutathione in PFC (p=0.0005), modestly, but significantly, elevated activities of MAO-A in PFC and MAO-B in PFC, ACC and hippocampus (+6-9%, p<0.05). Changes in estrogen and progesterone representing pseudopregnancy were associated with significantly elevated MAO-A activity in the ACC day 4-7 postpartum (10-18%, p<0.05 to p<0.0001) but not the PFC or hippocampus. Hence, our study provides data in support of strategies targeting glucocorticoid and glutathione systems, as well as changes in female sex hormones for normalization of MAO-A activities and thus treatment of mood disorders.

HIV-1 Nef expression in microglia disrupts dopaminergic and immune functions with associated mania-like behaviors

BACKGROUND

Neuropsychiatric disorders during HIV/AIDS are common although the contribution of HIV-1 infection within the brain, and in particular individual HIV-1 proteins, to the development of these brain disorders is unknown. Herein, an in vivo transgenic mouse model was generated in which the HIV-1 Nef protein was expressed in microglia cells, permitting investigation of neurobehavioral phenotypes and associated cellular and molecular properties.

METHODS

Transgenic (Tg) mice that expressed full length HIV-1 nef under the control of the c-fms promoter and wildtype (Wt) littermates were investigated using different measures of neurobehavioral performance including locomotory, forced swim (FST), elevated plus maze (EPM) and T-maze tests. Host gene and transgene expression were assessed by RT-PCR, immunoblotting, enzymatic activity and immunohistochemistry. Biogenic amine levels were measured by HPLC with electrochemical detection.

RESULTS

Tg animals exhibited Nef expression in brain microglia and cultured macrophages. Tg males displayed hyperactive behaviors including augmented locomotor activity, decreased immobility in the FST and increased open-arm EPM exploration compared to Wt littermates (p<0.05). Tg animals showed increased CCL2 expression with concurrent IFN-α suppression in striatum compared with Wt littermates (p<0.05). Dopamine levels, MAO activity and the dopamine transporter (DAT) expression were reduced in the striatum of Tg animals (p<0.05).

CONCLUSIONS

HIV-1 Nef expression in microglia induced CCL2 expression together with disrupting striatal dopaminergic transmission, resulting in hyperactive behaviors which are observed in mania and other psychiatric comorbidities among HIV-infected persons. These findings emphasize the selective effects of individual viral proteins in the brain and their participation in neuropathogenesis.

Synthesis, transport, and metabolism of serotonin formed from exogenously applied 5-HTP after spinal cord injury in rats

Spinal cord transection leads to elimination of brain stem-derived monoamine fibers that normally synthesize most of the monoamines in the spinal cord, including serotonin (5-hydroxytryptamine, 5-HT) synthesized from tryptophan by enzymes tryptophan hydroxylase (TPH, synthesizing 5-hydroxytryptophan, 5-HTP) and aromatic l-amino acid decarboxylase (AADC, synthesizing 5-HT from 5-HTP). Here we examine whether spinal cord caudal to transection remains able to manufacture and metabolize 5-HT. Immunolabeling for AADC reveals that, while most AADC is confined to brain stem-derived monoamine fibers in spinal cords from normal rats, caudal to transection AADC is primarily found in blood vessel endothelial cells and pericytes as well as a novel group of neurons (NeuN positive and GFAP negative), all of which strongly upregulate AADC with injury. However, immunolabeling for 5-HT reveals that there is no detectable endogenous 5-HT synthesis in any structure in the spinal cord caudal to a chronic transection, including in AADC-containing vessels and neurons, consistent with a lack of TPH. In contrast, when we applied exogenous 5-HTP (in vitro or in vivo), AADC-containing vessels and neurons synthesized 5-HT, which contributed to increased motoneuron activity and muscle spasms (long-lasting reflexes, LLRs), by acting on 5-HT2 receptors (SB206553 sensitive) located on motoneurons (TTX resistant). Blocking monoamine oxidase (MAO) markedly increased the sensitivity of the motoneurons (LLR) to 5-HTP, more than it increased the sensitivity of motoneurons to 5-HT, suggesting that 5-HT synthesized from AADC is largely metabolized in AADC-containing neurons and vessels. In summary, after spinal cord injury AADC is upregulated in vessels, pericytes, and neurons but does not endogenously produce 5-HT, whereas when exogenous 5-HTP is provided AADC does produce functional amounts of 5-HT, some of which is able to escape metabolism by MAO, diffuse out of these AADC-containing cells, and ultimately act on 5-HT receptors on motoneurons.

Comparison of phenelzine and geometric isomers of its active metabolite, β-phenylethylidenehydrazine, on rat brain levels of amino acids, biogenic amine neurotransmitters and methylamine

Phenelzine is a monoamine oxidase (MAO) inhibitor used in treatment of depression and anxiety disorders. It also elevates brain levels of γ-aminobutyric acid (GABA) and inhibits primary amine oxidase (PrAO), an enzyme whose activity and/or expression has been reported to be increased in diabetes mellitus, Alzheimer's disease and cardiovascular disorders. Phenelzine is not only an inhibitor of, but also a substrate for, MAO and it has been suggested that an active metabolite, namely β-phenylethylidenehydrazine (PEH), is responsible for phenelzine's effects on amino acids. PEH is also a strong inhibitor of PrAO but has weak effects on MAO. PEH has a double bond and can thus exist as (E)- and (Z)-geometric isomers, but to date the two isomers have not been compared with regard to their neurochemical effects. We have investigated the effects of phenelzine, (E)- and (Z)-PEH on rat whole brain levels of amino acids, biogenic amine neurotransmitters and methylamine (an endogenous substrate of PrAO). Under the conditions used in the study, (E)- and (Z)-PEH appear to be equivalent in their neurochemical properties. Both PEH isomers and phenelzine produced marked increases in rat brain levels of GABA and alanine while decreasing brain levels of glutamine. Phenelzine increased brain levels of biogenic amine neurotransmitters (noradrenaline, dopamine and serotonin), whereas neither PEH isomer altered levels of these neurotransmitters to a considerable extent. All three drugs significantly increased rat brain levels of methylamine, with (E)- and (Z)-PEH causing a greater increase than phenelzine. These results are discussed in relation to the possible therapeutic applications of these drugs.

Prodrugs for amines

The purpose of this work is to review the published strategies for the production of prodrugs of amines. The review is divided in two main groups of approaches: those that rely on enzymatic activation and those that take advantage of physiological chemical conditions for release of the drugs. A compilation of the most important approaches is presented in the form of a table, where the main advantages and disadvantages of each strategy are also referred.

Impaired hippocampal LTP in inbred mouse strains can be rescued by β-adrenergic receptor activation

Long-term potentiation (LTP), an activity-dependent enhancement of synaptic strength, and memory can be influenced by neuromodulatory transmitters such as norepinephrine (NE) and also by genetic background. beta-Adrenergic receptor activation can facilitate the expression of hippocampal CA1 LTP induced by weak stimulus patterns, but its influence on LTP induced by stronger stimulus patterns is unclear. We examined neural NE and dopamine (DA) levels, beta-adrenergic receptor expression and hippocampal LTP in genetically diverse inbred mouse strains. Brain tissue levels of NE were significantly lower in strains 129S1/SvImJ (129), BALB/cByJ (BALB) and C3H/HeJ (C3H) than in C57BL/6NCrlBR (B6). Western blot analysis showed that hippocampal beta(1)-adrenergic receptor expression was similar in strains B6, 129 and C3H, but was increased in BALB. LTP was induced in area CA1 of hippocampal slices by four trains of high-frequency stimulation (HFS) of the Schaeffer collaterals in the four inbred strains. Two hours after induction, LTP was significantly reduced in strains 129, BALB and C3H compared to B6, correlating with neural NE levels. We rescued hippocampal LTP in strains 129, BALB and C3H to levels seen in B6 by bath application of 1 microm isoproterenol, a beta-adrenergic receptor agonist, during HFS. Propranolol, a beta-adrenergic receptor antagonist, blocked this rescue in 129, BALB and C3H but did not affect LTP in strain B6. Thus, although this form of multitrain LTP does not rely on beta-adrenergic receptor activation, our data show that pharmacological activation of beta-adrenergic receptors during multiple trains of HFS can rescue CA1 LTP in genetically diverse strains with impaired LTP.

Effect of noradrenergic system on the anxiolytic-like effect of DOI (5-HT2A/2C agonists) in the four-plate test

RATIONALE

Selective serotonin reuptake inhibitors and serotonin and noradrenaline reuptake inhibitors demonstrated an anxiolytic-like effect in the four-plate test (FPT). (+/-)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane (DOI; a 5-HT2A receptor agonist) also possessed strong anxiolytic-like effect in the same test. A 5-HT2A mechanism seems to be implicated in the mechanism of action of both antidepressants and DOI in this test. On the other hand, the alpha-adrenergic ligands have also demonstrated an activity in other models of anxiety. A previous study demonstrated that the alpha2-adrenoceptor agonists abolished the anxiolytic-like effect of antidepressants.

OBJECTIVES

The aim of the present study was to evaluate the role of noradrenergic system on the regulation of 5-HT2 receptors implicated in the DOI anxiolytic-like activity in the FPT.

METHODS

First, the effect of noradrenergic and serotonergic lesions on DOI anxiolytic-like activity was studied in the FPT. Second, the effect of co-administration of alpha-adrenoceptor ligands and DOI was evaluated in the same test.

RESULTS

The noradrenergic and serotonergic lesions had no effect on DOI (1 mg/kg) anti-punishment activity in the FPT. Adrafinil 0.25 and 4 mg/kg (an alpha1-adrenoceptor agonist), prazosin 0.5 and 2 mg/kg (an alpha1-adrenoceptor antagonist) and idazoxan 1 and 4 mg/kg (an alpha2-adrenoceptor antagonist) did not modify the activity of DOI. Clonidine 0.06 mg/kg, guanabenz 0.125 and 0.5 mg/kg (two alpha2-adrenoceptor agonists) and guanfacine 0.06 and 0.125 mg/kg (a specific alpha2A-adrenoceptor agonist) completely abolished DOI-induced increase in punished passages.

CONCLUSION

These results indicate that the DOI seems to act on the 5-HT2 receptors post-synaptically located. The effect of DOI is regulated by the alpha2-adrenoceptors.

Effect of acute antipsychotic administration on dopamine synthesis in rodents and human subjects using 6-[18F]-L-m-tyrosine

Clinical effects of antipsychotic drugs are thought to be mediated primarily through antagonism of the dopamine D2 receptors. Recent studies have demonstrated increased aromatic decarboxylase activity following acute administration of dopamine D2 receptor antagonists both in vivo and ex vivo. However, this effect has never been demonstrated in human subjects. We studied the effect of acute antipsychotic administration on dopamine synthesis in rodents and healthy human subjects using 6-[18F]-L-m-tyrosine. In rats, we studied the effect of a single subcutaneous injection of haloperidol and risperidone on dopamine synthesis using 6-[18F]-L-m-tyrosine. In our human study, six healthy volunteers underwent two 6-[18F]-L-m-tyrosine PET scans, before and after 3 mg risperidone to measure the rate of accumulation of radioactivity in the striatum as an index of dopamine synthesis. The striatal/cerebellar radioactivity count ratio and the ratio of dopamine metabolites to dopamine concentration was significantly higher in all rodent treatment groups compared to controls. In the PET study we found no significant change in the rate of uptake in the striatum. Our results suggest that 6-[18F]-L-m-tyrosine PET may not be a useful tool in the study of the effect of antipsychotics on dopamine synthesis in human subjects.

Relationship of plasma amphetamine levels to physiological, subjective, cognitive and biochemical measures in healthy volunteers

Acute administration of the stimulant dextro-amphetamine produces multiple physiological, subjective cognitive and biochemical changes. These effects are similar to those seen in mania, and may be a useful model for mania. The aim of the present study was more fully to determine the multiple effects of dextro-amphetamine and to relate these to changes in plasma levels of the drug. In a double-blind, placebo-controlled crossover study in 25 healthy volunteers (ages 18-45), the effects of 25 mg of oral dextro-amphetamine were examined. Physiological, subjective, cognitive changes, concentrations of amino acids and metabolites of biogenic amines period were related to changes in plasma amphetamine concentrations over 500 min. Peak concentrations of dextro-amphetamine occurred at 2.5-3.5 h post-administration and levels decreased to 75% of peak value after 500 min. The results from the present study indicate that the subjective psychological, cognitive and blood pressure changes frequently did not mirror the time course of plasma levels of the drug. Thus, there was no clear-cut relationship between plasma levels and effects. In addition, dextro-amphetamine caused no significant changes in amino acids or amino metabolite concentrations. In conclusion, while dextro-amphetamine administration definitely causes several changes which are seen in mania, there remain some physiological and metabolic differences between these two conditions.

Antidepressant-like effects in various mice strains in the forced swimming test

RATIONALE

Strain differences in mice have been reported in response to drugs in the mouse forced swimming test (FST), even if few antidepressants were examined.

OBJECTIVES

The aim of the present study was to investigate the influence of genetic factors, using five antidepressants (imipramine, desipramine, citalopram, paroxetine and bupropion) in the mouse FST, in outbred strains (Swiss, NMRI) and inbred strains (DBA/2, C57BL/6J Rj). Moreover, whole brain levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT) in vehicle treated animals, which were or were not subjected to the FST, were measured by HPLC analysis in an attempt to explain behavioural differences.

METHODS

For each antidepressant, a dose range (1-16 mg/kg) was tested in the locomotor apparatus and only non-psychostimulant doses were then tested in the FST in order to detect antidepressant-like activity.

RESULTS

No baseline differences among Swiss, NMRI, DBA/2 and C57BL/6J Rj strains were observed in our experiments, allowing the comparison of different antidepressants in each strain. Imipramine (16 mg/kg), desipramine, citalopram (4-16 mg/kg) and paroxetine (8 and 16 mg/kg) treatment decreased the immobility time in the Swiss strain and the size of the effect reached more than 20% for each of these antidepressants. C57BL/6J Rj was the only strain sensitive to bupropion (2 and 4 mg/kg). In the NMRI strain, only paroxetine treatment decreased the immobility time (16 mg/kg).

CONCLUSION

Our study showed that drug sensitivity is genotype dependent. FST results have shown that Swiss mice are the most sensitive strain to detect 5-HT and/or NA treatment. The use of DBA/2 inbred mice may be limited, as an absence of antidepressant-like response was observed in the FST. The lack of sensitivity to antidepressant treatment in DBA/2 strains could be due to high DA, NA and 5-HT whole brain concentrations.

Time-dependent changes in brain monoamine oxidase activity and in brain levels of monoamines and amino acids following acute administration of the antidepressant/antipanic drug phenelzine

Phenelzine (PLZ) is a non-selective monoamine oxidase (MAO) inhibitor commonly used to treat depression and panic disorder. Acute administration of PLZ produces several neurochemical changes, including an increase in brain levels of the catecholamines norepinephrine (NE) and dopamine (DA), of 5-hydroxytryptamine (5-HT), and of the amino acids alanine and gamma-aminobutyric acid (GABA). The goal of the present series of experiments was to characterize the time course of these PLZ-induced changes. Male Sprague-Dawley rats were sacrificed 6, 24, 48, 96, 168, or 336 hr after acute PLZ administration (15 or 30 mg/kg, i.p., based on free base weight). Whole brain levels of monoamines and amino acids were determined using HPLC, and MAO A and B activities were determined using a radiochemical procedure. The results indicated that PLZ changed amino acid levels 6 and 24 hr after injection, but not 48 hr later. In contrast, the effects of PLZ on MAO activity and monoamines were longer-lasting. For example, PLZ-induced increases in dopamine and 5-HT were observed 1 week after injection, and PLZ-induced inhibition of MAO activity persisted for 2 weeks. Thus, in addition to demonstrating that the effects of PLZ on MAO activity and monoamines were long-lasting, these results indicate that the effects of PLZ on MAO activity and on brain levels of monoamines and amino acids are temporally dissociated. These findings regarding the long-term effects of PLZ on neurochemistry will have considerable critical implications for the design and interpretation of behavioral studies of the acute effects of PLZ.

Psychopharmacological profile of the selective serotonin reuptake inhibitor, paroxetine: implication of noradrenergic and serotonergic mechanisms

The present study was designed to evaluate the psychopharmacological profile of the selective serotonin reuptake inhibitor paroxetine, and thus assess potential noradrenergic and/or serotonergic activity. Paroxetine dose-dependently increased mobility time in the mouse forced swimming test (8, 16, 32 and 64 mg/kg, i.p.) and reduced spontaneous locomotor activity when administered at a high dose (64 mg/kg, i.p.). Prior administration of 8-hydroxy-2-(di-n-propylamino)tetralin (1 mg/kg, i.p.), (+/-) pindolol (32 mg/kg, i.p.) or 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridyl)-1H-indole (RU 24969) (1 mg/kg, i.p.) potentiated the antidepressant-like effects of subactive doses of paroxetine (1, 2 and 4 mg/kg, i.p.) in the mouse forced swimming test. These effects were antagonized by prior administration of 1-(2-methoxyphenyl)-4-[-(2-phthalimido)butyl]piperazine) (0.5 mg/kg, i.p.). Complementary studies suggested that RU24969-induced anti-immobility effects were a result of an increase in locomotor activity; other interactions were without increase/decrease in locomotor activity. Acute administration of paroxetine (8, 16, and 32 mg/kg, i.p.) antagonized the hypothermia induced by the D2/D1 receptor agonist, apomorphine (16 mg/kg, s.c.), while repeated treatment with paroxetine (32 mg/kg) attenuated clonidine-induced (0.5 mg/kg, i.p.) hypothermia. Pre-treatment with the serotonergic neurotoxin, para-chlorophenylalanine attenuated the anti-immobility effects of low doses of paroxetine (8 and 16 mg/kg, i.p.) in the forced swimming test, whereas a higher dose of paroxetine remained active (32 mg/kg, i.p.). The results of the present study indicated that paroxetine displayed both noradrenergic-like and serotonergic-like activity in the pre-clinical psychopharmacological tests employed.

Determination of phenethylamine, a phenethyl isothiocyanate marker, in dog plasma using solid-phase extraction and gas chromatography-mass spectrometry with chemical ionization

Phenethyl isothiocyanate is unstable in aqueous media and at low pH, and rapidly degrades to phenethylamine. Concentrations of phenethylamine, a phenethyl isothiocyanate marker, in dog plasma, were determined utilizing solid-phase extraction and gas chromatography-mass spectrometry with chemical ionization using acetone as the reagent gas. Deuterated d5-amphetamine was used as an internal standard. After extraction, phenethylamine and d5-amphetamine were derivatized using MBHFBA. Ions monitored for d5-amphetamine were m/z 337 and 338; and for phenethylamine were m/z 318 and 319. Precision and accuracy were studied using control solutions prepared in naive dog plasma (80 and 300 ng/ml). Intra-day variability was determined using six replicates of each control solution analyzed on a single day. The relative standard deviation for the 80 ng/ml control was 12.9% and for the 300 ng/ml it was 12.1%. Relative accuracy was 10.9% for the low control and -4.1% for the high control. Inter-day variability was determined over a 6-day period. For the 80 and 300 ng/ml control solutions, the relative standard deviations were 15.8 and 9.1%, respectively, and relative accuracy values were 10.1 and -5.2%, respectively. Standard curves were prepared in naive dog plasma and were linear over the range of phenethylamine assayed (10-500 ng/ml). The results of this study indicate that the proposed method is simple, precise, accurate and sensitive enough for analysis of large numbers of plasma samples.

Brain structures involved in the behavioral stimulant effect of central serotonin release

Drugs such as p-chloroamphetamine or a combination of tranylcypromine and tryptophan release serotonin in the central nervous system and produce a behavioral serotonin syndrome. However, in the presence of methysergide or following destruction of descending spinal serotonergic projections by 5,7-dihydroxytryptamine, central serotonin release produces hyperlocomotion. This supports the hypothesis that release of serotonin in the brain promotes locomotion but that the expression of this effect can be blocked by concomitant intraspinal effects of serotonin release. Hyperlocomotion induced by serotonin release is attenuated or blocked by: (a) pretreatment with p-chlorophenylalanine; (b) acute surgical lesions of the basal diencephalon; (c) chronic lesions of the ventromedial midbrain tegmentum by local injection of 5,7-dihydroxytryptamine; and (d) acute surgical decortication. Medial decortication tends to be more effective then lateral decortication. Hyperlocomotion produced by methamphetamine is also attenuated or blocked by acute basal diencephalic lesions or decortication. It is suggested that ascending serotonergic and dopaminergic projections collaborate in the generation of spontaneous voluntary motor activity.

The role of brain noradrenaline in cortical activation and behavior: a study of lesions of the locus coeruleus, medial thalamus and hippocampus-neocortex and of muscarinic blockade in the rat

Local injection of 6-hydroxydopamine in the locus coeruleus resulted in a 90% depletion of noradrenaline (NA) in the cerebral cortex as assessed by high-pressure liquid chromatography. This NA depletion had no effect on scopolamine-resistant hippocampal rhythmical slow activity and only an occasional effect on scopolamine-resistant neocortical low voltage fast activity. However, NE depletion resulted in a slight deficit in a behavioral swim-to-platform test and increased the deficit produced on the test by systemic treatment with scopolamine. Large surgical lesions of the medial thalamus or hippocampal formation plus posterior neocortex greatly increased the behavioral deficit produced by scopolamine. It is concluded that ascending noradrenergic projections play only a modest and possibly indirect role in the control of electrocortical activation and that a number of different brain lesions increase the behavioral impairment produced by central muscarinic blockade.

Inhibition of rat brain monoamine oxidase enzymes by fluoxetine and norfluoxetine

Fluoxetine and its primary metabolite, norfluoxetine, are inhibitors of neuronal uptake of 5-hydroxytryptamine. While fluoxetine has also been reported to inhibit monoamine oxidase (MAO) in vitro at concentrations much lower than those measured in brain following chronic fluoxetine treatment, neurochemical profiles are not consistent with substantial MAO inhibition in vivo. In an attempt to explain this inconsistency, we have examined the interactions of fluoxetine and norfluoxetine with rat brain MAO-A and -B by a radiochemical assay method. Fluoxetine and norfluoxetine were competitive inhibitors of MAO-A in vitro, with Ki values of 76.3 microM and 90.5 microM, respectively. Both compounds were non-competitive or uncompetitive inhibitors of MAO-B in vitro. Inhibition of MAO-B was time-dependent and was very slowly reversible by dialysis. IC50 values versus metabolism of 50 microM beta-phenylethylamine were 17.8 microM (fluoxetine) and 18.5 microM (norfluoxetine). Analysis of the time-dependence of MAO-B inhibition by fluoxetine revealed that an initial competitive interaction between the enzyme and the inhibitor (Ki 245 microM) was followed by tight-binding enzyme inactivation (K(inact) 0.071 min-1). Following administration of fluoxetine (20 mg kg-1 day-1) for 7 days, the cortical concentration of fluoxetine + norfluoxetine was estimated by gas-liquid chromatography to be 700 microM. Such drug treatment reduced MAO-A activity by 23% in 1:8 (w/v) cortical homogenates, but not in 1:80 homogenates. Inhibition of MAO-B in 1:8 homogenates was modest (12%) and was not significantly reduced by homogenate dilution. The concentration of 5-hydroxyindole-3-acetic acid, measured by high pressure liquid chromatography, was reduced by 47% in cortices from drug-treated rats, while concentrations of 5-hydroxytryptamine, noradrenaline, dopamine, 3,4-dihydroxyphenylacetic acid and homovanillic acid were unchanged. These results suggest that, following chronic drug administration leading to relatively high tissue concentrations of fluoxetine and norfluoxetine, inhibition of either form of MAO would be restricted by competition for the enzyme with intraneuronal amine substrates.

p-chlorophenylalanine-induced serotonin depletion: reduction in exploratory locomotion but no obvious sensory-motor deficits

Para-chlorophenylalanine (PCPA) depletes central serotonin (5-hydroxytryptamine, 5-HT) by inhibiting tryptophan hydroxylase, an enzyme necessary for the synthesis of 5-HT. The effects of a wide range of PCPA doses (150-1000 mg/kg) on spontaneous exploratory locomotor activity in a novel environment, activity in running wheels and a number of sensory-motor capacities were examined. Administration of 1000 mg/kg PCPA reduced whole brain levels of 5-HT and its metabolite 5-hydroxyindoleacetic acid to 9.4 and 8.2% of control levels, respectively. Treatment with PCPA produced a dose-dependent decrease in exploratory locomotion in an unfamiliar automated open field relative to vehicle-treated animals. Further, all measures of general, horizontal and vertical activity were suppressed by PCPA treatment. In contrast to previous work, hyperactivity of rats chronically housed in cages with running wheel access was not observed. In their home cages, some PCPA-treated rats exhibited hyper-reactivity to cutaneous stimulation. No other sensory-motor deficits were apparent. Previous theories of 5-HT function state that its action may be to inhibit motor activity or promote sleep. The present results challenge this view and suggest that 5-HT, at least in certain environments, may stimulate locomotor activity without directly controlling various sensory-motor capacities in rats.

Comparative behavioural and neurochemical studies with a psychomotor stimulant, an hallucinogen and 3,4-methylenedioxy analogues of amphetamine

Spontaneous behaviours were assessed in freely moving rats after treatment with equimolar doses of drugs that share a basic amphetamine structure. The drugs used included a psychomotor stimulant [(+)-amphetamine (AMPH)], an hallucinogen [para-methoxyamphetamine (PMA)] and the entactogens 3,4-methylenedioxymethamphetamine (MDMA), 3,4-methylenedioxyamphetamine (MDA) and 3,4-methylenedioxy-N-ethylamphetamine (MDE). A detailed analysis of the frequency and duration of 30 different behaviours and the temporal organization of the behaviours was conducted in addition to measuring motor activity with an automated device. Levels of the biogenic amines and their acid metabolites in discrete brain regions and brain drug levels were also obtained. The automated motor activity measures discriminated among entactogens, the stimulant and the hallucinogen, but failed to distinguish between the hallucinogen and vehicle. Principal components analysis and cluster analysis of the frequencies and durations of the behaviours did not improve the classification of the drugs over the automated motor activity measures. Only the cluster analysis of the transitions between individual behaviours succeeded in differentiating the drug classes from each other and from vehicle treatment. All the behavioural measures classified one entactogen (MDE) as an hallucinogen. Cortical 5-hydroxytryptamine (5-HT) measures grouped MDE with the other entactogens but did not distinguish AMPH from vehicle. However, striatal dopamine measures differentiated AMPH from vehicle treatment. Variations in the durations of behavioural effects across drugs were associated with large differences in drug levels 3 h after injection. Although the neurochemical data provided a classification system that most closely parallels human subjective effects of these drugs, both the neurochemical and the behavioural measures supported the existence of an entactogen class distinct from a psychomotor stimulant and an hallucinogen.

Lesions of central serotonin systems affect responding on a progressive ratio schedule reinforced either by intravenous cocaine or by food

The effect of intraventricular infusions of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), was examined in rats trained on a progressive ratio schedule for either IV cocaine or food reinforcement. Animals in the 5,7-DHT treatment group responded to significantly higher breaking points than vehicle-injected control animals, regardless of whether food or cocaine was used as the reinforcing stimulus. Analysis of the regional brain amines indicated that depletions of mesencephalic 5-HT correlated with postsurgical alterations in responding. These findings suggest that depletion of forebrain 5-HT produces a general effect on responding rather than a specific alteration in the reinforcing effects of psychomotor stimulant drugs.

Effect of 6-hydroxydopamine lesions of the amygdala on intravenous cocaine self-administration under a progressive ratio schedule of reinforcement

Bilateral six-hydroxydopamine (6-OHDA) lesions were placed in the amygdala of rats self-administering cocaine (1.5 mg/kg per injection i.v.) under a progressive ratio schedule of reinforcement. Post-lesion access to three doses of cocaine (1.5, 0.75 and 0.37 mg/kg per injection i.v.) revealed a lesion effect only at the highest dose. At this dose, the lesion caused a significant increase in breaking point. No change in the breaking point was produced at the lower two doses. The biochemical results show a significant reduction in dopamine and DOPAC levels within the amygdala and an increase in dopamine within the NACC. In contrast, noradrenaline and serotonin (5-HT) levels were unaffected by the lesion in any of the dissected areas. These results demonstrate that no specific effect on cocaine reinforcement was produced by 6-OHDA lesions of the amygdala. The possibility that the lesion may have attenuated the anxiogenic qualities of the high dose of cocaine is discussed.

Monoamine oxidase inhibitors: effects on tryptophan concentrations in rat brain

It has been suggested that inhibition of tryptophan (Trp) pyrrolase and a subsequent elevation of brain Trp may contribute to the actions of antidepressant drugs. In our laboratories, we have conducted a series of experiments measuring brain Trp levels in the rat after both acute and chronic administration of several monoamine oxidase (MAO) inhibitors. The drugs studied during the course of the long-term (28 day) experiments were phenelzine, N2-acetylphenelzine, tranylcypromine, 4-fluorotranylcypromine, 4-methoxytranylcypromine and (-)-deprenyl. High-pressure liquid chromatography with electrochemical detection was employed to measure Trp levels in brains of both MAO inhibitor- and vehicle-treated animals. No significant increases in brain Trp levels were observed as a consequence of MAO inhibitor treatment. Acute time-response (up to 24 h) and dose-response studies were conducted following the administration of phenelzine and tranylcypromine. Only after administration of high doses of these drugs was an elevation in brain Trp observed and the increase was relatively short-lived. These results suggest that elevation of brain Trp may be an important factor in the actions of MAO inhibitors only at high doses of these drugs.

Tranylcypromine does not enhance the effects of amitriptyline on 5-HT2 receptors in rat cerebral cortex

The combination of amitriptyline (a tricyclic antidepressant) and tranylcypromine (a monoamine oxidase inhibitor) has been reported to be effective for treatment of refractory depressed patients. In the study reported here, this drug combination was compared with amitriptyline administered alone on the number and affinity of 5-HT2 receptors in rat brain. Male Sprague-Dawley rats were given vehicle (distilled water), amitriptyline (3.5 mg/kg/day), or tranylcypromine and amitriptyline (0.5 and 3.5 mg/kg/day, respectively) in combination subcutaneously via osmotic minipumps for 4, 10, or 28 days. A membrane fraction prepared from whole cortex was employed for studying binding to 5-HT2 receptors ([3H]ketanserin as the radioligand). The combination of amitriptyline and tranylcypromine produced a small but significantly greater down-regulation (decrease in number) of 5-HT2 sites than did amitriptyline alone after 10 days of administration; at 4 and 28 days, both amitriptyline and the drug combination had produced down-regulation, but there was not a significant difference between the two treatments. These data suggest that the antidepressant efficacy observed with this combination is not likely due to an enhanced effect on 5-HT2 receptors.

Comparisons of the actions of high and low doses of the MAO inhibitor tranylcypromine on 5-HT2 binding sites in rat cortex

Tranylcypromine (TCP) is a commercially available antidepressant drug, and recent literature reports suggest that high doses of this drug may be particularly effective in treating refractory depression. Down-regulation of 5-HT2 receptors in rat cortex is an effect produced after chronic administration of several antidepressants, and we have conducted a chronic study comparing low- and high-dose TCP in this regard. Male Sprague-Dawley rats were administered TCP (0.5 or 2.5 mg/kg/day) or vehicle (distilled water) via Alzet minipumps implanted subcutaneously in the dorsal thoracic area. Groups of rats were killed 4, 10 or 28 days after pump implantation and whole cortex was dissected out and utilized for preparation of a membrane fraction. Binding studies were performed with this fraction using 3H-ketanserin as the radioligand. Down-regulation (decrease in Bmax) of the 5-HT2 binding site was observed in high-dose animals after 10 and 28 days but not after 4 days. Low-dose TCP had no effect on 5-HT2 densities at any time interval. The affinity of 3H-ketanserin for the 5-HT2 site was not affected by either dose at any time interval. These results suggest that down-regulation of the 5-HT2 site may contribute to the efficacy of high-dose TCP in the treatment of refractory depression.

Effects of two substituted hydrazine monoamine oxidase (MAO) inhibitors on neurotransmitter amines, gamma-aminobutyric acid, and alanine in rat brain

Time- and dose-response analyses were undertaken to investigate the effects of the substituted hydrazine monoamine oxidase (MAO) inhibitors iproniazid and nialamide on the following: MAO-A and -B activity; levels of gamma-aminobutyric acid (GABA), alanine (ALA), and the neurotransmitter amines dopamine, noradrenaline, and 5-hydroxytryptamine (serotonin) and their acid metabolites; and the activity of GABA-transaminase and ALA-transaminase. The results showed that these drugs are relatively potent MAO inhibitors but, unlike the unsubstituted hydrazine MAO inhibitor phenelzine, they do not produce increased GABA and ALA levels in brain. These experiments suggest that a free hydrazine group is necessary for MAO inhibitors to also have marked effects on GABA and ALA.

2-Phenylethylamine-induced changes in catecholamine receptor density: implications for antidepressant drug action

It is now established that (1) concentrations of 2-phenylethylamine (PEA) are greatly increased in brain following administration of monoamine oxidase inhibitor (MAOI) antidepressants; (2) PEA is a metabolite of the MAOI antidepressant phenelzine; and (3) PEA may be a neuromodulator of catecholamine activity. On the basis of these observations, the effects of long term increases in brain PEA on catecholamine receptors have been assessed. Both PEA and antidepressants induced a reduction in the behavioural response to the beta 2 adrenoceptor agonist salbutamol. Radioligand binding measurements revealed that 28 day administration of PEA in combination with the type B MAOI (-)-deprenyl results in a decrease in the density of beta 1 adrenoceptors but not beta 2 adrenoceptors in rat cerebral cortex and cerebellum. (-)-Deprenyl alone also induced a significant decrease in beta 1-adrenoceptors but when PEA was added to this treatment there was a further decrease in beta 1-adrenoceptor density. Only changes in beta 1 adrenoceptor density were evident following 28 day administration of MAOI antidepressants. PEA also induced a decrease in the density of D1-like dopamine (DA) receptors in the rat striatum. MAOI antidepressants induced a decrease in the density of both D1-like and D2-like DA receptors. These data are discussed in terms of a possible role of PEA-catecholamine interactions in antidepressant drug action.

Presynaptic dopaminergic neurotransmission mediates amphetamine-induced unconditioned but not amphetamine-conditioned locomotion and defecation in the rat

A series of experiments were conducted to investigate the role of presynaptic dopamine (DA) and noradrenaline (NA) neurotransmission in stimulant-unconditioned and conditioned locomotion and defecation. (+)-Amphetamine (AMP, 1.5 mg/kg, s.c.) increased both locomotion and defecation in rats, and both of these effects were conditioned to environmental stimuli. Some groups of rats were treated with DSP4 (50 mg/kg, i.p.), a selective, long-lasting NA neurotoxin, given 7 days prior to conditioning with AMP. This treatment depleted forebrain NA to between 1% and 54% of control levels, depending on the brain region, but did not attenuate either AMP-unconditioned or conditioned locomotion. These results indicate that NA does not mediate either AMP unconditioned or conditioned locomotion. alpha-Methyl-para-tyrosine methyl ester (alpha MPT, 25-50 mg/kg, s.c.), a selective inhibitor of catecholamine synthesis given during conditioning with AMP, attenuated unconditioned AMP-induced locomotion and defecation but did not influence AMP-conditioned locomotion and defecation. Thus, alpha MPT blocked AMP-induced unconditioned locomotion, supporting the hypothesis that the locomotor and defecation stimulant effects of AMP are mediated by DA release. In spite of the attenuation of the direct effects of AMP, alpha MPT did not attenuate AMP-conditioned locomotion or defecation. It is concluded that AMP-induced release of dopamine is responsible for the unconditioned behavioral effects of amphetamine but not for the conditioning of amphetamine-induced locomotion and defecation.

"Designer" amphetamines: effects on behavior and monoamines with or without reserpine and/or alpha-methyl-para-tyrosine pretreatment

Rats were given single injections of vehicle or one of three doses of (+)-amphetamine (AM), 4-methoxyamphetamine (MA) or 4-ethoxyamphetamine (EA) after pretreatment with vehicle or reserpine, and vehicle or alpha-methyl-para-tyrosine (AMPT). EA is a "designer" drug that was recently seized from an illicit laboratory in Canada. Locomotion of the rats was recorded after treatment with the drugs, and whole brain levels of the drugs as well as monoamine neurotransmitters and their major acidic metabolites were then determined. Neither of the ring-substituted AM analogues influenced locomotion. AM induced locomotion in a dose-dependent manner, and this effect was blocked by AMPT but potentiated by reserpine. Brain concentrations of EA were lower than those of the other two drugs. The brain levels of monoamines and their metabolites indicate that AM releases a newly synthesized pool of dopamine which is transferred to vesicles after re-uptake. A very low dose of AM, but not higher doses, was found to elevate serotonin (5-hydroxytryptamine: 5-HT) levels independently of effects on catecholamines. Both MA and EA affected monoamine metabolites in a manner consistent with actions as reversible inhibitors of monoamine oxidase-an effect which has been previously demonstrated to be true for MA. Both drugs increased 5-HT levels at a very low dose, as did AM, but also increased noradrenaline levels at this dose. It is concluded that EA is not a psychomotor stimulant, but is similar in many of its effects to MA, a potent hallucinogen.

Chronic treatment with D1 and D2 dopamine receptor agonists: combined treatments interact to differentially affect brain levels of monoamines

Recent evidence of functional interactions between D1 and D2 dopamine receptor subtypes has led to the concept that many of the behavioural effects of dopamine agonists occur only with activation of both receptor subtypes. Thus, combined treatment with dopamine agonists selective for each of the D1 and D2 receptors may be an effective therapy for Parkinson's disease, chiefly characterized by loss of central dopamine-containing neurons. In addition, recent hypotheses of the possible pathogenesis of this disorder have suggested that metabolism of dopamine by monoamine oxidase in the presynaptic terminal may contribute to the loss of dopaminergic cells, through the production of reactive by-products. Therefore, the effects of chronic (15 day) treatment of rats with different doses of (+)-4-propyl-9-hydroxynaphthoxazine (PHNO, a D2 receptor agonist), SKF 38393 (a D1 receptor partial agonist) or combinations of both drugs on levels of brain monoamines and some of their acidic metabolites were investigated. Little or no effects of the drugs were observed on measures of dopamine or noradrenaline when given separately, while each selective agonist dose-dependently reduced serotonin levels. Combined treatment with the two agonists produced profound effects on the catecholamines, but with no effect on 3,4-dihydroxyphenylacetic acid, the metabolite of dopamine produced by monoamine oxidase. In addition, the effects of combined treatment on serotonin levels were opposite of those of the drugs given independently. Concomitant treatment of animals with both D1 and D2 receptor agonists can therefore increase tissue levels of dopamine without increasing the potentially harmful metabolism of dopamine by monoamine oxidase.

The effects of serotonergic stimulation on hippocampal and neocortical slow waves and behavior

The effect of central serotonergic stimulation on hippocampal and neocortical electrical activity and behavior was studied in freely moving rats by administering: (a) tranylcypromine followed by tryptophan, (b) fluoxetine followed by 5-hydroxytryptophan, or (c) p-chloroamphetamine alone. In all rats, scopolamine-resistant hippocampal rhythmical slow activity (RSA), thought to be dependent on brain serotonin, maintained its normal relation to behavior, occurring in close correlation with Type 1 behaviors (postural changes, turning of the head, walking). This RSA was generally absent during stereotyped behavior (head weaving, forepaw treading, hindlimb splaying and tremor). Scopolamine-resistant neocortical low-voltage fast activity (LVFA), also though to be dependent on brain serotonin, was present during Type 1 behaviors and also during stereotyped behavior. Most rats that developed a full stereotyped behavior syndrome had behavioral and electrocortical seizures which were associated with a reduction in the amplitude of hippocampal activity. These seizures were suppressed by methysergide or benserazide. Metergoline (and methysergide to a lesser extent) suppressed the stereotypic behaviors of the serotonin syndrome, resulting in a striking increase in the locomotion caused by central serotonergic stimulation. Such locomotion was accompanied by RSA and LVFA. It was concluded that increased serotonergic activity in the CNS causes an increase in motor activity and a correlated increase in scopolamine-resistant hippocampal RSA and scopolamine-resistant neocortical LVFA and suggested that metergoline blocks serotonin receptors mediating stereotyped behaviors, thereby permitting the expression of serotonin-mediated locomotion.

Tissue levels and some pharmacological properties of an acetylated metabolite of phenelzine in the rat

The metabolic generation of N2-acetylphenelzine by rats treated with phenelzine, and the activity of this metabolite as an inhibitor of monoamine oxidase enzymes in vivo were confirmed. The isomeric amide N1-acetylphenelzine was not a metabolic product of phenelzine and also did not inhibit monoamine oxidase enzymes. Levels of N2-acetylphenelzine in rat blood, after treatment with a dose (0.1 mmol.kg-1) of N2-acetylphenelzine sufficient to inhibit monoamine oxidase enzymes but not to increase brain levels of dopamine or noradrenaline, were higher than those generated metabolically from a higher dose (0.38 mmol.kg-1) of phenelzine which did increase brain levels of these biogenic amines. Metabolically derived N2-acetylphenelzine, therefore, probably does not contribute in any significant way to monoamine oxidase inhibition by phenelzine.

N2-acetylphenelzine: effects on rat brain GABA, alanine and biogenic amines

The neurochemical properties of N2-acetylphenelzine were compared with those of phenelzine in a rat model. N2-Acetylphenelzine is a relatively potent inhibitor of monoamine oxidase-A and -B and causes increases in whole-brain levels of noradrenaline and 5-hydroxytryptamine, and decreases in homovanillic acid, 5-hydroxyindole-3-acetic acid, and 3,4-dihydroxyphenylacetic acetic after acute i.p. administration of the drug. Phenelzine is a more potent monoamine oxidase inhibitor than is N2-acetylphenelzine. The most marked difference in the profile was that N2-acetylphenelzine had no effect on whole brain levels of the amino acid neurotransmitters alanine and gamma-aminobutyric acid, whereas phenelzine caused dramatic increases. Acetylation of phenelzine at the N2 position presumably interferes with the inhibition of the transaminase enzymes for gamma-aminobutyric acid and alanine.

Effects of p-chlorophenylalanine and scopolamine on retention of habits in rats

Rats were trained on a conventional maze test or on a swim-to-platform test. Retention of swim-to-platform performance 7 days later was severely impaired by posttraining treatment with a combination of p-chlorophenylalanine (PCPA) and scopolamine although neither drug alone had any effect. Retention of the maze habit was moderately impaired by scopolamine alone and severely impaired by a combination of scopolamine and PCPA, but was unaffected by PCPA alone. Polygraphic recordings confirmed previous reports that a combination of PCPA and scopolamine can abolish neocortical low voltage fast activity and hippocampal rhythmical slow activity. Combined blockade of central cholinergic and serotonergic neurotransmission in rats may provide a useful animal model of Alzheimer's disease.

The role of serotonin in the control of cerebral activity: studies with intracerebral 5,7-dihydroxytryptamine

Intact rats treated with centrally acting antimuscarinic (atropinic) drugs display large amplitude irregular slow waves in both the neocortex and hippocampus during behavioral immobility and some stereotyped automatic behaviors (Type 2 behavior). However, rhythmical slow activity in the hippocampus and low voltage fast activity in the neocortex occur in close correlation with spontaneous changes in posture, head movement, walking, rearing, swimming or struggling when held (Type 1 behavior). It has previously been proposed that these waveforms, jointly referred to as atropine-resistant cerebral activation (ARCA) are dependent on ascending serotonergic projections. As a further test of this hypothesis, we have studied rats in which forebrain levels of serotonin and 5-hydroxyindoleacetic acid were reduced to 3-10% of control levels as a result of multiple intrabrainstem injections of 5,7-dihydroxytryptamine. This treatment strongly reduced or abolished ARCA in most cases but did not reduce atropine-sensitive cerebral activation which appears to be dependent on ascending cholinergic projections from the basal forebrain to the cerebral cortex. Therefore, ARCA appears to be dependent on ascending serotonergic inputs to the forebrain.

Functional interactions of 2-phenylethylamine and of tryptamine with brain catecholamines: implications for psychotherapeutic drug action

1. The relevance of trace amine research is outlined for PEA and T in the context of psychotherapeutic drug action, particularly in relation to the actions of MAO-inhibitor antidepressant drugs. 2. Evidence for the neuronal localization of these amines and their relationship to brain catecholamines is discussed with respect to possible co-localization with DA and their distribution within the nigro-striatal/striato-nigral system. 3. The results of recent experiments assessing the behavioural effects of prodrugs for PEA and T are described. The interactions of these compounds with MAO inhibitors are assessed and the actions of PEA prodrugs are discussed in relation to brain DA systems. 4. Recent evidence for functional decreases in beta-adrenergic receptors following chronic administration of MAO inhibitors is outlined. The lack of association of such effects with the percentage of MAO inhibition observed after these treatments indicates influences of these compounds (or metabolites) on factors other than MAO activity as mediators of these effects. The possible role of PEA (as a metabolite of PLZ) in this context is proposed. The possible involvement of PEA in emergent changes in beta-adrenergic receptors induced by chronic antidepressant drugs is hypothesized in relation to ongoing research.

N,N-dipropargyl-2-phenylethylamine, a potential prodrug of 2-phenylethylamine: neurochemical and neuropharmacological studies in rat

An evaluation of N,N-dipropargyl-2-phenylethylamine (DPGPEA) as a prodrug to increase brain levels of 2-phenylethylamine (PEA) was conducted in rats. A 0.1 mmol/kg (IP) dose of DPGPEA was administered and produced marked elevations of PEA levels in brain, liver and blood which were sustained for several hours. Ex vivo MAO inhibition data indicated DPGPEA to be an inhibitor of MAO-B, although the effect was much weaker than seen with tranylcypromine or pargyline. DPGPEA caused brain noradrenaline, dopamine and 3,4-dihydroxyphenylacetic acid levels to temporarily decrease significantly below controls. Concomitant increases in homovanillic acid and 5-hydroxyindole-3-acetic acid levels suggest that the PEA or N-propargylPEA formed and/or DPGPEA itself have significant effects on release and reuptake of neurotransmitters. DPGPEA was also shown to be metabolized to N-propargyl-PEA, another prodrug of PEA, in vivo.

Pharmacokinetic and neurochemical studies on N-propargyl-2-phenylethylamine, a prodrug of 2-phenylethylamine

Pharmacokinetic and neurochemical properties of N-propargyl-2-phenylethylamine, a known MAO inhibitor and a potential prodrug of the bioactive trace amine, 2-phenylethylamine, were studied in rats. Intraperitoneal (i.p.) administration of N-propargyl-2-phenylethylamine produced marked elevations in 2-phenylethylamine levels in rat brain, blood and liver; these levels remained significantly above controls for more than 4 h. Neurochemical studies showed the drug to be a preferential MAO-B inhibitor at a dose of 0.1 mmol/kg (i.p.). This selectivity was much more pronounced in liver than in brain. N-Propargyl-2-phenylethylamine produced a significant decrease in whole brain concentrations of noradrenaline, dopamine and DOPAC, but they returned to control values by 3 h after drug administration. Concentrations of 5-HT were unaffected, while 5-HIAA and HVA concentrations increased significantly above controls. Results indicate that N-propargyl-2-phenylethylamine is a prodrug of 2-phenylethylamine and a potentially useful pharmacological tool with which to study the functional role of 2-phenylethylamine in the mammalian central nervous system.