Inhibition of MAO-A fails to alter cocaine-induced increases in extracellular dopamine and norepinephrine in rat nucleus accumbens

Cocaine and desipramine elicit distinct striatal noradrenergic and behavioral responses in selectively bred obesity-resistant and obesity-prone rats

Previous studies have demonstrated a role for norepinephrine (NE) in energy regulation and feeding, and basal differences have been observed in hypothalamic NE systems in obesity-prone vs. obesity-resistant rats. Differences in the function of brain reward circuits, including in the nucleus accumbens (NAc), have been shown in obesity-prone vs. obesity-resistant populations, leading many researchers to explore the role of striatal dopamine in obesity. However, alterations in NE transmission also affect NAc mediated behaviors. Therefore, here we examined differences in striatal NE and the response to norepinephrine transporter blockers in obesity-prone and obesity-resistant rats. We found that striatal NE levels increase following systemic cocaine administration in obesity-prone, but not obesity-resistant rats. This could result from either blockade of striatal norepinephrine transporters (NET) by cocaine leading to reduced NE reuptake, or circuit-based responses following cocaine administration resulting in increased NE release. Retrodialysis of the NET inhibitor, desipramine, into the ventral striatum did not cause selective increases in striatal NE levels in obesity-prone rats, suggesting that circuit-based mechanisms underlie NE increases following systemic cocaine administration. Consistent with this, systemic desipramine treatment decreased locomotor activity in obesity-prone, but not obesity-resistant rats. Furthermore, obesity-prone rats were also more sensitive to desipramine-induced reductions in food intake compared to obesity-resistant rats. Taken together, these data expand our understanding of differences in NE systems of obesity-prone vs. resistant rats, and provide new insights into basal differences in striatal systems that may influence feeding behavior.

Noni (Morinda citrifolia L.) fruit extract attenuates the rewarding effect of heroin in conditioned place preference but not withdrawal in rodents

The present study was designed to investigate the effect of a methanolic extract of Morinda citrifolia Linn. fruit (MMC) on the rewarding effect of heroin in the rat conditioned place preference (CPP) paradigm and naloxone-precipitated withdrawal in mice. In the first experiment, following a baseline preference test (preconditioning score), the rats were subjected to conditioning trials with five counterbalanced escalating doses of heroin versus saline followed by a preference test conducted under drug-free conditions (post-conditioning score) using the CPP test. Meanwhile, in the second experiment, withdrawal jumping was precipitated by naloxone administration after heroin dependence was induced by escalating doses for 6 days (3×/ day). The CPP test results revealed that acute administration of MMC (1, 3, and 5 g/kg body weight (bw), p.o.), 1 h prior to the CPP test on the 12th day significantly reversed the heroin-seeking behavior in a dose-dependent manner, which was similar to the results observed with a reference drug, methadone (3 mg/kg bw, p.o.). On the other hand, MMC (0.5, 1, and 3 g/kg bw, p.o.) did not attenuate the heroin withdrawal jumps precipitated by naloxone. These findings suggest that the mechanism by which MMC inhibits the rewarding effect of heroin is distinct from naloxone-precipitated heroin withdrawal.

Update on the pharmacology of selective inhibitors of MAO-A and MAO-B: focus on modulation of CNS monoamine neurotransmitter release

Inhibitors of monoamine oxidase (MAO) were initially used in medicine following the discovery of their antidepressant action. Subsequently their ability to potentiate the effects of an indirectly-acting sympathomimetic amine such as tyramine was discovered, leading to their limitation in clinical use, except for cases of treatment-resistant depression. More recently, the understanding that: a) potentiation of indirectly-acting sympathomimetic amines is caused by inhibitors of MAO-A but not by inhibitors of MAO-B, and b) that reversible inhibitors of MAO-A cause minimal tyramine potentiation, has led to their re-introduction to clinical use for treatment of depression (reversible MAO-A inhibitors and new dose form MAO-B inhibitor) and treatment of Parkinson's disease (MAO-B inhibitors). The profound neuroprotective properties of propargyl-based inhibitors of MAO-B in preclinical experiments have drawn attention to the possibility of employing these drugs for their neuroprotective effect in neurodegenerative diseases, and have raised the question of the involvement of the MAO-mediated reaction as a source of reactive free radicals. Despite the long-standing history of MAO inhibitors in medicine, the way in which they affect neuronal release of monoamine neurotransmitters is still poorly understood. In recent years, the detailed chemical structure of MAO-B and MAO-A has become available, providing new possibilities for synthesis of mechanism-based inhibitors. This review describes the latest advances in understanding the way in which MAO inhibitors affect the release of the monoamine neurotransmitters dopamine, noradrenaline and serotonin (5-HT) in the CNS, with an accent on the importance of these effects for the clinical actions of the drugs.

Harmine augments electrically evoked dopamine efflux in the nucleus accumbens shell

Harmine is a β-carboline alkaloid and major component of ayahuasca, a traditional South American psychoactive tea with anecdotal efficacy for treatment of cocaine dependence. Harmine is an inhibitor of monoamine oxidase A (MAO-A) and interacts in vitro with several pharmacological targets which modulate dopamine (DA) neurotransmission. In vivo studies have demonstrated dopaminergic effects of harmine, attributed to monoamine oxidase inhibitor (MAOI) activity, however none have directly demonstrated a pharmacological mechanism. This study investigated the acute effects, and pharmacological mechanism(s), of harmine on electrically evoked DA efflux parameters in the nucleus accumbens both in the absence and presence of cocaine. Fast cyclic voltammetry in rat brain slices was used to measure electrically evoked DA efflux in accumbens core and shell. Harmine (300 nM) significantly augmented DA efflux (148±8% of baseline) in the accumbens shell. Cocaine augmented efflux in shell additive to harmine (260±35%). Harmine had no effect on efflux in the accumbens core or on reuptake in either sub-region. The effect of harmine in the shell was attenuated by the 5-HT(2A/2C) antagonist ketanserin. The MAOI moclobemide (10 µM) had no effect on DA efflux. These data suggest that harmine augments DA efflux via a novel, shell-specific, presynaptic 5-HT(2A) receptor-dependent mechanism, independent of MAOI activity. A DA-releasing 'agonist therapy' mechanism may thus contribute to the putative therapeutic efficacy of ayahuasca for cocaine dependence.

Developments in harmine pharmacology--implications for ayahuasca use and drug-dependence treatment

Ayahuasca is a hallucinogenic botanical mixture originating in the Amazon area where it is used ritually, but is now being taken globally. The 2 main constituents of ayahuasca are N,N-dimethyltryptamine (DMT), a hallucinogen, and harmine, a monoamine oxidase inhibitor (MAOI) which attenuates the breakdown of DMT, which would otherwise be broken down very quickly after oral consumption. Recent developments in ayahuasca use include the sale of these compounds on the internet and the substitution of related botanical (anahuasca) or synthetic (pharmahuasca) compounds to achieve the same desired hallucinogenic effects. One intriguing result of ayahuasca use appears to be improved mental health and a reduction in recidivism to alternate (alcohol, cocaine) drug use. In this review we discuss the pharmacology of ayahuasca, with a focus on harmine, and suggest pharmacological mechanisms for the putative reduction in recidivism to alcohol and cocaine misuse. These pharmacological mechanisms include MAOI, effects at 5-HT(2A) and imidazoline receptors and inhibition of dual-specificity tyrosine-phosphorylation regulated kinase 1A (DYRK1A) and the dopamine transporter. We also speculate on the therapeutic potential of harmine in other CNS conditions.

The neuropharmacology of relapse to food seeking: methodology, main findings, and comparison with relapse to drug seeking

Relapse to old, unhealthy eating habits is a major problem in human dietary treatments. The mechanisms underlying this relapse are unknown. Surprisingly, until recently this clinical problem has not been systematically studied in animal models. Here, we review results from recent studies in which a reinstatement model (commonly used to study relapse to abused drugs) was employed to characterize the effect of pharmacological agents on relapse to food seeking induced by either food priming (non-contingent exposure to small amounts of food), cues previously associated with food, or injections of the pharmacological stressor yohimbine. We also address methodological issues related to the use of the reinstatement model to study relapse to food seeking, similarities and differences in mechanisms underlying reinstatement of food seeking versus drug seeking, and the degree to which the reinstatement procedure provides a suitable model for studying relapse in humans. We conclude by discussing implications for medication development and future research. We offer three tentative conclusions: (1)The neuronal mechanisms of food-priming- and cue-induced reinstatement are likely different from those of reinstatement induced by the pharmacological stressor yohimbine. (2)The neuronal mechanisms of reinstatement of food seeking are possibly different from those of ongoing food-reinforced operant responding. (3)The neuronal mechanisms underlying reinstatement of food seeking overlap to some degree with those of reinstatement of drug seeking.

Chronic treatment with monoamine oxidase-B inhibitors decreases cocaine reward in mice

RATIONALE AND OBJECTIVE

Whether monoamine oxidase inhibitors (MAOIs) can be used to suppress the reinforcing effect of cocaine remains unknown. This study was undertaken to examine effects of a long-term dosing regimen with selective MAOIs on cocaine and food reward.

MATERIALS AND METHODS

Since single dose of clorgyline (2 mg/kg), deprenyl (1 mg/kg), and pargyline (10 mg/kg) did not acutely affect mouse locomotor activity, these doses were chosen to treat the male C57BL/6j mice on a daily basis.

RESULTS

Fourteen consecutive days of pretreatments with clorgyline, deprenyl, or pargyline (one injection per day) did not affect natural reward-supported operant behavior, since acquisition of the lever pressing responses for food pellets under an FR-1 protocol did not differ among these drug- and saline-treated mice. Likewise, 24 consecutive days of pretreatments with clorgyline did not alter acquisition of the cocaine (0.3 mg/kg per infusion)-supported operant responses under an FR-1 protocol. In contrast, 24 days of pretreatments with deprenyl and pargyline abolished the cocaine-supported operant responses under a similar protocol. Twenty-four days of clorgyline treatment enhanced serotonin contents in striatum, nucleus accumbens, and frontal cortex. Frontal cortical 3,4-dihydroxyphenylacetic acid and 5-hydroxyindoleacidic acid concentrations were decreased following 24 days of pretreatments with deprenyl and pargyline. These changes were not evident in mice pretreated with clorgyline.

CONCLUSIONS

We suggest that long-term treatments with MAO-B inhibitors may decrease cocaine-supported operant responses in cocaine-naïve mice by selectively decreasing frontal cortical metabolism of dopamine and serotonin.

Long-term administration of cocaine or serotonin reuptake inhibitors results in anatomical and neurochemical changes in noradrenergic, dopaminergic, and serotonin pathways

The catechol and indole pathways are important components underlying plasticity in the frontal cortex and basal ganglia. This study demonstrates that administering rats either cocaine or a selective serotonin (or 5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) for 16 weeks results in reduced density of dopaminergic and noradrenergic terminals in the striatum and olfactory bulb, respectively, reflecting pruning of the terminal arbor of ventral midbrain dopaminergic and locus coeruleus noradrenergic neurones. In the striatum of cocaine-treated animals, basal dopamine levels, as well as cocaine-induced dopamine release, is diminished compared with controls. In contrast, serotonergic fibers, projecting from the raphe, sprout and have increased terminal density in the lateral septal nucleus and frontal cortex, following long-term cocaine or SSRI treatment. This is associated with elevated basal 5-HT and enhanced cocaine-induced 5-HT release in the frontal cortex. The anatomical and neurochemical changes in serotonergic fibers following cocaine or SSRI treatment may be explained by attenuated 5-HT(1A) autoreceptor function in the raphe. This study demonstrates extensive plasticity in the morphology and neurochemistry of the catechol and indole pathways that contribute to drug-induced plasticity of the corticostriatal (and other) projections. Moreover, our data suggest that drug-induced plastic adaptation is anatomically widespread and consequently, likely to have multiple and complex consequences.

The role of amine oxidases in xenobiotic metabolism

The amine oxidases of mammalian tissues are a heterogeneous family of enzymes that metabolise various monoamines, diamines and polyamines produced endogenously, or being absorbed as dietary or xenobiotic substances. The heterogeneous class of amine oxidases can be divided on an arbitrary basis of the chemical nature of their cofactors into two types. Monoamine oxidase (MAO) and an intracellular form of polyamine oxidase (PAO) contain flavin adenine dinucleotide (FAD) as their cofactor, whereas a second group of amine oxidases without FAD contain a cofactor possessing one or more carbonyl groups, making them sensitive to inhibition by carbonyl reagents such as semicarbazide; this group includes semicarbazide-sensitive amine oxidase (SSAO) and the connective tissue enzyme, lysyl oxidase. This article focuses on the general aspects of MAO's contribution to the metabolism of foreign toxic substances including toxins and illegal drugs. Another main objective of this review is to discuss the properties of PAO and SSAO and their involvement in the metabolism of xenobiotics.

Effects of high-dose selegiline on morphine reinforcement and precipitated withdrawal in dependent rats

Selegiline is an irreversible inhibitor of monoamine oxidase (MAO) with psychostimulant and neuroprotective effects. Several lines of evidence suggest that treatment with selegiline at doses that exceed levels required for inhibition of MAO can produce distinct pharmacologic effects. The purpose of this study was to evaluate the effects of chronic treatment with high-dose selegiline on extinction responding, cue-induced reinstatement, morphine reinforcement and naloxone-precipitated withdrawal. After pretreatment with noncontingent morphine to establish opiate dependence, rats acquired self-administration of 3.2 mg/kg per injection of morphine under a progressive ratio schedule. Daily treatment with saline or 6.4 mg/kg per day of selegiline was then administered over extinction, reinstatement and re-acquisition of morphine self-administration. To enhance or diminish the potential for psychostimulant effects, selegiline was administered either immediately prior to (pre-session) or 1 h following (post-session) extinction, reinstatement and self-administration sessions. Pre-session selegiline decreased the number of ratios completed on days 2, 3 and 4 of extinction, and decreased morphine self-administration during all four re-acquisition sessions. When administered at the same dose level, post-session selegiline decreased responding on the fourth extinction session, and was ineffective in modifying re-acquisition of self-administration. Selegiline administered by either schedule did not modify cue-induced reinstatement. Daily treatment with 6.4 mg/kg per day of selegiline did not modify self-administration of food under a progressive ratio schedule. Acute treatment with single, 6.4 mg/kg doses of selegiline attenuated naloxone-induced increases in ptosis and global withdrawal score, but did not modify any other sign of withdrawal or global withdrawal score calculated without ratings of ptosis. In conclusion, high-dose selegiline can attenuate extinction responding and morphine-reinforced behavior, and these effects may be mediated by psychostimulant metabolites.

Determination of serotonin, noradrenaline, dopamine and their metabolites in rat brain extracts and microdialysis samples by column liquid chromatography with fluorescence detection following derivatization with benzylamine and 1,2-diphenylethylenediamine

A highly selective and sensitive column liquid chromatographic method for fluorescence determination of serotonin (5-HT), dopamine (DA), noradrenaline (NA) and their related metabolites 5-hydroxyindole-3-acetic acid (5-HIAA) and 3,4-dihydroxyphenylacetic acid (DOPAC) following derivatization with benzylamine and 1,2-diphenylethylenediamine (DPE) is described. The monoamines and the metabolites (20 microl samples) were derivatized in a two-step reaction, initiated with 20 microl of 0.3M benzylamine in 0.3M 3-cyclohexylaminopropanesulfonic acid (CAPS) buffer (pH 10.0), (for 5-HT, 5-HIAA, 2 min, 24 degrees C) and followed by 20 microl of 0.1M DPE in 0.3M glycine buffer (pH 10.0), (for DA, NA, DOPAC, 20 min, 50 degrees C). Both reagents contained 0.02 M potassium hexacyanoferrate(III) and 50% (v/v) methanol. The resulting highly fluorescent and stable benzoxazole derivatives were isocratically separated on a reversed-phase column (150 mm x 1.5 mm i.d., packed with C18 silica, 5 microm) within 45 min. Using fluorescence detection at ex. and em. wavelengths of 345 and 480 nm, respectively, the detection limit (signal-to-noise ratio of 3) for 5-HT, DA, NA, 5-HIAA, L-DOPA and DOPAC ranged between 0.08 and 5.65 fmol per 20-microl injection (12-847.5 pM in standard solution). The concentrations of monoamines (expressed in microg/g wet weight, mean +/- S.E.M., n=5) in tissue extracts from the rat striatum were: 0.45+/-0.05 (5-HT), 4.27+/-0.08 (DA), 0.27+/-0.04 (NA), 0.55+/-0.06 (5-HIAA), 1.26+/-0.16 (L-DOPA) and 1.62+/-0.11 (DOPAC). Microdialysis samples were collected in 20 min intervals from the probes implanted in the striatum of awake rats. The basal monoamine levels (in fmol/20 microl, mean +/- S.E.M., n=5) in the dialysates were: 4.0+/-0.7 (5-HT), 78.4+/-9.1 (DA), 6.4+/-0.8 (NA), 785.5+/-64.5 (5-HIAA) and 5504.5+/-136.5 (DOPAC). It is concluded that the new fluorescence derivatization protocol provides an excellent means for simultaneous determination of all three monoamines both in the complex samples (e.g. brain homogenates) and also at trace levels, such as those found in the microdialysis samples.

Evidence for alterations in α2-adrenergic receptor sensitivity in rats exposed to repeated cocaine administration

It is well established that cocaine stimulates monoamine transmission by blocking reuptake of norepinephrine (NE), dopamine and serotonin into nerve cells, yet few investigations have addressed the effects of chronic cocaine on NE function. In the present study, we examined the effects of repeated cocaine injections on neuroendocrine responses evoked by the alpha2-adrenergic receptor agonist, clonidine. Previous findings show that clonidine increases pituitary growth hormone (GH) secretion by a central mechanism involving postsynaptic alpha2-adrenergic receptors. Male rats previously fitted with indwelling jugular catheters received two daily injections of cocaine (15 mg/kg, i.p.) or saline for 7 days. At 42 h and 8 days after treatment, rats were challenged with clonidine (25 microg/kg, i.v.) or saline, and serial blood samples were withdrawn. Plasma GH and corticosterone levels were measured by radioimmunoassay. Prior cocaine exposure did not affect basal levels of either hormone. However, cocaine-pretreated rats displayed a significant reduction in clonidine-evoked GH secretion at 42 h, and this blunted response was still apparent 8 days later. Corticosterone responses produced by clonidine were similar regardless of pretreatment. The present data suggest that withdrawal from repeated cocaine injections may be accompanied by desensitization of postsynaptic alpha2-adrenoreceptors coupled to GH secretion. Since human patients with depression often exhibit blunted GH responses to clonidine, our findings provide evidence that cocaine withdrawal might produce depressive-like symptoms via dysregulation of NE mechanisms.

Monoamine transporters and psychostimulant drugs

Most psychostimulants interact with monoamine transport proteins. This paper reviews work our laboratory has conducted to investigate the interaction of psychostimulants with monoamine transporters in order to advance our understanding of how these drugs affect the brain. We review two topics: (1) characterization of multiple binding sites for cocaine-like drugs and (2) an examination of the mechanisms of action of amphetamine-type anorectic agents. We conclude that the brain contains high abundance nonclassical binding sites for cocaine-like drugs that have micromolar affinity for cocaine and that none of the clinically available amphetamine-type appetite suppressants are equipotent substrates for dopamine transporter (DAT) and serotonin transporter (SERT) proteins. Future medications discovery efforts should focus on identifying new compounds which possess the equipotent substrate activity at DAT and SERT, but which lack the adverse effects of stimulants developed decades ago.

Intravenous cocaine induced-activity and behavioural sensitization in norepinephrine-, but not dopamine-transporter knockout mice

Previously, it was reported that both norepinephrine transporter (NET) and dopamine transporter (DAT) knockout (KO) mice were sensitive to the reinforcing effects of cocaine. However, assessing the locomotor-stimulant effects of cocaine in these subjects has proven difficult due to significant differences in their baseline activity compared to wild-type controls. The present studies were designed to clarify the role of NET and DAT in the stimulant effects of acute and repeated cocaine utilizing these knockout mice, and thereby assess the role of these substrates in the locomotor stimulant effects of cocaine. Mice were habituated to the test environment for sufficient time to ensure equal baselines at the time of cocaine administration. Mice then received cocaine (3-25 mg/kg) intravenously according to a within-session cumulative dose-response design. Cocaine dosing was repeated at 48-h intervals for four sessions to assess behavioural sensitization. NET-KO mice exhibited a reduced response to acute cocaine administration compared to wild-type (WT) controls. However, comparable sensitization developed in NET-KO and WT mice. The DAT-KO and DAT-heterozygote (HT) mice displayed no locomotor activation following either acute or repeated cocaine administration. These data suggest a role for the NET in the acute response to cocaine, but no involvement in sensitization to cocaine. In contrast, DAT appears to be necessary for both the acute locomotor response to cocaine and the subsequent development of sensitization. In addition to existing data concerning the reinforcing effects of cocaine in DAT-KO mice, these data suggest a dissociation between the reinforcing and locomotor stimulant effects of cocaine.

Appetite suppressants as agonist substitution therapies for stimulant dependence

Several lines of evidence support a dual-deficit model of stimulant withdrawal in which decreases in synaptic dopamine (DA) and serotonin (5-HT) contribute to withdrawal symptoms, drug craving, and relapse. According to the dual-deficit model, DA dysfunction during withdrawal underlies anhedonia and psychomotor disturbances, whereas 5-HT dysfunction gives rise to depressed mood, obsessive thoughts, and lack of impulse control. The model suggests that medications capable of normalizing stimulant-induced DA and 5-HT deficits should be effective treatment adjuncts. Furthermore, the model may explain why medications targeting only one neurotransmitter system (i.e., DA) have failed to treat cocaine dependence. Amphetamine-type appetite suppressants are logical choices for neurochemical normalization therapy of stimulant dependence, yet few clinical studies have tested anorectics in this regard. The chief purpose of the present work is to profile the activity of various anorectic agents at DA, 5-HT, and NE transporters, in order to identify possible medications for stimulant dependence. Compounds were tested in vitro for their ability to stimulate release and inhibit uptake of [(3)H]DA, [(3)H]NE, and [(3)H]5-HT. Selected compounds were tested in vivo for their ability to elevate extracellular levels of DA and 5-HT in rat nucleus accumbens. The results show that clinically available appetite suppressants display a wide range of activities at monoamine transporters. However, no single medication possesses equal potency at DA and 5-HT transporters, suggesting that none of the anorectics is ideally suited for treatment of stimulant addictions. Future efforts should focus on developing new medications that possess the desired therapeutic activity but lack the adverse effects associated with older amphetamine-type anorectics.