On the Presence of p-Hydroxynorephedrine in the Rat Brain and Heart in Relation to Changes in Catecholamine Levels after Administration of Amphetamine

Toxicity of the amphetamine metabolites 4-hydroxyamphetamine and 4-hydroxynorephedrine in human dopaminergic differentiated SH-SY5Y cells

Amphetamine (AMPH) is a psychostimulant used worldwide by millions of patients in the clinical treatment of attention deficit hyperactivity disorder, narcolepsy or even obesity, and is also a drug of abuse. 4-Hydroxynorephedrine (4-OHNE) and 4-hydroxyamphetamine (4-OHAMPH) are two major metabolites known to persist in the brain longer than AMPH. The contribution of AMPH metabolites for its neurotoxicity is undetermined. We evaluated the toxicity of AMPH and its metabolites 4-OHNE and 4-OHAMPH, obtained by chemical synthesis, in human dopaminergic differentiated SH-SY5Y neurons. Cells were exposed to AMPH (concentration range 0-5mM) or 4-OHAMPH or 4-OHNE (concentration range 0-10mM) for 24 or 48h, and the viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) and lactate dehydrogenase (LDH) leakage assays. Results showed that for both AMPH and the metabolites a concentration-dependent toxicity was observed. The toxic concentration 50% (TC₅₀) for AMPH and 4-OHNE following 24h exposure was circa 3.5mM and 8mM, respectively. For 4-OHAMPH the TC₅₀ was not reached in the tested concentration range. N-acetyl cysteine, cycloheximide, l-carnitine, and methylphenidate were able to reduce cell death induced by AMPH TC₅₀. Acridine orange/ethidium bromide staining showed evident signs of late apoptotic cells and necrotic cells following 24h exposure to AMPH 3.50mM. The 4-OHAMPH metabolite at 8.00mM originated few late apoptotic cells, whereas 4-OHNE at 8.00mM resulted in late apoptotic cells and necrotic cells, in a scenario similar to AMPH. In conclusion, the AMPH metabolite 4-OHNE is more toxic than 4-OHAMPH, nonetheless both are less toxic than the parent compound in vitro. The most toxic metabolite 4-OHNE has longer permanence in the brain, rendering likely its contribution for AMPH neurotoxicity.

Current research on methamphetamine-induced neurotoxicity: animal models of monoamine disruption

Methamphetamine (METH)-induced neurotoxicity is characterized by a long-lasting depletion of striatal dopamine (DA) and serotonin as well as damage to striatal dopaminergic and serotonergic nerve terminals. Several hypotheses regarding the mechanism underlying METH-induced neurotoxicity have been proposed. In particular, it is thought that endogenous DA in the striatum may play an important role in mediating METH-induced neuronal damage. This hypothesis is based on the observation of free radical formation and oxidative stress produced by auto-oxidation of DA consequent to its displacement from synaptic vesicles to cytoplasm. In addition, METH-induced neurotoxicity may be linked to the glutamate and nitric oxide systems within the striatum. Moreover, using knockout mice lacking the DA transporter, the vesicular monoamine transporter 2, c-fos, or nitric oxide synthetase, it was determined that these factors may be connected in some way to METH-induced neurotoxicity. Finally a role for apoptosis in METH-induced neurotoxicity has also been established including evidence of protection of bcl-2, expression of p53 protein, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL), activity of caspase-3. The neuronal damage induced by METH may reflect neurological disorders such as autism and Parkinson's disease.

Involvement of hydroxylated metabolites in amphetamine-induced hypothermia in mice

1. The hydroxylated metabolites of amphetamine, p-hydroxyamphetamine (p-OHA) and p-hydroxynorephedrine (p-OHN), were administered intracerebroventricularly in mice in order to evaluate their ability to elicit hypothermia. 2. Intracerebroventricular (i.c.v.) administration of p-OHA and p-OHN (1, 3 and 9 micrograms/mouse) induced maximal hypothermia 30 min after injection. p-OHA and p-OHN (9 micrograms, i.c.v.) produced maximal decreases in rectal temperature of -6.48 +/- 0.44 degrees C and -3.82 +/- 0.42 degrees C, respectively. Both metabolites are more effective than amphetamine (at 9 micrograms, i.c.v., -3.32 +/- 0.75 degrees C). 3. Pretreatment with haloperidol (5 micrograms, i.c.v.) suppressed the fall in temperature produced by p-OHA (3 micrograms, i.c.v.) and reduced that produced by p-OHN (3 micrograms, i.c.v.), respectively. The selective dopaminergic D1 receptor antagonist, SCH 23390, and the D2 receptor antagonists, sultopride and metoclopramide, were without effect on the hypothermia induced by either metabolite. Similarly, amphetamine-induced hypothermia was only inhibited by haloperidol. Apomorphine (0.1 mg kg-1, i.p.) did not potentiate the hypothermia induced by either metabolite, whereas the selective dopaminergic D2 agonist, quinpirole (0.2 mg kg-1, i.p.) did. Amphetamine-induced hypothermia was potentiated by apomorphine and quinpirole. 4. Neither the 5-hydroxytryptamine (5-HT) receptor blocker, cyproheptadine, nor the 5-HT receptor agonist, quipazine, modified metabolite-induced hypothermia. In contrast, amphetamine-induced hypothermia was affected by these 5-HT drugs. 5. The neuropeptide CCK-8 (0.04 mg kg-1, i.p.) and gamma-butyrolactone (40 mg kg-1, i.p.) potentiated the hypothermia produced by amphetamine and its metabolites. Conversely, desipramine (20 mg kg-1, i.p.) antagonized it.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of brain catecholamines on 'drug taking behaviour' relative to oral self-administration of d-amphetamine by rats

Oral administration of 5 mg/kg of d-amphetamine to adult Wistar rats caused brain NE to decrease to approx. 80% of the control level during 4-24 h after acute treatment and slowly further to 65% after 24 days of self-administration via drinking water. The norepinephrine (NE)-reducing effect was first recognized at 1 mg/kg and appeared to peak at 5 mg/kg of d-amphetamine. Brain dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were only shortly affected. Neither iprindole nor desipramine altered the effect of amphetamine on brain NE. DA was decreased by both inhibitors depending on the duration of pre-treatment. Iproniazid and alpha-methyl-p-tyrosine antagonized and potentiated respectively the amphetamine effect on NE- and DA-concentration after 4 days of simultaneous treatment. In the free choice experiment (water vs. 0.005% d-amphetamine solution) rats developed an aversion to amphetamine. The number of rats taking the drug and the consumption by rats still drinking it declined gradually from 100% and approx. 3 mg/kg/day to 50% and approx. 1.5 mg/kg/day, respectively, during 18 days. The time course of the developing aversive reaction to oral amphetamine ran approximately parallel to that of NE-depletion. Iprindole and desipramine intensified, iproniazid and propranolol weakened, while alpha-methyl-p-tyrosine and haloperidol hardly influenced the aversive effect of amphetamine. It is concluded that the development of aversive behaviour in response to oral d-amphetamine is mediated not only through the depleting effect of amphetamine on NE stores but also by its direct stimulation at beta-adrenergic receptors in the CNS.

Determination of amphetamine, norephedrine, and their phenolic metabolites in rat brain by gas chromatography

A specific analytical procedure for the quantitation of amphetamine (I), norephedrine (III), and their amphoteric metabolites, p-hydroxy-amphetamine (II) and p-hydroxynorephedrine (IV), in biological samples using electron-capture gas chromatography (GC-EC) is described. The procedure utilizes the ion-pairing reagent, bis(2-ethylhexyl)phosphoric acid, which frees the amines from most contaminants and permits the efficient extraction of the amphoteric compounds (as acetates) from the aqueous solution. Amines I and III and acetylated amines II and IV were perfluoroacetylated prior to GC-EC analysis. Metabolism of I, II, and III in the rat brain was studied. Results indicate that both in vivo and in vitro amines I and III are p-hydroxylated to II and IV, respectively, and II is beta-hydroxylated to give IV. Norephedrine (III) was not detected as a rat brain metabolite of amphetamine (I).

Evidence that a behavioral augmentation following repeated amphetamine administration does not involve peripheral mechanisms

Repeated administration of amphetamine (AMPH) to rats results in an augmentation of the drug-induced locomotion and stereotypy. The studies reported below were directed at examining the potential role for some dispositional and peripheral sympathomimetic factors in mediating the enhanced stereotypy response. These included three factors associated with repeated AMPH administration: (1) the possible accumulation of AMPH in a peripheral mobilizable pool; (2) repeated sympathetic activation; and (3) AMPH metabolite-induced depletion of peripheral stores of norepinephrine. The approach utilized was to selectively reduce or mimic the peripheral actions of AMPH through the use of non-pharmacological or pharmacological manipulations which are relatively lacking in AMPH-like central stimulant effects. The results indicate that these factors cannot account for the augmentation of the behavioral response to AMPH and suggest that these behavioral alterations reflect changes in the responsiveness of brain mechanisms which mediate the behavioral effects of the drug.

Metabolic and experimental factors in the behavioral response to repeated amphetamine

Previous studies have shown that repeated administration of d-amphetamine results in a progressive augmentation of locomotor activity and stereotypy. The present studies demonstrate that rats also exhibit an enhanced behavioral response following multiple daily injections of l-amphetamine and methylphenidate. Furthermore, behavioral augmentation is shown to persist for at least six days after a single injection of d-amphetamine. These results demonstrate the generality of the reverse tolerance phenomenon and indicate that metabolic factors, such as the formation of false neurotransmitters, do not account for the enhanced behavioral responsiveness observed with multiple injections of these drugs. The role of experiential factors in the behavioral augmentation was studied by (1) varying the amount of continuous exposure to the experimental environment prior to d-amphetamine administration, and (2) examining the effects of repeated injections of saline or d-amphetamine in different environments prior to testing in the experimental chambers. The results, which revealed a behavioral augmentation independent of pretreatment condition, indicate that neither acclimation to the test chamber nor state-dependent conditioning to external stimuli accounts for the enhanced locomotor activity and stereotypy observed with repeated administration of psychomotor stimulants.

Does tolerance develop to low doses of d- and l-amphetamine on locomotor activity in rats?

An observational study of the behavioural effects of chronic regimens of d- and l-amphetamine was designed to investigate possible mechanisms underlying any parallel behavioural changes: (1) Accumulation of p-hydroxynorephedrine in noradrenergic nerve terminals; (2) Altered sensitivity of dopaminergic receptors. The study revealed that locomotor activity seen with low doses of both isomers (2.0 mg/kg d- and 6.0 mg/kg l-) decreased with chronic once daily treatment. However, this was accompanied by an increase in directed sniffing activity and the behaviour came to resemble that seen with higher doses of amphetamine (8.0 mg/kg d- and 16.0 mg/kg l-). Nonsignificant decreases in locomotor activity and increases in directed sniffing to apomorphine administration were observed during chronic amphetamine treatment. These findings suggest that (1) p-hydroxynorephedrine, a metabolite of d- but not 1- amphetamine, does not play an important role in these alterations in behaviour with chronic treatment and (2) the tolerance to amphetamine observed under these conditions is due to an increased, rather than decreased, sensitivity of the rats to amphetamine.

Microiontophoretic studies of the effects of false transmitter candidates and amphetamine on cerebellar Purkinje cells

The effects of microiontophoretic applications of equivalent doses (ejection times and currents) of noradrenaline, amphetamine, octopamine and p-hydroxynorephedrine on the spontaneous firing of Purkinje and unidentified cells in the cerebellum of rats were examined. In addition, the effects of amphetamine of Purkinje cells were examined in animals pretreated with the tyrosine hydroxylase inhibitor, alpha-methyltyrosine (alpha-MpT) or with a combination of reserpine plus alpha-MpT. The results indicate that the "false transmitters" are weak agonists when compared to noradrenaline in inhibiting the firing of Purkinje cells. The results of the iontophoretic studies with amphetamine are not consistent with a pre-synaptic releasing effect by amphetamine at noradrenergic synapses in the cns since the efficacy of amphetamine on Purkinje cells was unaltered after pretreatment with alpha-MpT or alpha-MpT plus reserpine.