Hyperaemia in rat neocortex produced by acute exposure to methylenedioxymethamphetamine

MDMA 'ecstasy' increases cerebral cortical perfusion determined by bolus-tracking arterial spin labelling (btASL) MRI

BACKGROUND AND PURPOSE

The purpose of this study was to assess cerebral perfusion changes following systemic administration of the recreational drug 3,4-methylendioxymethamphetamine (MDMA 'ecstasy') to rats.

EXPERIMENTAL APPROACH

Cerebral perfusion was quantified using bolus-tracking arterial spin labelling (btASL) MRI. Rats received MDMA (20 mg·kg(-1); i.p.) and were assessed 1, 3 or 24 h later. Rats received MDMA (5 or 20 mg·kg(-1); i.p.) and were assessed 3 h later. In addition, rats received MDMA (5 or 10 mg·kg(-1); i.p.) or saline four times daily over 2 consecutive days and were assessed 8 weeks later. Perfusion-weighted images were generated in a 7 tesla (7T) MRI scanner and experimental data was fitted to a quantitative model of cerebral perfusion to generate mean transit time (MTT), capillary transit time (CTT) and signal amplitude.

KEY RESULTS

MDMA reduces MTT and CTT and increases amplitude in somatosensory and motor cortex 1 and 3 h following administration, indicative of an increase in perfusion. Prior exposure to MDMA provoked a long-term reduction in cortical 5-HT concentration, but did not produce a sustained effect on cerebral cortical perfusion. The response to acute MDMA challenge (20 mg·kg(-1); i.p.) was attenuated in these animals indicating adaptation in response to prior MDMA exposure.

CONCLUSIONS AND IMPLICATIONS

MDMA provokes changes in cortical perfusion, which are quantifiable by btASL MRI, a neuroimaging tool with translational potential. Future studies are directed towards elucidation of the mechanisms involved and correlating changes in cerebrovascular function with potential behavioural deficits associated with drug use.

MDMA (Ecstasy) and human dopamine, norepinephrine, and serotonin transporters: implications for MDMA-induced neurotoxicity and treatment

RATIONALE

3,4-Methylenedioxymethamphetamine (MDMA, designated as "Ecstasy" if illicitly marketed in tablet form) induces significant decrements in neuronal serotonin (5-HT) markers in humans, nonhuman primates, and rats as a function of dosing and dosing regimen. In rats, MDMA-mediated effects are attributed, in part, to selective high-affinity transport of MDMA into 5-HT neurons by the 5-HT transporter (SERT), followed by extensive 5-HT release.

OBJECTIVES

To clarify whether SERT-selective effects of MDMA at human monoamine transporters can account for the reported MDMA-induced selective toxicity of serotonin neurons in primate brain.

METHODS

We investigated the interaction of [(3)H](+/-, RS)- (+, S)- and (-, R)-MDMA with the human SERT, dopamine (DA) transporter (DAT), and norepinephrine (NE) transporter (NET) in stably transfected human embryo kidney (HEK)-293 cells.

RESULTS

The human DAT, NET, and SERT actively transported [(3)H]RS(+/-)-MDMA saturably, stereoselectively, and in a temperature-, concentration-, and transporter-dependent manner. MDMA exhibited the highest affinity for the NET>>SERT>or=DAT, the same rank order for MDMA inhibition of [(3)H]DA, [(3)H]NE, and [(3)H]5-HT transport and stimulated release of the [(3)H]monoamines, which differed from reports derived from rodent monoamine transporters. The extent of MDMA-induced release of 5-HT was higher compared with release of DA or NE.

CONCLUSIONS

The affinity of MDMA for the human SERT in transfected cells does not clarify the apparent selective toxicity of MDMA for serotonin neurons, although conceivably, its higher efficacy for stimulating 5-HT release may be a distinguishing factor. The findings highlight the need to investigate MDMA effects in DAT-, SERT-, and NET-expressing neurons in the primate brain and the therapeutic potential of NET or DAT inhibitors, in addition to SERT-selective inhibitors, for alleviating the pharmacological effects of MDMA.

L-tyrosine contributes to (+)-3,4-methylenedioxymethamphetamine-induced serotonin depletions

The specific mechanisms underlying (+)-3,4-methylenedioxymethamphetamine (MDMA)-induced damage to 5-HT terminals are unknown. Despite the hypothesized role for dopamine (DA) and DA-derived free radicals in mediating this damage, it remains unclear why MDMA produces long-term depletions of 5-HT in brain regions that are sparsely innervated by DA neurons. We hypothesized that the precursor to DA biosynthesis, tyrosine, mediates MDMA-induced 5-HT depletions. Extracellular tyrosine concentrations increased fivefold in striatum and 2.5-fold in hippocampus during the administration of neurotoxic doses of MDMA. In vitro results show that L-tyrosine can be hydroxylated nonenzymatically to the DA precursor l-3,4-dihydroxyphenylalanine (DOPA) under pro-oxidant conditions. The local infusion of L-tyrosine into the striatum or hippocampus during MDMA administration potentiated the acute increase in extracellular DA and the long-term depletion of 5-HT after MDMA. Coinfusion of the aromatic amino acid decarboxylase (AADC) inhibitor m-hydroxybenzylhydrazine attenuated these effects in hippocampus and decreased basal extracellular DA in the striatum. In contrast, the reverse dialysis of the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine into the hippocampus did not affect MDMA-induced increases in extracellular DA or the long-term depletion in 5-HT. These results show that MDMA increases the concentration of tyrosine in the brain to cause a long-term depletion of 5-HT via the nonenzymatic, tyrosine hydroxylase-independent, hydroxylation of tyrosine to DOPA and subsequently to DA via AADC. Overall, the findings suggest that MDMA depletes 5-HT by increasing tyrosine and its eventual conversion to DA within 5-HT terminals.

Brain hyperthermia as physiological and pathological phenomena

Although brain metabolism consumes high amounts of energy and is accompanied by intense heat production, brain temperature is usually considered a stable, tightly "regulated" homeostatic parameter. Current research, however, revealed relatively large and rapid brain temperature fluctuations (3-4 degrees C) in animals during various normal, physiological, and behavioral activities at stable ambient temperatures. This review discusses these data and demonstrates that physiological brain hyperthermia has an intra-brain origin, resulting from enhanced neural metabolism and increased intra-brain heat production. Therefore, brain temperature is an important physiological parameter that both reflects alterations in metabolic neural activity and affects various neural functions. This work also shows that brain hyperthermia may be induced by various drugs of abuse that cause metabolic brain activation and impair heat dissipation. While individual drugs (i.e., heroin, cocaine, methamphetamine, MDMA) have specific, dose-dependent effects on brain and body temperatures, these effects are strongly modulated by an individual's activity state and environmental conditions, and change dramatically during the development of drug self-administration. Thus, brain thermorecording may provide new information on the central effects of various addictive drugs, drug-activity-environment interactions in mediating drugs' adverse effects, and alterations in metabolic neural activity associated with the development of drug-seeking and drug-taking behavior. While ambient temperatures and impairment of heat dissipation may also affect brain temperature, these environmental conditions strongly potentiate thermal effects of psychomotor stimulant drugs, resulting in pathological brain overheating. Since hyperthermia exacerbates drug-induced toxicity and is destructive to neural cells and brain functions, use of these drugs under activated conditions that restrict heat loss may pose a significant health risk, resulting in both acute life-threatening complications and chronic destructive CNS changes.

Brain hyperthermia induced by MDMA (ecstasy): modulation by environmental conditions

Drugs of abuse, such as 3,4-methylenedioxymethamphetamine (MDMA), often have more powerful effects during states of increased activation and under specific environmental conditions. Because hyperthermia is a major complication of MDMA use and a factor potentiating neurotoxicity, we examined the effects of this drug (9 mg/kg, sc; approximately one-fifth of the known LD(50) in rats) on brain [nucleus accumbens (Nacc) and hippocampus (Hippo)] and muscle (musculus temporalis) temperatures in male rats under conditions that either model human drug use (social interaction with female, warm temperature) or restrict heat dissipation from the brain (chronic occlusion of jugular veins). Under quiet resting conditions at 23 degrees C, MDMA induced a moderate but prolonged hyperthermia. Both NAcc and Hippo showed more rapid and stronger temperature increases than muscle, suggesting metabolic neural activation as a primary cause of brain hyperthermia. During social interaction with a female, brain hyperthermia induced by MDMA was significantly potentiated (+89%). Brain hyperthermia induced by MDMA was also strongly potentiated (+188%) in animals with chronically occluded jugular veins, suggesting impaired cerebral outflow enhances intrabrain heat accumulation. At 29 degrees C, MDMA pushed temperatures in the brain to its biological limits (>41 degrees C; +268%), resulting in fatalities in most (83%) tested animals. Therefore, by inducing metabolic brain activation and restricting heat dissipation, MDMA use under 'party' conditions may be much more dangerous than under standard laboratory conditions.

Actions of amphetamine derivatives and cathinone at the noradrenaline transporter

We have recently shown that methylenedioxymethamphetamine (MDMA), methylenedioxyamphetamine (MDA), cathinone and methylenedioxyethylamphetamine (MDEA) have a cocaine-like action to potentiate the contractile actions of noradrenaline but not isoprenaline in the 1-Hz paced rat right ventricle. The purpose of this study was to directly test the actions of these compounds at the noradrenaline transporter. In rat left ventricular slices, potency (-log IC50) values at inhibiting uptake of [3H]noradrenaline were: cocaine 6.16+/-0.15, cathinone 6.03+/-0.16, MDMA 6.05+/-0.07, MDA 5.68+/-0.06 and MDEA 5.56+/-0.08. MDEA and MDA were significantly less potent. In rat cerebral cortex membranes, MDMA was significantly less potent at displacing [3H]nisoxetine binding; -log EC50 values: cocaine 5.04+/-0.08, cathinone 5.40+/-0.14, MDA 4.66+/-0.11, MDEA 4.99+/-0.15, MDMA 4.22+/-0.07. The noradrenaline uptake studies showed that MDEA was least potent: MDEA was also least potent functionally in the paced rat right ventricle. The [3H]nisoxetine displacement studies did not compare with the functional studies.

Effects of amphetamine derivatives and cathinone on noradrenaline-evoked contractions of rat right ventricle

We have compared the effects of methylenedioxymethamphetamine (MDMA), methylenedioxyethylamphetamine (MDEA), methylenedioxyamphetamine (MDA) and cathinone on contractions to noradrenaline and isoprenaline in 1 Hz paced rat right ventricular strips. Noradrenaline increased the force of contraction of 1 Hz paced ventricular strips with a pD(2) (-log EC(50)) of 5.64 +/- 0.07 (n = 49). Cocaine (10 microM), MDMA (10 microM), MDA (10 microM) and cathinone (3 and 10 microM) significantly increased the potency of noradrenaline to 6.25 +/- 0.11, 6.48 +/- 0.13, 6.17 +/- 0.05, 6.15 +/- 0.07 and 6.27 +/- 0.10, respectively (n = 5-10 each) as compared with the effects of vehicle (5.42 +/- 0.08, n = 15). However, MDEA (10 microM) failed to affect the potency of noradrenaline, although MDEA (100 microM) significantly increased noradrenaline potency (5.98 +/- 0.12). The potency of the agonist isoprenaline, which is not a substrate for the noradrenaline transporter, was not increased by cocaine, MDMA, cathinone, MDA or MDEA. Hence, MDMA, cathinone, MDA and MDEA share with cocaine an ability to potentiate the actions of noradrenaline, an action which may involve competitive blockade of the noradrenaline transporter rather than simply displacement of noradrenaline. Since cocaine is linked to an increased incidence of myocardial infarction, these results may have implications in terms of cardiac morbidity of amphetamine derivatives and cathinone.

Vascular actions of MDMA involve alpha1 and alpha2-adrenoceptors in the anaesthetized rat

We have investigated the effects of methylenedioxymethamphetamine (MDMA, 'ecstasy'), i.v., on diastolic blood pressure (DBP) in pithed and pentobarbitone anaesthetized rats. In pithed rats, the non-selective 5-HT receptor antagonist methiothepin (0.1 mg kg(-1)) and the alpha2-adrenoceptor antagonists methoxyidazoxan and yohimbine (1 mg kg(-1)) showed significant alpha1-adrenoceptor antagonist potency, but methiothepin did not show alpha2-adrenoceptor antagonist potency. MDMA (1 and 5 mg kg(-1)) produced pressor responses which were significantly reduced by the alpha(1)-adrenoceptor antagonist prazosin (0.1 mg kg(-1)), yohimbine (1 mg kg(-1)) or methiothepin (0.1 mg kg(-1)), but not by the 5-HT2 receptor antagonist ritanserin (1 mg kg(-1)). In anaesthetized rats, antagonists revealed two phases with three components to the effects of MDMA (5 mg kg(-1)) on DBP: an initial pressor response, a later pressor component at 1 min, the sustained depressor response. Methoxyidazoxan, methiothepin or the combination ritanserin/prazosin significantly reduced the initial pressor response, although neither of the latter compounds alone had any effect. The pressor response to MDMA (5 mg kg(-1)) at 1 min was converted to a depressor response by prazosin and to a lesser extent methiothepin and methoxyidazoxan. The depressor response to MDMA (5 mg kg(-1)) was significantly reduced by methoxyidazoxan (0.1 mg kg(-1)), and by the noradrenaline re-uptake blocker cocaine 10 mg kg(-1) but not 1 mg kg(-1). However, the most marked reduction in the depressor response was produced by the combination of methoxyidazoxan and cocaine. It is concluded that the initial pressor response to MDMA (5 mg kg(-1)) in anaesthetized rats involves alpha2- and possibly alpha1-adrenoceptors and 5-HT2 receptors, the pressor component at 1 min is largely alpha1-adrenoceptor mediated, and the sustained depressor response involves alpha2-adrenoceptors.

Effects of methylenedioxymethamphetamine on noradrenaline-evoked contractions of rat right ventricle and small mesenteric artery

We have compared the effects of methylenedioxymethamphetamine (MDMA) and cocaine on contractions to noradrenaline in 1 Hz paced rat right ventricular strips, and in rat small mesenteric artery and aorta. Noradrenaline increased the force of contraction of 1 Hz paced ventricular strips with a pD(2) (-log EC(50)) of 5.64+/-0.07. Both cocaine (10 microM) and MDMA (10 microM) significantly increased the potency of noradrenaline to 6.31+/-0.11 and 6.42+/-0.13, respectively. However, in the presence of cocaine (10 microM) which increased the potency of noradrenaline to 6.78+/-0.15, MDMA (10 microM) no longer increased the potency of noradrenaline (pD(2) of 6.78+/-0.32). Likewise, following chemical sympathectomy, MDMA failed to increase the potency of noradrenaline. The potency of the agonist isoprenaline, which is not a substrate for the noradrenaline transporter, was not increased by either cocaine or MDMA. In rat small mesenteric artery, but not aorta, MDMA and cocaine significantly increased the potency of noradrenaline, but in the presence of cocaine, MDMA had no further effect. Hence, MDMA shares with cocaine an ability to potentiate the actions of noradrenaline, an action in the case of MDMA which may involve competitive blockade of the noradrenaline transporter, rather than simply displacement of noradrenaline. Since cocaine is linked to an increased incidence of myocardial infarction, these results may have implications in terms of cardiac morbidity of MDMA.

7-nitroindazole reduces cerebral blood flow following chronic nitric oxide synthase inhibition

Blood flow and glucose utilization were measured in rat brain after chronic L-NAME treatment followed by acute 7-nitroindazole. Following chronic L-NAME, blood flow was not significantly different from control. Treatment with acute 7-nitroindazole reduced blood flow to the same extent in both chronic saline and L-NAME groups. Glucose utilization was unaffected. These results suggest that residual NOS activity in brain is sufficient to provide tonic, NO-dependent cerebrovascular dilator tone.

Investigation of the prejunctional alpha2-adrenoceptor mediated actions of MDMA in rat atrium and vas deferens

1. We have investigated the effects of methylenedioxymethamphetamine (MDMA, 'ecstasy') on peripheral noradrenergic neurotransmission in the rat. 2. In rat atrial slices pre-incubated with [3H]-noradrenaline and in the presence of desipramine (1 micronM) to prevent effects of MDMA on basal outflow of tritium, MDMA (10 micronM) significantly inhibited the release of tritium evoked by short trains of six pulses at 100 Hz every 10 s for 3 min. This effect did not occur in the presence of the alpha2-adrenoceptor antagonist yohimbine (1 micronM). 3. In epididymal portions of rat vas deferens in the presence of nifedipine (10 micronM), MDMA produced a concentration-dependent inhibition of single pulse nerve stimulation-evoked contractions with a pD2 of 5.88+/-0.16 (n=4). Inhibitory effects of MDMA were antagonized by the alpha2-adrenoceptor antagonist yohimbine (0.3 micronM), but not by the 5-hydroxytryptamine receptor antagonist cyanopindolol in a concentration (1 micronM) which markedly antagonized the inhibitory actions of the 5-HT-1 receptor agonist 5-carboxamidotryptamine. 4. In prostatic portions of rat vas deferens in the presence of cocaine (3 micronM), MDMA produced a concentration-dependent inhibition of single pulse nerve stimulation-evoked contractions with a pD2 of 5. 12+/-0.21 (n=4). In the absence of cocaine, only the highest concentration of MDMA (30 micronM) produced an inhibition, but the alpha2-adrenoceptor antagonist yohimbine (0.3 micronM) converted the response to MDMA from inhibition to potentiation of the stimulation-evoked contraction. 5. In radioligand binding studies, MDMA showed similar affinities for alpha2B, alpha2C and alpha2D-adrenoceptor sites, with pKi values of 5.14+/-0.16, 5.11+/-0. 05 and 5.31+/-0.14, respectively. 6 It is concluded that MDMA has significant alpha2-adrenoceptor agonist actions.

Effect of chronic inhibition of nitric oxide synthase on ocular blood flow and glucose metabolism in the rat

AIMS

To investigate the effects of chronic administration of nitric oxide synthase inhibition on ocular blood flow and metabolic demand in the rat and to compare these effects with changes in the cerebral and peripheral circulation.

METHODS

Male Sprague-Dawley rats were injected with the nitric oxide synthase inhibitor L-NAME (75 mg/kg i.p.), either on a single occasion only or once daily for 10 consecutive days. Controls were injected with saline. Regional blood flow and glucose metabolism were measured from tissue samples, using [14C]-iodoantipyrine and [14C]-2-deoxyglucose respectively, 1 hour after either acute L-NAME injection or 1 hour after the last injection of the chronic treatment protocol.

RESULTS

Mean arterial pressure was significantly increased (+31%) following the acute injection (indicating peripheral vasoconstriction) and this effect was enhanced (+50%) following chronic treatment. In both the ocular and cerebral circulation, blood flow was decreased following acute treatment (-48% and -43% respectively). However, while this response was totally attenuated in the cerebral circulation following chronic L-NAME treatment (-4%), the ocular circulation remained responsive (-57%). Metabolic demand in brain and eye tissue, as reflected in the accumulation of 2-deoxyglucose, was unaffected by either acute or chronic treatment with L-NAME.

CONCLUSION

Homeostatic mechanisms appear to be activated in the cerebral circulation which re-establish flow metabolism homeostasis, and the effect of L-NAME on cerebral blood flow is attenuated following repeated exposure. This process does not seem to happen in the ocular circulation and, thus, the ocular vasculature appears to behave more like those blood vessels which determine total peripheral resistance than the cerebral circulation. It remains to be seen whether the sustained decrease in blood flow in the eye is sufficient to compromise ocular function and render the eye susceptible to damage from chronic L-NAME induced oligaemia.