Srihari R. Tella

Powerful Cocaine-Like Actions of 3,4-Methylenedioxypyrovalerone (MDPV), a Principal Constituent of Psychoactive ‘Bath Salts’ Products

The abuse of psychoactive ‘bath salts’ containing cathinones such as 3,4-methylenedioxypyrovalerone (MDPV) is a growing public health concern, yet little is known about their pharmacology. Here, we evaluated the effects of MDPV and related drugs using molecular, cellular, and whole-animal methods. In vitro transporter assays were performed in rat brain synaptosomes and in cells expressing human transporters, while clearance of endogenous dopamine was measured by fast-scan cyclic voltammetry in mouse striatal slices. Assessments of in vivo neurochemistry, locomotor activity, and cardiovascular parameters were carried out in rats. We found that MDPV blocks uptake of [3H]dopamine (IC50=4.1 nM) and [3H]norepinephrine (IC50=26 nM) with high potency but has weak effects on uptake of [3H]serotonin (IC50=3349 nM). In contrast to other psychoactive cathinones (eg, mephedrone), MDPV is not a transporter substrate. The clearance of endogenous dopamine is inhibited by MDPV and cocaine in a similar manner, but MDPV displays greater potency and efficacy. Consistent with in vitro findings, MDPV (0.1–0.3 mg/kg, intravenous) increases extracellular concentrations of dopamine in the nucleus accumbens. Additionally, MDPV (0.1–3.0 mg/kg, subcutaneous) is at least 10 times more potent than cocaine at producing locomotor activation, tachycardia, and hypertension in rats. Our data show that MDPV is a monoamine transporter blocker with increased potency and selectivity for catecholamines when compared with cocaine. The robust stimulation of dopamine transmission by MDPV predicts serious potential for abuse and may provide a mechanism to explain the adverse effects observed in humans taking high doses of ‘bath salts’ preparations.

Effects of monoamine reuptake inhibitors on cocaine self-administration in rats.

The objective of this study was to investigate the effects of acute administration of monoamine reuptake inhibitors on cocaine self-administration in rats. Pretreatment with GBR 12909 (1-5.6 mg/kg, IV), a dopamine-selective reuptake inhibitor, produced a dose-dependent and large reduction in the self-administration of cocaine (1 mg/kg/infusion). The 3- and 5.6-mg/kg doses of GBR 12909 produced downward shifts in the dose-response curves for cocaine (0.3-3 mg/kg/infusion) self-administration. Unlike GBR 12909, the norepinephrine-selective reuptake inhibitors, desipramine and nisoxetine, at a 10-mg/kg dose produced small, but significant, reductions in the self-administration of cocaine (1 mg/kg/infusion). The 10-mg/kg dose of fluoxetine, a serotonin-selective reuptake inhibitor, produced a small, but not significant, reduction in the self-administration of cocaine. The 10-mg/kg dose of desipramine, nisoxetine, or fluoxetine produced brief respiratory distress and motor abnormalities immediately following IV injections, thereby suggesting that this dose is close to the toxic range for all three drugs. Desipramine, nisoxetine, or fluoxetine at nontoxic doses of 1 and 3 mg/kg had no significant effects on cocaine self-administration. These data indicate that the acute enhancement of endogenous dopaminergic activity by pretreatment with dopamine reuptake inhibitor reduces the total intake of cocaine, thus supporting the hypothesis that the dopamine is critically involved in the reinforcing properties of cocaine. The data also suggest that the acute enhancements in the endogenous norepinephrine or serotonin systems by nontoxic doses of norepinephrine- or serotonin-selective reuptake inhibitors do not appear to alter the reinforcing properties of cocaine.

Differential blockade of chronic versus acute effects of intravenous cocaine by dopamine receptor antagonists

The objectives of this study were to investigate behavioral sensitization to repeated once daily IV injections of cocaine, and to determine whether dopamine receptor antagonists differentially block chronic versus acute cocaine effects. Acute cocaine (0.3-3.0 mg/kg) produced a dose-dependent increase in both horizontal and stereotypic movements in male Sprague-Dawley rats. Repeated once daily injections of 1 or 3 mg/kg of cocaine augmented these effects. Pretreatment with either the D2 dopamine receptor antagonist haloperidol (0.03-0.3 mg/kg) or the D1 dopamine receptor antagonist R(+)-SCH-23390 (0.003-0.1 mg/kg) dose dependently attenuated cocaines behavioral effects in both sensitized and cocaine-naive animals. There was a rightward shift in the dose-effect relationship of these antagonists in blocking the expression of behavioral sensitization as compared to their ability to block the acute behavioral effects of cocaine. These results indicate that repeated once daily IV injections of cocaine produced behavioral sensitization and both D1 and D2 dopamine receptor antagonists attenuated the expression of this sensitization. The data also suggest that dopamine receptor antagonists were more potent in blocking cocaines effects in cocaine-naive animals than in cocaine-sensitized animals.

Pharmacological mechanisms in cocaine's cardiovascular effects

The squirrel monkey is a reliable model for the cardiovascular effects of cocaine in that it mimics the human response to cocaine; low to moderate doses of cocaine produce a sustained pressor effect and tachycardia. Pretreatment experiments have indicated the importance of alpha-1 and beta-1 adrenoceptor mechanisms in mediating the pressor and tachycardiac effects of cocaine, respectively. Little support for a role of dopaminergic mechanisms in the hemodynamic effects of cocaine has been found. Toxicity to cocaine is often observed hours after its administration, pointing to a potential role of the cocaine metabolites. Studies on the direct effects of a variety of cocaine metabolites indicate that their cardiovascular effects do not necessarily mimic those produced by cocaine, and therefore these differing effects of the metabolites should be considered when evaluating the cardiovascular toxicity of cocaine. Further, as these metabolites are present in the body for long periods of time, these results suggest a role of the metabolites in producing toxicity long after cocaine administration. Finally, studies using both dopaminergic and calcium channel antagonists indicate that the pharmacological mechanisms involved in the cardiovascular effects of cocaine are not the same as those involved in its behavioral effects.

Pharmacological mechanisms in the cardiovascular effects of methamphetamine in conscious squirrel monkeys.

The effects of methamphetamine were studied on cardiovascular function in conscious squirrel monkeys. Methamphetamine (0.1-3.0 mg/kg, IV) produced a dose-dependent increase in blood pressure. Its effects on heart rate were more complex, with lower doses (0.1-0.3 mg/kg) producing increases in heart rate and higher doses (1.0-3.0 mg/kg) producing decreases. To determine the pharmacological mechanisms involved in methamphetamines effects, a number of drugs were tested as pretreatments to an injection of 0.2 mg/kg methamphetamine. This dose produced the maximal heart rate increase. The alpha 1-antagonist prazosin completely antagonized the effects of methamphetamine on blood pressure, while the nonselective beta-antagonist propranolol and beta 1-selective antagonist atenolol completely antagonized the tachycardiac effect of methamphetamine. The dopaminergic antagonists SCH 23390 and haloperidol antagonized some of the cardiovascular effects of methamphetamine. These results indicate that the pressor and tachycardiac effects of methamphetamine are mediated via alpha 1- and beta 1-adrenoceptor mechanisms, respectively. Dopaminergic mechanisms are also involved in methamphetamines cardiovascular effects.

Adrenoceptor mechanisms in the cardiovascular effects of cocaine in conscious squirrel monkeys

The purpose of the current experiment was to study the role of various adrenoceptor subtypes in the cardiovascular response to cocaine in conscious squirrel monkeys. A variety of adrenoceptor antagonists were administered i.v. prior to the administration of 0.3 mg/kg cocaine (i.v.). Cocaine alone produced an increase in both blood pressure and heart rate. The non-selective alpha adrenoceptor antagonist phentolamine produced a dose-dependent antagonism of the pressor effect of cocaine, as did the alpha-1 selective antagonist prazosin. The alpha-2 selective antagonist yohimbine had no effect on the pressor effect of cocaine. The non-selective beta antagonist propranolol enhanced the pressor effect of cocaine as did the beta-1 selective antagonist atenolol. However, the effect of atenolol was not dose-dependent. The beta-2 selective antagonist ICI 118,551 and labetalol, which blocks both alpha and beta adrenoceptors, did not alter the pressor effect of cocaine. Propranolol, atenolol, and labetalol all antagonized the tachycardiac effect of cocaine in a dose-dependent manner, while the beta-2 antagonist ICI 118,551 did not. Phentolamine, prazosin and yohimbine also reduced the tachycardiac effect of cocaine, although these effects were dose-dependent only for yohimbine, which also significantly elevated baseline heart rate. These results indicate that alpha-1 adrenoceptor mechanisms mediate the pressor effect of cocaine, while beta-1 adrenoceptor mechanisms are involved in the tachycardiac effect of cocaine in squirrel monkeys. Propranolol potentiated cocaines pressor effect through beta-2 independent mechanisms. Thus, neither alpha-2 nor beta-2 adrenoceptor mechanisms appear to be involved in cocaines cardiovascular effects.

Cardiovascular responses to cocaine self-administration: acute and chronic tolerance.

The nature and the mechanism of tolerance to the cardiovascular responses to cocaine self-administration were studied in rats implanted with telemetric devices. The first infusion of cocaine (1 mg/kg/infusion) on day 1 of testing produced rapid and brief increases in mean arterial blood pressure and in heart rate. Subsequent infusions in the same session produced minimal effects. With chronic testing, there were gradual reductions in cardiovascular responses to the first infusion in the daily session and enhancements in the daily cocaine intake, with significant changes occurring by the fourth week of the testing. Following saline extinction testing (for 5 days), reinstatement of cocaine during week 6 led to a partial and short lasting (</=3 sessions) recovery from the chronic tolerance to the rapid cardiovascular responses to cocaine. There were significant enhancements in cardiovascular responses to post-session norepinephrine during week 2 and marked reductions during week 6 as compared to corresponding control responses. There were marked reductions in the cardiovascular responses to post-session tyramine tested during week 3. These data indicate that self-administered cocaine produces rapid and brief cardiovascular responses which undergo both within-session acute tolerance and a between-session, reversible chronic tolerance. Adrenergic adaptive mechanisms mediate the chronic tolerance. The development of chronic cardiovascular tolerance to cocaine temporally parallels that of the apparent tolerance to its reinforcing effects. Doses of cocaine that maintain self-administration behavior inhibit the norepinephrine transporter at peripheral sympathetic nerve terminals.

Monoamine Transporter and Sodium Channel Mechanisms in the Rapid Pressor Response to Cocaine

Intravenous (I.V.) cocaine (0.03-3 mg/kg) produced dose-dependent, rapid, and brief increases in blood pressure (BP) in conscious rats pretreated with the dopamine receptor antagonist, SCH 23390. Monoamine uptake inhibitors structurally analogous to cocaine (cocaethylene, CFT, betaCIT, CPT, (+)-cocaine, norcocaine, and benztropine) also produced this rapid pressor response, whereas structurally unrelated uptake inhibitors with diverse monoamine transporter selectivities (BTCP, indatraline, GBR 12935, mazindol, nomifensine, and zimeldine) either did not produce a rapid pressor response or produced only a small pressor response. At nonconvulsant doses, the sodium channel blockers acetylprocainamide, dibucaine, dyclonine, prilocaine, proparacaine, quinidine, and tetracaine produced a small pressor response or no increase in BP. In rats implanted with telemetric devices, cocaine and its analog, CFT, produced a biphasic pharmacological response that consisted of an initial brief and abrupt behavioral arousal associated with a rapid, large increase in BP followed by prolonged, parallel increases in BP and locomotor activity. Pretreatment with SCH 23390 prevented the prolonged but not the initial rapid and brief pressor and activity responses to both cocaine and CFT administration. The present data suggest that the inhibition of dopamine, norepinephrine, or serotonin transporter functions, either alone or in combination, does not mediate the rapid pressor response to cocaine. The sodium channel-blocking action of cocaine per se does not appear to be involved in the rapid pressor response to cocaine. Finally, the present results confirm previous findings that dopaminergic mechanisms mediate the prolonged increases in BP and locomotor activity produced by cocaine.

Effects of cocaine and its quaternary derivative cocaine methiodide on cardiovascular function in squirrel monkeys

The effects of cocaine and its quaternary derivative cocaine methiodide, which does not cross the blood-brain barrier, were studied on cardiovascular function in squirrel monkeys. In conscious monkeys, cocaine produced clear dose-dependent increases in blood pressure and heart rate, while cocaine methiodide did not. Both cocaine and cocaine methiodide enhanced the effects of norepinephrine in anesthetized animals, suggesting that both inhibit neuronal uptake of norepinephrine; cocaine was approximately 30 times more potent than cocaine methiodide. In anesthetized monkeys both cocaine and cocaine methiodide produced small, short duration pressor effects, although cocaine was at least 10 times more potent than cocaine methiodide. Cocaines effects in anesthetized animals were clearly blunted in comparison to its effects in conscious animals. These effects of cocaine on blood pressure occurred at doses lower than those required to enhance norepinephrines effects, indicating that the norepinephrine uptake blocking effects of the drugs cannot fully account for their cardiovascular effects. The greatly enhanced effect of cocaine in conscious animals and the finding that cocaine methiodide had little effect in conscious animals indicates that central mechanisms are involved in the effects of cocaine on cardiovascular function in conscious animals.

Effects of cocaine alone and in combination with haloperidol and SCH 23390 on cardiovascular function in squirrel monkeys

The potential involvement of D1 and D2 dopamine receptors in the effects of cocaine on cardiovascular function in squirrel monkeys was evaluated. A low dose of cocaine (0.1 mg/kg i.v.) produced increases in both blood pressure and heart rate. At the higher doses of cocaine (1.0-3.0 mg/kg) the heart rate response was biphasic, consisting of an early decrease followed by an increase in heart rate 10-20 min following injection. The dopamine D2 antagonist haloperidol (0.1 mg/kg i.m.) attenuated the heart rate increasing effect of cocaine, but doses as high as 0.03 mg/kg did not alter the blood pressure increase. The D1 antagonist SCH 23390 (0.01-0.03 mg/kg i.m.) did not attenuate either the blood pressure or heart rate increasing effects of cocaine. The D2 agonist quinpirole (1.0 mg/kg i.v.) produced increases in heart rate similar to cocaine, with little effect on blood pressure. Although effective against the heart rate increasing effect of cocaine, haloperidol (0.01 mg/kg) did not antagonize the heart rate increasing effects of quinpirole. The D1 agonist SKF 38393 (3.0 mg/kg i.v.) decreased heart rate and increased blood pressure. The blood pressure increasing effect of SKF 38393 was antagonized by 0.01 mg/kg SCH 23390. Haloperidols ability to partially antagonize the tachycardiac response to cocaine suggests the involvement of D2 receptors in that response. However, the failure of haloperidol to antagonize quinpiroles tachycardiac effect suggests that non-dopaminergic mechanisms may also be involved in haloperidols antagonism of cocaines tachycardiac effect. The pressor effects of cocaine do not appear to be controlled by selective dopamine receptors.