Lisa M. McFadden

4-Methylmethcathinone (Mephedrone): Neuropharmacological Effects of a Designer Stimulant of Abuse

The designer stimulant 4-methylmethcathinone (mephedrone) is among the most popular of the derivatives of the naturally occurring psychostimulant cathinone. Mephedrone has been readily available for legal purchase both online and in some stores and has been promoted by aggressive Web-based marketing. Its abuse in many countries, including the United States, is a serious public health concern. Owing largely to its recent emergence, there are no formal pharmacodynamic or pharmacokinetic studies of mephedrone. Accordingly, the purpose of this study was to evaluate effects of this agent in a rat model. Results revealed that, similar to methylenedioxymethamphetamine, methamphetamine, and methcathinone, repeated mephedrone injections (4× 10 or 25 mg/kg s.c. per injection, 2-h intervals, administered in a pattern used frequently to mimic psychostimulant “binge” treatment) cause a rapid decrease in striatal dopamine (DA) and hippocampal serotonin (5-hydroxytryptamine; 5HT) transporter function. Mephedrone also inhibited both synaptosomal DA and 5HT uptake. Like methylenedioxymethamphetamine, but unlike methamphetamine or methcathinone, repeated mephedrone administrations also caused persistent serotonergic, but not dopaminergic, deficits. However, mephedrone caused DA release from a striatal suspension approaching that of methamphetamine and was self-administered by rodents. A method was developed to assess mephedrone concentrations in rat brain and plasma, and mephedrone levels were determined 1 h after a binge treatment. These data demonstrate that mephedrone has a unique pharmacological profile with both abuse liability and neurotoxic potential.

Methamphetamine Self-Administration Causes Persistent Striatal Dopaminergic Alterations and Mitigates the Deficits Caused by a Subsequent Methamphetamine Exposure

Preclinical studies have demonstrated that repeated methamphetamine (METH) injections (referred to herein as a “binge” treatment) cause persistent dopaminergic deficits. A few studies have also examined the persistent neurochemical impact of METH self-administration in rats, but with variable results. These latter studies are important because: 1) they have relevance to the study of METH abuse; and 2) the effects of noncontingent METH treatment do not necessarily predict effects of contingent exposure. Accordingly, the present study investigated the impact of METH self-administration on dopaminergic neuronal function. Results revealed that self-administration of METH, given according to a regimen that produces brain METH levels comparable with those reported postmortem in human METH abusers (0.06 mg/infusion; 8-h sessions for 7 days), decreased striatal dopamine transporter (DAT) uptake and/or immunoreactivity as assessed 8 or 30 days after the last self-administration session. Increasing the METH dose per infusion did not exacerbate these deficits. These deficits were similar in magnitude to decreases in DAT densities reported in imaging studies of abstinent METH abusers. It is noteworthy that METH self-administration mitigated the persistent deficits in dopaminergic neuronal function, as well as the increases in glial fibrillary acidic protein immunoreactivity, caused by a subsequent binge METH exposure. This protection was independent of alterations in METH pharmacokinetics, but may have been attributable (at least in part) to a pretreatment-induced attenuation of binge-induced hyperthermia. Taken together, these results may provide insight into the neurochemical deficits reported in human METH abusers.

Regulation of the Dopamine and Vesicular Monoamine Transporters: Pharmacological Targets and Implications for Disease

Dopamine (DA) plays a well recognized role in a variety of physiologic functions such as movement, cognition, mood, and reward. Consequently, many human disorders are due, in part, to dysfunctional dopaminergic systems, including Parkinson’s disease, attention deficit hyperactivity disorder, and substance abuse. Drugs that modify the DA system are clinically effective in treating symptoms of these diseases or are involved in their manifestation, implicating DA in their etiology. DA signaling and distribution are primarily modulated by the DA transporter (DAT) and by vesicular monoamine transporter (VMAT)-2, which transport DA into presynaptic terminals and synaptic vesicles, respectively. These transporters are regulated by complex processes such as phosphorylation, protein–protein interactions, and changes in intracellular localization. This review provides an overview of 1) the current understanding of DAT and VMAT2 neurobiology, including discussion of studies ranging from those conducted in vitro to those involving human subjects; 2) the role of these transporters in disease and how these transporters are affected by disease; and 3) and how selected drugs alter the function and expression of these transporters. Understanding the regulatory processes and the pathologic consequences of DAT and VMAT2 dysfunction underlies the evolution of therapeutic development for the treatment of DA-related disorders.

Effect of Methamphetamine Self-Administration on Neurotensin Systems of the Basal Ganglia

Methamphetamine (METH) dependence causes alarming personal and social damage. Even though many of the problems associated with abuse of METH are related to its profound actions on dopamine (DA) basal ganglia systems, there currently are no approved medications to treat METH addiction. For this reason, we and others have examined the METH-induced responses of neurotensin (NT) systems in the basal ganglia. This neuropeptide is associated with inhibitory feedback pathways to nigrostriatal DA projections, and NT tissue levels are elevated in response to high doses of noncontingent METH because of its increased synthesis in the striatonigral pathway. The present study reports the contingent responses of NT in the basal ganglia to self-administration of METH (SAM). Intravenous infusions of METH linked to appropriate lever-pressing behavior by rats significantly elevated NT content in both dorsal striatum (210%) and substantia nigra (202%). In these same structures, NT levels were also elevated in yoked METH animals (160 and 146%, respectively) but not as much as in the SAM rats. These effects were blocked by a D1, but not D2, antagonist. A NT agonist administered before the day 5 of operant behavior blocked lever-pressing behavior in responding rats, but a NT antagonist had no significant effect on this behavior. These are the first reports that NT systems associated with striatonigral pathway are significantly altered during METH self-administration, and our findings suggest that activation of NT receptors during maintenance of operant responding reduces the associated lever-pressing behavior.

Sex-dependent effects of chronic unpredictable stress in the water maze

Exposure to chronic predictable stress, such as restraint, can affect performance on spatial memory tasks and these effects have been shown to be sex-specific in rats. It is not known whether unpredictable stress has similar sex-specific effects on spatial memory and whether those effects are present after the stress procedure has ended. Therefore, the current study tested male and female rats in the Morris water maze either immediately or 3 weeks following exposure to 10 days of unpredictable stress (CUS). Male and female rats were exposed to 10 days of stressors that varied by type and time of stressor application. Exposure to CUS decreased the distance swam to locate the hidden platform during acquisition training in the water maze for female but not male rats. Overall, male rats performed better than female rats during the acquisition, probe and matching to place trials. These effects were observed when assessing spatial memory performance immediately or 3 weeks following the last stressor. Plasma corticosterone levels followed the behavioral differences during the acquisition trials in that control female rats had increased basal and swim-stimulated corticosterone levels compared to CUS female rats and control male rats. These data demonstrate that unpredictable stress influences performance on the water maze in a sex-specific manner, which parallel plasma corticosterone levels. The improved performance of female rats following CUS exposure was present 3 weeks after the termination of the stress procedures, suggesting that stress may have lasting effects on underlying neural systems.

Response of Limbic Neurotensin Systems to Methamphetamine Self-Administration

Methamphetamine (METH) abuse is personally and socially devastating. Although effects of METH on dopamine (DA) systems likely contribute to its highly addictive nature, no medications are approved to treat METH dependence. Thus, we and others have studied the METH-induced responses of neurotensin (NT) systems. NT is associated with inhibitory feedback action on DA projections, and NT levels are elevated in both the nucleus accumbens and dorsal striatum after noncontingent treatment with high doses of METH. In the present study, we used a METH self-administration (SA) model (linked to lever pressing) to demonstrate that substitution of an NT agonist for METH, while not significantly affecting motor activity, dramatically reduced lever pressing but was not self-administered per se. We also found that nucleus accumbens NT levels were elevated via a D1 mechanism after five sessions in rats self-administering METH (SAM), with a lesser effect in corresponding yoked rats. Extended (15 daily sessions) exposure to METH SA manifested similar NT responses; however, more detailed analyses revealed (i) 15 days of METH SA significantly elevated NT levels in the nucleus accumbens shell and dorsal striatum, but not the nucleus accumbens core, with a lesser effect in the corresponding yoked METH rats; (ii) the elevation of NT in both the nucleus accumbens shell and dorsal striatum significantly correlated with the total amount of METH received in the self-administering, but not the corresponding yoked METH rats; and (iii) an NT agonist blocked, but an NT antagonist did not alter, lever-pressing behavior on day 15 in SAM rats. After 5 days in SAM animals, NT levels were also elevated in the ventral tegmental area, but not frontal cortex of rats self-administering METH.

Response of Neurotensin Basal Ganglia Systems During Extinction of Methamphetamine Self Administration in Rat

Because of persistent social problems caused by methamphetamine (METH), new therapeutic strategies need to be developed. Thus, we investigated the response of central nervous system neurotensin (NT) systems to METH self-administration (SA) and their interaction with basal ganglia dopamine (DA) pathways. Neurotensin is a peptide associated with inhibitory feedback pathways to nigrostriatal DA projections. We observed that NT levels decreased in rats during extinction of METH SA when lever pressing resulted in intravenous infusions of saline rather than METH. Thus, 6 h after the first session of extinction, NT levels were 53, 42, and 49% of corresponding controls in the anterior dorsal striatum, posterior dorsal striatum, and globus pallidus, respectively. NT levels were also significantly reduced in corresponding yoked rats in the anterior dorsal striatum (64% of control), but not the other structures examined. The reductions in NT levels in the anterior dorsal striatum particularly correlated with the lever pressing during the first session of extinction (r =s; 0.745). These, and previously reported findings, suggest that the extinction-related reductions in NT levels were mediated by activation of D2 receptors. Finally, administration of the neurotensin receptor 1 (NTR1) agonist [PD149163 [Lys(CH2NH)Lys-Pro,Trp-tert-Leu-Leu-Oet]; 0.25 or 0.5 mg/kg] diminished lever pressing during the first extinction session, whereas the NTR1 antagonist [SR48692 [2-[(1-(7-chloro-4-quinolinyl)-5-(2,6-imethoxyphenyl)pyrazol-3-yl)carbonylamino]tricyclo(3.3.1.1.(3.7))decan-2-carboxylic acid]; 0.3 mg/kg per administration] attenuated the reduction of lever pressing during the second to fourth days of extinction. In summary, these findings support the hypothesis that some of the endogenous basal ganglia NT systems contribute to the elimination of contingent behavior during the early stages of the METH SA extinction process.

The effects of methamphetamine exposure during preadolescence on male and female rats in the water maze.

Exposure to methamphetamine early in life can have lasting effects on cognitive processes. The maturation of neurotransmitter systems targeted by methamphetamine differs by gender during childhood and preadolescence, which could lead to differential long-term effects of early drug exposure. Therefore, the current study assessed whether preadolescent exposure to methamphetamine has gender specific long-term effects on adult spatial memory in rodents. Male and female rats were given 1 daily injection of 0 or 2mg/kg methamphetamine or not handled from PD21-35 and then tested as adults (PD95) in the Morris water maze. In general, male rats performed better than female rats in the water maze task regardless of treatment group. Female rats exposed to methamphetamine from PD21-35 had shorter latencies and took more direct paths to the hidden platform compared to control females during the 4 days of acquisition training and when the hidden platform was moved each day on matching to place trials. Male rats exposed to methamphetamine swam a shorter distance to the hidden platform on the first day of acquisition training, similar to the methamphetamine exposed females. However, the methamphetamine exposed males performed more poorly compared to control males in the matching to place trials. Overall, the current study found that methamphetamine exposure during preadolescence has long-term effects on spatial memory in a gender specific manner. These findings may contribute to our general understanding of the long-term effects of psychostimulant exposure at early developmental stages.

Methamphetamine treatment during development attenuates the dopaminergic deficits caused by subsequent high-dose methamphetamine administration

Administration of high doses of methamphetamine (METH) causes persistent dopaminergic deficits in both nonhuman preclinical models and METH‐dependent persons. Noteworthy, adolescent [i.e., postnatal day (PND) 40] rats are less susceptible to this damage than young adult (PND90) rats. In addition, biweekly treatment with METH, beginning at PND40 and continuing throughout development, prevents the persistent dopaminergic deficits caused by a “challenge” high‐dose METH regimen when administered at PND90. Mechanisms underlying this “resistance” were thus investigated. Results revealed that biweekly METH treatment throughout development attenuated both the acute and persistent deficits in VMAT2 function, as well as the acute hyperthermia, caused by a challenge METH treatment. Pharmacokinetic alterations did not appear to contribute to the protection afforded by the biweekly treatment. Maintenance of METH‐induced hyperthermia abolished the protection against both the acute and persistent VMAT2‐associated deficits suggesting that alterations in thermoregulation were caused by exposure of rats to METH duringdevelopment. These findings suggest METH during development prevents METH‐induced hyperthermia and the consequent METH‐related neurotoxicity. Synapse 2011.

Methamphetamine self-administration acutely decreases monoaminergic transporter function.

Numerous preclinical studies have demonstrated that noncontingent methamphetamine (METH) administration rapidly decreases both dopamine (DA) transporter (DAT) and vesicular monoamine‐2 transporter (VMAT‐2) function. Because of the importance of transporter function to the abuse and neurotoxic liabilities of METH, and previous research indicating that the effects of noncontingent METH treatment do not necessarily predict effects of contingent exposure, the present study examined the acute impact of METH self‐administration on these transporters. Results revealed that five days of METH self‐administration (4 h/session; 0.06 mg/infusion) decreased DAT and VMAT‐2 activity, as assessed in synaptosomes and vesicles, respectively, prepared from striatal tissue 1 h after the final self‐administration session. METH self‐administration increased core body temperatures as well. Brain METH and amphetamine (AMPH) levels, assessed 1 h after the final self‐administration session, were approximately twice greater in high‐pressing rats compared to low‐pressing rats despite similar changes in DAT function. In conclusion, the present manuscript is the first to describe transporter function and METH/AMPH levels after self‐administration in rodents. These data provide a foundation to investigate complex questions including how the response of dopaminergic systems to METH self‐administration contributes to contingent‐related processes such as dependence. Synapse, 2012.