Protein kinase C and dopamine transport—1. Effects of amphetamine in vivo

Post-translational mechanisms in psychostimulant-induced neurotransmitter efflux

The availability of monoamine neurotransmitters in the brain is under the control of dopamine, norepinephrine, and serotonin transporters expressed on the plasma membrane of monoaminergic neurons. By regulating transmitter levels these proteins mediate crucial functions including cognition, attention, and reward, and dysregulation of their activity is linked to mood and psychiatric disorders of these systems. Amphetamine-based transporter substrates stimulate non-exocytotic transmitter efflux that induces psychomotor stimulation, addiction, altered mood, hallucinations, and psychosis, thus constituting a major component of drug neurochemical and behavioral outcomes. Efflux is under the control of transporter post-translational modifications that synergize with other regulatory events, and this review will summarize our knowledge of these processes and their role in drug mechanisms.

Cholesterol modulation of interactions between psychostimulants and dopamine transporters

The dopamine transporter (DAT) is a key site of action for cocaine and amphetamines. Dysfunctional DAT is associated with aberrant synaptic dopamine transmission and enhanced drug-seeking and taking behavior. Studies in cultured cells and ex vivo suggest that DAT function is sensitive to membrane cholesterol content. Although it is largely unknown whether psychostimulants alter cholesterol metabolism in the brain, emerging evidence indicates that peripheral cholesterol metabolism is altered in patients with psychostimulant use disorder and circulating cholesterol levels are associated with vulnerability to relapse. Cholesterol interacts with sphingolipids forming lipid raft microdomains on the membrane. These cholesterol-rich lipid raft microdomains serve to recruit and assemble other lipids and proteins to initiate signal transduction. There are two spatially and functionally distinct populations of the DAT segregated by cholesterol-rich lipid raft microdomains and cholesterol-scarce non-raft microdomains on the plasma membrane. These two DAT populations are differentially regulated by DAT blockers (e.g. cocaine), substrates (e.g. amphetamine), and protein kinase C providing distinct cholesterol-dependent modulation of dopamine uptake and efflux. In this chapter, we summarize the impact of depletion and addition of membrane cholesterol on DAT conformational changes between the outward-facing and the inward-facing states, lipid raft-associated DAT localization, basal and induced DAT internalization, and DAT function. In particular, we focus on how the interactions of the DAT with cocaine and amphetamine are influenced by membrane cholesterol. Lastly, we discuss the therapeutic potential of cholesterol-modifying drugs as a new avenue to normalize DAT function and dopamine transmission in patients with psychostimulant use disorder.

Overview of the structure and function of the dopamine transporter and its protein interactions

The dopamine transporter (DAT) plays an integral role in dopamine neurotransmission through the clearance of dopamine from the extracellular space. Dysregulation of DAT is central to the pathophysiology of numerous neuropsychiatric disorders and as such is an attractive therapeutic target. DAT belongs to the solute carrier family 6 (SLC6) class of Na⁺/Cl^- dependent transporters that move various cargo into neurons against their concentration gradient. This review focuses on DAT (SCL6A3 protein) while extending the narrative to the closely related transporters for serotonin and norepinephrine where needed for comparison or functional relevance. Cloning and site-directed mutagenesis experiments provided early structural knowledge of DAT but our contemporary understanding was achieved through a combination of crystallization of the related bacterial transporter LeuT, homology modeling, and subsequently the crystallization of drosophila DAT. These seminal findings enabled a better understanding of the conformational states involved in the transport of substrate, subsequently aiding state-specific drug design. Post-translational modifications to DAT such as phosphorylation, palmitoylation, ubiquitination also influence the plasma membrane localization and kinetics. Substrates and drugs can interact with multiple sites within DAT including the primary S1 and S2 sites involved in dopamine binding and novel allosteric sites. Major research has centered around the question what determines the substrate and inhibitor selectivity of DAT in comparison to serotonin and norepinephrine transporters. DAT has been implicated in many neurological disorders and may play a role in the pathology of HIV and Parkinson's disease via direct physical interaction with HIV-1 Tat and α-synuclein proteins respectively.

The protein kinase Cβ-selective inhibitor, enzastaurin, attenuates amphetamine-stimulated locomotor activity and self-administration behaviors in rats

RATIONALE

Pathological amphetamine (AMPH) use is a serious public health concern with no pharmacological treatment options. Protein kinase Cβ (PKCβ) has been implicated in the mechanism of action of AMPH, such that inhibition of PKCβ attenuates AMPH-stimulated dopamine efflux in vivo. With this in mind, inhibition of PKCβ may be a viable therapeutic target for AMPH use disorder.

OBJECTIVE

The purpose of this study is to demonstrate that selective pharmacological inhibition of PKCβ alters AMPH-stimulated behaviors in rats.

METHODS

Rats were administered intracerebroventricular (i.c.v.) injections of the PKCβ-selective inhibitor enzastaurin 0.5, 3, 6, or 18 h before evaluating AMPH-stimulated locomotion (0.32-3.2 mg/kg). Rats were trained to make responses for different doses of AMPH infusions or sucrose under a fixed ratio 5 schedule of reinforcement, and the effects of enzastaurin pretreatment 3 or 18 h prior to a self-administration session were determined. Also, the effect of enzastaurin on AMPH-stimulated PKC activity in the ventral striatum was evaluated.

RESULTS

A large dose of enzastaurin (1 nmol) decreased AMPH-stimulated locomotor activity 0.5 h following enzastaurin administration. Small doses of enzastaurin (10-30 pmol) attenuated AMPH-stimulated locomotor activity and shifted the AMPH dose-effect curve to the right following an 18-h pretreatment. Rats pretreated with enzastaurin 18 h, but not 3, prior to a self-administration session showed a decrease in the number of responses for AMPH, shifted the ascending limb of the amphetamine dose effect curve, and produced no change in responses for sucrose. AMPH-stimulated PKC activity was decreased following a 0.5- or 18-h pretreatment, but not a 3-h pretreatment of enzastaurin.

CONCLUSIONS

These results demonstrate that inhibition of PKCβ will decrease AMPH-stimulated behaviors and neurobiological changes and suggest that PKCβ is potentially a viable target for AMPH use disorder.

Ruboxistaurin Reduces Cocaine-Stimulated Increases in Extracellular Dopamine by Modifying Dopamine-Autoreceptor Activity

Cocaine is a highly abused drug, and cocaine addiction affects millions of individuals worldwide. Cocaine blocks normal uptake function at the dopamine transporter (DAT), thus increasing extracellular dopamine. Currently, no chemical therapies are available to treat cocaine abuse. Previous works showed that the selective inhibitors of protein kinase Cβ (PKCβ), enzastaurin and ruboxistaurin, attenuate dopamine overflow and locomotion stimulated by another psychostimulant drug, amphetamine. We now test if ruboxistaurin similarly affects cocaine action. Perfusion of 1 μM ruboxistaurin directly into the core of the nucleus accumbens via retrodialysis reduced cocaine-stimulated increases in dopamine overflow, measured using microdialysis sampling, with simultaneous reductions in locomotor behavior. Because cocaine activity is highly regulated by dopamine autoreceptors, we examined whether ruboxistaurin was acting at the level of the D2 autoreceptor. Perfusion of 5 μM raclopride, a selective D2-like receptor antagonist, before addition of ruboxistaurin, abrogated the effect of ruboxistaurin on cocaine-stimulated dopamine overflow and hyperlocomotion. Further, ruboxistaurin was inactive against cocaine-stimulated locomotor activity in mice with a genetic deletion in D2 receptors as compared to wild-type mice. In contrast, blockade or deletion of dopamine D2 receptors did not abolish the attenuating effect of ruboxistaurin on amphetamine-stimulated activities. Therefore, the inhibition of PKCβ reduces dopamine overflow and locomotor activity stimulated by both cocaine and amphetamine, but the mechanism of action differs for each stimulant. These data suggest that inhibition of PKCβ would serve as a target to reduce the abuse of either amphetamine or cocaine.

Dynamic interactions of ceftriaxone and environmental variables suppress amphetamine seeking

Extrasynaptic glutamate within the nucleus accumbens (NAc) is a driver of relapse. Cocaine, ethanol, and methamphetamine reduce the expression of cystine-glutamate antiporter (xCT) and primary glial glutamate transporter 1 (GLT1) leading to increased extrasynaptic glutamate. Ceftriaxone (CTX) restores xCT and GLT1 expression and effectively suppresses cocaine and ethanol reinstatement, however, the effects of CTX on amphetamine (AMP) reinstatement are not determined. Rodents were reared in an enriched condition (EC), isolated (IC), or standard condition (SC) and trained in AMP self-administration (0.1 mg/kg/infusion). EC, IC, and SC rats received injections of SAL or CTX (200 mg/kg) after daily extinction sessions. Then rats were tested in cue- and AMP-induced reinstatement tests. We hypothesized that EC rearing would reduce reinstatement by altering GLT1 or xCT expression in the NAc and medial prefrontal cortex (mPFC). In Experiment 2, pair-housed rats received once-daily AMP (1.0 mg/kg i.p.) or SAL for eight days followed by once-daily CTX (200 mg/kg i.p.) or SAL injections for 10 days. CTX treatment reduced cue-induced drug seeking in EC rats but not IC or SC rats. In an AMP-induced reinstatement test, CTX reduced AMP-induced drug seeking in EC and SC rats, but not IC rats. Western blot analyses revealed that AMP self-administration and non-contingent repeated AMP exposure did not downregulate GLT1 or xCT in the NAc or mPFC. Therefore, the ability for EC housing to reduce amphetamine seeking may work through other mechanisms.

Molecular Mechanisms of Amphetamines

There is a plethora of amphetamine derivatives exerting stimulant, euphoric, anti-fatigue, and hallucinogenic effects; all structural properties allowing these effects are contained within the amphetamine structure. In the first part of this review, the interaction of amphetamine with the dopamine transporter (DAT), crucially involved in its behavioral effects, is covered, as well as the role of dopamine synthesis, the vesicular monoamine transporter VMAT2, and organic cation 3 transporter (OCT3). The second part deals with requirements in amphetamine's effect on the kinases PKC, CaMKII, and ERK, whereas the third part focuses on where we are in developing anti-amphetamine therapeutics. Thus, treatments are discussed that target DAT, VMAT2, PKC, CaMKII, and OCT3. As is generally true for the development of therapeutics for substance use disorder, there are multiple preclinically promising specific compounds against (meth)amphetamine, for which further development and clinical trials are badly needed.

Phosphorylation of the Amino Terminus of the Dopamine Transporter: Regulatory Mechanisms and Implications for Amphetamine Action

Amphetamines (AMPHs) are potent psychostimulants that are widely used and abused, with profound medical and societal impact. Their actions at dopaminergic neurons are thought to mediate their therapeutic efficacy as well as their liability for abuse and dependence. AMPHs target the dopamine transporter (DAT), the plasmalemmal membrane protein that mediates the inactivation of released dopamine (DA) through its reuptake. AMPHs act as substrates for DAT and are known to cause mobilization of dopamine (DA) to the cell exterior via DAT-mediated reverse transport (efflux). It has become increasingly evident that the mechanisms that regulate AMPH-induced DA efflux are distinct from those that regulate DA uptake. Central to these mechanisms is the phosphorylation of the DAT amino (N)-terminus, which has been repeatedly demonstrated to facilitate DAT-mediated DA efflux, without impacting other aspects of DAT physiology. This review aims to summarize the current status of knowledge regarding DAT N-terminal phosphorylation and its regulation by protein modulators and the membrane microenvironment. A better understanding of these mechanisms may lead to the identification of novel therapeutic approaches that interfere selectively with the pharmacological effects of AMPHs without altering the physiological function of DAT.

Direct and Systemic Administration of a CNS-Permeant Tamoxifen Analog Reduces Amphetamine-Induced Dopamine Release and Reinforcing Effects

Amphetamines (AMPHs) are globally abused. With no effective treatment for AMPH addiction to date, there is urgent need for the identification of druggable targets that mediate the reinforcing action of this stimulant class. AMPH-stimulated dopamine efflux is modulated by protein kinase C (PKC) activation. Inhibition of PKC reduces AMPH-stimulated dopamine efflux and locomotor activity. The only known CNS-permeant PKC inhibitor is the selective estrogen receptor modulator tamoxifen. In this study, we demonstrate that a tamoxifen analog, 6c, which more potently inhibits PKC than tamoxifen but lacks affinity for the estrogen receptor, reduces AMPH-stimulated increases in extracellular dopamine and reinforcement-related behavior. In rat striatal synaptosomes, 6c was almost fivefold more potent at inhibiting AMPH-stimulated dopamine efflux than [3H]dopamine uptake through the dopamine transporter (DAT). The compound did not compete with [3H]WIN 35,428 binding or affect surface DAT levels. Using microdialysis, direct accumbal administration of 1 μM 6c reduced dopamine overflow in freely moving rats. Using LC-MS, we demonstrate that 6c is CNS-permeant. Systemic treatment of rats with 6 mg/kg 6c either simultaneously or 18 h prior to systemic AMPH administration reduced both AMPH-stimulated dopamine overflow and AMPH-induced locomotor effects. Finally, 18 h pretreatment of rats with 6 mg/kg 6c s.c. reduces AMPH-self administration but not food self-administration. These results demonstrate the utility of tamoxifen analogs in reducing AMPH effects on dopamine and reinforcement-related behaviors and suggest a new avenue of development for therapeutics to reduce AMPH abuse.

Application of a Combined Approach to Identify New Psychoactive Street Drugs and Decipher Their Mechanisms at Monoamine Transporters

Psychoactive compounds can cause acute and long-term health problems and lead to addiction. In addition to well-studied and legally controlled compounds like cocaine, new psychoactive substances (NPS) are appearing in street drug markets as replacement strategies and legal alternatives. NPS are effectively marketed as "designer drugs" or "research chemicals" without any knowledge of their underlying pharmacological mode of action and their potential toxicological effects and obviously devoid of any registration process. As of 2016, the knowledge of structure-activity relationships for most NPS is scarce, and predicting detailed pharmacological activity of newly emerging drugs is a challenging task. Therefore, it is important to combine different approaches and employ biological test systems that are superior to mere chemical analysis in recognizing novel and potentially harmful street drugs. In this chapter, we provide a detailed description of techniques to decipher the molecular mechanism of action of NPS that target the high-affinity transporters for dopamine, norepinephrine, and serotonin. In addition, this chapter provides insights into a combined approach to identify and characterize new psychoactive street drugs of unknown content in a collaboration with the Austrian prevention project "checkit!."

PKCβ Inhibitors Attenuate Amphetamine-Stimulated Dopamine Efflux

Amphetamine abuse afflicts over 13 million people, and there is currently no universally accepted treatment for amphetamine addiction. Amphetamine serves as a substrate for the dopamine transporter and reverses the transporter to cause an increase in extracellular dopamine. Activation of the beta subunit of protein kinase C (PKCβ) enhances extracellular dopamine in the presence of amphetamine by facilitating the reverse transport of dopamine and internalizing the D2 autoreceptor. We previously demonstrated that PKCβ inhibitors block amphetamine-stimulated dopamine efflux in synaptosomes from rat striatum in vitro. In this study, we utilized in vivo microdialysis in live, behaving rats to assess the effect of the PKCβ inhibitors, enzastaurin and ruboxistaurin, on amphetamine-stimulated locomotion and increases in monoamines and their metabolites. A 30 min perfusion of the nucleus accumbens core with 1 μM enzastaurin or 1 μM ruboxistaurin reduced efflux of dopamine and its metabolite 3-methoxytyramine induced by amphetamine by approximately 50%. The inhibitors also significantly reduced amphetamine-stimulated extracellular levels of norepinephrine. The stimulation of locomotor behavior by amphetamine, measured simultaneously with the analytes, was comparably reduced by the PKCβ inhibitors. Using a stable isotope label retrodialysis procedure, we determined that ruboxistaurin had no effect on basal levels of dopamine, norepinephrine, glutamate, or GABA. In addition, normal uptake function through the dopamine transporter was unaltered by the PKCβ inhibitors, as measured in rat synaptosomes. Our results support the utility of using PKCβ inhibitors to reduce the effects of amphetamine.

Amphetamine-induced appetitive 50-kHz calls in rats: a marker of affect in mania?

RATIONALE

Animal models aimed to mimic mania have in common the lack of genuine affective parameters. Although rodent amphetamine-induced hyperlocomotion is a frequently used behavioral model of mania, locomotor activity is a rather unspecific target for developing new pharmacological therapies, and does not necessarily constitute a cardinal symptom in bipolar disorder (BD). Hence, alternative behavioral markers sensitive to stimulants are required.

OBJECTIVES

Since D-amphetamine induces appetitive 50-kHz ultrasonic vocalizations (USV) in rats, we asked whether established or potential antimanic drugs would inhibit this effect, thereby possibly complementing traditional analysis of locomotor activity.

METHODS

Amphetamine-treated rats (2.5 mg/kg) were systemically administered with the antimanic drugs lithium (100 mg/kg) and tamoxifen (1 mg/kg). Since protein kinase C (PKC) activity has been implicated in the pathophysiology of bipolar disorder and the biochemical effects of mood stabilizers, the new PKC inhibitor myricitrin (10, 30 mg/kg) was also evaluated.

RESULTS

We demonstrate for the first time that drugs with known or potential antimanic activity were effective in reversing amphetamine-induced appetitive 50-kHz calls. Treatments particularly normalized amphetamine-induced increases of frequency-modulated calls, a subtype presumably indicative of positive affect in the rat.

CONCLUSIONS

Our findings suggest that amphetamine-induced 50-kHz calls might constitute a marker for communicating affect that provides a useful model of exaggerated euphoric mood and pressured speech. The antimanic-like effects of the PKC inhibitors tamoxifen and myricitrin support the predictive and etiological validity of both drugs in this model and highlight the role of PKC signaling as a promising target to treat mania and psychosis-related disorders.

Tamoxifen use for the management of mania: a review of current preclinical evidence

RATIONALE

Preliminary data on the efficacy of tamoxifen in reducing manic symptoms of bipolar disorder (BD) suggest that this agent may be a potential treatment for the management of this psychiatric disorder. However, the antimanic properties of tamoxifen have not been fully elucidated, hampering the development and/or use of mood-stabilising drugs that may share its same therapeutic mechanisms of action. Notably, we may gain a greater understanding of the neurobiological and therapeutic properties of tamoxifen by using suitable animal models of mania.

OBJECTIVES

Here, we review the preclinical studies that have evaluated the effects of tamoxifen to provide an overview of the current progress in our understanding of its antimanic actions, highlighting the critical role of protein kinase C (PKC) as a therapeutic target for the treatment of BD.

CONCLUSIONS

To date, this field has struggled to make significant progress, and the organisation of an explicit battery of tests is a valuable tool for assessing a number of prominent facets of BD, which may provide a greater understanding of the entire scope of this disease.

How addictive drugs disrupt presynaptic dopamine neurotransmission

The fundamental principle that unites addictive drugs appears to be that each enhances synaptic dopamine by means that dissociate it from normal behavioral control, so that they act to reinforce their own acquisition. This occurs via the modulation of synaptic mechanisms that can be involved in learning, including enhanced excitation or disinhibition of dopamine neuron activity, blockade of dopamine reuptake, and altering the state of the presynaptic terminal to enhance evoked over basal transmission. Amphetamines offer an exception to such modulation in that they combine multiple effects to produce nonexocytic stimulation-independent release of neurotransmitter via reverse transport independent from normal presynaptic function. Questions about the molecular actions of addictive drugs, prominently including the actions of alcohol and solvents, remain unresolved, but their ability to co-opt normal presynaptic functions helps to explain why treatment for addiction has been challenging.

The effect of protein kinase C-delta knockdown on anti-free radical enzyme and neuropeptide Y gene expression in phenylpropanolamine-treated rats

Hypothalamic neuropeptide Y (NPY) has been reported to involve in regulating behavioral response of phenylpropanolamine (PPA), a sympathomimetic agent. This study explored if protein kinase C (PKC)-delta signaling participated in this regulation. Moreover, possible roles of anti-free radical enzyme catalase (CAT) and nitrogen oxide synthase (NOS) were also examined. Rats were treated daily with PPA for 4 days. Changes in food intake and hypothalamic NPY, PKCdelta, CAT, and NOS contents were assessed and compared. Results showed that PKCdelta and CAT increased during PPA treatment, which were concomitant with decreases in NPY content and food intake, while the change of NOS was expressed differently. Moreover, PKCdelta knockdown could modify PPA anorexia as well as NPY and CAT expression, while NOS expression remained unchanged. Furthermore, pre-treatment with NOS inhibitor could modify both PPA anorexia and NPY content. It is suggested that PKCdelta participates in the anorectic response of PPA via the modulation of NPY and CAT, while NOS contribute to this modulation via a different mechanism during PPA treatment. Results provide molecular mechanism of NPY-mediated PPA anorexia and may aid the therapeutic research of PPA and other anti-obesity drugs.

The reverse operation of Na(+)/Cl(-)-coupled neurotransmitter transporters--why amphetamines take two to tango

Sodium-chloride coupled neurotransmitter transporters achieve reuptake of their physiological substrate by exploiting the pre-existing sodium-gradient across the cellular membrane. This terminates the action of previously released substrate in the synaptic cleft. However, a change of the transmembrane ionic gradients or specific binding of some psychostimulant drugs to these proteins, like amphetamine and its derivatives, induce reverse operation of neurotransmitter:sodium symporters. This effect eventually leads to an increase in the synaptic concentration of non-exocytotically released neurotransmitters [and - in the case of the norepinephrine transporters, underlies the well-known indirect sympathomimetic activity]. While this action has long been appreciated, the underlying mechanistic details have been surprisingly difficult to understand. Some aspects can be resolved by incorporating insights into the oligomeric nature of transporters, into the nature of the accompanying ion fluxes, and changes in protein kinase activities.

Rapid substrate-induced down-regulation in function and surface localization of dopamine transporters: rat dorsal striatum versus nucleus accumbens

The dopamine transporter (DAT) substrates dopamine, d-amphetamine (AMPH), and methamphetamine are known to rapidly and transiently reduce DAT activity and/or surface expression in dorsal striatum and heterologous expression systems. We sought to determine if similar substrate-induced regulation of DATs occurs in rat nucleus accumbens. In dorsal striatum synaptosomes, brief (15-min) in vitro substrate pre-exposure markedly decreased maximal [(3)H]dopamine uptake velocity whereas identical substrate pre-exposure in nucleus accumbens synaptosomes produced a smaller, non-significant reduction. However, 45 min after systemic AMPH administration, maximal ex vivo [(3)H]dopamine uptake velocity was significantly reduced in both brain regions. Protein kinase C inhibition blocked AMPH's down-regulation of DAT activity. DAT synaptosomal surface expression was not modified following either the brief in vitro or in vivo AMPH pre-exposure but was reduced after a longer (1-h) in vitro pre-exposure in both brain regions. Together, our findings suggest that relatively brief substrate exposure results in greater down-regulation of DAT activity in dorsal striatum than in nucleus accumbens. Moreover, exposure to AMPH appears to regulate striatal DATs in a biphasic manner, with an initial protein kinase C-dependent decrease in DAT-mediated uptake velocity and then, with longer exposure, a reduction in DAT surface expression.

Involvement of nicotinic receptors in methamphetamine- and MDMA-induced neurotoxicity: pharmacological implications

During the last years, we have focused on the study of the neurotoxic effects of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (METH) on the central nervous system (CNS) and their pharmacological prevention methods. In the process of this research, we have used a semipurified synaptosomal preparation from striatum of mice or rats as a reliable in vitro model to study reactive oxygen species (ROS) production by these amphetamine derivatives, which is well-correlated with their dopaminergic injury in in vivo models. Using this preparation, we have demonstrated that blockade of alpha7 nicotinic receptors with methyllycaconitine (MLA) prevents ROS production induced by MDMA and METH. Consequently, in vivo, MLA significantly prevents MDMA- and METH-induced neurotoxicity at dopaminergic level (mouse striatum), without affecting hyperthermia induced by these amphetamines. Additionally, when neuroprotection was assayed with memantine (MEM), a dual antagonist of NMDA and alpha7 receptors, an effective neuroprotection was obtained also ahead of serotonergic injury induced by MDMA in rats. MEM also prevents MDMA effect on serotonin transporter functionality and METH effect on dopamine transporter (DAT), suggesting that behavioral effects of these psychostimulants can also be modulated by MEM. Finally, we have demonstrated that MEM prevents the impaired memory function induced by MDMA, and also, using binding studies with radioligands, we have characterized the interaction of these substances with nicotinic receptors. Studies at molecular level showed that both MDMA and METH displaced competitively the binding of radioligands with homomeric alpha7 and heteromeric nicotinic acetylcholine receptors (nAChRs), indicating that they can directly interact with them. In all the cases, MDMA displayed higher affinity than METH and it was higher for heteromeric than for alpha7 subtype. Pre-incubation of differentiated PC12 cells with MDMA or METH induces nAChR upregulation in a concentration- and time-dependent manner, as many nicotinic ligands do, supporting their functional interaction with nAChRs. Such interaction expands the pharmacological profile of amphetamines and can account for some of their effects.

Protein kinase C inhibitors: rationale for use and potential in the treatment of bipolar disorder

Bipolar disorder is one of the most severely debilitating of all medical illnesses. For a large number of patients, outcomes are quite poor. The illness results in tremendous suffering for patients and their families and commonly impairs functioning and workplace productivity. Risks of increased morbidity and mortality, unfortunately, are frequent occurrences as well. Until recently, little has been known about the specific molecular and cellular underpinnings of bipolar disorder. Such knowledge is crucial for the prospect of developing specific targeted therapies that are more effective and that have a more rapid onset of action than currently available treatments. Exciting recent data suggest that regulation of certain signalling pathways may be involved in the aetiology of bipolar disorder and that these pathways may be profitably targeted to treat the disorder. In particular, mania is associated with overactive protein kinase C (PKC) intracellular signalling, and recent genome-wide association studies of bipolar disorder have implicated an enzyme that reduces the activation of PKC. Importantly, the current mainstays in the treatment of mania, lithium (a monovalent cation) and valproate (a small fatty acid) indirectly inhibit PKC. In addition, recent clinical studies with the relatively selective PKC inhibitor tamoxifen add support to the relevance of the PKC target in bipolar disorder. Overall, a growing body of work both on a preclinical and clinical level indicates that PKC signalling may play an important role in the pathophysiology and treatment of bipolar disorder. The development of CNS-penetrant PKC inhibitors may have considerable benefit for this devastating illness.

A review of the preclinical and clinical evidence for protein kinase C as a target for drug development for bipolar disorder

In this article, we review preclinical studies investigating the role of protein kinase C (PKC) as it pertains to mania and effective antimanic agents. We then discuss clinical studies conducted with tamoxifen, a relatively selective PKC inhibitor, in acute bipolar mania. We conclude that PKC is an important target-arguably the first mechanistically distinct drug target for bipolar disorder. PKC holds considerable promise as a novel target for developing a new line of treatments for bipolar disorder.

Verapamil augmentation of lithium treatment improves outcome in mania unresponsive to lithium alone: preliminary findings and a discussion of therapeutic mechanisms

OBJECTIVES

Attenuation of protein kinase C (PKC) is a mechanism common to both established (lithium, valproate) and some novel (tamoxifen) antimanic agents. Verapamil, although primarily known as a calcium channel blocker, also has PKC inhibitory activity. Verapamil has shown antimanic activity in some but not all studies. Therefore, we investigated verapamil, used alone or as an adjunctive treatment, in manic patients who did not respond to an initial adequate trial of lithium.

METHODS

Each study phase lasted three weeks. Subjects were treated openly with lithium in Phase 1 (n = 45). Those who failed to respond were randomly assigned to double-blind treatment in Phase 2 with either verapamil (n = 10) or continued-lithium (n = 8). Phase 2 nonresponders (n = 10) were assigned to combined verapamil/lithium in Phase 3.

RESULTS

Response in Phase 2 did not differ significantly between verapamil and continued-lithium. During Phase 3, response to combined treatment was significantly better than overall response to monotherapy in Phase 2 (Fisher's Exact test, p = 0.043). Mania ratings improved during combined treatment in Phase 3 by 88.2% (linear mixed model analysis, F = 4.34, p = 0.013), compared with 10.5% improvement during Phase 2.

CONCLUSIONS

In this preliminary investigation, verapamil monotherapy did not demonstrate antimanic efficacy. By contrast, the combination of verapamil plus lithium was highly efficacious. Our findings thus suggest that verapamil may have potential utility as an adjunct to lithium. This effect may be mediated by additive actions on PKC inhibition, which may be an important mechanism for antimanic agents in general.

Glutamate receptors as targets of protein kinase C in the pathophysiology and treatment of animal models of mania

Considerable biochemical evidence suggests that the protein kinase C (PKC) signaling cascade may be a convergent point for the actions of anti-manic agents, and that excessive PKC activation can disrupt prefrontal cortical regulation of thinking and behavior. To date, however, brain protein targets of PKC's anti-manic effects have not been fully identified. Here we showed that PKC activity was enhanced in the prefrontal cortex of animals treated with the psychostimulant amphetamine. Phosphorylation of MARCKS, a marker of PKC activity, was increased in the prefrontal cortex of animals treated with the psychostimulant amphetamine, as well as in sleep-deprived animals (another animal model of mania), but decreased in lithium-treated animals. The antidepressant imipramine, which shows pro-manic properties in patients with bipolar disorder (BPD), also enhanced phospho-MARCKS in prefrontal cortex in vivo. We further explored the functional targets of PKC in mania-associated behaviors. Neurogranin is a brain-specific, postsynaptically located PKC substrate. PKC phosphorylation of neurogranin was robustly increased by pro-manic manipulations and decreased by anti-manic agents. PKC phosphorylation of the NMDA receptor site GluN1S896 and the AMPA receptor site GluA1T840 was also enhanced in the prefrontal cortex of animals treated with the antidepressant imipramine, as well as in behaviorally sleep-deprived animals, in striking contrast to the reduced activity seen in lithium-treated animals. These results suggest that PKC may play an important role in regulating NMDA and AMPA receptor functions. The biochemical profile of the PKC pathway thus encompasses both pro- and anti-manic effects on behavior. These results suggest that PKC modulators or their intracellular targets may ultimately represent novel avenues for the development of new therapeutics for mood disorders.

Protein kinase C inhibition by tamoxifen antagonizes manic-like behavior in rats: implications for the development of novel therapeutics for bipolar disorder

RATIONALE

In the context of bipolar disorder (BPD) research it was demonstrated that administration of the structurally dissimilar mood stabilizers lithium and valproate produced a striking reduction in protein kinase C (PKC) in rat brain. In a small clinical study, tamoxifen (a PKC inhibitor) had antimanic efficacy. However, both lithium and valproate exert many biochemical changes and attribution of therapeutic relevance to any molecular findings needs to be based on linking them to behavioral effects.

OBJECTIVES

The present study was designed to explore such relationship by studying the effects of PKC inhibition in amphetamine-induced behavioral animal models of mania and changes in GAP-43.

METHODS

The effects of two daily tamoxifen (1 mg/kg) i.p. injections on acute or chronic (7 injections) amphetamine (0.5 mg/kg) -induced behaviors and GAP-43 phosphorylation were tested.

RESULTS

The study demonstrates that tamoxifen significantly reduced amphetamine-induced hyperactivity in a large open field without affecting spontaneous activity levels and normalized amphetamine-induced increase in visits to the center of an open field (representing risk-taking behavior). Tamoxifen also attenuated amphetamine-induced phosphorylation of GAP-43, a result that is consistent with the behavioral findings.

CONCLUSIONS

These results support the possibility that PKC signaling may play an important role in the pathophysiology and treatment of BPD. These findings may have direct clinical implications as they offer a new avenue for attempts to develop more specific drugs for the disorder.

Efficacy of a protein kinase C inhibitor (tamoxifen) in the treatment of acute mania: a pilot study

OBJECTIVES

Considerable preclinical biochemical and behavioral data suggest that protein kinase C inhibition would bring about antimanic effects. Notably, the structurally highly dissimilar antimanic agents lithium and valproate, when administered in therapeutically relevant paradigms, attenuate protein kinase C [corrected] function. There is currently only one relatively selective protein kinase C inhibitor that crosses the blood-brain barrier available for human use--tamoxifen. Our group recently conducted a single-blind study with tamoxifen in acute mania and found that it significantly decreased manic symptoms within a short period of time (3-7 days). In this study, we investigated whether antimanic effects can be achieved with a protein kinase C inhibitor in subjects with mania.

METHODS

In a double-blind, placebo-controlled study, 16 subjects with bipolar disorder, manic or mixed, with or without psychotic features, were randomly assigned to receive tamoxifen (20-140 mg/day; n = 8) or placebo (n = 8) for three weeks. Primary efficacy was assessed by the Young Mania Rating Scale.

RESULTS

Subjects on tamoxifen showed significant improvement in mania compared to placebo as early as five days, an effect that remained significant throughout the three-week trial. The effect size for the drug difference was very large (d = 1.08, 95% confidence interval 0.45-1.71) after three weeks (p = 0.001). At study endpoint, response rates were 63% for tamoxifen and 13% for placebo (p = 0.12).

CONCLUSIONS

Antimanic effects resulted from a protein kinase C inhibitor; onset occurred within five days. Large, controlled studies with selective protein kinase C inhibitors in acute mania are warranted.

Currents in neurotransmitter transporters

Traditionally, substrate translocation by neurotransmitter transporters has been described by the alternate access model. Recent structural data obtained with three distantly related transporters have also been interpreted as supportive of this model, because conformational correlates were visualized (inward-facing conformation, occluded state). However, the experimental evidence is overwhelmingly in favour of a more complex mode of operation: Transporters also exist in conformations that do not seal the permeation pathway. These conformations support a channel-like activity, including random permeation of substrate and co-substrate ions in a single-file mode. It is likely that the channel-like activity is modified by the interaction of the transporters with accessory proteins and regulatory kinases. Finally, channel-like activity is instrumental to understand the mechanism of action of amphetamines.

Regulation of dopamine transporter trafficking by intracellular amphetamine

The dopamine (DA) transporter (DAT) mediates the removal of released DA. DAT is the major molecular target responsible for the rewarding properties and abuse potential of the psychostimulant amphetamine (AMPH). AMPH has been shown to reduce the number of DATs at the cell surface, and this AMPH-induced cell surface DAT redistribution may result in long-lasting changes in DA homeostasis. The molecular mechanism by which AMPH induces trafficking is not clear. Because AMPH is a substrate, we do not know whether extracellular AMPH stimulates trafficking through its interaction with DAT and subsequent alteration in DAT function, thereby triggering intracellular signaling or whether AMPH must be transported and then act intracellularly. In agreement with our previous studies, extracellular AMPH caused cytosolic redistribution of the wild-type human DAT (WT-hDAT). However, AMPH did not induce cytosolic redistribution in an uptake-impaired hDAT (Y335A-hDAT) that still binds AMPH. The divalent cation zinc (Zn(2+)) inhibits WT-hDAT activity, but it restores Y335A-hDAT uptake. Coadministration of Zn(2+) and AMPH consistently reduced WT-hDAT trafficking but stimulated cytosolic redistribution of Y335A-hDAT. Furthermore, direct intracellular application of AMPH, via a whole-cell patch pipette, stimulated the trafficking of Y335A-hDAT. Taken together, these data suggest that the DAT transport cycle is not required for AMPH-induced down-regulation and that an increase of intracellular AMPH is an essential component of DAT redistribution.

Transcriptional involvement of protein kinase C-alpha isozyme in amphetamine-mediated appetite suppression

Amphetamine (AMPH) is known as an anorectic agent. The anorectic action of AMPH has been attributed to its inhibitory action on hypothalamic neuropeptide Y (NPY), an appetite stimulant in the brain. The molecular mechanisms behind this anorectic action of AMPH are still unclear. This study investigated the possible role of protein kinase C (PKC) isotypes in this anorectic action. Results revealed that most PKC isotypes (alpha, betaII, gamma, delta, eta, lambda and zeta), except betaI and epsilon isotypes, were stimulated during a repeated treatment of AMPH. Among these stimulated isotypes, three isotypes (alpha, delta, lambda) were activated and expressed in a similar manner, while the other isotypes were expressed differently and specifically. To determine if PKCalpha was involved in the anorectic response of AMPH, the infusions of antisense oligonucleotide into the brain were performed 1 h before daily AMPH treatment in freely moving rats, and the results showed that PKCalpha knock down could block the anorectic response and restore NPY mRNA levels in AMPH-treated rats. These results suggest that PKC isotypes- (at least the alpha isotype), related modification of NPY gene expression in hypothalamus might play an essential role in the central regulation of AMPH-mediated feeding suppression.

Mechanisms of neurotransmitter release by amphetamines: a review

Amphetamine and substituted amphetamines, including methamphetamine, methylphenidate (Ritalin), methylenedioxymethamphetamine (ecstasy), and the herbs khat and ephedra, encompass the only widely administered class of drugs that predominantly release neurotransmitter, in this case principally catecholamines, by a non-exocytic mechanism. These drugs play important medicinal and social roles in many cultures, exert profound effects on mental function and behavior, and can produce neurodegeneration and addiction. Numerous questions remain regarding the unusual molecular mechanisms by which these compounds induce catecholamine release. We review current issues on the two apparent primary mechanisms--the redistribution of catecholamines from synaptic vesicles to the cytosol, and induction of reverse transport of transmitter through plasma membrane uptake carriers--and on additional drug effects that affect extracellular catecholamine levels, including uptake inhibition, effects on exocytosis, neurotransmitter synthesis, and metabolism.

Emerging experimental therapeutics for bipolar disorder: insights from the molecular and cellular actions of current mood stabilizers

Bipolar disorder afflicts approximately 1-3% of both men and women, and is coincident with major economic, societal, medical, and interpersonal consequences. Current mediations used for its treatment are associated with variable rates of efficacy and often intolerable side effects. While preclinical and clinical knowledge in the neurosciences has expanded at a tremendous rate, recent years have seen no major breakthroughs in the development of novel types of treatment for bipolar disorder. We review here approaches to develop novel treatments specifically for bipolar disorder. Deliberate (ie not by serendipity) treatments may come from one of two general mechanisms: (1) Understanding the mechanism of action of current medications and thereafter designing novel drugs that mimics these mechanism(s); (2) Basing medication development upon the hypothetical or proven underlying pathophysiology of bipolar disorder. In this review, we focus upon the first approach. Molecular and cellular targets of current mood stabilizers include lithium inhibitable enzymes where lithium competes for a magnesium binding site (inositol monophosphatase, inositol polyphosphate 1-phosphatase, glycogen synthase kinase-3 (GSK-3), fructose 1,6-bisphosphatase, bisphosphate nucleotidase, phosphoglucomutase), valproate inhibitable enzymes (succinate semialdehyde dehydrogenase, succinate semialdehyde reductase, histone deacetylase), targets of carbamazepine (sodium channels, adenosine receptors, adenylate cyclase), and signaling pathways regulated by multiple drugs of different classes (phosphoinositol/protein kinase C, cyclic AMP, arachidonic acid, neurotrophic pathways). While the task of developing novel medications for bipolar disorder is truly daunting, we are hopeful that understanding the mechanism of action of current mood stabilizers will ultimately lead clinical trials with more specific medications and thus better treatments those who suffer from this devastating illness.

Effects of fenfluramine and antidepressants on protein kinase C activity in rat cortical synaptoneurosomes

Fenfluramine releases serotonin (5-HT) via the 5-HT transporter (SERT). Previous work has shown that amphetamine increases particulate protein kinase C (PKC) activity in striatal synaptoneurosomes. The increased PKC activity is linked to the outward transport of dopamine, and when release is diminished, the inward transport of amphetamine inhibits PKC instead. Since there is homology among monoamine transporters, this study was undertaken to determine if D-fenfluramine has similar effects on PKC. The role of 5-HT receptors and endogenous 5-HT were also examined. Naive rats and rats pretreated with p-chlorophenylalanine (PCPA), a 5-HT synthesis inhibitor, were sacrificed. Cortical synaptoneurosomes were prepared and incubated with fenfluramine. PKC activity was determined by thiophosphorylation of endogenous substrates. It was found that 5-HT, D/L-fenfluramine, and D-fenfluramine increased PKC activity in a time- and dose-dependent manner. The 5-HT-mediated increase in PKC activity was attenuated by pretreatment with the 5-HT(2) antagonist ketanserin, but not with the SERT inhibitor fluoxetine. The D-fenfluramine-induced increase in PKC activity was completely prevented, however, by pretreatment with SERT inhibitors and partially with ketanserin. It was also attenuated by pretreatment with PCPA, resulting in a dose-dependent inhibition of PKC instead. Thus, when 5-HT release was diminished the uptake of D-fenfluramine inhibited PKC. Similar effects have been observed with amphetamine. Unlike D-fenfluramine, the D/L-fenfluramine-induced increase in PKC activity was partially resistant to PCPA pretreatment but was attenuated with bupropion, a dopamine transporter (DAT) inhibitor. SERT inhibitors (sertraline, paroxetine, citalopram, and fluoxetine) also increased PKC activity. Nefazodone and bupropion increased PKC activity, but mirtazapine was relatively inactive. The SERT inhibitor-induced increase in PKC was unaffected by pretreatment with PCPA but was inhibited by calcium. Similar effects on PKC activity have been observed with DAT inhibitors. These results, showing that D-fenfluramine altered PKC activity similar to D-amphetamine, suggest that the topographic homology between DAT and SERT may extend to their effects on PKC activity.

Repeated amphetamine treatment induces neurite outgrowth and enhanced amphetamine-stimulated dopamine release in rat pheochromocytoma cells (PC12 cells) via a protein kinase C- and mitogen activated protein kinase-dependent mechanism

Repeated intermittent treatment with amphetamine (AMPH) induces both neurite outgrowth and enhanced AMPH-stimulated dopamine (DA) release in PC12 cells. We investigated the role of protein kinases in the induction of these AMPH-mediated events by using inhibitors of protein kinase C (PKC), mitogen activated protein kinase (MAP kinase) or protein kinase A (PKA). PKC inhibitors chelerythrine (100 nm and 300 nm), Ro31-8220 (300 nm) and the MAP kinase kinase inhibitor, PD98059 (30 micro m) inhibited the ability of AMPH to elicit both neurite outgrowth and the enhanced AMPH-stimulated DA release. The direct-acting PKC activator, 12-O-tetradecanoyl phorbol 13-acetate (TPA, 250 nm) mimicked the ability of AMPH to elicit neurite outgrowth and enhanced DA release. On the contrary, a selective PKA inhibitor, 100 micro m Rp-8-Br-cAMPS, blocked only the development of AMPH-stimulated DA release but not the neurite outgrowth. Treatment of the cells with acute AMPH elicited an increase in the activity of PKC and MAP kinase but not PKA. These results demonstrated that AMPH-induced increases in MAP kinase and PKC are important for induction of both the enhancement in transporter-mediated DA release and neurite outgrowth but PKA was only required for the enhancement in AMPH-stimulated DA release. Therefore the mechanisms by which AMPH induces neurite outgrowth and the enhancement in AMPH-stimulated DA release can be differentiated.

The effect of phosphorylation on amphetamine-mediated outward transport

Amphetamine elicits its locomotor-activating and drug-reinforcing effects by releasing the catecholamines dopamine and norepinephrine into the synapse. Amphetamine is a substrate of the plasmalemmal transporters for both dopamine and norepinephrine. As such, it binds to the transporters in conjunction with Na+ and Cl-, facilitating a conformational change leading the transporter to face inward. The subsequent binding of intracellular catecholamine results in an outward transport and release of the catecholamine into the synapse. Both inward and outward transport through the catecholamine transporters are regulated by protein kinases, particularly protein kinase C, but the effect of the enzyme on the two processes appears to be asymmetric. The purpose of this review is to discuss the evidence showing that protein kinase C activation facilitates outward transport through the catecholamine plasmalemmal transporters which may mediate amphetamine action in intact tissue.

Implication of brain-derived neurotrophic factor in the release of dopamine and dopamine-related behaviors induced by methamphetamine

It is widely recognized that methamphetamine enhances the release of dopamine at dopaminergic neuron terminals of the mesolimbic system, which induces dopamine-related behaviors. Brain-derived neurotrophic factor (BDNF), a neurotrophin, binds to and activates its specific receptor tyrosine kinase, TrkB. BDNF has been shown to influence the release of dopamine in the mesolimbic dopamine system. The present study was designed to investigate roles of BDNF and TrkB in the expression of methamphetamine-induced dopamine release in the nucleus accumbens and dopamine-related behaviors induced by methamphetamine in rats. Methamphetamine (1 mg/kg, s.c.) produced a substantial increase in the extracellular levels of dopamine and induced a progressive augmentation of dopamine-related behaviors such as rearing and sniffing. In contrast, both the stimulation of dopamine release and induction of dopamine-related behaviors by methamphetamine were significantly suppressed by pretreatment with intra-nucleus accumbens injection of either BDNF (2.0 microl/rat, 1:1000, 1:300 and 1:100) or TrkB (2.0 microl/rat, 1:1000 and 1:100) antibody. Furthermore, the basal level of dopamine in the nucleus accumbens was not affected by treatment with both BDNF and TrkB antibodies. These findings provide further evidence that BDNF/TrkB pathway is implicated in the methamphetamine-induced release of dopamine and the induction of dopamine-related behaviors.

Differential effects of amphetamine transport vs. dopamine reverse transport on particulate PKC activity in striatal synaptoneurosomes

Amphetamine has been shown to increase striatal particulate protein kinase C (PKC) activity [Giambalvo (1992b) Neuropharmacology 31:1211-1222]. The present study examined possible mechanisms involved. Specifically, the effects of calcium, endogenous DA, and DA receptors on the amphetamine-induced increase in PKC activity in striatal synaptoneurosomes were examined. Naïve rats and rats pretreated with N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinolone (EEDQ, i.p.), a nonselective irreversible receptor antagonist, or with alpha-methyl-p-tyrosine (AMPT, i.p.), a DA synthesis inhibitor, were sacrificed and striatal synaptoneurosomes were prepared. The tissue was incubated with amphetamine, with and without calcium, and PKC activity was then determined by the endogenous phosphorylation of endogenous substrate proteins, as described previously [Giambalvo (1988a) Biochem Pharmacol 37:4001-4007]. It was found that calcium enhanced the effect of amphetamine on PKC activity, even in rats pretreated with EEDQ. Intracellular calcium was required since pretreatment with 1,2-bis (2-aminophenoxy) ethane-N, N, N, N-tetracetic acid acetoxymethyl ester (BAPTA-AM) in vitro attenuated the amphetamine-induced increase in PKC activity, resulting in an inhibition of PKC activity instead. Likewise, endogenous DA was essential since pretreatment with AMPT resulted in a similar amphetamine-induced inhibition of PKC activity. Pretreatment with AMPT did not alter the inhibitory effect of the D2 DA agonist, LY 171555, on PKC activity. It did, however, abolish the calcium-dependent stimulatory effect of the D1 agonist SKF 38393 on PKC activity, rendering it inhibitory regardless of calcium. Considering that both BAPTA-AM and AMPT pretreatments, which diminished DA release without affecting uptake via different mechanisms, produced similar inhibitory effects on PKC activity by amphetamine, these results suggest that the inward transport of amphetamine had an inhibitory effect on PKC activity. In contrast, the outward transport of DA seemed to have a stimulatory effect on PKC activity since incubation with low sodium or with ouabain, conditions that promote DA reverse-transport, increased PKC activity. These results showed that PKC activity was altered differently during inward vs. outward transport. The amphetamine-induced increase in PKC activity was attenuated by pretreatment with DA uptake blockers (nomifensine, GBR 12935, and bupropion), even though these drugs by themselves also increased PKC activity. This effect was diminished by calcium and persisted in rats pretreated with EEDQ. Thus, calcium had a differential effect on the PKC activity induced by a transported substrate (amphetamine) vs. nontransported inhibitors (DA uptake blockers).

Mechanisms underlying the effects of amphetamine on particulate PKC activity

Amphetamine stimulates particulate protein kinase C (PKC) activity that is associated with the outward-transport of dopamine (DA) (Giambalvo [2003] Synapse 49:125-133). This stimulatory effect requires intracellular calcium ([Ca](i)) and endogenous DA and when DA release is diminished, the inward-transport of amphetamine inhibits PKC activity. This study examines the mechanisms involved. It was found that synaptoneurosomes incubated with amphetamine showed a dose-dependent increase in phospholipase C and A(2) activities. Furthermore, pretreatments with the phospholipase C inhibitor D609 or the phospholipase A(2) inhibitors quinacrine or p-bromophenacylbromide attenuated the amphetamine-induced increase in PKC activity. This suggests that both phospholipases were essential for the amphetamine-induced increase in PKC activity. The Na/Ca antiporter was also involved, since pretreatment with amiloride or benzamil attenuated the amphetamine-induced increase in PKC activity. Since these drugs by themselves increased PKC activity, the return to basal activity after addition of amphetamine suggests that, in the absence of Na/Ca exchange, amphetamine had an inhibitory effect on PKC activity. This inhibitory effect might be due to the activation of phospholipase A(2) through an increase in intracellular pH induced by amphetamine. This was supported by the finding that pretreatment with dimethylamiloride, an inhibitor of the Na/H antiporter that increases intracellular [H(+)], attenuated the effects of amphetamine on PKC activity. Other drugs that decrease intracellular [H(+)] (ammonia, monensin) also inhibited PKC activity without Ca. In contrast to amphetamine, monensin had no effect on PKC activity with Ca. This could be related to its large differential effects on phospholipase A(2) vs. phospholipase C activity. Thus, the monensin-mediated decrease in PKC activity seen without Ca was partially attenuated by pretreatment with quinacrine. Furthermore, when Na/Ca antiporter was inhibited with benzamil, monensin inhibited PKC activity. These results suggest that amphetamine, as well as monensin, may have dual effects on PKC activity, a Ca-dependent stimulatory effect via phospholipase C, and an inhibitory effect via phospholipase A(2).

Methamphetamine-induced deficits of brain monoaminergic neuronal markers: distal axotomy or neuronal plasticity

We examined the effects of methamphetamine (METH) on monoaminergic (i.e. dopamine and serotonin) axonal markers and glial cell activation in the rat brain. Our findings indicate that the loss of dopamine transporters (DAT), serotonin transporters (5-HTT), vesicular monoamine transporter type-2 (VMAT-2) and glial cell activation induced by METH in the striatum and in the central gray are consistent with a degenerative process. Our novel finding of METH effects on monoaminergic neurons in the central gray may have important implications on METH-induced hyperthermia. In other brain regions examined, DAT and 5-HTT deficits after METH administration were present in the absence of lasting changes in VMAT-2 levels or glial cell activation. Brain regions exhibiting protracted deficits in DAT and/or 5-HTT and VMAT-2 levels also expressed increased levels of [(3)H]-R-PK11195 binding to peripheral benzodiazepine receptors, a quantitative marker of glial cell activation. Immunohistochemical assessment of microglia and astrocytes confirmed the PBR results. Microglia activation was more pronounced than astrocytosis in affected regions in most METH-exposed brains with the exception of a small number of rats that were most severely affected by METH based on loss of body weight. In these rats, both microglia and astrocytes were highly activated and expressed a distinct regional pattern suggestive of widespread brain injury. The reason for the pattern of glial cell activation in this group of rats is not currently known but it may be associated with METH-induced hyperthermia. In summary, our findings suggest two neurotoxic endpoints in the brain of METH-exposed animals. Brain regions exhibiting DAT and 5-HTT deficits that co-localize with decreased VMAT-2 levels and glial cell activation may represent monoaminergic terminal degeneration. However, the DAT and 5-HTT deficits in brain regions lacking a deficit in VMAT-2 and glial cell activation may reflect drug-induced modulation of these plasma membrane proteins.

Mood stabilizer psychopharmacology

Mood stabilizers represent a class of drugs that are efficacious in the treatment of bipolar disorder. The most established medications in this class are lithium, valproic acid, and carbamazepine. In addition to their therapeutic effects for treatment of acute manic episodes, these medications often are useful as prophylaxis against future episodes and as adjunctive antidepressant medications. While important extracellular effects have not been excluded, most available evidence suggests that the therapeutically relevant targets of this class of medications are in the interior of cells. Herein we give a prospective of a rapidly evolving field, discussing common effects of mood stabilizers as well as effects that are unique to individual medications. Mood stabilizers have been shown to modulate the activity of enzymes, ion channels, arachidonic acid turnover, G protein coupled receptors and intracellular pathways involved in synaptic plasticity and neuroprotection. Understanding the therapeutic targets of mood stabilizers will undoubtedly lead to a better understanding of the pathophysiology of bipolar disorder and to the development of improved therapeutics for the treatment of this disease. Furthermore, the involvement of mood stabilizers in pathways operative in neuroprotection suggests that they may have utility in the treatment of classical neurodegenerative disorders.

Differential effects of stimulants on monoaminergic transporters: pharmacological consequences and implications for neurotoxicity

Many psychostimulants alter plasmalemmal monoaminergic transporter function. Some, such as cocaine, prevent the reuptake of newly released dopamine, serotonin or norepinephrine into their associated neurons. Others, such as the amphetamines, facilitate release of these transmitters into the extraneuronal space by causing a reversal of function of these carrier proteins. An understanding of how psychostimulants regulate the function of not only plasmalemmal, but also vesicular monoamine transporter function is important to appreciate the pharmacological and sometimes neurotoxic consequences of administering these drugs, as well as the physiological regulation of these carrier proteins. Hence, this review will describe recent ex vivo studies investigating the rapid and differential affects of several stimulants on both plasmalemmal and vesicular monoamine transporter function.

The effects of methamphetamine on serotonin transporter activity: role of dopamine and hyperthermia

Multiple administrations of methamphetamine (METH) rapidly decreased serotonin (5HT) transporter (SERT) function in rat striatum and hippocampus. The purpose of this study was to identify the mechanisms/ factors contributing to this METH-induced decrease in SERT function. Multiple high-dose METH injections rapidly decreased 5HT uptake without altering binding of the 5HT transporter ligand paroxetine. Hyperthermia contributed to this deficit in transporter function in striatum and hippocampus, as prevention of METH-induced hyperthermia attenuated this decrease. A role for dopamine (DA) was suggested by findings that pretreatment with the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine, the D1 antagonist SCH-23390, or the D2 antagonist eticlopride attenuated the METH-induced decrease in striatal, but not hippocampal, SERT activity. These effects were independent of the ability of these DA-antagonizing drugs to prevent METH-induced hyperthermia. These results suggest that DA contributes to the decrease in SERT function caused by multiple METH injections in the striatum, but not hippocampus, and that hyperthermia facilitates these deficits in SERT function in both brain regions. In contrast, the response of SERT to a single administration of METH was DA and hyperthermia independent. These findings suggest that the mechanisms/ factors involved in decreasing SERT activity after a single administration of METH are distinct from that caused by multiple administrations.

Lithium regulates PKC-mediated intracellular cross-talk and gene expression in the CNS in vivo

It has become increasingly appreciated that the long-term treatment of complex neuropsychiatric disorders like bipolar disorder (BD) involves the strategic regulation of signaling pathways and gene expression in critical neuronal circuits. Accumulating evidence from our laboratories and others has identified the family of protein kinase C (PKC) isozymes as a shared target in the brain for the long-term action of both lithium and valproate (VPA) in the treatment of BD. In rats chronically treated with lithium at therapeutic levels, there is a reduction in the levels of frontal cortical and hippocampal membrane-associated PKC alpha and PKC epsilon. Using in vivO microdialysis, we have investigated the effects of chronic lithium on the intracellular cross-talk between PKC and the cyclic AMP (cAMP) generating system in vivo. We have found that activation of PKC produces an increase in dialysate cAMP levels in both prefrontal cortex and hippocampus, effects which are attenuated by chronic lithium administration. Lithium also regulates the activity of another major signaling pathway the c-Jun N-terminal kinase pathway--in a PKC-dependent manner. Both Li and VPA, at therapeutically relevant concentrations, increase the DNA binding of activator protein 1 (AP-1) family of transcription factors in cultured cells in vitro, and in rat brain ex vivo. Furthermore, both agents increase the expression of an AP-1 driven reporter gene, as well as the expression of several endogenous genes known to be regulated by AP-1. Together, these results suggest that the PKC signaling pathway and PKC-mediated gene expression may be important mediators of lithium's long-term therapeutic effects in a disorder as complex as BD.

A protein kinase inhibitor, H-7, blocks naloxone-precipitated changes in dopamine and its metabolites in the brains of opioid-dependent rats

The influence of an inhibitor of cAMP-dependent protein kinase and protein kinase C, H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine], on naloxone (an opioid receptor antagonist)-precipitated withdrawal signs and changes in levels of dopamine (DA) and its metabolites in morphine- or butorphanol-dependent rats was investigated. Animals were infused continuously with morphine (a mu-opioid receptor agonist) or butorphanol (a mu/delta/kappa mixed opioid receptor agonist) for 3 days. Naloxone precipitated withdrawal syndrome and decreased the levels of DA in the cortex, striatum, and midbrain; 3, 4-dihydroxyphenylacetic acid (DOPAC) in the cortex, striatum, limbic areas, and midbrain; and homovanilic acid (HVA) in the striatum, limbic areas, and midbrain regions. In animals rendered dependent on butorphanol, the results obtained were similar to those of morphine-dependent rats except for the changes in DOPAC levels. Concomitant infusion of H-7 and opioid blocked both the expression of withdrawal signs and the decreases in DA, DOPAC, and HVA levels in a dose-dependent manner. These results suggest that the enhancement of cAMP-dependent protein kinase and/or protein kinase C activity accompanying the increase of DA neuron activity during continuous infusion of opioids leads to an abrupt reduction in levels of DA and its metabolites precipitated by naloxone, which is intimately involved in the expression of physical dependence on opioids.

Ziskind-Somerfeld Research Award. Protein kinase C signaling in the brain: molecular transduction of mood stabilization in the treatment of manic-depressive illness

Understanding the biology of the pharmacological stabilization of mood will undoubtedly serve to provide significant insight into the pathophysiology of manic-depressive illness (MDI). Accumulating evidence from our laboratories and those of other researchers has identified the family of protein kinase C isozymes as a shared target in the brain for the long-term action of both lithium and valproate. In rats chronically treated with lithium, there is a reduction in the hippocampus of the expression of two protein kinase isozymes, alpha and epsilon, as well as a reduction in the expression of a major PKC substrate, MARCKS, which has been implicated in long-term neuroplastic events in the developing and adult brain. In addition, we have been investigating the down-stream impact of these mood stabilizers on another kinase system, GSK-3 beta and on the AP-1 family of transcription factors. Further studies have generated promising preliminary data in support of the antimanic action of tamoxifen, and antiestrogen that is also a PKC inhibitor. Future studies must address the therapeutic relevance of these protein targets in the brain using innovative strategies in both animal and clinical investigations to ultimately create opportunities for the discovery of the next generations of mood stabilizers for the treatment of MDI.

Protein kinase A activity may kinetically upregulate the striatal transporter for dopamine

The neuronal dopamine transporter (DAT) plays a key role in terminating dopaminergic chemical neurotransmission; thus, the study of the regulation of DAT activity is important in defining parameters relevant to the control of dopaminergic neurotransmission. Interpretation of the results from previous work of this laboratory suggests that occupation of presynaptic autoreceptors increases DAT activity. Second messenger signaling related to kinetic upregulation of DAT has not been examined previously. However, others have shown that protein kinase C activity may downregulate DAT activity, whereas protein kinase A has shown variable results. Herein it is shown that protein kinase A activity mediates the kinetic upregulation of DAT. Quinpirole increased DAT activity that was blocked by sulpiride and the protein kinase A selective inhibitor H-89. Brief incubations with forskolin and 8-bromo-cAMP (8-Br-cAMP) were found to stimulate striatal DAT activity by increasing the Vmax of transport without affecting the Km. Exposures >15 min had no effect. The 8-Br-cAMP-stimulated increases in DAT activity were blocked by pre-exposure to H-89. Thus, second messenger signaling via the cAMP cascade may mediate kinetic upregulation of DAT. Kinetic analyses of the results suggest that either insertion of DAT into the membrane or activation of pre-existing DAT within the membrane mediates the regulation.

Injection of the protein kinase C inhibitor Ro31-8220 into the nucleus accumbens attenuates the acute response to amphetamine: tissue and behavioral studies

The ability of amphetamine to produce heightened locomotor activity is thought to be due to its ability to enhance dopamine release from mesolimbic dopamine neurons. The mechanism by which amphetamine increases dopamine release is not well understood, but is thought to involve exchange diffusion with synaptosomal dopamine through the dopamine transporter. We recently reported that amphetamine-mediated dopamine release in the striatum is also dependent on protein kinase C activity. In the current study, we investigated the role of protein kinase C activity in the acute neurochemical and behavioral response to amphetamine in the nucleus accumbens. Consistent with previous results in the striatum, amphetamine-stimulated dopamine release from nucleus accumbens tissue was inhibited by the specific protein kinase C inhibitor Ro31-8220, but not by the relatively inactive analog bisindoylmaleimide V. In addition, the effects of protein kinase C activity on the acute behavioral response to amphetamine was examined by injecting Ro31-8220 into the nucleus accumbens 15 min prior to intra-accumbens amphetamine. Pretreatment with Ro31-8220 attenuated the motor-stimulant effects of intra-accumbens amphetamine relative to control subjects pretreated with vehicle. Bisindoylmaleimide V did not significantly inhibit the motor-stimulant effects of intra-accumbens amphetamine. These results suggest that the action of amphetamine in the nucleus accumbens in increasing dopamine release and locomotor activity is dependent on protein kinase C activity.

Nature of methamphetamine-induced rapid and reversible changes in dopamine transporters

The nature of methamphetamine-induced rapid and transient decreases in dopamine transporter activity was investigated. Regional specificity was demonstrated, since [3H]dopamine uptake was decreased in synaptosomes prepared from the striatum, but not nucleus accumbens, of methamphetamine-treated rats. Differences among effects on dopamine transporter activity and ligand binding were also observed, since a single methamphetamine administration decreased [3H]dopamine uptake without altering [3H]WIN35428 ([3H](-)-2-beta-carbomethoxy-3-beta-(4-fluorophenyl)tropane 1,5-naphthalenedisulfonate) binding in synaptosomes prepared 1 h after injection. Moreover, multiple methamphetamine injections caused a greater decrease in [3H]dopamine uptake than [3H]WIN35428 binding in synaptosomes prepared I h after dosing. Finally, decreases in [3H]dopamine uptake, but not [3H]WIN35428 binding, were partially reversed 24 h after multiple methamphetamine injections. Western blotting indicated that saline- and methamphetamine-affected dopamine transporters co-migrated on sodium dodecyl sulfate (SDS) gels at approximately 80 kDa, and that acute, methamphetamine-induced decreases in [3H]dopamine uptake were not due to loss of dopamine transporter protein. These findings demonstrate heretofore-uncharacterized features of the acute effect of methamphetamine on dopamine transporters.

D2-Like dopamine autoreceptor activation reduces quantal size in PC12 cells

D2-like dopamine autoreceptors regulate dopamine release and are implicated in important actions of antipsychotic drugs and rewarding behaviors. To directly observe the effects of D2 autoreceptors on exocytic neurotransmitter release, we measured quantal release of dopamine from pheochromocytoma PC12 cells that express D2 and D4 autoreceptors. High potassium-evoked secretion in PC12 cells produced a unimodal population of quantal sizes. We found that exposures to the D2-like agonist quinpirole that inhibited tyrosine hydroxylase activity by approximately 50% also reduced quantal size by approximately 50%. The reduced quantal size was blocked by the D2 antagonist sulpiride and reversed by L-DOPA. Quinpirole also decreased the frequency of stimulation-evoked quantal release. Together, these findings indicate effects on quantal neurotransmission by D2-like dopamine autoreceptors previously distinguished as synthesis-modulating autoreceptors that regulate tyrosine hydroxylase activity versus impulse-regulating autoreceptors that modulate membrane potential. The results also provide an initial demonstration of a receptor-mediated mechanism that alters quantal size.

Protein kinase C in the parabrachial nucleus of rats during conditioned taste aversion induced by amphetamine

D-Amphetamine (AM) is a potent inducer of conditioned taste aversion (CTA) the mechanism of which differs from that induced by lithium. The aim of the present communication is to see whether AM-induced CTA will produce shift in the protein kinase (PKC) activity in the parabrachial nucleus (PBN). Activity of PKC was measured in PBN of rats during AM-induced CTA. In the control experiments a single intraperitoneal (i.p.) injection of AM (3 mg/kg) alone (not paired with saccharin drinking) resulted in rise of particulate bound PKC by 77% and a tendency to decrease its activity in cytosol 60 min but not 24 and 48 h after AM administration. The results suggest translocation of the enzyme from cytosol to membrane. Cytosolic PKC increased by 17 and 50%, 24 and 48 h, respectively, after acquisition of CTA (15 min after the retrieval test), when the direct effect of AM on PKC had already disappeared. Particulate PKC did not change at either of the two time intervals. Thus the total PKC activity was increased. Since we have previously observed the same PKC shifts using LiCl or CuSO4 as CTA unconditioned stimuli, we assume that any CTA inducer will elicit the same alteration of PKC in PBN.

Amphetamine increases the phosphorylation of neuromodulin and synapsin I in rat striatal synaptosomes

Amphetamine is taken up through the dopamine transporter in nerve terminals and enhances the release of dopamine. We previously found that incubation of rat striatal synaptosomes increases phosphorylation of the presynaptic neural-specific protein, neuromodulin (Gnegy et al., Mol. Brain Res. 20:289-293, 1993). Using a state-specific antibody, we now demonstrate that incubation of rat striatal synaptosomes with amphetamine increases levels of neuromodulin phosphorylated at ser41, the protein kinase C substrate site. Phosphorylation was maximal at 5 min at 37 degrees C at concentrations from 100 nM to 10 microM amphetamine. The effect of amphetamine on the phosphorylation of synapsin I at a site specifically phosphorylated by Ca2+/calmodulin-dependent protein kinase II (site 3), was examined using a state-specific antibody for site 3-phosphosynapsin I. Incubation with concentrations of amphetamine from 1 to 100 nM increased the level of site 3-phospho-synapsin I at times from 30 sec to 2 min. The effect of amphetamine on synapsin I phosphorylation was blocked by nomifensine. The presence of calcium in the incubating buffer was required for amphetamine to increase the level of site 3-phospho-synapsin I. The amphetamine-mediated increase in the content of phosphoser41-neuromodulin was less sensitive to extrasynaptosomal calcium. The amphetamine-mediated increase in the content of site 3-phospho-synapsin I persisted in the presence of 10 microM okadaic acid and was not significantly altered by D1 or D2 dopamine receptor antagonists. Preincubation of striatal synaptosomes with 10 microM of the protein kinase C inhibitor, Ro-31-8220, blocked the amphetamine-mediated increases in the levels of both phosphoser41-neuromodulin and site 3-phospho-synapsin I. Our results demonstrate that amphetamine can alter phosphorylation-related second messenger activities in the synaptosome.