Neuronal dopamine transporter activity, density and methamphetamine inhibition are differentially altered in the nucleus accumbens and striatum with no changes in glycosylation in rats behaviorally sensitized to methamphetamine

Berberine hydrochloride attenuates voluntary methamphetamine consumption and anxiety-like behaviors via modulation of oxytocin receptors in methamphetamine addicted rats

OBJECTIVE

Methamphetamine (METH) addiction is recognized as one of the major public health concerns, with no approved pharmacological agents for treatment. Berberine hydrochloride, an isoquinoline alkaloid in plants, induces antipsychotic and anxiolytic effects. Hence, we hypothesized that berberine may modulate the METH-induced rewarding effects.

MATERIALS AND METHODS

In this study, three groups of rat including control (N = 10), METH + vehicle (N = 10), and METH + berberine (N = 10) were kept in separate cages one day before expriments. METH (20 mg/L) was dissolved in tap water inside a bottle, while there was only tap water in the control bottle. Two groups received free METH solutions for two weeks (up to 12 mg/kg). Afterwards, they were abstianced for three weeks. Only one group received 100 mg/kg/day of berberine. After three weeks, locomotor activity and anxiety (elevated plus maze test) were evaluated, then the two-bottles choice model was used for one week to evaluate drug preferences. Finally, the brain of rats was removed for evaluation of oxytocin receptor expression via immunofluorescence staining method.

RESULTS

The results showed that METH preference was lower in the berberine + METH group during drug intake compared to the METH group (P < .05). During withdrawal, berberine reduced anxiety-like behaviors (P < .05) and decreased locomotor activity versus the METH group (P < .001). Also, berberine increased numbers of oxytocin receptors in comparison with the METH group (P < .01).

CONCLUSION

Considering the modulation of oxytocin receptors, berberine may be considered as a potential therapeutic agent for METH addiction.

Regionally Specific Effects of Oxytocin on Reinstatement of Cocaine Seeking in Male and Female Rats

BACKGROUND

Oxytocin reduces cued reinstatement of cocaine seeking in male and female rats, but the underlying neurobiology has not been uncovered. The majority of effort on this task has focused on oxytocin and dopamine interactions in the nucleus accumbens core. The nucleus accumbens core is a key neural substrate in relapse, and oxytocin administration in the nucleus accumbens core reduces reinstatement to methamphetamine cues. Further, the nucleus accumbens core has strong glutamatergic innervation from numerous regions including the prefrontal cortex. Thus, we hypothesize that oxytocin regulates presynaptic glutamate terminals in the nucleus accumbens core, thereby affecting reinstatement.

METHODS

To begin to evaluate this hypothesis, we examined the effects of intra-nucleus accumbens core oxytocin on extracellular glutamate levels in this region. We next determined if direct infusion of oxytocin into the nucleus accumbens core could attenuate cued reinstatement of cocaine seeking in a manner dependent on metabotropic glutamate 2/3 receptors. Finally, we tested if site-specific application of oxytocin in the prefrontal cortex reduced cued reinstatement of cocaine seeking.

RESULTS

We found an increase in nucleus accumbens core extracellular glutamate for several minutes following reverse dialysis of oxytocin. In male and female rats with a history of cocaine self-administration, site-specific application of oxytocin in the nucleus accumbens core and prefrontal cortex had opposing effects, decreasing and increasing cued reinstatement, respectively. The mGlu2/3 antagonist LY-341495 reversed oxytocin's ability to attenuate cued reinstatement.

CONCLUSIONS

While the precise mechanism by which oxytocin increases nucleus accumbens core glutamate is yet to be determined, the present results clearly support oxytocin mediation of glutamate neurotransmission in the nucleus accumbens core that impacts cued cocaine seeking.

Effects of chronic methamphetamine on psychomotor and cognitive functions and dopamine signaling in the brain

Methamphetamine (MA) studies in animals usually involve acute, binge, or short-term exposure to the drug. However, addicts take substantial amounts of MA for extended periods of time. Here we wished to study the effects of MA exposure on brain and behavior, using an animal model analogous to this pattern of MA intake. MA doses, 4 and 8mg/kg/day, were based on previously reported average daily freely available MA self-administration levels. We examined the effects of 16 week MA treatment on psychomotor and cognitive function in the rat using open field and novel object recognition tests and we studied the adaptations of the dopaminergic system, using in vitro and in vivo receptor imaging. We show that chronic MA treatment, at doses that correspond to the average daily freely available self-administration levels in the rat, disorganizes open field activity, impairs alert exploratory behavior and anxiety-like state, and downregulates dopamine transporter in the striatum. Under these treatment conditions, dopamine terminal functional integrity in the nucleus accumbens is also affected. In addition, lower dopamine D1 receptor binding density, and, to a smaller degree, lower dopamine D2 receptor binding density were observed. Potential mechanisms related to these alterations are discussed.

The neurocircuitry involved in oxytocin modulation of methamphetamine addiction

The role of oxytocin in attenuating the abuse of licit and illicit drugs, including the psychostimulant methamphetamine, has been examined with increased ferocity in recent years. This is largely driven by the potential application of oxytocin as a pharmacotherapy. However, the neural mechanisms by which oxytocin modulates methamphetamine abuse are not well understood. Recent research identified an important role for the accumbens core and subthalamic nucleus in this process, which likely involves an interaction with dopamine, glutamate, GABA, and vasopressin. In addition to providing an overview of methamphetamine, the endogenous oxytocin system, and the effects of exogenous oxytocin on drug abuse, we propose a neural circuit through which exogenous oxytocin modulates methamphetamine abuse, focusing on its interaction with neurochemicals within the accumbens core and subthalamic nucleus. A growing understanding of exogenous oxytocin effects at a neurochemical and neurobiological level will assist in its evaluation as a pharmacotherapy for drug addiction.

Nucleus accumbens invulnerability to methamphetamine neurotoxicity

Methamphetamine (Meth) is a neurotoxic drug of abuse that damages neurons and nerve endings throughout the central nervous system. Emerging studies of human Meth addicts using both postmortem analyses of brain tissue and noninvasive imaging studies of intact brains have confirmed that Meth causes persistent structural abnormalities. Animal and human studies have also defined a number of significant functional problems and comorbid psychiatric disorders associated with long-term Meth abuse. This review summarizes the salient features of Meth-induced neurotoxicity with a focus on the dopamine (DA) neuronal system. DA nerve endings in the caudate-putamen (CPu) are damaged by Meth in a highly delimited manner. Even within the CPu, damage is remarkably heterogeneous, with ventral and lateral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared the damage that accompanies binge Meth intoxication, but relatively subtle changes in the disposition of DA in its nerve endings can lead to dramatic increases in Meth-induced toxicity in the CPu and overcome the normal resistance of the NAc to damage. In contrast to the CPu, where DA neuronal deficiencies are persistent, alterations in the NAc show a partial recovery. Animal models have been indispensable in studies of the causes and consequences of Meth neurotoxicity and in the development of new therapies. This research has shown that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of Meth to include brain structures not normally targeted for damage. The resistance of the NAc to Meth-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of Meth neurotoxicity by alterations in DA homeostasis is significant in light of the numerous important roles played by this brain structure.

Sex-Dependent Changes in Striatal Dopamine Transport in Preadolescent Rats Exposed Prenatally and/or Postnatally to Methamphetamine

Methamphetamine (MA) is the most commonly used psychostimulant drug, the chronic abuse of which leads to neurodegenerative changes in the brain. The global use of MA is increasing, including in pregnant women. Since MA can cross both placental and haematoencephalic barriers and is also present in maternal milk, children of chronically abused mothers are exposed prenatally as well as postnatally. Women seem to be more vulnerable to some aspects of MA abuse than men. MA is thought to exert its effects among others via direct interactions with dopamine transporters (DATs) in the brain tissue. Sexual dimorphism of the DAT system could be a base of sex-dependent actions of MA observed in behavioural and neurochemical studies. Possible sex differences in the DATs of preadolescent offspring exposed to MA prenatally and/or postnatally have not yet been evaluated. We examined the striatal synaptosomal DATs (the activity and density of surface expressed DATs and total DAT expression) in preadolescent male and female Wistar rats (31-35-day old animals) exposed prenatally and/or postnatally to MA (daily 5 mg/kg, s.c. to mothers during pregnancy and lactation). To distinguish between specific and nonspecific effects of MA on DATs, we also evaluated the in vitro effects of lipophilic MA on the fluidity of striatal membranes isolated from preadolescent and young adult rats of both sexes. We observed similar changes in the DATs of preadolescent rats exposed prenatally or postnatally (MA-mediated drop in the reserve pool but no alterations in surface-expressed DATs). However, prenatal exposure evoked significant changes in males and postnatal exposure in females. A significant decrease in the activity of surface-expressed DATs was found only in postnatally exposed females sensitized to MA via prenatal exposure. MA applied in vitro increased the fluidity of striatal membranes of preadolescent female but not male rats. In summary, DATs of preadolescent males are more sensitive to prenatal MA exposure via changes in the reserve pool and those of preadolescent females to postnatal MA exposure via the same mechanism. The combination of prenatal and postnatal MA exposure increases the risk of dopaminergic deficits via alterations in the activity of surface-expressed DATs especially in preadolescent females. MA-mediated changes in DATs of preadolescent females could be still enhanced via nonspecific disordering actions of MA on striatal membranes.

Expression of dopamine transporter in the different cerebral regions of methamphetamine-dependent rats

Objectives:

To observe the expression of the dopamine transporter (DAT) in six cerebral regions of a methamphetamine (MA)-dependent rat, which were frontal cortex, nucleus accumbens septi, striatum, hippocampus, substantia nigra and ventral tegmental area.

Methods:

The rats were administrated intraperitoneally with 10 mg/kg/day of MA for 10 days consecutively; the behaviour changes were measured via the conditioned place preference (CPP), and the scores of stereotyped behaviour (SB) were used to confirm animal addiction. Then, the animals were further injected with MA respectively for 1, 2, 4 and 8 weeks to establish different periods of MA-dependent models. The expressions of DAT and DAT messenger RNA in six cerebral regions were detected.

Results:

The results of CPP and SB scores were significant different when comparing all four experimental groups with the control group ( p < 0.05). Comparing between different experimental groups, the expression of DAT mainly decreased and had dynamic changes in the same regions ( p < 0.05). Comparing the different regions with each other in the same experimental group, the expression of DAT also had significant difference in several regions p < 0.05).

Conclusions:

The expression of DAT mainly decreased and had different in the six cerebral regions at the same MA-dependent time period as well as at different time periods in the same cerebral region. It was speculated that DAT might play a crucial role in the mechanism of MA dependence.

Involvement of the brain renin-angiotensin system (RAS) in the neuroadaptive responses induced by amphetamine in a two-injection protocol

A single or repeated exposure to psychostimulants induces long-lasting neuroadaptative changes. Different neurotransmitter systems are involved in these responses including the neuropeptide angiotensin II. Our study tested the hypothesis that the neuroadaptative changes induced by amphetamine produce alterations in brain RAS components that are involved in the expression of the locomotor sensitization to the psychostimulant drug. Wistar male rats, pretreated with amphetamine were used 7 or 21 days later to study AT1 receptors by immunohistochemistry and western blot and also angiotensinogen mRNA and protein in caudate putamen and nucleus accumbens. A second group of animals was used to explore the possible role of Ang II AT1 receptors in the expression of behavioral sensitization. In these animals treated in the same way, bearing intra-cerebral cannula, the locomotor activity was tested 21 days later, after an amphetamine challenge injection and the animals received an AT1 blocker, losartan, or saline 5min before the amphetamine challenge. An increase of AT1 receptor density induced by amphetamine was found in both studied areas and a decrease in angiotensinogen mRNA and protein only in CPu at 21 days after treatment; meanwhile, no changes were established in NAcc. Finally, the increased locomotor activity induced by amphetamine challenge was blunted by losartan administration in CPu. No differences were detected in the behavioral sensitization when the AT1 blocker was injected in NAcc. Our results support the hypothesis of a key role of brain RAS in the neuroadaptative changes induced by amphetamine.

Methamphetamine-induced dopamine terminal deficits in the nucleus accumbens are exacerbated by reward-associated cues and attenuated by CB1 receptor antagonism

Methamphetamine (METH) exposure is primarily associated with deleterious effects to dopaminergic neurons. While several studies have implicated the endocannabinoid system in METH's locomotor, rewarding and neurochemical effects, a role for this signaling system in METH's effects on dopamine terminal dynamics has not been elucidated. Given that CB1 receptor blockade reduces the acute potentiation of phasic extracellular dopamine release from other psychomotor stimulant drugs and that the degree of acute METH-induced increases in extracellular dopamine levels is related to the severity of dopamine depletion, we predicted that pretreatment with the CB1 receptor antagonist rimonabant would reduce METH-induced alterations at dopamine terminals. Furthermore, we hypothesized that administration of METH in environments where reward associated-cues were present would potentiate METH's acute effects on dopamine release in the nucleus accumbens and exacerbate changes in dopamine terminal activity. Fast-scan cyclic voltammetry was used to measure electrically-evoked dopamine release in the nucleus accumbens and revealed markers of compromised dopamine terminal integrity nine days after a single dose of METH. These were exacerbated in animals that received METH in the presence of reward-associated cues, and attenuated in rimonabant-pretreated animals. While these deficits in dopamine dynamics were associated with reduced operant responding on days following METH administration in animals treated with only METH, rimonabant-pretreated animals exhibited levels of operant responding comparable to control. Moreover, dopamine release correlated significantly with changes in lever pressing behavior that occurred on days following METH administration. Together these data suggest that the endocannabinoid system is involved in the subsecond dopaminergic response to METH.

Oxytocin directly administered into the nucleus accumbens core or subthalamic nucleus attenuates methamphetamine-induced conditioned place preference

Accumulating evidence indicates that the neuropeptide oxytocin (OXY) may modulate reward-related behavioural responses to methamphetamine (METH) administration. Limited research has examined the effect of OXY on METH-induced conditioned place preference (CPP) and little is known about the neural mechanisms involved. A Fos immunohistochemistry study recently demonstrated that peripheral OXY administration reduced METH-induced Fos expression within the nucleus accumbens (NAc) core and subthalamic nucleus (STh) in rats. The current study aimed to (i) investigate the effect of systemically administered OXY on METH-induced CPP, (ii) determine the effectiveness of a single-trial CPP procedure with METH, in order to (iii) evaluate whether pretreatment with OXY injected directly into the NAc core or the STh attenuates METH-induced CPP. Results showed that male Sprague Dawley rats learned to associate unique compartmental cues with METH (1 mg/kg, i.p.) such that they spent more time in the METH-paired compartment and less time in the saline-paired compartment. Pretreatment with systemic OXY (0.6 mg, i.p.), or OXY (0.6 ng, i.c.) microinjected into the NAc core or the STh prior to METH administration attenuated the formation of a CPP to METH. This provides further evidence that OXY acts within either the NAc core or the STh to reduce the rewarding effects of METH administration.

Mass transport at rotating disk electrodes: effects of synthetic particles and nerve endings

An unstirred layer (USL) exists at the interface of solids with solutions. Thus, the particles in brain tissue preparations possess a USL as well as at the surface of a rotating disk electrode (RDE) used to measure chemical fluxes. Time constraints for observing biological kinetics based on estimated thicknesses of USLs at the membrane surface in real samples of nerve endings were estimated. Liposomes, silica, and Sephadex were used separately to model the tissue preparation particles. Within a solution stirred by the RDE, both diffusion and hydrodynamic boundary layers are formed. It was observed that the number and size of particles decreased the following: the apparent diffusion coefficient excluding Sephadex, boundary layer thicknesses excluding silica, sensitivity excluding diluted liposomes (in agreement with results from other laboratories), limiting current potentially due to an increase in the path distance, and mixing time. They have no effect on the detection limit (6 ± 2 nM). The RDE kinetically resolves transmembrane transport with a timing of approximately 30 ms.

Extended methamphetamine self-administration in rats results in a selective reduction of dopamine transporter levels in the prefrontal cortex and dorsal striatum not accompanied by marked monoaminergic depletion

Chronic abuse of methamphetamine leads to cognitive dysfunction and high rates of relapse, paralleled by significant changes of brain dopamine and serotonin neurotransmission. Previously, we found that rats with extended access to methamphetamine self-administration displayed enhanced methamphetamine-primed reinstatement of drug-seeking and cognitive deficits relative to limited access animals. The present study investigated whether extended access to methamphetamine self-administration produced abnormalities in dopamine and serotonin systems in rat forebrain. Rats self-administered methamphetamine (0.02-mg/i.v. infusion) during daily 1-h sessions for 7 to 10 days, followed by either short- (1-h) or long-access (6-h) self-administration for 12 to 14 days. Lever responding was extinguished for 2 weeks before either reinstatement testing or rapid decapitation and tissue dissection. Tissue levels of monoamine transporters and markers of methamphetamine-induced toxicity were analyzed in several forebrain areas. Long-access methamphetamine self-administration resulted in escalation of daily drug intake ( approximately 7 mg/kg/day) and enhanced drug-primed reinstatement compared with the short-access group. Furthermore, long-, but not short-access to self-administered methamphetamine resulted in persistent decreases in dopamine transporter (DAT) protein levels in the prefrontal cortex and dorsal striatum. In contrast, only minor alterations in the tissue levels of dopamine or its metabolites were found, and no changes in markers specific for dopamine terminals or glial cell activation were detected. Our findings suggest that persistent methamphetamine seeking is associated with region-selective changes in DAT levels without accompanying monoaminergic neurotoxicity. Greater understanding of the neuroadaptations underlying persistent methamphetamine seeking and cognitive deficits could yield targets suitable for future therapeutic interventions.

Methamphetamine-induced behavioral sensitization is enhanced in the HIV-1 transgenic rat

Methamphetamine (METH) addiction is prevalent among individuals with HIV infection. We hypothesize that HIV-positive individuals are more prone to METH use and to the development of METH dependence. To test this hypothesis, we examined the effects of METH (daily intraperitoneal injection 2.5 mg/kg for 6 days) on rearing and head movement in 12-13-week-old male HIV-1 transgenic (HIV-1Tg) rats compared to F344 control rats as an indicator of behavioral sensitization, also representing neural adaptation underlying drug dependence and addiction. Body and brain weights were also recorded. The involvement of the dopaminergic system was investigated by examining dopamine receptors 1 (D1R) and 2 (D2R) and dopamine transporter (DAT) expression in the striatum and prefrontal cortex. METH increased rearing number and duration in both F344 and HIV-1Tg rats. Rearing number was attenuated over time, whereas rearing duration remained constant. METH also induced a progressive increase in stereotypical head movement in both F344 and HIV-1Tg rats, but it was greater in the HIV-1Tg rats than in the F344 animals. The brain to body weight ratio was significantly lower in METH-treated HIV-1Tg rats compared to F344 controls. There was no significant difference in striatal D1R, D2R, or DAT messenger RNA in HIV-1Tg and F344 rats. However, D1R expression was greater in the prefrontal cortex of HIV-1Tg rats than F344 rats and was attenuated by METH. Our results indicate that METH-induced behavioral sensitization is greater in the presence of HIV infection and suggest that D1R expression in the prefrontal cortex may play a role in METH addiction in HIV-positive individuals.

Increases in cytoplasmic dopamine compromise the normal resistance of the nucleus accumbens to methamphetamine neurotoxicity

Methamphetamine (METH) is a neurotoxic drug of abuse that damages the dopamine (DA) neuronal system in a highly delimited manner. The brain structure most affected by METH is the caudate-putamen (CPu) where long-term DA depletion and microglial activation are most evident. Even damage within the CPu is remarkably heterogenous with lateral and ventral aspects showing the greatest deficits. The nucleus accumbens (NAc) is largely spared of the damage that accompanies binge METH intoxication. Increases in cytoplasmic DA produced by reserpine, L-DOPA or clorgyline prior to METH uncover damage in the NAc as evidenced by microglial activation and depletion of DA, tyrosine hydroxylase (TH), and the DA transporter. These effects do not occur in the NAc after treatment with METH alone. In contrast to the CPu where DA, TH, and DA transporter levels remain depleted chronically, DA nerve ending alterations in the NAc show a partial recovery over time. None of the treatments that enhance METH toxicity in the NAc and CPu lead to losses of TH protein or DA cell bodies in the substantia nigra or the ventral tegmentum. These data show that increases in cytoplasmic DA dramatically broaden the neurotoxic profile of METH to include brain structures not normally targeted for damage by METH alone. The resistance of the NAc to METH-induced neurotoxicity and its ability to recover reveal a fundamentally different neuroplasticity by comparison to the CPu. Recruitment of the NAc as a target of METH neurotoxicity by alterations in DA homeostasis is significant in light of the important roles played by this brain structure.