Amphetamine-induced locomotion and gene expression are altered in BDNF heterozygous mice

Administration of amphetamine overstimulates medium spiny neurons (MSNs) by releasing dopamine and glutamate from afferents in the striatum. However, these afferents also release brain-derived neurotrophic factor (BDNF) that protects striatal MSNs from overstimulation. Intriguingly, all three neurochemicals increase opioid gene expression in MSNs. In contrast, striatal opioid expression is less in naive BDNF heterozygous (BDNF(+/-)) vs. wild-type (WT) mice. This study was designed to determine whether partial genetic depletion of BDNF influences the behavioral and molecular response to an acute amphetamine injection. An acute injection of amphetamine [5 mg/kg, intraperitoneal (i.p.)] or saline was administered to WT and BDNF(+/-) mice. WT and BDNF(+/-) mice exhibited similar locomotor activity during habituation, whereas BDNF(+/-) mice exhibited more prolonged locomotor activation during the third hour after injection of amphetamine. Three hours after amphetamine injection, there was an increase of preprodynorphin mRNA in the caudate putamen and nucleus accumbens (Acb) and dopamine D(3) receptor mRNA levels were increased in the Acb of BDNF(+/-) and WT mice. Striatal/cortical trkB and BDNF, and mesencephalic tyrosine hydroxylase mRNA levels were only increased in WT mice. These results indicate that BDNF modifies the locomotor responses of mice to acute amphetamine and differentially regulates amphetamine-induced gene expression.     

Brain-derived neurotrophic factor modulates dopaminergic deficits in a transgenic mouse model of Huntington's disease

Dysfunction of dopaminergic neurons may contribute to motor impairment in Huntington's disease. Here, we study the role of brain-derived neurotrophic factor (BDNF) in alterations of the nigrostriatal system associated with transgenics carrying mutant huntingtin. Using huntingtin-BDNF+/- double-mutant mice, we analyzed the effects of reducing the levels of BDNF expression in a model of Huntington's disease (R6/1). When compared with R6/1 mice, these mice exhibit an increased number of aggregates in the substantia nigra pars compacta. In addition, reduction of BDNF expression exacerbates the dopaminergic neuronal dysfunction seen in mutant huntingtin mice, such as the decrease in retrograde labelling of dopaminergic neurons and striatal dopamine content. However, mutant huntingtin mice with normal or lowered BDNF expression show the same decrease in the anterograde transport, number of dopaminergic neurons and nigral volume. In addition, reduced BDNF expression causes decreased dopamine receptor expression in mutant huntingtin mice. Examination of changes in locomotor activity induced by dopamine receptor agonists revealed that, in comparison with R6/1 mice, the double mutant mice exhibit lower activity in response to amphetamine, but not to apomorphine. In conclusion, these findings demonstrate that the decreased BDNF expression observed in Huntington's disease exacerbates dopaminergic neuronal dysfunction, which may participate in the motor disturbances associated with this neurodegenerative disorder.     

The effect of chronic levodopa on haloperidol induced behavioral supersensitivity in the guinea pig

The effect of chronic levodopa-carbidopa administration (200 mg/kg for 21 days) on guinea pigs rendered behaviorally supersensitive by the prior administration of haloperidol (.5 mg/kg for 21 days) was examined. Animals who showed an increased behavioral response to apomorphine after chronic haloperidol administration were treated with levodopa-carbidopa and then apomorphine - induced stereotypy was reexamined. Although the chronic levodopa control groups and the chronic haloperidol control remained supersensitive to the behavioral effect of apomorphine, the haloperidol-levodopa group's behavioral response to apomorphine returned to normal. Both chronic dopaminergic antagonist and agonist administration have been demonstrated to induce heightened apomorphine-induced stereotypy and this has been interpreted as a reflection of altered striatal dopamine receptor site sensitivity. The finding that the serial administration of a chronic dopaminergic antagonist followed by a chronic dopaminergic agonist results in a return to normal of a striatal dopamine receptor-dependent behavior suggests that these chronic treatments affect dopamine receptor sites by different mechanisms of action. Since neuroleptic induced dopaminergic supersensitivity in animals is an accepted model of tardive dyskinesia, levodopa may also reverse dopaminergic supersensitivity in patients and might be a potential therapeutic agent in tardive dyskinesia.     

Physiological and behavioral effects of amphetamine in BACE1−/− mice

β‐Site APP‐cleaving Enzyme 1 (BACE1) is a protease that has been linked to schizophrenia, a severe mental illness that is potentially characterized by enhanced dopamine (DA) release in the striatum. Here, we used acute amphetamine administration to stimulate neuronal activity and investigated the neurophysiological and locomotor‐activity response in BACE1‐deficient (BACE1^?/?) mice. We measured locomotor activity at baseline and after treatment with amphetamine (3.2 and 10 mg/kg). While baseline locomotor activity did not vary between groups, BACE1^?/? mice exhibited reduced sensitivity to the locomotor‐enhancing effects of amphetamine. Using high‐performance liquid chromatography (HPLC) to measure DA and DA metabolites in the striatum, we found no significant differences in BACE1^?/? compared with wild‐type mice. To determine if DA neuron excitability is altered in BACE1^?/? mice, we performed patch‐clamp electrophysiology in putative DA neurons from brain slices that contained the substantia nigra. Pacemaker firing rate was slightly increased in slices from BACE1^?/? mice. We next measured G protein‐coupled potassium currents produced by activation of D2 autoreceptors, which strongly inhibit firing of these neurons. The maximal amplitude and decay times of D2 autoreceptor currents were not altered in BACE1^?/? mice, indicating no change in D2 autoreceptor‐sensitivity and DA transporter‐mediated reuptake. However, amphetamine (30 µm )‐induced potassium currents produced by efflux of DA were enhanced in BACE1^?/? mice, perhaps indicating increased vesicular DA content in the midbrain. This suggests a plausible mechanism to explain the decreased sensitivity to amphetamine‐induced locomotion, and provides evidence that decreased availability of BACE1 can produce persistent adaptations in the dopaminergic system.     

The kappa-opioid agonist, U-69593, decreases acute amphetamine-evoked behaviors and calcium-dependent dialysate levels of dopamine and glutamate in the ventral striatum

The effects of a kappa-opioid receptor agonist on acute amphetamine-induced behavioral activation and dialysate levels of dopamine and glutamate in the ventral striatum were investigated. Amphetamine (2.5 mg/kg i.p.) evoked a substantial increase in rearing, sniffing, and hole-poking behavior as well as dopamine and glutamate levels in the ventral striatum of awake rats. U-69593 (0.32 mg/kg s.c.) significantly decreased the amphetamine-evoked increase in behavior and dopamine and glutamate levels in the ventral striatum. Reverse dialysis of the selective kappa-opioid receptor antagonist, nor-binaltorphimine, into the ventral striatum antagonized the effects of U-69593 on amphetamine-induced behavior and dopamine and glutamate levels. Reverse dialysis of low calcium (0.1 mM) into the ventral striatum decreased basal dopamine, but not glutamate, dialysate levels by 91% 45 min after initiation of perfusion. Strikingly, 0.1 mM calcium perfusion significantly reduced the 2.5 mg/kg amphetamine-evoked increase in dopamine and glutamate levels in the ventral striatum, distinguishing a calcium-dependent and a calcium-independent component of release. U-69593 did not alter the calcium-independent component of amphetamine-evoked dopamine and glutamate levels. These data are consistent with the view that a transsynaptic mechanism augments the increase in dopamine and glutamate levels in the ventral striatum evoked by a moderately high dose of amphetamine and that stimulation of kappa-opioid receptors suppresses the calcium-dependent component of amphetamine's effects.     

Low-dose apomorphine reduces serum homovanillic acid concentrations in schizophrenic patients

This study was carried out to evaluate the postulated dopaminergic autoreceptor regulatory effect in man of low-dose apomorphine. Behavior and serum homovanillic acid concentrations following low-dose apomorphine were investigated. Five medicated chronic schizophrenic patients had serum homovanillic acid concentrations measured by mass fragmentography before and after 0.005 mg/kg of apomorphine or saline placebo. Results demonstrate significant reductions in serum homovanillic acid concentrations in all five subjects following apomorphine as compared with placebo. These findings present direct evidence of a specific dopamine autoreceptor effect of low-dose apomorphine in schizophrenic patients.     

Endogenous Dopamine Functionally Activates D-1 and D-2 Receptors in Striatum

Rat striatal slices incubated with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine at 1 mM were exposed to different concentrations (1-100 microM) of the catecholamine-releasing drug amphetamine. This produced both a concentration-dependent release of endogenous dopamine and accumulation of cyclic AMP in the slices. The cyclic AMP accumulation due to amphetamine was greatly increased when slices were coincubated with the selective dopamine D-2 antagonist (-)-sulpiride (30 microM), but the amphetamine-induced release of dopamine from the slices was the same in the presence or absence of (-)-sulpiride. Pretreatment of animals with reserpine (5 mg/kg s.c., 18 h before death) and in vitro incubation with alpha-methyl-p-tyrosine (50 microM for 90 min), respectively, reduced the ability of amphetamine (1-100 microM) [in the presence of 30 microM (-)-sulpiride] to induce release of dopamine and to elevate cyclic AMP accumulation in striatal slices. A similar reduction in amphetamine-induced dopamine release and cyclic AMP accumulation in striatal slices was observed 7 days following unilateral 6-OHDA lesions of the medial forebrain bundle of rats. These results suggest that amphetamine induces release of endogenous dopamine from the terminals of nigrostriatal dopamine neurones. Released dopamine is then able functionally and concomitantly to activate D-1 and D-2 receptors, seen as stimulation and inhibition of cyclic AMP accumulation, respectively.     

Dose Response for Amphetamine-Induced Changes in Dopamine Levels in Push-Pull Perfusates of Rat Striatum

Levels of dopamine were determined in push-pull perfusates of striatum in chloral hydrate-anesthetized rats as a function of increasing systemic doses of amphetamine over the range 0.5-5.0 mg/kg. In the absence of amphetamine administration, basal dopamine levels remained stable for at least 6 h. Perfusate levels of dopamine responded in a quantitatively predictable fashion to increasing doses of amphetamine: (1) the maximal increase in perfusate levels of dopamine after amphetamine, relative to predrug levels, was directly proportional to the dose of the drug up to 3 mg/kg (fivefold after 0.5 mg/kg to 30-fold after 3 mg/kg); (2) the duration over which perfusate levels of dopamine were significantly elevated, with respect to preamphetamine levels, was proportional to the dose of amphetamine up to 5 mg/kg; and (3) each successively higher dose of amphetamine significantly increased the perfusate level of dopamine over that observed at the next lower dose up to 3 mg/kg amphetamine. However, maximal levels of dopamine in striatal perfusates were achieved following 3 mg/kg amphetamine and were not increased further at higher doses of the drug. The data suggest that, at higher doses of amphetamine, extraneuronal metabolism of dopamine may be of sufficient capacity to limit increases in synaptic levels of dopamine. The absence of further increases in perfusate levels of dopamine as the dose of amphetamine is increased beyond 3 mg/kg is discussed in terms of potential relevance to mechanisms of amphetamine-induced stereotyped behaviors.     

Amphetamine-Induced Glutamate Efflux in the Rat Ventral Tegmental Area Is Prevented by MK-801, SCH 23390, and Ibotenic Acid Lesions of the Prefrontal Cortex

We showed previously that amphetamine challenge produces a delayed increase in glutamate efflux in the ventral tegmental area of both naive and chronic amphetamine-treated rats. The present study examined the mechanisms underlying this response. The NMDA receptor antagonist MK-801 (0.1 mg/kg, i.p.) or the D1 dopamine receptor antagonist SCH 23390 (0.1 mg/kg, i.p.), given 30 min before acute amphetamine (5 mg/kg, i.p.), prevented amphetamine-induced glutamate efflux. Neither antagonist by itself altered glutamate efflux. Ibotenic acid lesions of the prefrontal cortex similarly prevented amphetamine-induced glutamate efflux, while producing a trend toward decreased basal glutamate levels (82.8% of sham group). Previous work has shown that the doses of NMDA and D1 receptor antagonists used in this study prevent the induction of behavioral sensitization when coadministered repeatedly with amphetamine, and that identical prefrontal cortex lesions performed before repeated amphetamine prevent the induction of ambulatory sensitization. Thus, treatments that prevent acute amphetamine from elevating glutamate efflux in the ventral tegmental area also prevent repeated amphetamine from eliciting behavioral sensitization. These findings suggest that repeated elevation of glutamate levels during a chronic amphetamine regimen may contribute to the cascade of neuroadaptations within the ventral tegmental area that enables the induction of sensitization.     

Lergotrile and lisuride: in vivo dopaminergic agonists which do not stimulate the presynaptic dopamine autoreceptor.

The presynaptic dopaminergic autoreceptor which regulates tyrosine hydroxylase is unaffected by the dopaminergic ergots and their specific antagonists. Apomorphine inhibits the conversion of tyrosine to catechols by striatal synaptosomes; neither lergotrile nor lisuride mimic this action of apomorphine. Furthermore metoclopramide, a potent dopaminergic antagonist in the anterior pituitary, does not block the inhibitory effect of apomorphine. The data demonstrate that the several categories pharmacologically distinct dopamine receptors exist.     

Enhanced striatal 3H-spiroperidol binding induced by chronic haloperidol treatment inhibited by peptides administered during the withdrawal phase

Chronic intragastric administration of haloperidol (1.5 mg/kg/day) for 21 days followed by a 3-day withdrawal period resulted in the development of enhanced locomotor activity response to apomorphine, and an increase in the number of binding sites for 3H-spiroperidol in the striatal membranes of the rat brain. Subcutaneous administration of Pro-Leu-Gly-NH2 or cyclo(Leu-Gly) in doses of 2 mg/kg/day given for 3-days after termination of haloperidol treatment inhibited the enhanced response to apomorphine, as well as the increases in the number of 3H-spiroperidol binding sites in the striatum. If indeed, the supersensitivity of striatal dopamine receptors is one of the mechanisms in the development of tardive dyskinesia symptoms, the present results suggest that the above peptides may be helpful in ameliorating some of the symptoms of tardive dyskinesia induced by neuroleptic drugs.     

Amperozide, a Novel Antipsychotic Drug, Inhibits the Ability of d-Amphetamine to Increase Dopamine Release In Vivo in Rat Striatum and Nucleus Accumbens

The in vivo effects of amperozide, a novel atypical antipsychotic drug, on the release of dopamine (DA) and the output of its metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), were investigated in the striatum and the nucleus accumbens of awake, freely moving rats using microdialysis. Amperozide (2-10 mg/kg, s.c.) significantly increased extracellular levels of DA in both the striatum and nucleus accumbens in a dose-dependent manner. It had a similar but lesser effect on extracellular DOPAC levels in both regions. d-Amphetamine (2 mg/kg, s.c.) alone produced a very large (43-fold) increase in DA release, together with a 70% decrease in DOPAC levels in both the striatum and the nucleus accumbens. Amperozide (1-5 mg/kg, s.c.) 30 min before d-amphetamine (2 mg/kg) dose-dependently attenuated d-amphetamine-induced DA release but had no effect on the d-amphetamine-induced decrease in extracellular DOPAC levels in both regions. The effect of amperozide on d-amphetamine-induced DA release in the nucleus accumbens may explain the inhibitory effect of amperozide on amphetamine-induced locomotor activity. However, the failure of amperozide to block amphetamine-induced stereotypy, despite marked inhibition of striatal DA release, suggests the need to reexamine the importance of striatal DA for amphetamine-induced stereotypy.     

Phencyclidine-Induced Dysregulation of Dopamine Response to Amphetamine in Prefrontal Cortex and Striatum

Phencyclidine (PCP) administration in rodents has been used to model aspects of schizophrenia. One aspect of such treatment has been the enhancement of amphetamine-induced increase of dopamine in the prefrontal cortex and striatum. To further characterize this mechanism rats were treated for 2 weeks with continuous PCP (15 mg/kg per day via Alzet minipump). Rats were implanted with a microdialysis probe into the prefrontal cortex (PFC) or striatum. Amphetamine was administered locally via the dialysis probe during one collection period and changes in extracellular dopamine were monitored. The effect of local administration of the dopamine uptake blocker nomifensine was also measured. Amphetamine (10 M) and nomifensine (10 M) increased the level of dopamine in both the PFC and striatum. PCP administration did not alter the response to amphetamine or nomifensine in the PFC, but reduced this response about 2-fold in striatum. To examine effects of continuous PCP administration on dopamine autoreceptor function, release of [³H]dopamine in response to electrical stimulation and in the presence of a dopamine agonist or antagonist was tested in striatal and prefrontal cortical tissue. Autoreceptor responses were similar in control and PCP-treated tissues. We conclude that the brain region-specific enhancement of dopamine release by peripheral amphetamine administration in rats after PCP is not likely mediated by alterations in the dopamine autoreceptors or changes in the dopamine transporter. The selective local responses of amphetamine indicates heterogeneous regional effects of continuous PCP on NMDA receptor function; effects that influence both regional excitatory responses and the overall dynamics of tonic excitatory/inhibitory inputs to the PFC and striatum.     

Effects of Daily Cocaine and Morphine Treatment on Somatodendritic and Terminal Field Dopamine Release

Daily injections of cocaine or morphine into rodents produces behavioral sensitization such that the last daily injection results in a greater motor stimulant effect than the first injection. To evaluate a role for brain dopamine in behavioral sensitization to cocaine and morphine, tissue slices from the ventromedial mesencephalon (containing dopamine cell bodies), the nucleus accumbens, and striatum (dopamine terminal fields) were obtained from rats pretreated with daily cocaine, morphine, or saline 2-3 weeks earlier. When the tissue slices were depolarized by increasing potassium concentration in the superfusate, the release of endogenous dopamine from the ventromedial mesencephalon of cocaine- and morphine-pretreated rats was significantly decreased. In contrast, the release of dopamine from the nucleus accumbens and striatum was either unaltered or slightly enhanced in rats pretreated with cocaine and morphine. When dopamine was released by amphetamine, a significant decrease in dopamine release from the ventromedial mesencephalon of cocaine-pretreated rats was measured. No other significant changes were measured after amphetamine-induced release. It is postulated that the decrease in dopamine release from the ventromedial mesencephalon of cocaine- and morphine-sensitized rats results in less somatodendritic autoreceptor stimulation, and thereby produces an increase in dopamine neuronal activity.     

Differential Effects of δ- and μ-Opioid Receptor Antagonists on the Amphetamine-Induced Increase in Extracellular Dopamine in Striatum and Nucleus Accumbens

Abstract: The specific opioid receptor antagonist naloxone attenuates the behavioral and neurochemical effects of amphetamine. Furthermore, the amphetamine-induced increase in locomotor activity is attenuated by intracisternally administered naltrindole, a selective δ-opioid receptor antagonist, but not by the irreversible μ-opioid receptor antagonist β-funaltrexamine. Therefore, this research was designed to determine if naltrindole would attenuate the neurochemical response to amphetamine as it did the behavioral response. In vivo microdialysis was used to monitor the change in extracellular concentrations of dopamine in awake rats. Naltrindole (3.0, 10, or 30 µg) or vehicle was given 15 min before and β-funaltrexamine (10 µg) or vehicle 24 h before the start of cumulative dosing, intracisternally in a 10-µl volume, while the rats were lightly anesthetized with methoxyflurane. Cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) followed pretreatment injections at 30-min intervals. Dialysate samples were collected every 10 min from either the striatum or nucleus accumbens and analyzed for dopamine content by HPLC. Amphetamine dose-dependently increased dopamine content in both the striatum and nucleus accumbens, as reported previously. Naltrindole (3.0, 10, and 30 µg) significantly reduced the dopamine response to amphetamine in the striatum. In contrast, 30 µg of naltrindole did not modify the dopamine response to amphetamine in the nucleus accumbens. On the other hand, β-funaltrexamine (10 µg) had no effect in the striatum but significantly attenuated the amphetamine-induced increase in extracellular dopamine content in the nucleus accumbens. These data suggest that δ-opioid receptors play a relatively larger role than μ-opioid receptors in mediating the amphetamine-induced increase in extracellular dopamine content in the striatum, whereas μ-opioid receptors play a larger role in mediating these effects in the nucleus accumbens.     

Calmodulin kinase II interacts with the dopamine transporter C terminus to regulate amphetamine-induced reverse transport

Efflux of dopamine through the dopamine transporter (DAT) is critical for the psychostimulatory properties of amphetamines, but the underlying mechanism is unclear. Here we show that Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) plays a key role in this efflux. CaMKIIalpha bound to the distal C terminus of DAT and colocalized with DAT in dopaminergic neurons. CaMKIIalpha stimulated dopamine efflux via DAT in response to amphetamine in heterologous cells and in dopaminergic neurons. CaMKIIalpha phosphorylated serines in the distal N terminus of DAT in vitro, and mutation of these serines eliminated the stimulatory effects of CaMKIIalpha. A mutation of the DAT C terminus impairing CaMKIIalpha binding also impaired amphetamine-induced dopamine efflux. An in vivo role for CaMKII was supported by chronoamperometry measurements showing reduced amphetamine-induced dopamine efflux in response to the CaMKII inhibitor KN93. Our data suggest that CaMKIIalpha binding to the DAT C terminus facilitates phosphorylation of the DAT N terminus and mediates amphetamine-induced dopamine efflux.     

Ibogaine reduces amphetamine-induced locomotor stimulation in C57BL/6By mice, but stimulates locomotor activity in rats.

The effect of ibogaine hydrochloride on locomotor stimulation induced by d-amphetamine sulfate was tested in male C57BL/6By mice and in female Sprague-Dawley rats. In mice, locomotor stimulation induced by d-amphetamine at 1 or 5 mg/kg s.c. was reduced by prior administration of one or two injections of ibogaine (40 mg/kg), given 2 or 18 hours earlier. This reduction in locomotor activity persisted for two days. Locomotor stimulation induced by a higher dose (10 mg/kg) of d-amphetamine was not reduced by such prior administration of ibogaine. A lower dose of ibogaine (20 mg/kg) did not reduce the subsequent locomotor activity induced by d-amphetamine. Ibogaine decreased striatal dopamine levels, while d-amphetamine increased them. Ibogaine treatment (2 x 40 mg/kg, 18 hours apart) induced a decrease by 30% in the level of striatal dopamine and its metabolites measured in tissue extracts 3 hours after the second ibogaine injection. One hour after d-amphetamine (5 mg/kg) administration, the level of striatal dopamine increased by 26%. Although the level of striatal dopamine was initially lower in the ibogaine-pretreated mice, d-amphetamine (5 mg/kg) administration induced an increase in striatal dopamine and its metabolites. The effect of ibogaine seems to be species specific, since in rats pretreated with ibogaine 18 hours before d-amphetamine, locomotor stimulation induced by d-amphetamine was further increased. In addition, the in vitro electrical-evoked release of [3H]dopamine from striatal tissue was either unchanged or inhibited in the presence of d-amphetamine, and after ibogaine pretreatment in vivo, the release of tritium in the presence of d-amphetamine was inhibited or stimulated in mice and rats, respectively.     

In Vitro Release of Endogenous Dopamine from the Striatum of the Weaver Mutant Mouse

The weaver mutant mouse has a genetically determined defect in the nigrostriatal dopaminergic system. The present study was undertaken to test the hypothesis that in the weaver mutant mouse, striatal nerve terminals undergo compensatory changes in response to this deficiency. To test this hypothesis, we studied the basal and stimulated release of dopamine from striatal slices of weaver mutant mice and matched controls. By using a superfusion system and concentrating the superfusate by passage over alumina, resting dopamine release could be determined in the weaver mutant despite the fact that striatal tissue content of dopamine in these mice is reduced by greater than 75% compared with control mice. Fractional resting release of dopamine in weaver striatal slices was significantly elevated compared with that in controls, suggesting that the release mechanisms in the weaver may be adapting to overcome the dopamine deficit. Potassium-evoked release (24 and 48 mM potassium) was not significantly different between the two genotypes. In contrast, amphetamine-evoked release (1 microM) was significantly greater in the weaver mice than in controls. In both genotypes, release evoked by amphetamine was completely inhibited by cocaine, implicating the dopamine uptake carrier in this release process. These findings suggest that fundamental differences in dopamine release mechanisms exist between weaver and control mice and support the hypothesis that compensatory mechanisms may develop in neurons in response to dopamine deficits.     

Repeated treatment with the kappa opioid receptor agonist U69593 reverses enhanced K+ induced dopamine release in the nucleus accumbens, but not the expression of locomotor sensitization in amphetamine-sensitized rats

The effects after the acute activation of the kappa opioid receptor (KOR) can be distinguished from the effect after repeated administration of KOR agonist. Here, we report the effect of repeated administration of U69593 during abstinence after amphetamine-induced locomotor sensitization. Rats were injected once daily with amphetamine for five consecutive days. From day 6 to 9, rats that developed locomotor sensitization, received once daily injection of U69593 or vehicle. On day 10, all rats were injected with a challenging dose of amphetamine and locomotor activity was measured to assess the expression of sensitization. Microdialysis studies were carried out to assess dopamine extracellular levels in NAc. Rats that develop and express horizontal locomotor sensitization to amphetamine show increased dopamine release in the NAc induced by high K(+). The repeated treatment with U69593 reverses the sensitized depolarization-stimulated dopamine release in the NAc, but not the expression of locomotor sensitization induced by amphetamine. Thus, repeated activation of KORs during early amphetamine withdrawal dissociates the behavioral responses and the neurochemical responses that accompany the expression of sensitization to amphetamine.     

Effects of amphetamine on dopamine release in the rat nucleus accumbens shell region depend on cannabinoid CB1 receptor activation

The psychostimulant drug amphetamine is often prescribed to treat Attention-Deficit/Hyperactivity Disorder. The behavioral effects of the psychostimulant drug amphetamine depend on its ability to increase monoamine neurotransmission in brain regions such as the nucleus accumbens (NAC) and medial prefrontal cortex (mPFC). Recent behavioral data suggest that the endocannabinoid system also plays a role in this respect. Here we investigated the role of cannabinoid CB1 receptor activity in amphetamine-induced monoamine release in the NAC and/or mPFC of rats using in vivo microdialysis. Results show that systemic administration of a low, clinically relevant dose of amphetamine (0.5mg/kg) robustly increased dopamine and norepinephrine release (to ～175-350% of baseline values) in the NAC shell and core subregions as well as the ventral and dorsal parts of the mPFC, while moderately enhancing extracellular serotonin levels (to ～135% of baseline value) in the NAC core only. Although systemic administration of the CB1 receptor antagonist SR141716A (0-3mg/kg) alone did not affect monoamine release, it dose-dependently abolished amphetamine-induced dopamine release specifically in the NAC shell. SR141716A did not affect amphetamine-induced norepinephrine or serotonin release in any of the brain regions investigated. Thus, the effects of acute CB1 receptor blockade on amphetamine-induced monoamine transmission were restricted to dopamine, and more specifically to mesolimbic dopamine projections into the NAC shell. This brain region- and monoamine-selective role of CB1 receptors is suggested to subserve the behavioral effects of amphetamine.