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

Paradoxical effects of prodynorphin gene deletion on basal and cocaine-evoked dopaminergic neurotransmission in the nucleus accumbens

Quantitative and conventional microdialysis were used to investigate the effects of constitutive deletion of the prodynorphin gene on basal dopamine (DA) dynamics in the nucleus accumbens (NAc) and the responsiveness of DA neurons to an acute cocaine challenge. Saline- and cocaine-evoked locomotor activity were also assessed. Quantitative microdialysis revealed that basal extracellular DA levels were decreased, while the DA extraction fraction, an indirect measure of DA uptake, was unchanged in dynorphin (DYN) knockout (KO) mice. The ability of cocaine to increase NAc DA levels was reduced in KO. Similarly, cocaine-evoked locomotor activity was decreased in KO. The selective kappa opioid receptor agonist U-69593 decreased NAc dialysate DA levels in wildtype mice and this effect was enhanced in KO. Administration of the selective kappa opioid receptor (KOPr) antagonist nor-binaltorphimine to KO mice attenuated the decrease in cocaine-induced DA levels. However, it was ineffective in altering the decreased locomotor response to cocaine. These studies demonstrate that constitutive deletion of prodynorphin is associated with a reduction of extracellular NAc DA levels and a decreased responsiveness to acute cocaine. Data regarding the effects of U-69593 and nor-binaltorphimine in KO suggest that the kappa opioid receptor is up-regulated as a consequence of prodynorphin gene deletion and that this adaptation underlies the decrease in basal DA dynamics and cocaine-evoked DA levels observed in DYN KO mice. These findings suggest that the phenotype of DYN KO mice is not solely due to loss of endogenous opioid peptide but also reflects developmental compensations that occur at the level of the opioid receptor.

Hyperthermia influences excitatory and inhibitory amino acid neurotransmitters in the central nervous system. An experimental study in the rat using behavioural, biochemical, pharmacological, and morphological approaches

Role of excitatory amino acids, glutamate, aspartate, and inhibitory amino acids, gamma aminobutyric acid (GABA) and glycine in brain damage caused by heat stress was examined in a rat model. Subjection of rats to 4 h heat stress at 38 degrees C in a biological oxygen demand (BOD) incubator resulted in a marked increase in glutamate and aspartate in some brain regions, whereas a significant decline in GABA and glycine was observed in several brain areas. Profound behavioural alterations and impairment of motor and cognitive functions were seen at this time. Breakdown of the blood-brain barrier (BBB), reduction in regional cerebral blood flow (CBF), edema formation and cell injuries are prominent in several parts of the brain. Pretreatment with multiple opioid receptor antagonist, naloxone (10 mg/kg, i.p.) significantly restored the heat stress induced decline in GABA and glycine and thwarted the elevation of glutamate and aspartate in various brain areas. The motor or cognitive deficits were also attenuated. A significant reduction in BBB permeability, cerebral blood flow abnormalities, edema formation and cell injuries was evident. These novel observations suggest that (i) glutamate, aspartate, GABA and glycine are involved in the pathophysiology of heat stress, and (ii) a balance between excitatory and inhibitory amino acids in brain is crucial in hyperthermia induced brain injuries or repair.

Extracellular signal-regulated mitogen-activated protein kinase inhibitors decrease amphetamine-induced behavior and neuropeptide gene expression in the striatum

The aim of this study was to determine whether inhibition of the extracellular-regulated kinase signaling pathway decreases acute amphetamine-induced behavioral activity and neuropeptide gene expression in the rat striatum. Western blotting revealed that extracellular-regulated kinase1/2 phosphorylation was highly induced in the rat striatum 15 min after an acute amphetamine (2.5 mg/kg, i.p.) injection without altering the total amount of extracellular-regulated kinase protein. In a separate experiment, the systemic injection of SL327, a selective inhibitor of extracellular regulated kinase kinase that crosses the blood-brain barrier, 1 h prior to amphetamine administration decreased amphetamine-induced vertical and horizontal activity. Quantitative in situ hybridization histochemistry showed that SL327 abolished the high levels of preproenkephalin and preprodynorphin mRNA induced by amphetamine in the striatum with no alteration of their basal levels. In another set of experiments, the hyperlocomotor activity induced by amphetamine was reduced by pretreatment with intra-striatal infusion of U0126. U0126 also blocked the amphetamine-induced increases in phospho-extracellular-regulated kinase and preproenkephalin and preprodynorphin gene expression in the striatum. These data indicate that activation of the extracellular-regulated kinase cascade contributes to the behavioral effects and changes in striatal neuropeptide gene expression induced by acute amphetamine.

Comparison of basal and D-1 dopamine receptor agonist-stimulated neuropeptide gene expression in caudate-putamen and nucleus accumbens of ad libitum fed and food-restricted rats

Behavioral studies have demonstrated that chronic food restriction augments the rewarding and motor-activating effects of centrally injected psychostimulants and direct dopamine (DA) receptor agonists. Recently, it has been shown that intracerebroventricular (i.c.v.) injection of the D-1 DA receptor agonist, SKF-82958, produces an enhanced locomotor-activating effect as well as increased activation of striatal ERK 1/2 MAP kinase, CaM kinase II, CREB, and c-fos in food-restricted (FR) relative to ad libitum fed (AL) rats. Striatal neurons that express the D-1 DA receptor coexpress dynorphin and substance P, and CREB is known to couple D-1 DA receptor stimulation to preprodynorphin (ppD) gene expression. The purpose of the present study was to examine possible genomic consequences of FR using real-time quantitative RT-PCR to measure striatal neuropeptide gene expression 3 h after i.c.v. injection of SKF-82958 (20 microg). Results indicate that, in nucleus accumbens (NAc), basal levels of ppD and preprotachykinin (ppT) mRNA are lower in FR than AL rats. This may reflect a decrease in tonic DA transmission during FR which precedes the compensatory upregulation of postsynaptic D-1 DA receptor-mediated cell signaling. In response to SKF-82958 challenge, however, FR subjects displayed greater levels of ppD and ppT mRNA in NAc than did AL subjects. A similar trend was seen in caudate-putamen (CPu). SKF-82958 also increased preproenkephalin (ppE) mRNA in Nac, but not CPu, with no difference between feeding groups. The present findings regarding ppD and ppT are consistent with prior findings of increased behavioral and cellular responses to acute D-1 DA agonist challenge in FR rats. The functional consequences of increased neuropeptide gene expression in response to acute drug challenge remain to be investigated but may include modulation of behavioral effects that emerge with repeated drug exposure, including sensitization, tolerance, and addiction.

Intracerebral baclofen administration decreases amphetamine-induced behavior and neuropeptide gene expression in the striatum

In a previous study, systemic administration of the GABA(B) receptor agonist, R-(+)-baclofen (2.5 mg/kg, i.p.) blocked acute amphetamine (2.5 mg/kg, i.p.)-induced rearing and neuropeptide (preprodynorphin (PPD), preprotachykinin (PPT), preproenkephalin (PPE), and secretogranin II (SGII)) mRNA expression in the striatum (Zhou et al, 2004). The purpose of the present study was to investigate the site(s) of action of these baclofen effects in the dorsal and ventral striatal circuitries. Infusion of baclofen (75 ng/side) into the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NA), caudate-putamen (Cpu), or medial prefrontal cortex (mPFC) had no effect on behavioral activity in saline-treated rats habituated to a photocell apparatus. However, intra-VTA infusion of baclofen (75 ng/side) completely blocked, whereas intra-NA and intra-SN infusion of baclofen attenuated, amphetamine-induced vertical activity without affecting amphetamine-induced total distance traveled. In contrast, intramedial PFC and intra-CPu infusion of baclofen had no effect on behavioral activity in amphetamine-treated rats. Infusion of baclofen into the VTA, NA, or SN decreased amphetamine-induced neuropeptide gene expression in the striatum. These results indicate that GABA(B) receptor stimulation within the ventral striatal circuitry is involved in mediating acute amphetamine-induced behaviors and neuropeptide gene expression in the dorsal and ventral striatum. The present study provides information on the potential targets in the brain for baclofen in the initial behavioral and genomic response to amphetamine.

Blockade of stimulant-induced preprodynorphin mRNA expression in the striatal matrix by serotonin depletion

Cocaine and methamphetamine (METH) induce preprodynorphin (PPD) mRNA expression in the striatum. Cocaine induces PPD expression in both the patch and matrix compartments of the rostral striatum, whereas METH induces PPD expression in the patch compartment of the rostral striatum. In middle striatum, both stimulants increase PPD expression in the patch and matrix compartments. METH and cocaine treatment also increase extracellular serotonin (5-HT). Several studies have shown that 5-HT receptors are present on striatonigral neurons that express PPD mRNA, and that 5-HT is a positive regulator of striatal neuropeptide expression. The current study examined whether 5-HT plays a role in the patch/matrix expression of PPD mRNA induced by cocaine and METH in striatum. Male Sprague-Dawley rats were treated with p-chloroamphetamine (PCA; 8 mg/kg, i.p), a serotonin neurotoxin, 1 week prior to cocaine (30 mg/kg, i.p) and METH (15 mg/kg, s.c.) treatment. The 80% loss of 5-HT induced by PCA-pretreatment blocked cocaine-induced PPD expression in the rostral matrix compartment. Cocaine- and METH-induced PPD expression in the rostral patch compartment was unaffected by PCA-pretreatment. PCA-pretreatment also decreased both cocaine- and METH-induced PPD expression in the matrix, but not patch of middle striatum. PCA-induced 5-HT depletion did not affect stimulant-induced increases in PPT mRNA expression in the striatum. These data suggest that 5-HT plays a role in stimulant-induced PPD expression in the matrix compartment of rostral and middle striatum. Thus, 5-HT innervation may play a critical role in basal ganglia function.

3,4-Methylenedioxymethamphetamine increases neuropeptide messenger RNA expression in rat striatum

The amphetamine analog 3,4-methylenedioxymethamphetamine (MDMA) is also known as the recreational drug of abuse, Ecstasy. Several neuropeptides are found in striatal neurons postsynaptic to dopamine and serotonin nerve terminals, and changes in neuropeptide neurotransmission may be important for behavioral effects of 3,4-methylenedioxymethamphetamine. This study used in situ hybridization to characterize the effects of 3,4-methylenedioxymethamphetamine on four neuropeptide mRNAs: preprodynorphin, preprotachykinin, neurotensin/neuromedin N, and preproenkephalin. Male, Sprague-Dawley rats received a single administration of 10 mg/kg 3,4-methylenedioxymethamphetamine and were sacrificed 30 min or 3 h later. Three hours after administration, 3,4-methylenedioxymethamphetamine increased preprodynorphin, preprotachykinin, and neurotensin/neuromedin N mRNAs. These increases were most prominent in ventral and medial aspects of the rostral-middle striatum, and then became more dorsally restricted in the caudal striatum. At the 30-minute time point, MDMA significantly decreased the signal for preproenkephalin mRNA in a general manner but did not affect the signal for the other neuropeptide precursors. These data suggest that 3,4-methylenedioxymethamphetamine has a generalized, transient, inhibitory effect on striatopallidal neuron gene expression, and then preferentially influences striatonigral neuropeptide systems at the later time point in a regionally selective manner.

Morphine-induced c-fos mRNA expression in striatofugal circuits: modulation by dose, environmental context, and drug history

Opiates and psychostimulants produce many shared behavioral and neurobiological adaptations, such as behavioral sensitization and the induction of immediate early genes in the caudate-putamen (CPu). Previous studies indicate that factors such as dose, the environmental context surrounding drug administration and drug history can influence both morphine- and psychostimulant-induced behavioral sensitization. In addition, these factors can modulate the ability of psychostimulants to engage striatofugal circuits in the CPu. The present study, therefore, sought to examine whether these factors have similar influences over the ability of morphine to engage cortico-striatofugal circuits. We report that, when given in the home cage, morphine produced a small, but significant increase in the number of c-fos+ striatonigral cells and c-fos+ cells in cingulate cortex, but had no effect on the number of c-fos+ striatopallidal cells. When given in a novel test environment, however, morphine dramatically increased the number of c-fos+ striatonigral cells in a dose-dependent fashion, and this effect was maintained following repeated treatment. Unexpectedly, morphine treatment in a novel environment produced a dose-dependent reduction in the number of c-fos+ striatopallidal cells and c-fos+ cells in cingulate cortex, relative to exposure to novelty alone-effects that were reversed by repeated morphine treatment. We suggest that alterations in c-fos expression patterns in striatofugal circuits following morphine administration may be involved in drug-experience-dependent plasticity.

mGluR5-dependent increases in immediate early gene expression in the rat striatum following acute administration of amphetamine

Metabotropic glutamate receptor 5 (mGluR5) is densely expressed in medium-sized spiny projection neurons of the rat striatum. Activation of mGluR5 increases intracellular Ca2+, resulting in Ca(2+)-dependent cellular responses. Acute administration of the psychostimulant amphetamine (AMPH) induces immediate early gene (IEG) expression in the striatum, which is considered an important molecular event for the development of striatal neuroplasticity related to the addictive properties of drugs of abuse. This study investigated the role of mGluR5 in the mediation of IEG expression in the rat striatum induced by a single dose of AMPH (4 mg/kg, i.p.) in vivo. We found that systemic administration of the mGluR5-selective antagonist 2-methyl-6-(phenylethynyl) pyridine hydrochloride (MPEP) at a dose of 10 mg/kg, i.p. reduced AMPH-stimulated c-fos mRNA levels in the dorsal (caudoputamen) and ventral (nucleus accumbens) striatum as revealed by quantitative in situ hybridization. Similar results were observed in the three areas of cerebral cortex (cingulate, sensory, and piriform cortex). In contrast to c-fos mRNAs, AMPH-stimulated mRNA expression of another IEG, zif/268, was not significantly altered by the blockade of mGluR5 with MPEP in the entire striatum and the three areas of cortex. Treatment with MPEP alone had no effect on basal levels of c-fos and zif/268 mRNAs in the striatal and cortical areas. These results indicate that an mGluR5-dependent mechanism selectively contributes to c-fos expression in the striatum and cortex in response to acute exposure to AMPH.

In vivo evidence that genetic background controls impulse-dependent dopamine release induced by amphetamine in the nucleus accumbens

Amphetamine is known to increase dopamine (DA) release by acting directly on dopamine transporters (DAT), primarily through a mechanism that is independent of impulse flow. We present evidence to show that impulse-dependent increase in DA outflow in the nucleus accumbens (NAc) is produced by amphetamine depending on genetic background. Systemic amphetamine produced higher accumbal DA release in the widely exploited C57BL/6J background than in the DBA/2J. By contrast, intra-accumbens perfusion using increasing doses of amphetamine dramatically increased DA outflow in the DBA/2J background, whereas very low DA outflow was evident in C57BL/6J mice. The fast sodium channel blocker tetrodotoxin infused through the microdialysis probe abolished accumbal DA release induced by systemic amphetamine only in the C57BL/6J background. Finally, medial prefrontal excitotoxic lesion abolished amphetamine-induced mesoaccumbens DA release in C57BL/6J mice, without significantly affecting it in the DBA/2J background. These results represent the first functional evidence in an in vivo study that amphetamine can increase DA release in the NAc mainly through an impulse-dependent mechanism regulated by prefronto-cortical glutamatergic transmission. Moreover, they point to a genetic control of impulse-dependent DA release in the accumbens, providing an exploitable tool to investigate aetiological factors involved in psychopathology and drug addiction.

GABAB receptor stimulation decreases amphetamine-induced behavior and neuropeptide gene expression in the striatum

The purpose of this study was to investigate whether GABA(B) receptor activation blocks acute amphetamine-induced behavioral activity, dopamine release, and neuropeptide mRNA expression in the striatum. Systemic administration of R-(+)-baclofen (1.25 mg/kg, i.p.) did not alter total distance traveled or vertical rearing induced by amphetamine (2.5 mg/kg, i.p.). At 2.5 mg/kg, baclofen did not alter spontaneous motor activity or total distance traveled, but completely blocked vertical rearing induced by amphetamine. At 5.0 mg/kg, baclofen completely blocked both total distance traveled and vertical rearing induced by amphetamine. Quantitative in situ hybridization histochemistry revealed that baclofen (2.5 mg/kg, i.p.) decreased the ability of amphetamine to increase preprodynorphin (PPD), preprotachykinin (PPT), preproenkephalin (PPE), and secretogranin II (SGII) mRNA levels in the striatum without altering the basal levels of these signals. Baclofen also blocked the amphetamine-induced rise in SGII mRNA in the core and shell of the nucleus accumbens and cingulate cortex. In a separate experiment, systemic baclofen (2.5 mg/kg) decreased the amphetamine-induced increase in dialysate dopamine levels in the striatum. These results suggest that reduced striatal dopamine release contributes to the ability of GABA(B) receptor activation to decrease acute amphetamine-induced behavioral activity and striatal neuropeptide gene expression.

Distinct expression of phosphorylatedN-methyl-D-aspartate receptor NR1 subunits by projection neurons and interneurons in the striatum of normal and amphetamine-treated rats

N-methyl-D-aspartate (NMDA) receptors are heteromeric assemblies of subunits (NR1 and NR2A-D), and are enriched in the striatum. Receptor phosphorylation has recently been demonstrated on the NR1 subunit at three serine residues, 897, 896, and 890, which appear to correspond to the level of receptor activity. In this study, expression of phospho-specific NR1 subunits at serine 897 (pNR1S897), serine 896 (pNR1S896), or serine 890 (pNR1S890) in neurochemically identified neurons of the adult rat striatum was detected by using double-immunofluorescent labeling or combined in situ hybridization and immunohistochemistry. In both the dorsal and ventral striatum, pNR1S897 was expressed at high levels in projection neurons containing >55% dynorphin (striatonigral) and >90% enkephalin (striatopallidal) and in interneurons that were 100% positive for choline, >90% positive for parvalbumin, and >45% positive for somatostatin (co-containing neuropeptide Y and neuronal nitric oxide synthase). Low levels of pNR1S896 were present in a small portion of projection neurons (<15% for both populations of projection neurons) and were almost lacking in the three types of interneurons. Interestingly, pNR1S890 was exclusively expressed in most parvalbumin-containing interneurons (70-80%). Acute administration of a psychostimulant, amphetamine, increased the number of dynorphin-containing projection neurons and parvalbumin interneurons showing detectable levels of pNR1S896 and pNR1S890, respectively. These results demonstrate the distinct expression of phospho-NR1 subunits in different populations of striatal projection neurons and interneurons at variable levels in normal rats; they also demonstrate that phosphorylation of NR1, at least on serine 896 and 890 sites, is sensitive to drug exposure.

Local μ and δ opioid receptors regulate amphetamine-induced behavior and neuropeptide mRNA in the striatum

The purpose of this study was to investigate the role that mu and delta opioid receptor blockade has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. Intrastriatal infusion of the mu opioid antagonist, H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP), or the delta opioid antagonist, H-Tyr-Tic[CH(2)NH]-Phe-Phe-OH (TIPPpsi), significantly decreased amphetamine-induced vertical activity. However, only CTAP reduced amphetamine-induced distance traveled. Quantitative in situ hybridization histochemistry revealed that CTAP blocked amphetamine-induced preprodynorphin and substance P mRNA. However, preproenkephalin mRNA levels in the dorsal striatum were increased to the same extent by CTAP, amphetamine, or a combination of the two drugs. In contrast, TIPPpsi significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that both mu and delta receptor subtypes differentially regulate amphetamine-induced behavior and neuropeptide gene expression in the rat striatum.

Effect of acute and chronic administration of U50,488, a kappa opioid receptor agonist, in 6-OHDA-lesioned rats chronically treated with levodopa

To evaluate the possible involvement of kappa opioid receptor-mediated mechanisms in levodopa-induced motor fluctuations, we have investigated the effects of U50,488, a selective kappa opioid agonist, on levodopa-induced motor alterations in rats with unilateral 6-OHDA lesion. Acute and chronic administration of U50,488 has been studied to evaluate the possible reversion or prevention of these levodopa effects. In a first set of experiments, rats were treated with levodopa (25 mg/kg with benserazide, twice daily, ip) for 22 days and, on Day 23 U50,488 (0.5, 1, or 3 mg/kg, i.p.) was administered immediately before levodopa. In a second set of experiments, rats were treated daily for 22 days with levodopa and U50,488 (1 or 3 mg/kg/day, i.p.). The duration of the rotational behavior induced by chronic levodopa decreased after 22 days (P < 0.05). Acute administration of U50,488 on Day 23 reversed this effect when low doses were administered (P < 0.05). Chronic U50,488 administration did not prevent the shortening in response duration induced by levodopa. Our results demonstrate that the kappa opioid receptor agonist U50,488 reverses but does not prevents levodopa-induced motor alterations in parkinsonian rats. These results suggest a role for kappa opioid receptor-mediated mechanisms in the pathophysiology of levodopa-induced motor response complications. These findings suggest that the stimulation of kappa opioid receptors might confer clinical benefit to parkinsonian patients under levodopa therapy suffering from motor complication syndrome.

Contribution of ionotropic glutamate receptors to acute amphetamine-stimulated preproenkephalin mRNA expression in the rat striatum in vivo

The present study evaluated the possible role of ionotropic glutamate receptors in mediating the inducible preproenkephalin (PPE) mRNA expression in the rat striatum in response to systemic administration of a psychostimulant amphetamine. A single injection of amphetamine (5 mg/kg, i.p.) increased PPE mRNA levels in the dorsal striatum as revealed via quantitative in situ hybridization. Pretreatment with the NMDA receptor antagonist, MK801 (0.5 mg/kg, i.p.) or 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (10 mg/kg, i.p.), attenuated the amphetamine-induced PPE mRNA expression. A similar attenuation of the amphetamine-induced PPE expression, although to a lesser extent, was also observed following pretreatment with a kainate/AMPA receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (100-200 mg/kg, i.p.). Three antagonists alone had no significant effect on basal levels of PPE mRNAs in the dorsal striatum. These data indicate that both NMDA and kainate/AMPA receptors are involved in coupling extracellular signals evoked by acute stimulant exposure to PPE mRNA expression in striatal neurons.

Neurotensin: dual roles in psychostimulant and antipsychotic drug responses

Central administration of neurotensin (NT) results in a variety of neurobehavioral effects which, depending upon the administration site, resemble the effects of antipsychotic drugs (APDs) and psychostimulants. All clinically effective APDs exhibit significant affinities for dopamine D(2) receptors, supporting the hypothesis that an increase in dopaminergic tone contributes to schizophrenic symptoms. Psychostimulants increase extracellular dopamine (DA) levels and chronics administration can produce psychotic symptoms over time. APDs and psychostimulants induce Fos and NT expression in distinct striatal subregions, suggesting that changes in gene expression underlie some of their effects. To gain insight into the functions of NT, we analyzed APD and psychostimulant induction of Fos in NT knockout mice and rats pretreated with the NT antagonist SR 48692. In both NT knockout mice and rats pretreated with SR 48692, haloperidol-induced Fos expression was markedly attenuated in the dorsolateral striatum; amphetamine-induced Fos expression was reduced in the medial striatum. These results indicate that NT is required for the activation of specific subpopulations of striatal neurons in distinct striatal subregions in response to both APDs and psychostimulants. This review integrates these new findings with previous evidence implicating NT in both APD and psychostimulant responses.

Brain neurotransmitter turnover rates during rat intravenous cocaine self-administration

The turnover rates of dopamine, norepinephrine, serotonin, aspartate, glutamate and GABA were measured in 27 brain regions of rats self-administering cocaine and in yoked cocaine- and yoked vehicle-infused controls using radioactive pulse-labeling procedures to identify brain neuronal systems underlying self-administration. Changes in the activity of heretofore unrecognized dopamine, norepinephrine, serotonin, glutamate and GABA innervations of the forebrain specific to cocaine self-administration were found. This included innervations of the nucleus accumbens, ventral pallidum, lateral hypothalamus and the anterior and posterior cingulate, entorhinal-subicular and visual cortices. Turnover rates also were calculated using metabolite/neurotransmitter ratios which were inconsistent with the pulse-label technologies indicating that ratio procedures are not accurate measures of neurotransmitter utilization. Results with the pulse-label technique provide evidence of the involvement of neuronal systems in cocaine self-administration not previously known, some of which may have a broader role in brain reinforcement processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are thought to produce reinforcing effects by modulating activity in these endogenous systems.

Kappa opioid receptor activation in the nucleus accumbens inhibits glutamate and GABA release through different mechanisms

Through their actions in the nucleus accumbens (NAc), kappa opioid (KOP) receptors and their endogenous ligand, dynorphin, modify behaviors associated with the administration of drugs of abuse and are regulated by exposure to such drugs. Despite their demonstrated behavioral significance, the synaptic actions of KOP receptor ligands in the NAc are not clearly understood. Using whole-cell voltage-clamp recordings of NAc medium spiny neurons, we have found that, in addition to suppressing glutamate release, the KOP receptor agonist also inhibits GABA release. Interestingly, the mechanism of inhibition of the release of glutamate differs from that controlling GABA. reduces the frequency of Ca(2+)-independent miniature excitatory postsynaptic currents, but not miniature inhibitory postsynaptic currents. Furthermore, while the inhibition of GABAergic transmission is blocked by the N-type Ca(2+) channel blocker omega-CgTx, the inhibition of excitatory glutamatergic transmission by is unaffected by N-type Ca(2+) channel blockade. These results indicate that KOP receptor activation inhibits GABA release by reducing Ca(2+) influx, but inhibits glutamate release at a step downstream of Ca(2+) entry.

Lack of effect of kappa-opioid receptor agonism on long-term methamphetamine-induced neurotoxicity in rats

High-dose methamphetamine treatment induces long-term deficits in central monoamine systems. However, the mechanisms underlying these effects are unknown. Previous work has shown that the Kappa-opioid receptor agonist U-69593 [(+)-(5alpha,7alpha,8b)-(+)-N-methyl-N[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl] benzeneacetamide] attenuates the neurotoxic effects of methamphetamine on extracellular dopamine levels in mice, suggesting that endogenous Kappa-opioid receptor ligands, such as dynorphin, may protect against methamphetamine-induced toxicity and play a role in mediating the long-term consequences of methamphetamine. To further examine the role that dynorphin systems play in methamphetamine-induced neurotoxicity, we administered to male rats a total of four injections of methamphetamine (7.5 mg/kg, s.c.), with a 2-h interval between each dose. Rats were pretreated with either the Kappa-agonist U-69593 (0.32 mg/kg, s.c.) or vehicle, 15 min prior to the first and third methamphetamine injection. Furthermore, cages containing the U-69593 + methamphetamine-treated rats were placed on heating pads for 30 min after the first U-69593 injection to prevent the drug from blocking methamphetamine-induced hyperthermia. Rats were sacrificed 7 days after treatment. Striatal dopamine and serotonin contents were decreased approximately 75% and 55%, respectively, in the methamphetamine-treated rats and approximately 88% and 65%, respectively, in rats receiving the U-69593 + methamphetamine combination. There was a approximately 20% mortality rate in the rats treated with methamphetamine compared to approximately 75% mortality rate in rats treated with both U-69593 and methamphetamine. A similar rate of mortality was observed when combining a different Kappa-agonist, U-50488 [trans-(-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamine], with methamphetamine. These data suggest that Kappa-agonists do not protect against methamphetamine-induced toxicity to monoamines in rats, and may potentiate mortality when co-administered with methamphetamine.

D-amphetamine fails to increase extracellular dopamine levels in mice lacking alpha 1b-adrenergic receptors: relationship between functional and nonfunctional dopamine release

It was found recently that locomotor and rewarding effects of psychostimulants and opiates were dramatically decreased or suppressed in mice lacking alpha1b-adrenergic receptors [alpha1b-adrenergic receptor knock-outs (alpha1bAR-KOs)] (Drouin et al., 2002). Here we show that blunted locomotor responses induced by 3 and 6 mg/kg d-amphetamine in alpha1bAR-KO mice [-84 and -74%, respectively, when compared with wild-type (WT) mice] are correlated with an absence of d-amphetamine-induced increase in extracellular dopamine (DA) levels in the nucleus accumbens of alpha1bAR-KO mice. Moreover, basal extracellular DA levels in the nucleus accumbens are lower in alpha1bAR-KO than in WT littermates (-28%; p < 0.001). In rats however, prazosin, an alpha1-adrenergic antagonist, decreases d-amphetamine-induced locomotor hyperactivity without affecting extracellular DA levels in the nucleus accumbens, a finding related to the presence of an important nonfunctional release of DA (Darracq et al., 1998). We show here that local d-amphetamine releases nonfunctional DA with the same affinity but a more than threefold lower amplitude in C57BL6/J mice than in Sprague Dawley rats. Altogether, this suggests that a trans-synaptic mechanism amplifies functional DA into nonfunctional DA release. Our data confirm the presence of a powerful coupling between noradrenergic and dopaminergic neurons through the stimulation of alpha1b-adrenergic receptors and indicate that nonfunctional DA release is critical in the interpretation of changes in extracellular DA levels. These results suggest that alpha1b-adrenergic receptors may be important therapeutic pharmacological targets not only in addiction but also in psychosis because most neuroleptics possess anti-alpha1-adrenergic properties.

Kappa-Opioid and NMDA glutamate receptors are differentially targeted within rat medial prefrontal cortex

Activation of kappa-opioid receptors (KOR) in the medial prefrontal cortex (mPFC) modulates excitatory transmission, which may involve interactions with N-methyl-D-aspartate (NMDA) glutamate receptors. We investigated possible anatomical correlates of this modulation by using dual labeling electron microscopy to examine the cellular distributions of antibodies raised against KOR and the R1 subunit of the NMDA receptor (NR1). KOR immunoreactivity primarily was localized to plasma and vesicular membranes of axons and axon terminals that were morphologically heterogeneous. A small proportion of KOR immunoreactivity was associated with cytosolic compartments of dendrites and membranes of glial processes. NR1 labeling was mainly postsynaptic, associated most often with membranes of cytoplasmic organelles in cell bodies and large dendrites and plasmalemmal surfaces of distal dendrites. The remaining NR1-labeled profiles were axonal profiles and glial processes. Of all cellular associations between labeled profiles, the majority were KOR-labeled axons that contacted NR1-immunoreactive dendrites or cell bodies. Occasionally the two antigens were colocalized in axon terminals that formed either asymmetric synapses or displayed varicose morphology. KOR and NR1 also were colocalized within dendrites, and rarely were observed in the same cell bodies. Occasionally glial processes coursing adjacent to axo-spinous appositions expressed both KOR and NR1 immunoreactivity. These results indicate that ligand activation of KOR or NMDA receptors differentially modulates excitatory transmission in the mPFC through pre- and postsynaptic mechanisms, respectively. The data also suggest more minor roles for colocalized KOR and NMDA receptors in shared regulation of presynaptic transmitter release, postsynaptic responsivity, and glial function.

Sensitization of midbrain dopamine neuron reactivity promotes the pursuit of amphetamine

Stimulant drugs such as amphetamine are readily self-administered by humans and laboratory animals by virtue of their actions on dopamine (DA) neurons of the midbrain. Repeated exposure to this drug systemically or exclusively in the cell body region of these neurons in the ventral tegmental area (VTA) leads to long-lasting changes in dopaminergic function that can be assessed by increased locomotor activity and enhanced DA overflow in the nucleus accumbens (NAcc) after re-exposure to the drug. Three experiments were conducted to evaluate the possibility that this enduring sensitized reactivity underlies compulsive drug self-administration. In all experiments, rats were pre-exposed to amphetamine and, starting 10 d later, their intravenous self-administration of the drug was assessed. In the first experiment, rats previously exposed to amphetamine systemically or exclusively in the VTA subsequently worked harder than untreated animals to obtain the drug when the work required to obtain successive infusions was increased progressively. In the second experiment, this progressively increasing workload was found to decrease the magnitude of amphetamine-induced DA overflow observed with successive infusions until responding ceased. Rats previously exposed to amphetamine were more resistant to this decline and more apt to maintain responding. Finally, in experiment three, a noncontingent priming injection of the drug produced a greater NAcc DA response and a greater parallel increase in lever pressing in drug compared with saline pre-exposed rats. Together, these results demonstrate a direct relation between drug-induced sensitization of midbrain dopamine neuron reactivity and the excessive pursuit and self-administration of an abused substance.

The effect of fluvoxamine and sertraline on the opioid withdrawal syndrome: a combined in vivo cerebral microdialysis and behavioural study

A microdialysis study was undertaken to determine the effect of acute and sub-chronic administration of the selective serotonin reuptake inhibitor, fluvoxamine, and the acute effect of the selective serotonin reuptake inhibitor sertraline, on the naloxone precipitated opioid withdrawal induced increase in hippocampal noradrenaline levels. This study also determined the effect of fluvoxamine and sertraline on opioid withdrawal-induced physical behaviours. Naloxone (1 mg kg(-1); i.p.) increased noradrenaline levels in the hippocampus of morphine dependent rats 20 min after administration, with peak levels of 267+/-13% of baseline, occurring 40 min after administration of naloxone. Opioid withdrawal-induced physical behaviours were evident in morphine dependent rats 5 min after a naloxone injection (1 mg kg(-1); i.p.). Acute fluvoxamine or sertraline (10 mg kg(-1); i.p.) given 40 min before naloxone (1 mg kg(-1); i.p.) did not modify the increased hippocampal noradrenaline levels (242+/-15 and 242+/-19%, respectively), observed in morphine dependent rats following an naloxone injection. Acute fluvoxamine and sertraline (10 mg kg(-1); i.p.) reduced the severity of the naloxone precipitated opioid withdrawal syndrome. Sub-chronic treatment with fluvoxamine (10 mg kg(-1); i.p.) prevented the naloxone precipitated increase in hippocampal noradrenaline levels in morphine dependent rats. Furthermore, sub-chronic fluvoxamine produced a significantly reduced baseline level of noradrenaline in these rats which was 52.5+/-8% of baseline 40 min after naloxone.

Impaired preprodynorphin, but not preproenkephalin, mRNA induction in the striatum of mGluR1 mutant mice in response to acute administration of the full dopamine D(1) agonist SKF-82958

Metabotropic glutamate receptor 1 (mGluR1) is highly expressed in striatonigral projection neurons of rat striatum. To define the role of mGluR1 in the regulation of striatal gene expression, the responsiveness of the three neuropeptide gene expression to a single injection of the dopamine D(1) agonist SKF-82958 was compared between mGluR1 mutant and wild-type control mice. We found that acute injection of SKF-82958 increased preprodynorphin (PPD), substance P (SP), and preproenkephalin (PPE) mRNAs in the dorsal and ventral striatum of mutant and wild-type mice in a dose-dependent manner (0.125, 0.5, and 2 mg/kg, i.p.) as revealed by quantitative in situ hybridization. However, the induction of PPD mRNA in both the dorsal and ventral striatum of mGluR1 minus sign/minus sign mice was significantly less than that of wild-type +/+ mice in response to the two higher doses of SKF-82958. In contrast to PPD, SP and PPE in the dorsal and ventral striatum of mGluR1 mutant mice were elevated to a similar level as that of wild-type mice. There were no differences in basal levels and distribution patterns of all three mRNAs between the two genotypes of mice treated with saline. These results indicate that mGluR1 selectively participates in striatonigral PPD induction in response to D(1) receptor stimulation.

NK-1 receptor blockade decreases amphetamine-induced behavior and neuropeptide mRNA expression in the striatum

The effect of intrastriatal administration of LY306740, a specific NK-1 receptor antagonist, on the behavior and changes in gene expression elicited by the psychomotor stimulant, amphetamine, was studied. Acute administration of amphetamine (2.5 mg/kg, i.p.) caused an increase in behavioral activity and preproenkephalin, preprodynorphin and substance P mRNA expression in the striatum. When amphetamine-treated rats were pretreated with LY306740 (35 and 20 nmoles per side, intrastriatally), there was a significant decrease in amphetamine-induced behavioral activity. Quantitative in situ hybridization histochemistry revealed that both concentrations of LY306740 significantly decreased amphetamine-induced mRNA expression of all three neuropeptides. These data indicate that striatal NK-1 receptors modulate amphetamine-induced behavior and mRNA expression of neuropeptides in the rat striatum.

Augmented motor activity and reduced striatal preprodynorphin mRNA induction in response to acute amphetamine administration in metabotropic glutamate receptor 1 knockout mice

Metabotropic glutamate receptor 1 (mGluR1) is a G-protein-coupled receptor and is expressed in the medium spiny projection neurons of mouse striatum. To define the role of mGluR1 in actions of psychostimulant, we compared both motor behavior and striatal neuropeptide mRNA expression between mGluR1 mutant and wild-type control mice after a single injection of amphetamine. We found that acute amphetamine injection increased motor activity in both mutant and control mice in a dose-dependent manner (1, 4, and 12 mg/kg, i.p.). However, the overall motor responses of mGluR1 -/- mice to all three doses of amphetamine were significantly greater than those of wild-type +/+ mice. Amphetamine also induced a dose-dependent elevation of preprodynorphin mRNA in the dorsal and ventral striatum of mutant and wild-type mice as revealed by quantitative in situ hybridization. In contrast to behavioral responses, the induction of dynorphin mRNA in both the dorsal and ventral striatum of mutant mice was significantly less than that of wild-type mice in response to the two higher doses of amphetamine. In addition, amphetamine elevated basal levels of substance P mRNA in the dorsal and ventral striatum of mGluR1 mutant mice to a similar level as that of wild-type mice. There were no differences in basal levels and distribution patterns of the two mRNAs between the two genotypes of mice treated with saline. These results demonstrate a clear augmented behavioral response of mGluR1 knockout mice to acute amphetamine exposure that is closely correlated with reduced dynorphin mRNA induction in the same mice. It appears that an intact mGluR1 is specifically critical for full dynorphin induction, and impaired mobilization of inhibitory dynorphin system as a result of lacking mGluR1 may contribute to an augmentation of motor stimulation in response to acute administration of psychostimulant.

Major coexpression of kappa-opioid receptors and the dopamine transporter in nucleus accumbens axonal profiles

The behavioral effects of psychostimulants, which are produced at least in part through inhibition of the dopamine transporter (DAT), are modulated by kappa-opioid receptors (KOR) in the nucleus accumbens (Acb). Using electron microscopic immunocytochemistry, we reveal that in the Acb KOR labeling is mainly, and DAT immunoreactivity is exclusively, presynaptic. From 400 KOR-labeled presynaptic structures, including axon terminals, intervaricosities, and small axons, 51% expressed DAT and 29% contacted another population of terminals exclusively labeled for DAT. Within axonal profiles that contained both antigens, DAT and KOR were prominently localized to plasma membrane segments that showed overlapping distributions of the respective immunogold-silver and immunoperoxidase markers. KOR labeling was also localized to membranes of small synaptic vesicles in terminals with or without DAT immunoreactivity. In addition, from 24 KOR-immunoreactive dendritic spines 42% received convergent input from DAT-containing varicosities and unlabeled terminals forming asymmetric, excitatory-type synapses. Our results provide the first ultrastructural evidence that in the Acb, KOR is localized to strategic sites for involvement in the direct presynaptic release and/or reuptake of dopamine. These data also suggest a role for KOR in the presynaptic modulation of other neurotransmitters and in the postsynaptic excitatory responses of single spiny neurons in the Acb. Dual actions on dopamine terminals and their targets in the Acb may account for KOR-mediated attenuation of drug reinforcement and sensitization.

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Gliogenesis in the striatum of the adult rat: alteration in neural progenitor population after psychostimulant exposure

Cytogenesis from proliferating progenitor cells is present in the rat brain throughout adulthood, and is regulated by a variety of environmental stimuli. To determine whether adult cytogenesis occurs in the intact rat striatum and to explore the possible regulatory role of psychostimulant exposure on striatal cytogenesis, immunohistochemistry with the thymidine analog bromodeoxyuridine (BrdU), a marker of DNA synthesis that labels dividing cells and their terminal progeny, was performed on the brain sections of normal adult rats and rats treated with a psychostimulant, amphetamine (AMPH). Scattered cells that incorporated BrdU were consistently seen throughout the dorsal (caudate putamen) and ventral (nucleus accumbens) striatum 24 h after BrdU injection. Three to four weeks after BrdU injection, approximately 10-20% of surviving newborn cells differentiated into astroglia according to their radial morphology of glia and co-expression of an astroglial marker, S100beta. However, none of BrdU-positive cells were found to co-localize with a neuronal marker, neuronal nuclear antigen (NeuN). Acute injection of AMPH at a behaviorally active dose (10 mg/kg, i.p.) produced a rapid and transient decrease in the number of BrdU-labeled cells in both the dorsal (70.6% of control) and ventral (66.7% of control) striatum, but not in the subventricular zone and the hippocampal dentate gyrus. However, the fraction of differentiated astrocytes was not altered 3-4 weeks after AMPH treatment. These results indicate an existence of active gliogenesis (both proliferation and differentiation) in the adult rat striatum. Vulnerability of striatal cytogenesis to psychostimulant exposure indicates a new approach to elucidate brain mechanisms responsible for addictive properties of drugs of abuse.

Kappa opioid receptor stimulation decreases amphetamine-induced behavior and neuropeptide mRNA expression in the striatum

The purpose of this study was to investigate the role that kappa opioid receptor stimulation has upon stimulant-induced behavior and neuropeptide gene expression in the striatum. Acute administration of amphetamine (2.5 mg/kg i.p.) caused an increase in behavioral activity and preprodynorphin, substance P, and preproenkephalin mRNA expression. When amphetamine-treated rats were pretreated with U69593, a kappa agonist (0.16 or 0.32 mg/kg s.c.), there was a significant decrease in behavioral activity. Quantitative in situ hybridization histochemistry revealed that 0.32 mg/kg U69593 significantly decreased amphetamine-induced mRNA expression of all three neuropeptides; however, only the induction of preproenkephalin mRNA was decreased by 0.16 mg/kg. These data suggest that stimulation of kappa receptors decreases acute amphetamine-induced behavior and mRNA expression of neuropeptides in the rat striatum.

Differentially altered mGluR1 and mGluR5 mRNA expression in rat caudate nucleus and nucleus accumbens in the development and expression of behavioral sensitization to repeated amphetamine administration

Altered glutamatergic transmission in the striatum may be implicated in behavioral sensitization to repeated amphetamine (AMPH) administration. Quantitative in situ hybridization histochemistry was performed to define the effects of acute and chronic AMPH exposures on mRNA expression of Group I metabotropic glutamate receptors (mGluRs) in the striatum. Behavioral ratings indicated that the motor activity of rats was significantly higher after the final of five daily AMPH injections (4 mg/kg, i.p.) than that after the first of five daily AMPH, indicative of the development of behavioral sensitization. Moreover, the motor activity of rats treated with five daily AMPH was significantly greater than that of rats treated with five daily saline in response to a 2 mg/kg challenge dose of AMPH 7, 14, 28, and 60 days after the discontinuation of drug treatments, indicative of the persistent expression of behavioral sensitization. Three hours after acute administration of AMPH to naive rats, mGluR1 and mGluR5 mRNA expression in the dorsal (caudatoputamen) and ventral (nucleus accumbens) striatum showed no change as compared to acute saline injection. In rats that developed behavioral sensitization to repeated AMPH, mGluR1 levels in the dorsal and ventral striatum were increased by 53% and 43%, respectively, 3 h after the final AMPH treatment. However, this change did not persist during withdrawal since it was not observed 7, 14, and 28 days after the discontinuation of AMPH treatment. Conversely, mGluR5 levels were markedly reduced 3 h after the final of five daily AMPH treatments in the entire striatum of sensitized rats (34% and 77% of controls in the dorsal and ventral striatum, respectively). The reduction persisted at 7, 14, and 28 days of withdrawal. These results reveal a close linkage between striatal Group I mGluR gene expression and behavioral sensitization to AMPH. This may indicate functional implications of the two subtypes of Group I mGluRs in the regulation of behavioral sensitization to the dopamine stimulant.

Modulation of the behavioral and neurochemical effects of psychostimulants by kappa-opioid receptor systems

The repeated, intermittent use of cocaine and other drugs of abuse produces profound and often long-lasting alterations in behavior and brain chemistry. It has been suggested that these consequences of drug use play a critical role in drug craving and relapse to addiction. This article reviews the effects of psychostimulant administration on dopaminergic and excitatory amino acid neurotransmission in brain regions comprising the brain's motive circuit and provides evidence that the activation of endogenous kappa-opioid receptor systems in these regions opposes the behavioral and neurochemical consequences of repeated drug use. The role of this opioid system in mediating alterations in mood and affect that occur during abstinence from repeated psychostimulant use are also discussed.

Glutamate regulates the spontaneous and evoked release of dopamine in the rat striatum

Resting and evoked extracellular dopamine levels in the striatum of the anesthetized rat were measured by fast-scan cyclic voltammetry in conjunction with carbon fiber microelectrodes. Identification of the substance detected in vivo was achieved by inspection of background-subtracted voltammograms. Intrastriatal microinfusion of kynurenate, a broad-spectrum antagonist of ionotropic glutamate receptors, caused a decrease in the resting extracellular level of dopamine. The kynurenate-induced decrease was unaffected by systemic pretreatment with pargyline, an inhibitor of monoamine oxidase, but was significantly attenuated by systemic pretreatment with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase. Although glutamate by itself did not affect resting extracellular dopamine levels, glutamate did attenuate the kynurenate-induced decrease. Kynurenate decreased dopamine release in response to electrical stimulation of the medial forebrain bundle, an effect that was also attenuated by glutamate. These results suggest that both spontaneous and evoked dopamine release in the rat striatum are under the local tonic excitatory influence of glutamate. Interactions between central dopamine and glutamate systems that have been implicated in the etiologies of Parkinson's disease, schizophrenia, stress, and substance abuse. The precise nature of those interactions, however, remains a matter of some controversy.

Effect of the kappa-opioid receptor agonist, U69593, on reinstatement of extinguished amphetamine self-administration behavior

Previous research has indicated that pretreatment with the kappa-opioid receptor agonist, U69593, decreased the ability of experimenter-administered cocaine to reinstate extinguished cocaine self-administration behavior. This effect was specific to cocaine-produced drug seeking since U69593 failed to attenuate the ability of experimenter-administered amphetamine to reinstate extinguished cocaine self-administration behavior. One possibility is that U69593 selectively attenuates the behavioral effects of the drug that was originally self-administered. In order to test this hypothesis, the present study examined the effect of U69593 (0.0 or 0.32 mg/kg) on the reinstatement of extinguished amphetamine self-administration behavior produced by experimenter-administered injections of cocaine and amphetamine. Following extinction of amphetamine self-administration (0.04 mg/kg/infusion) the ability of cocaine (0.0, 5.0, 10.0 or 20.0 mg/kg) or amphetamine (0.0, 0.3, 1.0 or 3.0 mg/kg) to reinstate extinguished self-administration behavior was measured. Both drugs reinstated extinguished responding and the reinstatement was attenuated by pretreatment with U69593. The data indicate that the ability of U69593 to decrease drug seeking is not restricted to subjects experienced with cocaine self-administration. Self-administration history does, however, determine the effect of U69593 on amphetamine-produced drug seeking.

N-Methyl-D-aspartate receptors and p38 mitogen-activated protein kinase are required for cAMP-dependent cyclase response element binding protein and Elk-1 phosphorylation in the striatum

In vivo cyclic adenosine monophosphate (cAMP)-induced N-methyl-D-aspartate receptor and mitogen-activated protein kinase activation was investigated in the dorsal striatum by semiquantitative immunocytochemistry. Intracerebroventricular infusion of 8-bromo-adenosine 3',5'-cyclic monophosphorothioate, Sp isomer (Sp-8-Br-cAMPS), increased phosphorylated cAMP-responsive element binding protein, phosphorylated Elk-1 and Fos immunoreactivity in a dose-dependent manner. Intracerebroventricular infusion of the N-methyl-D-aspartate antagonist, MK801, decreased, but tetrodotoxin or the mitogen-activated extracellular-regulated kinase inhibitor, PD98059, did not affect Sp-8-Br-cAMPS-induced phosphorylated c-AMP-responsive element binding protein, phosphorylated Elk-1, phosphorylated extracellular-signal-regulated kinase and Fos immunoreactivity. The p38 mitogen-activated protein kinase inhibitor, SB203580, decreased the Sp-8-Br-cAMPS-induced increase in all markers, except phosphorylated extracellular-signal-regulated kinase, in a dose-dependent manner. We suggest that N-methyl-D-aspartate receptors couple c-AMP to phosphorylation events and immediate early gene induction in the nucleus of striatal medium spiny neurons. These events are mediated by crosstalk between protein kinase A and mitogen-activated protein kinase cascades in vivo.

Kappa opioid receptor inhibition of glutamatergic transmission in the nucleus accumbens shell

Microinjection of kappa-opioid receptor agonists into the nucleus accumbens produces conditioned place aversion. While attention has focused primarily on the inhibition of dopamine release by kappa-receptor agonists as the synaptic mechanism underlying this effect, recent anatomical studies have raised the possibility that regulation of noncatecholaminergic transmission also contribute. We have investigated the effects of kappa-receptor activation on fast excitatory synaptic transmission in an in vitro slice preparation using whole cell voltage-clamp or extracellular field recordings in the shell region of the nucleus accumbens. The kappa-receptor agonist U69593 produces a pronounced, dose-dependent inhibition of glutamatergic excitatory postsynaptic currents (EPSCs) that can be reversed by 100 nM nor-BNI. Furthermore, U69593 causes an increase in the paired-pulse ratio as well as a decrease in the frequency of spontaneous miniature events, suggesting a presynaptic site of action. Despite anatomical evidence for kappa-receptor localization on dendritic spines of nucleus accumbens neurons, no electrophysiological evidence of a postsynaptic effect was found. This presynaptic inhibition of excitatory synaptic transmission in the nucleus accumbens shell provides a novel mechanism that may contribute to the kappa-receptor-mediated aversion observed in intact animals.

Differential effects of endomorphin-1 and -2 on amphetamine sensitization: neurochemical and behavioral aspects

Mu-opioid receptors are known to modulate mesolimbic dopaminergic activity in the ventral tegmental area via disinhibition of GABA-containing neurons. Recently, two novel tetrapeptides, endomorphin-1 and endomorphin-2, were identified in the mammalian brain and reported to have high binding affinities toward mu-opioid receptors. To determine if endomorphins would modulate the development of amphetamine sensitization, we administered endomorphins locally into the rat brain followed by behavioral and neurochemical examinations. The results indicate that rats pretreated with endomorphin-1 or -2 (5 microg per side for 7 days) in the ventral tegmental area developed locomotor sensitization to the challenge injection of amphetamine (1 mg/kg). On the other hand, when endomorphins were given in the lateral ventricle (20 microg for 5 days) of amphetamine-sensitized rats (5 mg/kg x 14 days) during the withdrawal period (w5-w9), neither peptide had a modulatory effect on locomotor sensitization. Biochemical analyses revealed that treatment with endomorphins in the ventral tegmental area significantly increased the levels of glutamate in the medial prefrontal cortex and ventral and dorsal striatum to levels comparable to those observed in the amphetamine-sensitized rats. In the same animals, endomorphins also caused decreases in the levels of serotonin and its metabolite, 5-hydroxyindoleacetic acid, in the medial prefrontal cortex. Interestingly, although there was no behavioral significance, endomorphin-1 treatment in the lateral ventricle of control and amphetamine-sensitized rats during withdrawal resulted in decreases of GABA, aspartate, dopamine, and its metabolite 3,4-dihydroxyphenylacetic acid in the ventral striatum. We conclude that endomorphins, by stimulating the mu-opioid receptors in the ventral tegmental area, could sensitize the behavioral response to amphetamine. The results also demonstrate that there are differential responses between endomorphin-1 and -2 on behavioral amphetamine sensitization and the underlying neurochemical substrates.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Activation of group III metabotropic glutamate receptors inhibits basal and amphetamine-stimulated dopamine release in rat dorsal striatum: an in vivo microdialysis study

Group III metabotropic glutamate (mGlu) receptors are negatively coupled to adenylate cyclase and are distributed pre-synaptically in the striatum. A behavioral study previously conducted in this laboratory shows that activation of this group of mGlu receptors attenuates acute amphetamine-stimulated motor activity. By administering a group III selective agonist or antagonist via the dialysis probe, the present study employed in vivo microdialysis to evaluate the capacity of the group III selective agents to alter extracellular levels of dopamine in the dorsal striatum of normal and amphetamine-treated rats. It was found that the group III agonist L-2-amino-4-phosphonobutyrate (L-AP4) dose-dependently (1, 10 and 100 microM) reduced basal levels of extracellular dopamine. In contrast, the group III antagonist alpha-methyl-4-phosphonophenylglycine (MPPG) dose-dependently (10, 50 and 250 microM) elevated the basal release of extracellular dopamine. This elevation was antagonized by co-perfusion of L-AP4. Perfusion of 5-microM amphetamine through the dialysis probe increased extracellular dopamine in the dorsal striatum. Co-perfusion of L-AP4 (100 microM) significantly reduced amphetamine-stimulated dopamine levels, whereas co-perfusion of L-AP4 (100 microM) and MPPG (100 microM) did not alter the capacity of amphetamine to elicit dopamine release. The data obtained from this study demonstrate the presence of a tonically active glutamatergic tone on group III mGlu receptors in the dorsal striatum to pre-synaptically regulate basal dopamine release in an inhibitory fashion. Moreover, activation of L-AP4-sensitive group III mGlu receptors can suppress the phasic release of dopamine induced by a dopamine stimulant amphetamine.

Kappa opioid receptor immunoreactivity in the nucleus accumbens and caudate-putamen is primarily associated with synaptic vesicles in axons

A rabbit polyclonal antiserum, raised against a C-terminal oligopeptide of the mouse kappa opioid receptor, was used to localize the cellular distribution of kappa receptors in the dorsal and ventral striatum of rats with light and electron microscopic immunocytochemistry. Prominent, diffuse kappa receptor immunoreactivity was present in the nucleus accumbens, particularly in the shell, ventral caudate-putamen and olfactory tubercle. The density of receptor immunoreactivity decreased in more dorsal areas of the caudate-putamen. In contrast, neuronal cell bodies stained clearly in the dorsal endopiriform nucleus, claustrum and layer VI of the adjacent cerebral cortex. Observations at the electron microscopic level in the dorsomedial shell of the nucleus accumbens and caudate-putamen revealed that the kappa receptor immunoreactivity was predominantly located in axons, often associated with synaptic vesicles, remote from the terminal or preterminal area. The few terminals which were labeled made slightly more asymmetrical than symmetrical contacts and the percentage of asymmetrical contacts observed was greater in the caudate than in the accumbens. A small number of postsynaptic spines was labeled; most of them were contacted by asymmetrical terminals. No labeling was observed in dendritic shafts.Thus, the predominant localization of kappa receptor immunoreactivity in axons is consistent with its role as a major inhibitor of glutamate and dopamine release in the dorsal and ventral striatum.

Delta opioid receptors regulate calcium-dependent, amphetamine-evoked glutamate levels in the rat striatum: an in vivo microdialysis study

Blockade of opioid receptors decreases amphetamine-induced behaviors and dopamine release in the striatum. Use of selective opioid receptor ligands has indicated that these effects are mediated by delta opioid receptors (DORs). However, the site of action of delta receptors and the influence of delta receptor antagonists on other neurotransmitters released by amphetamine are unknown. Therefore, the effect of reverse microdialysis of the selective delta opioid antagonist, naltrindole, on extracellular striatal glutamate levels evoked by amphetamine (2.5 mg/kg, i.p.) was investigated. Naltrindole (10-100 microM) decreased amphetamine-evoked glutamate levels in a concentration-dependent manner. The selective delta agonist, [D-Pen(2,5)]-enkephalin (100, 500 microM), reversed the effect of naltrindole, confirming that delta receptors mediated this effect. The amphetamine-evoked increase in extracellular glutamate levels was determined to be 39% calcium-sensitive by lowering the calcium concentration in the perfusate. Under these conditions, naltrindole had no effect on the calcium-independent component of amphetamine-evoked glutamate levels. These data indicate that intrastriatal DORs modulate a calcium-dependent, amphetamine-evoked component of extracellular glutamate levels that may depend on activation of a transsynaptic basal ganglia-thalamo-cortical loop.

Modulation of the neurotoxic effects of methamphetamine by the selective kappa-opioid receptor agonist U69593

Kappa-opioid receptor agonists prevent alterations in dopamine neurotransmission that occur in response to repeated cocaine administration. The present microdialysis study examined whether administration of the selective kappa-opioid receptor agonist U69593 with methamphetamine prevents alterations in dopamine levels produced by neurotoxic doses of methamphetamine. Swiss Webster mice were injected intraperitoneally with methamphetamine (10.0 mg/kg) or saline, four times in 1 day, at 2-h intervals. Prior to the first and third injection, they received U69593 (0.32 mg/kg s.c.) or vehicle. Microdialysis was conducted 3, 7, or 21 days later. Basal and K+-evoked (60 and 100 mM) dopamine overflow were reduced 3 days after methamphetamine administration. These effects were long-lasting in that they were still apparent 7 and 21 days after methamphetamine treatment. Intrastriatal (5.0 and 50 microM) or systemic (1.0-10.0 mg/kg) administration of methamphetamine increased dopamine concentrations in control animals. In mice preexposed to methamphetamine, methamphetamine-evoked dopamine overflow was reduced. In animals that had received methamphetamine with U69593, basal dopamine levels did not differ from those of vehicle-treated controls. U69593 treatment attenuated the decrease in K+-evoked dopamine produced by prior methamphetamine exposure. The reduction in methamphetamine-evoked dopamine levels was also attenuated. The administration of U69593 alone did not modify basal or stimulus-evoked dopamine levels. These data demonstrate that repeated methamphetamine administration reduces presynaptic dopamine neuronal function in mouse striatum and that co-administration of a selective kappa-opioid receptor agonist with methamphetamine attenuates these effects. U69593 treatment did not modify the hyperthermic effects of methamphetamine, indicating that this kappa-opioid receptor agonist selectively attenuates methamphetamine-induced alterations in dopamine neurotransmission.

Presynaptic kappa-opioid and muscarinic receptors inhibit the calcium-dependent component of evoked glutamate release from striatal synaptosomes

In addition to cytosolic efflux, reversal of excitatory amino acid (EAA) transporters evokes glutamate exocytosis from the striatum in vivo. Both kappa-opioid and muscarinic receptor agonists suppress this calcium-dependent response. These data led to the hypothesis that the calcium-independent efflux of striatal glutamate evoked by transporter reversal may activate a transsynaptic feedback loop that promotes glutamate exocytosis from thalamo- and/or corticostriatal terminals in vivo and that this activation is inhibited by presynaptic kappa and muscarinic receptors. Corollaries to this hypothesis are the predictions that agonists for these putative presynaptic receptors will selectively inhibit the calcium-dependent component of glutamate released from striatal synaptosomes, whereas the calcium-independent efflux evoked by an EAA transporter blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), will be insensitive to such receptor ligands. Here we report that a muscarinic agonist, oxotremorine (0.01-10 microM), and a kappa-opioid agonist, U-69593 (0.1-100 microM), suppressed the calcium-dependent release of glutamate that was evoked by exposing striatal synaptosomes to the potassium channel blocker 4-aminopyridine. The presynaptic inhibition produced by these ligands was concentration dependent, blocked by appropriate receptor antagonists, and not mimicked by the delta-opioid agonist [D-Pen2,5]-enkephalin. The finding that glutamate efflux evoked by L-trans-PDC from isolated striatal nerve endings was entirely calcium independent supports the notion that intact basal ganglia circuitry mediates the calcium-dependent effects of this agent on glutamate efflux in vivo. Furthermore, because muscarinic or kappa-opioid receptor activation inhibits calcium-dependent striatal glutamate release in vitro as it does in vivo, it is likely that both muscarinic and kappa receptors are inhibitory presynaptic heteroceptors expressed by striatal glutamatergic terminals.