NR2A and NR2B subunit containing NMDA receptors differentially regulate striatal output pathways

Triple probe microdialysis was employed to investigate whether striatal NR2A and NR2B subunit containing NMDA receptors regulate the activity of striato-pallidal and striato-nigral projection neurons. Probes were implanted in the striatum, ipsilateral globus pallidus and substantia nigra reticulata. Intrastriatal perfusion with the NR2A subunit selective antagonist ( R )-[( S )-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydroquinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) reduced pallidal GABA and increased nigral glutamate (GLU) release whereas perfusion with the NR2B subunit selective antagonist ( R -( R *, S *)-α-(4-hydroxyphenyl)-β-methyl-4-(phenylmethyl)-1-piperidinepropanol (Ro 25-6981) reduced nigral GABA and elevated striatal and pallidal GLU release. To confirm that changes in GABA levels were because of blockade of (GLUergic-driven) tonic activity of striatofugal neurons, tetrodotoxin was perfused in the striatum. Tetrodotoxin reduced both pallidal and nigral GABA release without changing GLU levels. To investigate whether striatal NR2A and NR2B subunits were also involved in phasic activation of striatofugal neurons, NVP-AAM077 and Ro 25-6981 were challenged against a NMDA concentration able to evoke GABA release in the three areas. Both antagonists prevented the NMDA-induced striatal GABA release. NVP-AAM077 also prevented the NMDA-induced surge in GABA release in the globus pallidus, whereas Ro 25-6981 attenuated it in the substantia nigra. We conclude that striatal NMDA receptors containing NR2A and NR2B subunits preferentially regulate the striato-pallidal and striato-nigral projection neurons, respectively.     

Diurnal rhythm of the in vivo release of enkephalin from the globus pallidus of the rat

The in vivo spontaneous release of enkephalin in the globus pallidus of the rat increases from noon to evening by 100%; during this period the local release of exogenous gamma-aminobutyric acid (GABA) decreases by 60%. These diurnal rhythms are more marked in the K+-stimulated release: enkephalin-induced output increases 6-fold while GABA decreases 10-fold during the afternoon and evening hours. Since pallidal enkephalin and GABA are involved in the control of locomotor activity we suggest that these rhythms may be linked to the circadian changes of activity in the rat.     

Pallidal hyperdopaminergic innervation underlying D2 receptor-dependent behavioral deficits in the schizophrenia animal model established by EGF

Epidermal growth factor (EGF) is one of the ErbB receptor ligands implicated in schizophrenia neuropathology as well as in dopaminergic development. Based on the immune inflammatory hypothesis for schizophrenia, neonatal rats are exposed to this cytokine and later develop neurobehavioral abnormality such as prepulse inhibition (PPI) deficit. Here we found that the EGF-treated rats exhibited persistent increases in tyrosine hydroxylase levels and dopamine content in the globus pallidus. Furthermore, pallidal dopamine release was elevated in EGF-treated rats, but normalized by subchronic treatment with risperidone concomitant with amelioration of their PPI deficits. To evaluate pathophysiologic roles of the dopamine abnormality, we administered reserpine bilaterally to the globus pallidus to reduce the local dopamine pool. Reserpine infusion ameliorated PPI deficits of EGF-treated rats without apparent aversive effects on locomotor activity in these rats. We also administered dopamine D1-like and D2-like receptor antagonists (SCH23390 and raclopride) and a D2-like receptor agonist (quinpirole) to the globus pallidus and measured PPI and bar-hang latencies. Raclopride (0.5 and 2.0 μg/site) significantly elevated PPI levels of EGF-treated rats, but SCH23390 (0.5 and 2.0 μg/site) had no effect. The higher dose of raclopride induced catalepsy-like changes in control animals but not in EGF-treated rats. Conversely, local quinpirole administration to EGF-untreated control rats induced PPI deficits and anti-cataleptic behaviors, confirming the pathophysiologic role of the pallidal hyperdopaminergic state. These findings suggest that the pallidal dopaminergic innervation is vulnerable to circulating EGF at perinatal and/or neonatal stages and has strong impact on the D2-like receptor-dependent behavioral deficits relevant to schizophrenia.     

GABA-opioid interactions in the globus pallidus: [D-Ala2]-Met-enkephalinamide attenuates potassium-evoked GABA release after nigrostriatal lesion

The motor signs of Parkinson's disease have been partly attributed to an overinhibition of the external globus pallidus (GP) that results from hyperactivity of striatopallidal GABA/enkephalinergic neurons. The goals of this study were to measure basal levels of extracellular fluid GABA in the GP of normal cats, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated parkinsonian cats and cats spontaneously recovered from MPTP-induced parkinsonism, and to examine the effects of opioid receptor activation on potassium (K+)-evoked GABA release in the GP in these animals. Basal GP GABA levels were increased 75% from normal in parkinsonian animals 1 week after MPTP administration and returned to control levels in recovered animals 6 weeks after MPTP administration. No significant differences were observed in K+-evoked GABA release across conditions. The opioid receptor agonist [D-Ala2]-Met-Enkephalinamide (DALA) significantly attenuated K+-evoked GABA release in the GP of MPTP-treated symptomatic and recovered cats, but had no significant effect on GABA release in normal animals. These data show that basal GP GABA levels are elevated coincident with expression of parkinsonian signs and return to normal in animals that have functionally compensated for a nigrostriatal lesion. DALA-induced inhibition of pallidal GABA release after a dopamine-depleting lesion, suggests that enkephalin may attenuate GABA release in the GP specifically after striatal dopamine loss.     

Changes in extracellular dopamine in the rat globus pallidus induced by typical and atypical antipsychotic drugs

Typical antipsychotic drugs with a high extrapyramidal motor side-effects liability markedly increase extracellular dopamine in the caudate-putamen, while atypical antipsychotic drugs with a low incidence of extrapyramidal motor side-effects have less pronounced stimulating actions on striatal dopamine. Therefore, it has been suggested that the extrapyramidal motor side-effects liability of antipsychotic drugs (APD) is correlated with their ability to increase extracellular dopamine in the caudate-putamen. The globus pallidus (GP) is another basal ganglia structure probably mediating extrapyramidal motor side-effects of typical antipsychotic drugs. Therefore, the present study sought to determine whether extracellular dopamine in the globus pallidus might be a further indicator to differentiate neurochemical actions of typical and atypical antipsychotic drugs. Using in vivo microdialysis we compared effects on pallidal dopamine induced by typical and atypical antipsychotic drugs in rats. Experiment I demonstrated that systemic administration of haloperidol (1 mg/kg; i.p.) and clozapine (20 mg/kg; i.p.) induced a significant pallidal dopamine release to about 160 and 180% of baseline, respectively. Experiment II revealed that reverse microdialysis of raclopride and clozapine using a cumulative dosing regimen did not stimulate extracellular dopamine in the globus pallidus if low (1microM) or intermediate (10 and 100 microM) concentrations were used. Only at a high concentration (1,000 microM), raclopride and clozapine induced a significant pallidal dopamine release to about 130 and 300% of baseline values, respectively. Thus, effects of typical and atypical antipsychotic drugs on pallidal dopamine were similar and thus, may not be related to their differential extrapyramidal motor side-effects liability. Furthermore, the finding that reverse microdialysis of raclopride over a wide range of concentrations did not stimulate pallidal dopamine concentrations tentatively suggests that pallidal dopamine release under basal conditions is not regulated by D2 autoreceptors.     

Stimulation of delta opioid receptors located in substantia nigra reticulata but not globus pallidus or striatum restores motor activity in 6‐hydroxydopamine lesioned rats: new insights into the role of delta receptors in parkinsonism

The delta opioid peptide (DOP) receptor has been proposed as a target in the symptomatic therapy of Parkinson’s disease. However, the circuitry underlying the antiparkinsonian action of DOP receptor agonists and their site of action have never been adequately investigated. Systemic administration of the DOP receptor agonist (+)‐4‐[(αR)‐α‐(2S,5R)‐allyl‐2,5‐dimethyl‐1‐piperazinyl)‐3‐methoxy‐benzyl]‐N‐N‐diethylbenzamide (SNC‐80) attenuated akinesia/bradykinesia and improved motor activity in 6‐hydroxydopamine hemilesioned rats. Opposite effects were produced by the selective DOP receptor antagonist naltrindole (NTD), suggesting that endogenous enkephalins tonically sustain movement under parkinsonian conditions. Microdialysis revealed that SNC‐80 reduced GABA release in globus pallidus (GP) while NTD elevated it. Moreover, SNC‐80 reduced GABA and glutamate release in substantia nigra reticulata (SNr) whereas NTD reduced GABA without affecting glutamate release. The bar test coupled to microdialysis showed that perfusion with NTD in SNr but not GP or striatum prevented the antiakinetic effect of systemic SNC‐80 and its neurochemical correlates. Consistently, microinjections of SNC‐80 into SNr or bicuculline in GP attenuated parkinsonian‐like symptoms while SNC‐80 microinjections in GP or striatum were ineffective. This study demonstrates that nigral DOP receptors mediate antiparkinsonian actions of SNC‐80 and challenges the common view that DOP receptor agonists solely attenuate parkinsonism via pallidal mechanisms.     

GABA Alters GABAA Receptor mRNAs and Increases Ligand Binding

Abstract: Adenosine A_2a receptors have been localized to GABAergic striatopallidal neurons, but their functional role is unknown. To address this question, the modulation of endogenous GABA release by adenosine A_2a receptors was examined in slices of rat globus pallidus. The selective adenosine A_2a receptor agonist CGS-21680 (3.0–10 n M ) significantly increased electrically stimulated release (overflow) of GABA, with 10 n M CGS-21680 resulting in a 44% increase compared with the control. Both the nonselective adenosine receptor antagonist 8-phenyltheophylline (10 μ M ) and the selective A_2a receptor antagonist KF-17837 (100 n M ) abolished the CGS-21680-induced increase in GABA overflow. Higher concentrations of CGS-21680 (0.10–1.0 μ M ) decreased GABA overflow by ˜25%.8-Phenyltheophylline (10 μ M ) antagonized these effects, whereas KF-17837 (100 n M ) did not, suggesting actions of CGS-21680 on other adenosine receptors at these concentrations. These results demonstrate that activation of adenosine A_2a receptors augments electrically stimulated release of GABA from globus pallidus slices and suggest a mechanism by which adenosine may modulate GABAergic output from the striatopallidal efferent system.     

Effects of physiological and pharmacological stimuli on dopamine release in the rat globus pallidus

A major aspect of understanding functions of the globus pallidus (GP) within the basal ganglia is the significance of its dopamine innervation. Here, we used in vivo-microdialysis in rats to characterize pallidal dopamine release in response to a number of physiological and pharmacological stimuli known to activate dopamine neurons. Results reveal that an aversive stimulus, i.e. handling for 20 min, significantly increased dialysate dopamine in the globus pallidus to about 130% of baseline levels. Likewise, a novel and appetitive stimulus, i.e. presentation of unfamiliar, palatable food, significantly elevated pallidal dopamine to about 150% of baseline levels both in rats which did and did not consume the food reward. These findings provide evidence that increases of dopamine (DA) efflux may largely reflect stimulus saliency implicating an involvement of pallidal dopamine signalling in control of behaviour governed by salient stimuli. Results further showed that reverse microdialysis of D-amphetamine and cocaine in augmenting concentrations of 0.1-100 microM elevated dialysate dopamine in a concentration-dependent manner suggesting a role of pallidal dopamine in mediating behavioural effects of psychostimulant drugs.     

The K(+)-evoked release of [3H]acetylcholine from slices of rat globus pallidus: modulation by delta-opioid receptors.

Previous investigations have shown that the activation of delta-opioid receptors depresses the release of acetylcholine (ACh) in the rat caudate putamen. This finding raised the possibility that the release of ACh is similarly modulated in the globus pallidus, a region containing a distinct population of cholinergic neurons and enriched in enkephalinergic nerve terminals. In the present study the pallidal release of ACh was characterized and the effects of delta-opioid receptor activation on this release were examined. The results show that this release is stimulated by high K+ in a concentration- and Ca(2+)-dependent manner. D-Pen2,L-Pen5-enkephalin (0.1-10 microM), a selective delta-opioid receptor agonist, produced a dose-related inhibition of the 25 mM K(+)-evoked tritium release. The maximal inhibitory effect, representing a 34% decrease in the K(+)-induced tritium release, was observed at a concentration of 1 microM. This opioid effect was attenuated by the selective delta-opioid receptor antagonist, ICI 174864 (1 microM). These findings support the role of a delta-opioid receptor in the modulation of ACh release in the rat globus pallidus.     

苍白球γ-氨基丁酸能神经传递及其与神经系统疾病的关系

苍白球是基底神经节间接环路的重要核团,在机体运动功能调节中发挥重要作用。近年来,苍白球在基底神经节正常及异常功能调节中的重要性已日渐受到重视。然而,目前对苍白球内各种神经递质系统的功能活动了解较少。GABA是苍白球主要的神经递质。采用电生理记录、免疫组织化学及行为测试等实验方法,人们对大鼠苍白球GABA能神经传递系统的受体分布及功能活动有了新的认识。形态学研究揭示,苍白球存在GABAA受体及其苯二氮卓结合位点和GABAB受体。在亚细胞水平,GABAA受体主要位于对称性突触(GABA能突触)的突触后膜,而GABAB受体则位于对称性突触和非对称性突触(兴奋性突触)的突触前膜及突触后膜。功能学研究进一步揭示,激活苍白球突触前膜GABAB自身和异源性受体可分别减少GABA和谷氨酸释放;激活突触后膜GABAB受体,可引起苍白球神经元超极化。除GABAB受体外,激活苍白球GABAA受体苯二氮卓结合位点及阻断GABA重摄取可延长GABA电流持续时间,从而改变苍白球神经元兴奋性。与离体实验结果相一致,激活苍向球GABAB受体和苯二氮卓结合位点及阻断GABA重摄取可引起整体动物旋转行为。苍白球GABA神经递质系统与帕金森病病因学及癫痫发病有关。已证实,苍白球神经元放电频率的降低及簇状放电的产生与帕金森病运动减少及静止性震颤等症状直接相关。此外,电牛理及行为学实验发现,新型抗癫痫药物替加平可调节苍白球神经元功能活动．这为进一步了解苍白球与癫痫发病的关系提供了新的理论及实验依据。     

Biotransformation of l-DOPA to Dopamine in the Substantia Nigra of Freely Moving Rats: Effect of Dopamine Receptor Agonists and Antagonists

Abstract: We investigated the effects of continuous intranigral perfusion of dopamine D1 and D2 receptor agonists and antagonists on the biotransformation of locally applied l -DOPA to dopamine in the substantia nigra of freely moving rats by means of in vivo microdialysis. The "dual-probe" mode was used to monitor simultaneously changes in extracellular dopamine levels in the substantia nigra and the ipsilateral striatum. Intranigral perfusion of 10 µ M l -DOPA for 20 min induced a significant 180-fold increase in extracellular nigral dopamine level. No effect of the intranigral l -DOPA administration was observed on dopamine levels in the ipsilateral striatum, suggesting a tight control of extracellular dopamine in the striatum after enhanced nigral dopamine levels. Continuous nigral infusion with the D1 receptor agonist CY 208243 (10 µ M ) and with the D2 receptor agonist quinpirole at 10 µ M (a nonselective concentration) attenuated the l -DOPA-induced increase in dopamine in the substantia nigra by 85 and 75%, respectively. However, perfusion of the substantia nigra with a lower concentration of quinpirole (1 µ M ) and the D1 antagonist SCH 23390 (10 µ M ) did not affect the nigral l -DOPA biotransformation. The D2 antagonist (−)-sulpiride (10 µ M ) also attenuated the l -DOPA-induced dopamine release in the substantia nigra to ∼10% of that of the control experiments. We confirm that there is an important biotransformation of l -DOPA to dopamine in the substantia nigra. The high concentrations of dopamine formed after l -DOPA administration may be the cause of dyskinesias or further oxidative stress in Parkinson's disease. Simultaneous administration of D1 receptor agonists with l -DOPA attenuates the biotransformation of l -DOPA to dopamine in the substantia nigra. The observed effects could occur via changes in nigral GABA release that in turn influence the firing rate of the nigral dopaminergic neurons.     

NMDA-mediated release of glutamate and GABA in the subthalamic nucleus is mediated by dopamine: an in vivo microdialysis study in rats

The present study investigated the effects of N-methyl-D-aspartic acid.H2O (NMDA) on the dopamine, glutamate and GABA release in the subthalamic nucleus (STN) by using in vivo microdialysis in rats. NMDA (100 micromol/L) perfused through the microdialysis probe evoked an increase in extracellular dopamine in the STN of the intact rat of about 170%. This coincided with significant increases in both extracellular glutamate (350%) and GABA (250%). The effect of NMDA perfusion on neurotransmitter release at the level of the STN was completely abolished by co-perfusion of the selective NMDA-receptor antagonist MK-801 (10 micromol/L), whereas subthalamic perfusion of MK-801 alone had no effect on extracellular neurotransmitter concentrations. Furthermore, NMDA induced increases in glutamate were abolished by both SCH23390 (8 micromol/L), a selective D1 antagonist, and remoxipride (4 micromol/L), a selective D2 antagonist. The NMDA induced increase in GABA was abolished by remoxipride but not by SCH23390. Perfusion of the STN with SCH23390 or remoxipride alone had no effect on extracellular neurotransmitter concentrations. The observed effects in intact animals depend on the nigral dopaminergic innervation, as dopamine denervation, by means of 6-hydroxydopamine lesioning of the substantia nigra, clearly abolished the effects of NMDA on neurotransmitter release at the level of the STN. Our work points to a complex interaction between dopamine, glutamate and GABA with a crucial role for dopamine at the level of the STN.     

Amantadine attenuates levodopa-induced dyskinesia in mice and rats preventing the accompanying rise in nigral GABA levels

Amantadine is the only drug marketed for treating levodopa-induced dyskinesia. However, its impact on basal ganglia circuitry in the dyskinetic brain, particularly on the activity of striatofugal pathways, has not been evaluated. We therefore used dual probe microdialysis to investigate the effect of amantadine on behavioral and neurochemical changes in the globus pallidus and substantia nigra reticulata of 6-hydroxydopamine hemi-lesioned dyskinetic mice and rats. Levodopa evoked abnormal involuntary movements (AIMs) in dyskinetic mice, and simultaneously elevated GABA release in substantia nigra reticulata (～3-fold) but not globus pallidus. Glutamate levels were unaffected in both areas. Amantadine (40 mg/kg, i.p.), ineffective alone, attenuated (～50%) AIMs expression and prevented the GABA rise. Moreover, it unraveled a facilitatory effect of levodopa on pallidal glutamate levels. Levodopa also evoked AIMs expression and a GABA surge (～2-fold) selectively in the substantia nigra of dyskinetic rats. However, different from mice, glutamate levels rose simultaneously. Amantadine, ineffective alone, attenuated (～50%) AIMs expression preventing amino acid increase and leaving unaffected pallidal glutamate. Overall, the data provide neurochemical evidence that levodopa-induced dyskinesia is accompanied by activation of the striato-nigral pathway in both mice and rats, and that the anti-dyskinetic effect of amantadine partly relies on the modulation of this pathway.     

Convergent evidence from microdialysis and presynaptic immunolabeling for the regulation of gamma-aminobutyric acid release in the globus pallidus following acute clozapine or haloperidol administration in rats

Antipsychotic drugs (APDs) have been primarily characterized for their effects on dopaminergic terminal regions in the brain, especially within the corpus striatum. Efferent GABA pathways are the primary outflow of striatal processing via their projections to the substantia nigra and the globus pallidus (GP). In the current study, we analyzed changes in pallidal GABA function following acute APD administration by means of in vivo microdialysis, followed by immunolabeling of presynaptic GABA terminal density in the contralateral hemisphere of the same animals. Acute administration of the atypical APD, clozapine (10 or 30 mg/kg, s.c.), produced a dose-dependent decrease in extracellular GABA. A corresponding dose-dependent increase in the density of presynaptic terminal GABA immunolabeling in the GP was found. In contrast, the typical APD, haloperidol (1 or 3 mg/kg, s.c.), had no significant effects on either measure, although a non-significant increase in extracellular GABA and decrease in the density of GABA terminal immunolabeling was noted. Paw retraction tests conducted during the time of microdialysis showed that haloperidol produced a typical pattern of highly pronounced motor impairment, while clozapine showed an atypical profile of minimal catalepsy. These complementary results obtained from in vivo neurochemistry and presynaptic neurotransmitter labeling suggest that systemic clozapine suppresses neuronal GABA release within the GP. This decrease in released pallidal GABA may play a role in the low motor side-effect liability of atypical APDs.     

Differential Transmitter Release from Nerve Terminals Isolated from Basal Ganglia and Substantia Nigra

Abstract: The K⁺-induced release of amino acids and dopamine from synaptosomes of basal ganglia and substantia nigra of sheep was studied. K⁺ (56 mM) caused an increase in the release of GABA from caudate, putamen, globus pallidus, and substantia nigra, the increased release being 227, 171, 198, and 366%, respectively, compared with samples incubated without stimulation. The release of glutamate was also increased by 56 mM-K⁺ (136–183%) from all regions except the globus pallidus, and a significant release of aspartate was only seen in response to K⁺ stimulation of synaptosomes from putamen (50%). Veratrine (75 μM) also stimulated a similar pattern of amino acid release from these regions. Regional correlation was shown between the presence of an uptake system for an amino acid and its evoked release. [¹⁴C]Dopamine formed from L-[U-¹⁴C]tyrosine was released only from caudate and putamen synaptosomes by K⁺ stimulation, the increases being 105% and 74%, respectively. Synthesis of [¹⁴C]dopamine from L-[U-¹⁴C]tyrosine occurred only in synaptosomes prepared from these two regions and was not detected in synaptosomes from substantia nigra or globus pallidus although whole-tissue homogenates of substantia nigra were able to synthesise dopamine.     

Possible mechanism of cannabinoid-mediated modulation of signal transduction through the basal ganglia

A possible mechanism of cannabinoid-mediated akinesia is suggested. This effect is proposed to be the consequence of a decrease in LTD/LTP in cortical inputs to striatopallidal/striatonigral cells in the matrix due to CB1 receptor activation. In addition, cannabinoids can attenuate locomotor activity due to a reducing of glutamate/GABA release from axon terminals of subthalamic nucleus/striatonigral cells of matrix and subsequent decrease/increase in the activity of neurons of globus pallidus/substantia nigra pars reticulata. Cannabinoid-mediated rise of dopamine release might be a result of a decrease of dopamine neuron inhibition by striatonigral cells of striosomes. It follows from the suggested mechanism that an inactivation (activation) of CB1 receptors leading to rise (lowering) of the motor activity can be useful for treatment of Parkinson (Huntington) disease.     

Opioid peptide effects on insulin release and c-AMP in islets of Langerhans

The time course and specificity of the effect of opioid peptides on c-AMP production in the islets of Langerhans was examined. An enkephalin analogue, d-Ala2Me Phe4 Met(O)-ol enkephalin (DAMME, Sandoz) produced a significant stimulation of basal c-AMP levels, with a peak of stimulation at 5 minutes and a decline thereafter. These changes in intracellular c-AMP levels were of the same order of magnitude as those induced by other secretagogues, but did not coincide in time with the more rapid peak of enkephalin-induced insulin release. The rise in islet c-AMP and insulin secretion induced by DAMME and alpha-endorphin but not leu enkephalin was antagonised by naloxone. The effects of high and low concentrations of a variety of opioid peptides and naloxone on insulin release and islet c-AMP levels were determined, alpha-endorphin, dynorphin, leu enkephalin and met enkephalin all stimulated insulin secretion significantly, though not to the same extent. Higher concentrations of alpha-endorphin, dynorphin and met enkephalin inhibited insulin release relative to effects at low opiate concentrations. However, higher concentrations of leu enkephalin stimulated insulin release further. We conclude from these results that the mode of action of opioid peptides in stimulating insulin release is not via increased islet c-AMP exclusively. Furthermore, the results obtained with different classes of opioid suggest the presence of distinctive types of opiate receptor in islets of Langerhans.     

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.     

The effects of local perfusion of DAMGO on extracellular GABA and glutamate concentrations in the rostral ventromedial medulla

Electrophysiological data suggest an involvement of rostral ventromedial medulla (RVM) GABA and glutamate (GLU) neurons in morphine analgesia. Direct evidence that extracellular concentrations of GABA or GLU are altered in response to mu opioid receptor (MOP-R) activation is, however, lacking. We used in vivo microdialysis to investigate this issue. Basal GABA overflow increased in response to intra-RVM perfusion of KCl (60 mmol/L). Reverse microdialysis of the MOP-R agonist d -Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) (20–500 μmol/L) produced a concentration-dependent decrease of RVM GABA overflow. Behavioral testing revealed that concentrations that decreased GABA levels increased thermal withdrawal thresholds. A lower agonist concentration that did not increase GABA failed to alter thermal thresholds. DAMGO did not alter GLU concentrations. However, KCl also failed to modify GLU release. Since rapid, transporter-mediated uptake may mask the detection of changes in GLU release, the selective excitatory amino acid transporter inhibitor pyrrolidine-2,4-dicarboxylic acid (tPDC, 0.6 mmol/L) was added to the perfusion medium for subsequent studies. tPDC increased GLU concentrations, confirming transport inhibition. KCl increased GLU dialysate levels in the presence of tPDC, demonstrating that transport inhibition permits detection of depolarization-evoked GLU overflow. In the presence of tPDC, DAMGO increased GLU overflow in a concentration-dependent manner. These data demonstrate that MOP-R activation decreases GABA and increases GLU release in the RVM. We hypothesize that the opposing effects of MOP-R on GLU and GABA transmission contribute to opiate antinociception.