ABSTRACT
We recently demonstrated that dopaminergic (DA) neurons of the rat substantia nigra constitutively expressed CXCR4, receptor for the chemokine stromal cell-derived factor-1 (SDF-1)/CXCL12 (SDF-1). To check the physiological relevance of such anatomical observation, in vitro and in vivo approaches were used. Patch clamp recording of DA neurons in rat substantia nigra slices revealed that SDF-1 (10 nmol/L) induced: (i) a depolarization and increased action potential frequency; and (ii) switched the firing pattern of depolarized DA neurons from a tonic to a burst firing mode. This suggests that SDF-1 could increase DA release from neurons. Consistent with this hypothesis, unilateral intranigral injection of SDF-1 (50 ng) in freely moving rat decreased DA content and increased extracellular concentrations of DA and metabolites in the ipsilateral dorsal striatum, as shown using microdialysis. Furthermore, intranigral SDF-1 injection induced a contralateral circling behavior. These effects of SDF-1 were mediated via CXCR4 as they were abrogated by administration of a selective CXCR4 antagonist. Altogether, these data demonstrate that SDF-1, via CXCR4, activates nigrostriatal DA transmission. They show that the central functions of chemokines are not restricted, as originally thought, to neuroinflammation, but extend to neuromodulatory actions on well-defined neuronal circuits in non-pathological conditions.
Subject(s)
Chemokines, CXC/pharmacology , Corpus Striatum/drug effects , Dopamine/metabolism , Substantia Nigra/drug effects , Action Potentials/drug effects , Animals , Behavior, Animal/drug effects , Brain Chemistry/drug effects , Chemokine CXCL12 , Dose-Response Relationship, Drug , Functional Laterality , Male , Microdialysis/methods , Motor Activity/drug effects , Rats , Rats, Wistar , Receptors, CCR4 , Receptors, Chemokine/metabolism , Tyrosine 3-Monooxygenase/metabolismABSTRACT
In rat substantia nigra (SN), Chemokine (CXC motif) receptor 4 (CXCR4) for the chemokine stromal cell-derived factor (SDF)-1alpha is expressed on dopaminergic (DA) neurones, but also on non-DA cells, suggesting presynaptic actions. Using whole-cell patch-clamp recordings in DA neurones of rat SN slices at a holding potential of -60 mV, we showed here that SDF-1alpha exerts multiple presynaptic effects. First, SDF-1alpha (10 nm) induced an increase in the frequency of spontaneous and miniature GABA(A) postsynaptic currents by presynaptic mechanisms, consistent with the presence of CXCR4 on GABAergic neurones of the SN, as revealed by immunocytochemistry. Second, SDF-1alpha (0.1-1 nm) induced a glutamatergic inward current resistant to tetrodotoxin (TTX), most probably the result of glutamate release from non-neuronal cells. This inward current was not blocked by the CXCR4 antagonist AMD 3100 (1 microm), consistent with the lack of CXCR4 on astrocytes as shown by immunocytochemistry under basal conditions. Finally, SDF-1alpha (10 nm) induced, via CXCR4, an outward G protein-activated inward rectifier (GIRK) current, which was TTX sensitive and prevented by application of the GABA(B) antagonist CGP55845A, suggesting GABA spillover on to GABA(B) receptors. Our results show that SDF-1alpha induces, via presynaptic mechanisms, alterations in the excitability of DA neurones as confirmed by current-clamp experiments.
Subject(s)
Chemokines, CXC/metabolism , Dopamine/metabolism , Neural Inhibition/physiology , Presynaptic Terminals/metabolism , Substantia Nigra/metabolism , Synaptic Transmission/physiology , Animals , Benzylamines , Chemokine CXCL12 , Chemokines, CXC/pharmacology , Cyclams , G Protein-Coupled Inwardly-Rectifying Potassium Channels/drug effects , G Protein-Coupled Inwardly-Rectifying Potassium Channels/metabolism , GABA Antagonists/pharmacology , Glutamic Acid/metabolism , Heterocyclic Compounds/pharmacology , Ion Channels/drug effects , Ion Channels/metabolism , Male , Membrane Potentials/drug effects , Membrane Potentials/physiology , Neural Inhibition/drug effects , Organ Culture Techniques , Patch-Clamp Techniques , Presynaptic Terminals/drug effects , Rats , Rats, Wistar , Receptors, CXCR4/drug effects , Receptors, CXCR4/metabolism , Receptors, GABA-A/drug effects , Receptors, GABA-A/metabolism , Sodium Channel Blockers/pharmacology , Substantia Nigra/drug effects , Synaptic Transmission/drug effects , Up-Regulation/drug effects , Up-Regulation/physiologyABSTRACT
Recent studies demonstrated that the chemokine monocyte chemoattractant protein-1 (MCP-1)/CCL2 and its receptor, CCR2, play important roles in various brain diseases. In this study, using quantitative autoradiography, we studied the pharmacological properties of [125l]MCP-1/CCL2 binding in rat brain and we clearly showed the distribution of CCR2 receptors in cerebral cortex, nucleus accumbens, striatum, amygdala, thalamus, hypothalamus, hippocampus, substantia nigra, mammillary bodies and raphe nuclei. Moreover, using double fluorescent immunohistochemistry, we showed that CCR2 receptors were constitutively expressed on neurons and astrocytes. Using RT-PCR methods, we demonstrated that CCR2 mRNA is present in various brain areas described above. Four hours after an acute intraperitoneal lipopolysaccharide injection, we showed that MCP-1/CCL2 binding was up-regulated in several brain structures; this effect took place on both CCR2B labelled neurons and astrocytes and to a lesser extent on activated microglia. To explore neurobiological function of CCR2, actimetric study was carried out. After intracerebroventricular injections of MCP-1/CCL2, we showed that motor activity was markedly decreased. Our results provide the first evidence for constitutive CCR2 receptor expression with precise neuroanatomical and cellular localizations in the brain, and its regulation during an inflammatory process, suggesting that MCP-1/CCL2 and CCR2 play important physiological and pathophysiological role(s) in the CNS.
