Presynaptic Facilitation of Glutamatergic Synapses to Dopaminergic Neurons of the Rat Substantia Nigra by Endogenous Stimulation of Vanilloid Receptors
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3136 • The Journal of Neuroscience, April 15, 2003 • 23(8):3136–3144 Presynaptic Facilitation of Glutamatergic Synapses to Dopaminergic Neurons of the Rat Substantia Nigra by Endogenous Stimulation of Vanilloid Receptors Silvia Marinelli,1 Vincenzo Di Marzo,4 Nicola Berretta,1 Isabel Matias,4 Mauro Maccarrone,3 Giorgio Bernardi,1,2 and Nicola B. Mercuri1,2 1Istituto di Ricovero e Cura a Carattere Scientifico Fondazione Santa Lucia and Departments of 2Neuroscience and 3Experimental Medicine and Biochemical Sciences, University of Tor Vergata, 00179 Rome, Italy, and 4Endocannabinoid Research Group, Istituto di Chimica Biomolecolare, Consiglio Nazionale delle Ricerche, 80078 Pozzuoli, Italy Growing evidence regarding the function of vanilloid receptor-1 (VR1) in the brain suggests potential central roles of this receptor, previously described to occur primarily in peripheral sensory neurons. In the present study, we used electrophysiological and biochem- ical techniques to investigate the function and the endogenous stimulation of VR1 in dopaminergic neurons of the substantia nigra pars compacta (SNc). The VR1 agonist capsaicin increased the frequency of both TTX-sensitive and -insensitive spontaneous EPSCs (sEPSCs) without affecting their amplitude, suggesting a presynaptic site of action. In contrast, no effect was detected with regard to GABAergic transmission. No increase in sEPSC frequency was observed in the presence of cadmium chloride, while the voltage-dependent calcium channel antagonist -conotoxin MVIIC did not prevent capsaicin action. The VR1 antagonists capsazepine and iodoresiniferatoxin (IRTX) blocked the effects of capsaicin. Importantly, IRTX per se reduced sEPSC frequency, suggesting a tonic activity of VR1. The endogenous VR1 agonist anandamide (AEA) produced an IRTX-sensitive increase in the frequency of sEPSCs on dopaminergic neurons that was more pronounced when protein kinase A had been activated. Furthermore, mass spectrometric analyses and binding experi- ments revealed high levels of endogenous AEA and specific binding of AEA to VR1 receptors in the SNc. These data suggest a tonic facilitation of glutamate release exerted by VR1 in the SNc through a stimulation of VR1 by endovanilloids, including anandamide. The increase in sEPSC frequency by VR1 onto midbrain dopaminergic neurons suggests the involvement of these receptors in motor and cognitive functions involving the dopaminergic system. Key words: capsaicin; VR1; EPSCs; substantia nigra; anandamide; presynaptic mechanisms Introduction 12-hydroperoxyeicosatetraenoic acid, and leukotriene B4 (Shin et The recent demonstration of a widespread distribution of the al., 2002) and, more potently, N-arachidonoyl-dopamine (NADA) mRNA for vanilloid receptor subtype-1 [VR1, also known as (Huang et al., 2002) can act as endogenous VR1 agonists. transient receptor potential vanilloid 1 (TRPV1)] and of VR1- With regard to the role of VR1 in the CNS, little is known of its like immunoreactivity in the CNS (Sasamura et al., 1998; Mezey function in the brain. Recent evidence suggests distinct actions in et al., 2000; Schumacher et al., 2000; Szabo et al., 2002) strongly the hypothalamus, locus ceruleus, and hippocampus. In particu- suggests that these receptors are involved not only in peripheral lar, capsaicin has been reported to increase the firing rate of hy- functions such as perception of inflammatory and thermal pain pothalamic preoptic neurons recorded in vivo (Hori et al., 1988) (Caterina et al., 1997; Julius and Basbaum, 2001) but also in and the release of glutamate from hypothalamic slices (Sasamura central responses. It is known that the “hot” chili pepper constit- et al., 1998). In the locus ceruleus, capsaicin activated firing uent capsaicin (Caterina et al., 1997; Tominaga et al., 1998; Kress (Hajos et al., 1987) and increased the frequency of excitatory and Zeilhofer, 1999; Smart et al., 2000) and the irritant extract postsynaptic currents (Marinelli et al., 2002a). However, capsa- from the Euphorbia resinifera, resiniferatoxin (RTX) activate icin, anandamide, and NADA potentiate the GABA-dependent these receptors (Acs et al., 1994). In addition protons, heat, and paired-pulse depression in the CA1 region of the hippocampus endogenous fatty acid derivatives such as anandamide (AEA) (Al-Hayani et al., 2001; Huang et al., 2002). (Zygmunt et al., 1999; Smart et al., 2000; Di Marzo et al., 2001b), The basal ganglia is endowed with VR1-immunopositive cells, with the substantia nigra being an area with a strong signal (Mezey et al., 2000). Behavioral experiments have reported that Received Sept. 10, 2002; revised Jan. 13, 2003; accepted Jan. 13, 2003. We thank Dr. Marco Capogna for helpful comments on this manuscript, Drs. Monica Bari and Natalia Battista for an intranigral injection of capsaicin enhanced motor behavior their skillful assistance in the binding experiments, Dr. Luciano De Petrocellis for valuable assistance in the RT-PCR (Dawbarn et al., 1981; Hajos et al., 1988), suggesting a functional experiments, and the VolkswagenStiftung for partial support (V.D.M.). role of these receptors in this area, but no direct investigations Correspondence should be addressed to Dr. Silvia Marinelli, Fondazione Santa Lucia Istituto di Ricovero e have been performed so far to elucidate VR1 function in the Cura a Carattere Scientifico, Experimental Neurology Laboratory, Via Ardeatina 306, 00179 Rome, Italy. E-mail: [email protected]. ventral midbrain. Copyright © 2003 Society for Neuroscience 0270-6474/03/233136-09$15.00/0 We now present direct evidence of VR1 function in the sub- Marinelli et al. • Presynaptic Increase in sEPSCs by VR1 in the SNc J. Neurosci., April 15, 2003 • 23(8):3136–3144 • 3137 stantia nigra and demonstrate an excitatory role of these recep- vitrogen, San Diego, CA). After extraction, RNA was precipitated using tors, exerted through an increase in synaptic glutamate release ice-cold isopropanol and resuspended in diethyl pyrocarbonate (Sigma, onto the dopaminergic neurons. Moreover, we provide evidence St. Louis, MO)-treated water; its integrity was verified after separation by for an endogenous activation of VR1 that tonically stimulates electrophoresis into a 1% agarose gel containing ethidium bromide. glutamate release in the dopaminergic system. Part of this work RNA was further treated with RNase-free DNase I (DNA-free kit; Am- has been published previously in abstract form (Marinelli et al., bion, Austin, TX) to digest contaminating genomic DNA and to subse- quently remove the DNase and divalent cations. 2002b). The expression of mRNAs for glyceraldehyde-3-phosphate dehydro- genase (GAPDH) and VR1 receptors was examined by reverse transcrip- Materials and Methods tion (RT)-PCR. Total RNA was reverse-transcribed using oligo(dT) Slice preparation. Wistar rats (12–24 d of age) were anesthetized with halo- primers. DNA amplifications were performed in PCR buffer (Q-Biogen) thane and killed by decapitation. All experiments followed international containing 2 l of cDNA, 500 M deoxyNTP, 2 mM MgCl2, 0.8 M of each guidelines on the ethical use of animals from the European Communities primer, and 0.5 U of Taq polymerase (Q-Biogen). The thermal reaction Council (EEC) Directive of November 24, 1986 (86/609/EEC). The brain profile consisted of a denaturation step at 94°C for 1 min, annealing at was rapidly removed from the skull, and horizontal midbrain slices (300 60°C for 1 min, and an extension step at 72°C for 1 min. A final extension m) were cut in cold artificial CSF (ACSF) using a vibratome and left to step of 10 min was performed at 72°C. The PCR cycles were 35 for VR1 recover at 33°C for at least 1 hr. Slices were placed in a recording chamber and GAPDH and were observed to be optimal and in the linear portion of and submerged in continuously flowing (2.5 ml/min, 33–33.5°C) ACSF. the amplification curve (data not shown). Reaction was performed in a ACSF composition was as follows (in mM): 126 NaCl, 2.5 KCl, 1.2 MgCl2, 2.4 PE Gene Amp PCR System 2400 (PerkinElmer Life Sciences, Emeryville, CaCl2, 1.2 NaH2PO4, 19 NaHCO3, and 10 glucose. In experiments using CA). After reaction, the PCR products were electrophoresed on a 2% cadmium, NaH2PO4 and CaCl2 were omitted from ACSF. agarose gel containing ethidium bromide for UV visualization. The spe- Neurons were visualized using infrared Nomarski optics on an upright cific oligonucleotides were synthesized on the basis of cloned human microscope (BX50WI; Olympus Optical, Hataguya, Shibuya-Ku, Japan). cDNA sequences of rat GAPDH and VR1. For GAPDH, the primer se- Patch-clamp recordings were obtained using glass electrodes (3–4M⍀) quences were CCCTTCATTGACCTCAACTACATGGT (sense) and filled with (in mM): 150 K-methylsulfate, 2 MgCl2, 0.1 EGTA, 10 HEPES, GAGGGGCCATCCACAGTCTTCTG (antisense). The VR1 sense and anti- 2 MgATP, and 0.3 Na3GTP, pH 7.4, with KOH. sense primers were TCTATGATCGCAGGAGCATCTTCG and TCTGT- Whole-cell voltage-clamp recordings (Ϫ60 mV holding potential) GTAGCTGGCATTGACAAAC, respectively. The expected sizes of the were obtained using an Axopatch 1D (Axon Instruments, Foster City, amplicons were 470 bp for GAPDH and 256 bp for VR1. CA). Spontaneous EPSCs (sEPSCs) and spontaneous IPSCs (sIPSCs) Binding of [3H]anandamide to rat midbrain slices. The binding of 200 3 were filtered at 1 kHz, digitized at 10 kHz, and recorded on computer nM [ H]AEA to substantia nigra membrane fractions (50 g protein/ using pClamp8 software (Axon Instruments). Series resistances (8–15