title: Effects of the Parkinsonian toxin MPP+ on electrophysiological properties of nigral dopaminergic neurons.
abstract: Although MPP(+) (1-methyl-4-phenylpyridinium) has been widely used to damage dopaminergic neurons of the Substantia Nigra pars compacta (SNc) and produce animal and cellular models of Parkinson's disease, the action of this toxin on ion channels and electrophysiological properties of these neurons remains controversial. Previous work has attributed the early effects of MPP(+) on the membrane potential and firing frequency of SNc neurons either to block of hyperpolarisation-activated (Ih) current, or to activation of ATP-sensitive K(+) (KATP) channels. Using a combination of electrophysiological and pharmacological techniques, we investigated the acute effects of MPP(+) (20 μM) on SNc neurons in rat midbrain slices. Our results show that MPP(+) inhibits the activity of these neurons in distinct stages involving different mechanisms. The early phase of inhibition was dependent on D2 autoreceptors, but [(3)H]raclopride membrane binding and cAMP production assays demonstrated that the toxin (0.001-100 μM) did not directly bind to these receptors nor activated the Gi-linked signalling pathway. Depletion of vesicular dopamine with Ro4-1284 attenuated the early inhibitory effect, indicating that D2 autoreceptors were activated by dopamine released from the somato-dendritic region. After longer exposure (>10-20 min), MPP(+) produced a late phase of inhibition which mainly involved activation of KATP channels, and required uptake of the toxin via dopamine transporter. Although Ih current mediated by hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels was reduced by MPP(+), neither inhibition of firing nor membrane potential hyperpolarisation was significantly attenuated by blocking HCN channels with ZD7288. Our results indicate that the initial cellular events that lead to activation of cell death pathways by MPP(+) are complex and include KATP, and dopamine-dependent components, and show that the inhibitory effect of the toxin is independent of Ih block.
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: andrew.yee@auckland.ac.nz.
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Yee
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: slee544@aucklanduni.ac.nz.
authors.forename: Sun-Min
authors.initials: SM
authors.lastname: Lee
authors.affiliation: Department of Pharmacology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: m.hunter@auckland.ac.nz.
authors.forename: Morag R
authors.initials: MR
authors.lastname: Hunter
authors.affiliation: Department of Pharmacology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: m.glass@auckland.ac.nz.
authors.forename: Michelle
authors.initials: M
authors.lastname: Glass
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: peter.s.freestone@gmail.com.
authors.forename: Peter S
authors.initials: PS
authors.lastname: Freestone
authors.affiliation: Department of Physiology, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand; Centre for Brain Research, Faculty of Medical and Health Sciences, University of Auckland, Auckland 1142, New Zealand. Electronic address: j.lipski@auckland.ac.nz.
authors.forename: Janusz
authors.initials: J
authors.lastname: Lipski
mesh_terms: D015655:1-Methyl-4-phenylpyridinium / Q000633:toxicity*; D000818:Animals; D004195:Disease Models, Animal; D004298:Dopamine / Q000378:metabolism; D050483:Dopamine Plasma Membrane Transport Proteins / Q000378:metabolism; D059290:Dopaminergic Neurons / Q000187:drug effects* / Q000378:metabolism; D055724:Electrophysiological Phenomena; D054086:KATP Channels / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects*; D009433:Neural Inhibition / Q000187:drug effects; D020734:Parkinsonian Disorders / Q000139:chemically induced* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D017448:Receptors, Dopamine D2 / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D013378:Substantia Nigra / Q000187:drug effects* / Q000378:metabolism
chemical_list: D050483:Dopamine Plasma Membrane Transport Proteins; D054086:KATP Channels; D017448:Receptors, Dopamine D2; D015655:1-Methyl-4-phenylpyridinium; D004298:Dopamine
keywords: Brain slice; D2 autoreceptors; Dopamine; HCN channels; K(ATP) channels