The conductance underlying the parallel fibre slow EPSP in rat cerebellar Purkinje neurones studied with photolytic release of L-glutamate.
1. Tetanic stimulation of parallel fibres (PFs) produces a slow EPSP (sEPSP) or slow EPSC (sEPSC) in Purkinje neurones (PNs), mediated by type 1 metabotropic glutamate receptors (mGluR1). The conductance change underlying the sEPSP was investigated with rapid photolytic release of L-glutamate from nitroindolinyl (NI)-caged glutamate with ionotropic glutamate receptors blocked, and showed a slow mGluR1-activated cation channel. 2. In cerebellar slices rapid photolytic release (t (1/2) < 0.7 ms) of 7--70 microM L-glutamate on PNs voltage clamped at -65 mV activated first a transient inward current, peaking in 8 ms, followed by a slow inward current with time course similar to the PF sEPSP, peaking at -1 nA in 700 ms. 3. The initial current was inhibited by 300 microM threo-hydroxyaspartate (THA) and did not reverse as the potential was made positive up to +50 mV, suggesting activation of electrogenic glutamate uptake. 4. The slow current was inhibited reversibly by 1 mM (R,S)-MCPG or the non-competitive mGluR1 antagonist CPCCOEt (20 microM), indicating activation of metabotropic type 1 glutamate receptors. The mGluR current was associated with increases of input conductance and membrane current noise, and reversed close to 0 mV, indicating activation of channels permeant to Na(+) and K(+). 5. The sEPSC was not blocked by Cd(2+), Co(2+), Mg(2+) or Gd(3+) ions, by the inhibitor of hyperpolarisation-activated current (I(H)) ZD7288, or by the purinoceptor inhibitor PPADS. Activation was not affected by inhibitors of phospholipase C (PLC) or protein kinase C (PKC), nor mimicked by photorelease of InsP(3) or Ca(2+). The results show that mGluR1 in PNs produces a slow activation of cation-permeable ion channels which is not mediated by PLC activation, Ca(2+) release from stores, or via the activation of PKC.
Canepari
M
M
National Institute for Medical Research, Mill Hill, London NW7 1AA, UK.
Papageorgiou
G
G
Corrie
J E
JE
Watkins
C
C
Ogden
D
D
D000818:Animals; D002463:Cell Membrane Permeability / Q000502:physiology; D002867:Chromones / Q000494:pharmacology; D004553:Electric Conductivity; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology*; D018698:Glutamic Acid / Q000737:chemistry / Q000378:metabolism*; D066298:In Vitro Techniques; D007211:Indoles / Q000737:chemistry / Q000378:metabolism; D007473:Ion Channels / Q000037:antagonists & inhibitors; D008564:Membrane Potentials / Q000502:physiology; D009412:Nerve Fibers / Q000502:physiology*; D010716:Phosphatidylinositols / Q000037:antagonists & inhibitors; D010782:Photolysis; D011689:Purkinje Cells / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017208:Rats, Wistar; D011930:Reaction Time / Q000187:drug effects / Q000502:physiology; D018094:Receptors, Metabotropic Glutamate / Q000037:antagonists & inhibitors / Q000378:metabolism
C117789:7-(hydroxyimino)cyclopropan(b)chromen-1a-carbxoylic acid ethyl ester; D002867:Chromones; D018691:Excitatory Amino Acid Antagonists; D007211:Indoles; D007473:Ion Channels; D010716:Phosphatidylinositols; D018094:Receptors, Metabotropic Glutamate; C104077:metabotropic glutamate receptor type 1; D018698:Glutamic Acid