The conductance underlying the parallel fibre slow EPSP in rat cerebellar Purkinje neurones studied with photolytic release of L-glutamate

Authors


Corresponding author D. Ogden: National Institute for Medical Research, Mill Hill, London NW7 1AA, UK. Email: dogden@nimr.mrc.ac.uk

Abstract

  • 1Tetanic 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.
  • 2In cerebellar slices rapid photolytic release (t1/2 < 0.7 ms) of 7-70 μM 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.
  • 3The initial current was inhibited by 300 μM threo-hydroxyaspartate (THA) and did not reverse as the potential was made positive up to +50 mV, suggesting activation of electrogenic glutamate uptake.
  • 4The slow current was inhibited reversibly by 1 mM (R,S)-MCPG or the non-competitive mGluR1 antagonist CPCCOEt (20 μM), 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+.
  • 5The sEPSC was not blocked by Cd2+, Co2+, Mg2+ or Gd3+ ions, by the inhibitor of hyperpolarisation-activated current (IH) 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 InsP3 or Ca2+. The results show that mGluR1 in PNs produces a slow activation of cation-permeable ion channels which is not mediated by PLC activation, Ca2+ release from stores, or via the activation of PKC.

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