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 metab otropic glutamate receptors (mGluR1). The conductance change underlying the sEPSP was investigated with rapid photolytic release of L-glutamate from n itroindolinyl (NI)-caged glutamate with ionotropic glutamate receptors bloc ked, and showed a slow mGluR1-activated cation channel. 2. In cerebellar slices rapid photolytic release (t(1/2) < 0.7 ms) of 7-70 <mu>M L-glutamate on PNs voltage clamped at -65 mV activated first a transi ent 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 muM threo-hydroxyaspartate (THA ) and did not reverse as the potential was made positive up to +50 mV, sugg esting 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 muM), indicating activation of m etabotropic type 1 glutamate receptors. The mGluR current was associated wi th 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 Cd2+, Co2+, Mg2+ or Gd3+ ions, by the inhib itor of hyperpolarisation-activated current (I-H) ZD7288, or by the purinoc eptor inhibitor PPADS. Activation was not affected by inhibitors of phospho lipase C (PLC) or protein kinase C (PKC), nor mimicked by photorelease of I nsP(3) or Ca2+. The results show that mGluR1 in PNs produces a slow activat ion of cation-permeable ion channels which is not mediated by PLC activatio n, Ca2+ release from stores, or via the activation of PKC.