Responses to Metabotropic Glutamate Receptor Activation in Cerebellar Purkinje Cells: Induction of an Inward Current

The responses to activation of metabotropic glutamate receptors (mGluRs) of Purkinje cells in rat cerebellar slice cultures were investigated using intracellular recordings in single-electrode voltage-clamp mode combined with microfluorometric measurements of cytosolic free calcium using fura-2. Purkinje cells were perfused with saline containing 0.5 microM tetrodotoxin and 10 microM bicuculline and voltage-clamped at -60 mV. Bath-applied trans-(+/-)-1-amino-1,3-cyclopentanedicarboxylic acid (t-ACPD, 50 - 100 microM), a selective agonist of mGluRs, induced a transient inward current that was followed by an outward current. The response induced by t-ACPD was not affected by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, up to 40 microM). In contrast, inward currents caused by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA, 1 - 2 microM) were completely abolished, while inward currents caused by quisqualate (0.25 microM) were only partially depressed by CNQX (5 - 40 microM). The inward current induced by t-ACPD was unaffected by external Ba2+ (1 mM), tetraethylammonium (10 mM) and Cs+ (1 mM), and was associated with an increase in apparent input conductance of the cell membrane. The extrapolated reversal potential of inward currents induced by t-ACPD was +18 mV while Cl- currents induced by muscimol reversed at -66 mV. Inward currents induced by t-ACPD, but not those induced by AMPA, were associated with a rise in cytosolic Ca2+ concentration and suppressed by intracellular injection of a calcium chelator. Replacement of external Na+ by choline or Li+ depressed the inward current and resulted in a slower decay of the Ca2+ signal.