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 μM tetrodotoxin and 10 μM bicuculline and voltage‐clamped at –60 mV. Bath‐applied trans‐(±)‐1‐amino‐1,3‐cyclopentanedicarboxylic acid (t‐ACPD, 50–100 μM), 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 μM). In contrast, inward currents caused by (RS)‐α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionic acid (AMPA, 1–2 μM) were completely abolished, while inward currents caused by quisqualate (0.25 μM) were only partially depressed by CNQX (5–40μM). 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.