Characterization of the mGluR₁-mediated electrical and calcium signaling in purkinje cells of mouse cerebellar slices

The metabotropic glutamate receptor 1 (mGluR1) plays a fundamental role in postnatal development and plasticity of ionotropic glutamate receptor-mediated synaptic excitation of cerebellar Purkinje cells. Synaptic activation of mGluR₁ by brief tetanic stimulation of parallel fibers evokes a slow excitatory postsynaptic current and an elevation of intracellular calcium concentration ([Ca²+]_i) in Purkinje cells. The mechanism underlying these responses has not been identified yet. Here we investigated the responses to synaptic and direct activation of mGluR₁ using whole ceil patch-clamp recordings in combination with microfluorometric measurements of [Ca²⁺]_i in mouse Purkinje cells. Following pharmacological block of ionotropic glutamate receptors, two to six stimuli applied to parallel fibers at 100 Hz evoked a slow inward current that was associated with an elevation of [Ca²⁺]_i. Both the inward current and the rise in [Ca²⁺]_i increased in size with increasing number of pulses albeit with no clear difference between the minimal number of pulses required to evoke these responses. Application of mGluR₁ agonist (S)-3,5-dihydroxy-phenylglycine (3,5-DHPG) by means of short-lasting (5-100 ms) pressure pulses delivered through an agonist-containing pipette positioned over the Purkinje cell dendrite, evoked responses resembling the synaptically induced inward current and elevation of [Ca²⁺]_i. No increase in [Ca²⁺]_i was observed with inward currents of comparable amplitudes induced by the ionotropic glutamate receptor agonist AMPA. The 3,5-DHPG-induced inward current but not the associated increase in [Ca²⁺]_i was depressed when extracellular Na⁺ was replaced by choline, but, surprisingly, both responses were also depressed when bathing the tissue in a low calcium (0.125 mM) or calcium-free/EGTA solution, gargin (10 μM) and cyclopiazonic acid (30 μM), inhibitors of sarco-endoplasmic reticulum Ca²⁺-ATPase, had little effect on either the inward current or the elevation in [Ca²⁺]_i induced by 3,5-DHPG. Furthermore, the inward current induced by 3,5-DHPG was neither blocked by 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy] ethyl-1H-imidazole, an inhibitor of store operated calcium influx, nor by nimodipine or omega-agatoxin, blockers of voltage-gated calcium channels. These electrophysiological and Ca²⁺-imaging experiments suggest that the mGluR₁-mediated inward current, although mainly carried by Na⁺, involves influx of Ca²⁺ from the extracellular space.