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Ca²⁺ store-dependent potentiation of Ca²⁺-activated non-selective cation channels in rat hippocampal neurones in vitro

L. Donald Partridge and C. Fernando Valenzuela

1. Potentiation of calcium-activated non-selective cation (CAN) channels was studied in rat hippocampal neurones. CAN channels were activated by IP₃-dependent Ca²⁺ release following metabotropic glutamate receptor (mGluR) stimulation either by Schaffer collateral input to CA1 neurones in brain slices in which ionotropic glutamate and GABA_A receptors, K⁺ channels, and the Na⁺-Ca²⁺ exchanger were blocked or by application of the mGluR antagonist ACPD in cultured hippocampal neurones.

2. The CAN channel-dependent depolarization (V_CAN) was potentiated when [Ca²⁺]_i was increased in neurones impaled with Ca²⁺-containing microelectrodes.

3. Fura-2 measurements revealed a biphasic increase in [Ca²⁺]_i when 200 µM ACPD was bath applied to cultured hippocampal neurones. This increase was greatly attenuated in the presence of Cd²⁺.

4. Thapsigargin (1 µM) caused marked potentiation of V_CAN in CA1 neurones in the slices and of the CAN current (I_CAN) measured in whole cell-clamped cultured hippocampal neurones.

5. Ryanodine (20 µM) also led to a potentiation of V_CAN while neurones pretreated with 100 µM dantrolene failed to show potentiation of V_CAN when impaled with Ca²⁺-containing microelectrodes.

6. The mitochondrial oxidative phosphorylation uncoupler carbonyl cyanide m-chlorophenyl hydrazone (2 µM) also caused a potentiation of V_CAN.

7. CAN channels are subject to considerable potentiation following an increase in [Ca²⁺]_i due to Ca²⁺ release from IP₃-sensitive, Ca²⁺-sensitive, or mitochondrial Ca²⁺ stores. This I_CAN potentiation may play a crucial role in the 'amplification' phase of excitotoxicity.

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