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Paradoxical Ca²⁺ rises induced by low external Ca²⁺ in rat hippocampal neurones

Andrea Burgo, Giorgio Carmignoto*, Paola Pizzo, Tullio Pozzan and Cristina Fasolato

Confocal Ca²⁺ imaging of rat hippocampal slices shows a paradoxical effect of acute reductions of the [Ca²⁺]_o. Upon slice perfusion with low-Ca²⁺ media, a prompt intracellular Ca²⁺ rise selectively occurs in neurones. This response is observed only in slices challenged with agonists of group I metabotropic glutamate or M₁ muscarinic receptors. In contrast, the intracellular Ca²⁺ level of non-stimulated neurones is insensitive to reductions of [Ca²⁺]_o. The phenomenon is observed in 20-25 % of cultured cortical neurones. Evidence is provided demonstrating that: (1) this paradoxical response is not due to a non-specific decrease in divalent cation concentration but it is selectively activated by a reduction in [Ca²⁺]_o, being maximal with [Ca²⁺]_o between 0.25 and 0.5 mM; (2) upon maximal stimulation, 70-90 % of CA1-CA3 pyramidal neurones sense a reduction in [Ca²⁺]_o; a weaker response is observed in neurones from the neocortex, whereas neurones from the dentate gyrus and granule cells from the cerebellum fail to respond; (3) conditions that elicit paradoxical Ca²⁺ responses cause depolarisation and increase the firing rate of hippocampal neurones; (4) paradoxical Ca²⁺ rises depend, primarily, on Ca²⁺ influx through L-type voltage-operated Ca²⁺ channels and to a lesser extent on release from intracellular Ca²⁺ stores. Inhibition of phospholipase C or protein kinase C failed to suppress the neuronal response, whereas a selective inhibitor of the Src-family of tyrosine kinases abolishes the paradoxical neuronal Ca²⁺ rise. A model is presented to explain how this response is elicited by contemporaneous reduction of the [Ca²⁺]_o and metabotropic receptor stimulation; implications for the pathophysiology of the CNS are also discussed.

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