Metabotropic glutamate receptor activation enhances the activities of two types of Ca²⁺-activated K⁺ channels in rat hippocampal astrocytes

The influence of activation of glutamate receptor (GluR) on outward K⁺ current in cultured neonate rat hippocampal astrocytes was investigated. Patch-clamp analysis of K⁺ channel currents in cultured astrocytes identified the existence of 71 ± 6 and 161 ± 11 pS single-channel K⁺ currents that were sensitive to changes in voltage and [Ca²⁺]_i and blocked by external TEA but not by charybdotoxin, iberiotoxin, apamin, or 4-aminopyridine. Reverse transcriptase (RT)-PCR and Northern blot analysis revealed transcripts of the Ca²⁺-activated K⁺ channel (K_Ca) β₄-subunit (β4) (KCNMB4) in cultured astrocytes. Expression of the metabotropic glutamate receptor (mGluR) subtypes mGluR1 and mGluR5 and the ionotropic glutamate receptor (iGluR) subtypes iGluR1 and iGluR4 were detected by RT-PCR and immunofluorescence analysis in cultured astrocytes. The mGluR agonists L-glutamate and quisqualate increased the open state probability (NP_o) of the 71 and 161 pSK⁺ channel currents that were prevented by the mGluR receptor antagonists 1-aminoindan-1,5-dicarboxylic acid or L-(+)-2-amino-3-phosphonopropionic acid and not by the iGluR antagonists (+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate or CNQX. Activation of the two types of K⁺ channel currents by mGluR agonists was attenuated by pertussis toxin and by inhibition of phospholipase C (PLC) or cytochrome P450 arachidonate epoxygenase. These results indicate that brain astrocytes contain the KCNMB4 transcript and express two novel types of K_Ca channels that are gated by activation of a G-protein coupled metabotropic glutamate receptor functionally linked to PLC and cytochrome P450 arachidonate epoxygenase activity.