Intracellular inositol 1,3,4,5-tetrakisphosphate enhances the calcium current in hippocampal CA1 neurones of the gerbil after ischaemia

1. To examine the role of the phosphoinositide cascade triggered by disturbed Ca2+ homeostasis in ischaemic neurones, inositol 1,3,4,5-tetrakisphosphate (InsP(4)) was applied to the cytoplasmic face of membrane patches isolated from CA1 pyramidal neurones in the gerbil hippocampus. 2. In outside-out recordings, InsP(4) induced an inward current which was increased by raising the extracellular [Ca2+]. In contrast, no clear channel openings could be observed in patches from neurones of sham-operated gerbils. 3. Open probabilities of InsP(4)-activated channels were significantly decreased upon application of omega-conotoxin but were not affected by omega-agatoxin or nifedipine. 4. In inside-out patches using high concentrations of Ca2+, Ba2+ or Sr2+ in the pipette solution, InsP(4) enhanced inward currents. 5. Application of the isomers of InsP(4) slightly enhanced the currents, but inositol 1,4,5-trisphosphate (InsP(3)) had no effect. 6. In the absence of InsP(4) there was a single main Ba2+ current peak of 4.0 pA in amplitude, whereas upon its application two main peaks of 3.0 and 7.2 pA were present. 7. The open probabilities of these channels mere apparently increased by InsP(4). 8. These findings support the view that a disturbed phosphoinositide cascade occurs in the hippocampal pyramidal neurones after ischaemia and the InsP(4) thus formed plays an important role in promoting the Ca2+ accumulation which results in neuronal death.