Mapping second messenger systems in the rat hippocampus after transient forebrain ischemia: in vitro [3H]forskolin and [3H]inositol 1,4,5-trisphosphate binding.

Autoradiographic imaging demonstrated predominant and reciprocal localization of forskolin and inositol 1,4,5-trisphosphate (IP3) binding sites in synaptic areas in the hippocampus. We produced selective damage to the CA1 pyramidal cells in the rat hippocampus by means of transient forebrain ischemia and analyzed the alteration of the intracellular signal transduction using quantitative autoradiography of these second messenger systems. The dendritic fields (stratum oriens, radiatum and lacunosummoleculare) in the CA1 showed 20% decrease in [3H]IP3 binding activity 3 h after ischemia, when no morphological abnormalities were obvious. Thereafter, grain density in these layers decreased and half of the binding sites were lost 2 days after ischemia. By contrast, the stratum pyramidale of the CA1 showed no significant change until 2 days after recirculation. Seven days after ischemia, when CA1 pyramidal cells were depleted, all layers in the CA1 subfield lost 85% of [3H]IP3 binding sites. In the CA3 subfield, only a small and transient alteration in the [3H]IP3 binding was noticed during recirculation. Postischemic reduction of [3H]forskolin binding sites was obvious in the CA1 only 1 h after ischemia followed by loss of 50% of binding activity 7 days after recirculation. These results suggest that forskolin and IP3 binding sites are predominantly distributed on the pyramidal cells in the CA1 subfield and that marked alteration of intracellular signal transduction precedes the delayed CA1 pyramidal cell death.