Characterisation and distribution of inositol polyphosphate and Ryanodine receptors in the rat brain

The regional distribution of inositol 1,4,5-trisphosphate (InsP3), inositol 1,3,4,5-tetrakisphosphate (InsP4), and ryanodine binding sites has been characterised and compared in the rat brain using radioligand binding assays. Cortical [3H]InsP3 binding indicated similar positional and stereospecificity as observed in other tissues, with 100-fold selectivity for InsP3 over InsP4. Similarly, high-affinity [32P]InsP4 binding also showed a high degree of positional specificity, with a 1,000-fold selectivity for InsP4 over InsP3. Initial characterisation of [3H]ryanodine binding to cortical membranes demonstrated that specific binding was highly dependent on high salt and micromolar Ca2+ concentrations and inhibited by Ca2+ levels of > 1 mM. [3H]-Ryanodine binding was also enhanced by beta, gamma-methylene-adenosine 5'-trisphosphate and caffeine and inhibited by magnesium and ruthenium red (Ki = 0.81 microM). However, dantrolene (300 microM) was ineffective on the binding. Therefore, although the results indicate a greater similarity to the binding properties of the Ca(2+)-induced Ca2+ release channel isoform present in skeletal, rather than cardiac, muscle, it does not appear to be identical. Detailed binding analysis of ryanodine and polyphosphate sites, with the exception of ruthenium red, indicated no interaction between binding sites. Ruthenium red markedly enhanced the binding of both [3H]InsP3 and [32P]InsP4, an effect most probably due to nonspecific complex formation. Regional binding of InP3, InsP4, and ryanodine in the rat brain was of similar affinity for each ligand in each area, but the density profile for each ligand was clearly different. The highest density of InsP3 sites was in the cerebellum, whereas the highest density of ryanodine sites was in the hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)