Ca2+‐Guanine Nucleotide Interactions in Brain Membranes. II. Characteristics of [3H]Guanosine Triphosphate and [3H]β, γ‐Imidoguanosine 5′‐Triphosphate Binding and Catabolism in the Rat Hippocampus and Striatum

Abstract

Abstract: With [3H]guanosine triphosphate ([3H]GTP) and [3H]β, γ ‐imidoguanosine 5′‐triphosphate ([3H]GppNHp) as the labelled substrates, both the binding and the catabolism of guanine nucleotides have been studied in various brain membrane preparations. Both labelled nucleotides bound to a single class of noninteracting sites (KD= 0.1‐0.5 μm) in membranes from various brain regions (hippocampus, striatum, cerebral cortex). Unlabelled GTP, GppNHp, and guanosine diphosphate (GDP) but not guanosine monophosphate (GMP) and guanosine competitively inhibited the specific binding of [3H]guanine nucleotides. Calcium (0.1–5 mm) partially prevented the binding of [3H]GTP and [3H]GppNHp to hippocampal and striatal membranes. This resulted from both an increased catabolism of [3H]GTP (into [3H]guanosine) and the likely formation of Ca‐guanine nucleotide2‐ complexes. The blockade of guanine nucleotide catabolism was responsible for the enhanced binding of [3H]GTP to hippocampal membranes in the presence of 0.1 mm‐ATP or 0.1 mm‐GMP. Striatal lesions with kainic acid produced both a 50% reduction of the number of specific guanine nucleotide binding sites and an acceleration of [3H]GTP and [3H]GppNHp catabolism (into [3H]guanosine) in membranes from the lesioned striatum. This suggests that guanine nucleotide binding sites were associated (at least in part) with intrinsic neurones whereas the catabolising enzyme(s) would be (mainly) located to glial cells (which proliferate after kainic acid lesion). The characteristics of the [3H]guanine nucleotide binding sites strongly suggest that they may correspond to the GTP subunits regulating neurotransmitter receptors including those labelled with [3H]5‐hydroxytryptamine ([3H]5‐HT) in the rat brain.