Bicuculline- and Baclofen-Insensitive γ-Aminobutyric Acid Binding to Rat Cerebellar Membranes

Up to 60% of gamma-[3H]aminobutyric acid ([3H]GABA) bound specifically to rat cerebellar membranes in the absence of Ca2+ was insensitive to the GABAA antagonist bicuculline and to the GABAB agonist baclofen. This indicates that a significant component of specifically bound [3H]GABA is associated with non-GABAA, non-GABAB binding sites. The presence of this binding component appeared seasonal, peaking in the month of September (early spring) each year over a 4-year period. The calcium independence and bicuculline and baclofen insensitivity of the binding indicate that this binding is not to the classical GABAA and GABAB binding sites. High concentrations of muscimol and isoguvacine inhibited non-GABAA, non-GABAB binding. Scatchard analysis of the non-GABAA, non-GABAB binding sites indicated two kinetic components: KD1 = 42 nM and KD2 = 9.2 microM; Bmax1 = 1.6 pmol/mg of protein and Bmax2 = 28 pmol/mg of protein.     

Isoguvacine Binding, Uptake, and Release: Relation to the GABA System

Isoguvacine (1,2,3,6-tetrahydropyridine-4-car-boxylic acid) is a GABA (γ-aminobutyric acid) agonist with limited conformational flexibility. In these studies we investigated the binding, uptake, and release of [³H] isoguvacine by use of tissue preparations of rat CNS, comparing the results with similar studies of [³H]GABA. The results from these investigations indicate that isoguvacine binds to membrane preparations of rat forebrain with pharmacological characteristics similar to the post-synaptic GABA recognition site; that it is transported into synaptosomal preparations by an uptake system similar to the high-affinity GABA uptake system; and that recently accumulated isoguvacine is released in a Ca²⁺-dependent manner and by heteroexchange with external GABA. The ability of isoguvacine and γ-hydroxybutyric acid to decrease the K⁺-stimulated Ca²⁺-dependent release process was also investigated. The results indicate that isoguvacine interactions have many of the biochemical features of GABA synaptic function, isoguvacine being, however, less potent than GABA.