Phylogenetic distribution of bicuculline-sensitive gamma-amino-butyric acid (GABA) receptor binding

Bicuculline-sensitive [3H]GABA receptor binding was studied in membrane fractions prepared from vertebrate whole brain or invertebrate cephalic ganglia. In tissue not treated with Triton X-100, a significant amount of bicuculline-displaceable [3H]GABA binding was detected in the brains of all vertebrates studied, with the hagfish brain binding over twice as much [3H]GABA as the spiny dogfish, the next oldest species. All other vertebrates bound similar amounts of [3H]GABA, being one-third to one-fourth that observed in the hagfish. In contrast, after Triton treatment, the hagfish displayed the least amount of bicuculline-sensitive [3H]GABA binding and, under those conditions, the amount of binding observed increased in an evolutionary fashion. No measurable bicuculline-sensitive GABA receptor binding was noted in any invertebrate studied. These results suggest that bicuculline-sensitive GABA receptors are present in the brains of all vertebrates and that during the course of evolution there developed a Triton-sensitive substance(s) whose presence modifies the kinetic properties of this receptor site.