Maximal GABA and muscimol binding to high-affinity sites differ in physiological and in non-physiological buffers

The sensitivity of [(3)H]GABA and [(3)H]muscimol high-affinity binding sites to physiological (Krebs-Ringer's bicarbonate) and non-physiological (Tris-citrate) buffers was examined using synaptosomal membranes from bovine retinas. The maximum number of sites (B(max)) for [(3)H]GABA was present when the tissue was assayed in KRB. With only one exception, this effect was independent of the washing conditions used or a small change in pH. In contrast, [(3)H]muscimol binding sites were maximally present when the tissue was washed in Tris, regardless of the assaying conditions or the small change in pH. Neither [(3)H]GABA nor [(3)H]muscimol was displaced by ( - )baclofen. The apparent dissociation constants (K(d)) of the ligands did not change under any of the conditions tested. These findings demonstrate a fundamental difference between GABA and muscimol binding sites.