Specific [3H]Ro 5-4864 binding to rat spinal cord membranes: evidence for peripheral type benzodiazepine recognition sites.

The binding of [3H]Ro 5-4864 and [3H]methylclonazepam to membranes prepared from adult rat spinal cord has been studied in vitro. Scatchard analysis of saturation isotherms suggested that both 3H-labeled ligands bind to single binding sites ([3H]Ro 5-4864 Kd = 2 nM, [3H]methylclonazepam Kd = 3.5 nM), although [3H]Ro 5-4864 bound to 3 times the number of sites labeled by [3H]methylclonazepam (respective Bmax values were 15 vs 5.3 pmol/g tissue). Displacement experiments with clonazepam, flunitrazepam and Ro 5-4864 indicated that [3H]Ro 5-4864 and [3H]methylclonazepam binding had the expected pharmacologic specificity for peripheral and central benzodiazepine recognition sites respectively (i.e. [3H]methylclonazepam binding was sensitive to clonazepam but not Ro 5-4864 whereas [3H]Ro 5-4864 binding was potently inhibited by Ro 5-4864 but not clonazepam. Flunitrazepam had similar affinities for both sites). Thus, in addition to central type benzodiazepine receptors, the rat spinal cord contains comparatively high concentrations of peripheral benzodiazepine recognition sites.