Characterization of benzodiazepine receptors in the bovine pineal gland: evidence for the presence of an atypical binding site.

Bovine and rat pineal benzodiazepine receptors were characterized using ligands with high affinities for either 'central-type' (CBR) or 'peripheral-type' (PBR) benzodiazepine receptors. The characteristics (Bmax = 83 +/- 10 fmol/mg protein, Kd = 3.88 +/- 0.46 nM) of benzodiazepine receptors in bovine pineal membranes measured with [3H]flunitrazepam (using flunitrazepam to define non-specific binding) were consistent with previously reported values. However, if non-specific binding was defined using Ro 15-1788 (a selective CBR ligand), the Bmax and Kd of [3H]flunitrazepam decreased 51 and 58%, respectively. In addition, when using PK 11195 to determine non-specific binding, the Bmax of [3H]flunitrazepam binding to bovine pineal decreased further (approximately 80%, Kd decreased approximately 39%). Together, these observations strongly suggested the presence of PBR in the bovine pineal. Bovine pineal PBR characterized with [3H]PK 11195 revealed a high density (relative to CBR) of high affinity binding sites (Kd = 1.08 +/- 0.30, Bmax = 776 +/- 33.0 fmol/mg protein). In contrast, when [3H]Ro 5-4864 (1-20 nM) was used to define PBR, no binding was detectable. These observations are in sharp contrast to the rat pineal gland, in which both [3H]Ro 5-4864 and [3H]PK 11195 bind to a large number of PBR with high affinity (Kd approximately equal to 1.9 nM, Bmax approximately equal to 26 pmol/mg protein). Bovine pineal PBR were further characterized with compounds structurally related to either Ro 5-4864 or PK 11195.(ABSTRACT TRUNCATED AT 250 WORDS)