Buprenorphine: Characteristics of binding sites in the rat central nervous system

The binding of the mixed opiate agonist-antagonist ³H-buprenorphine to rat CNS membranes was stereospecific, saturable and had high affinity. Maximal specific binding of ³H-buprenorphine at 25°C was reached by 30 minutes and dissociation from the receptor was slow. ³H-Buprenorphine labelled a single class of high affinity binding sites (K_D = 0.86nM, B_max = 30.2pmole/g tissue). The B_max for ³H-buprenorphine was about two times that for the μ-opiate receptor drugs ³H-naloxone and ³H-dihydromorphine, and three times the B_max for the σ-opiate receptor ligand ³H-D-Ala², L-Met⁵-enkephalinamide. The regional distribution of ³H-buprenorphine binding was qualitatively similar to the distribution of ³H-naloxone and ³H-dihydromorphine binding. Changing the incubation temperature from 25°C to 37°C increased ³H-buprenorphine binding in all regions of the CNS yet decreased ³H-naloxone and ³H-dihydromorphine binding in most regions. These effects of increasing temperature were a result of changes in ³H-opiate affinity for the receptor with no significant changes in receptor number. Sodium chloride (154mM) enhanced both ³H-buprenorphine and ³H-naloxone binding, and decreased ³H-dihydromorphine binding. The potency of opiate alkaloids and peptides in displacing ³H-buprenorphine was relatively weak with IC₅₀ values ranging between 40nM and 600nM. Furthermore displacement curves were shallow, yielding curvilinear Scatchard plots. Buprenorphine was very potent in displacing ³H-naloxone (IC₅₀ = 0.52nM), ³H-dihydromorphine (IC₅₀ = 1.17nM) and ³H-D-Ala², L-Met⁵-enkephalinamide (IC₅₀ = 0.47nM). These findings suggest that buprenorphine binds to both μ- and δ-opiate receptors.