BRAIN AND SPINAL-CORD DISTRIBUTION OF BIPHALIN - CORRELATION WITH OPIOID RECEPTOR DENSITY AND MECHANISM OF CNS ENTRY

Biphalin [(Tyr-D-Ala-Gly-Phe-NH)(2)] is a bivalent, opioid peptide con taining two pharmacophores linked by a hydrazine bridge, When administ ered intracerebroventricularly, it has been shown to be more potent th an morphine and etorphine at eliciting antinociception. Biphalin has a lso been shown to cross both the blood-brain and blood-cerebrospinal f luid barriers. To understand the basis of biphalin's potency, regional brain and spinal cord distribution studies with [I-125-Tyr(1)]biphali n were performed 5, 20, and 40 min after intravenous bolus injections. A statistically greater amount of [I-125-Tyr(1)]biphalin was detected in the nucleus accumbens compared with other brain regions (p < 0.05) ,This correlates with the high density of delta- and mu-opioid recepto r mRNA and binding sites shown to be expressed in the nucleus accumben s. Also, a statistically greater amount of [I-125]-Tyr(1)] biphalin wa s detected in two other circumventricular organs, the choroid plexus a nd pituitary, when compared with other brain regions. These studies pr ovide evidence that biphalin can reach not only brain sites, but also spinal sites to elicit antinociception. The overall CNS distribution o f [I-125-Tyr(1)]biphalin was decreased with naloxone, D-Phe-Cys-Tyr-D- Trp-Brg-Thr-Pen-Thr-NH2, or naltrindole pretreatment, showing that bip halin detected in the brain and spinal cord is binding to delta- and m u-opioid recep'rors. Additional in situ brain perfusion experiments id entified a saturable component contributing to CNS entry of [I-125-Tyr (1)]biphalin, which could be described by Michaelis-Menten kinetics wi th a K-m of 2.6 +/- 4.8 mu M, V-max of 14.6 +/- 2.89 pmol(-1).min(-1). g(-1), and K-d of 0.568 +/- 0.157 mu l.min(-1).g(-1). Brain entry of [ I-125-Tyr(1)]biphalin was sensitive to 2-aminobicyclo[2.2.1] heptane-2 -carboxylic acid and L-phenylalanine, suggesting use of the large neut ral amino acid carrier, This work provides evidence that biphalin is a promising, potent analgesic that has a unique mechanism for reaching both spinal and supraspinal opioid receptor sites.