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Vol. 286, Issue 1, 376-381, July 1998
The Cotzias Laboratory of Neuro-Oncology (G.P.B., G.W.P.),
Memorial
Sloan-Kettering Cancer Center and Departments of Neurology and
Neuroscience (G.P.B., G.W.P.) and
Pharmacology (G.W.P.), Cornell U. Medical College, New York, New York
Naloxone benzoylhydrazone (NalBzoH) is a potent mu
antagonist in vivo. In a cell line stably transfected
with MOR-1 (CHO/MOR-1), NalBzoH also was an antagonist when examined in
adenylyl cyclase studies. In binding studies, it displayed high
affinity for the mu receptor, confirming its earlier
characterization in brain membranes. In competition studies under
equilibrium conditions, NalBzoH and diprenorphine both retained their
potency in the presence of the stable GTP analog
5'-guanylylimidophosphate, consistent with their mu
antagonist properties, whereas the agonist DAMGO showed more than a
3-fold loss of affinity. The dissociation of H-diprenorphine was monophasic. However, kinetic studies
revealed biphasic dissociations for both 3H-NalBzoH and
3H-DAMGO. The slow component of 3H-NalBzoH
dissociation, corresponding to the higher affinity state, was dependent
on coupling to G-proteins. It is selectively abolished by guanine
nucleotides, leaving only the rapid dissociation phase. Furthermore,
the slow dissociation component is eliminated by treatment of the cells
with pertussis toxin, but not cholera toxin. In conclusion, NalBzoH is
an unusual opioid. Functionally it is an antagonist, a classification
consistent with its equilibrium binding in the presence of guanine
nucleotides. Yet, kinetic studies reveal that it labels a G-protein
coupled state of the receptor with high affinity.
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