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Relationship Between Opioid‐Receptor Occupancy and Stimulation of Low‐ K m GTPase in Brain Membranes
Author(s) -
Clark Mary J.,
Nordby Gordon L.,
Medzihradsky Fedor
Publication year - 1989
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1989.tb01862.x
Subject(s) - ligand (biochemistry) , chemistry , stimulation , receptor , membrane , opioid receptor , agonist , antagonist , gtpase , biophysics , stereochemistry , biochemistry , endocrinology , biology
Treatment of rat brain membranes with the irreversible opioid ligand cis ‐3‐methylfentanylisothiocyanate (Superfit) was used to reduce gradually the number of available binding sites for the δ‐selective agonist [ 3 H][ d ‐Ser 2 , Leu 5 lenkephalin‐Thr 6 ([ 3 H]DSLET). Subsequently, the correlation between ligand binding and low‐ K m GTPase was investigated. Alkylation with 10 μM and 25 μM Superfit inactivated 66% and 71% of high‐affinity ( K D , 1 n M ) binding sites without decreasing the affinity of the remaining sites and the stimulation of low‐. K m GTPase by DSLET. Following exposure of the membranes to 50 μM and 75 μM Superfit, ligand binding was confined to the low‐affinity ( K D , 20 n M ) sites. In these membranes, the δ‐agonists DSLET and [ d ‐Pen 2 ,D‐Pen 5 ]enkephalin still stimulated low‐ K m GTPase, and these effects were blocked by ICI 174864 ( N,N‐ diallyl‐Tyr‐AIB‐AIB‐Phe‐Leu‐OH; AIB, α‐aminoisobutyric acid), a δ‐selective antagonist. A similar relationship between low‐affinity ligand binding and GTPase stimulation was observed following alkylation of the δ‐opioid receptor with the nonselective irreversible antagonist β‐chlomaltrexamine in the presence of protective concentrations of DSLET. The results reveal spare receptor sites in the coupling of the δ‐opioid receptor to low‐ K m GTPase in brain and identify low‐affinity ligand binding as a functional component in the process.