Alkylation with β‐funaltrexamine suggests differences between μ‐opioid receptor systems in guinea‐pig brain and myenteric‐plexus

Abstract

The effects of pre‐incubation with β‐funaltrexamine (β‐FNA) on the binding of [3H]‐[d‐Ala2, MePhe4, Gly‐ol5]enkephalin ([3H]‐DAMGO) to homogenates of guinea‐pig brain and myenteric‐plexus longitudinal muscle have been studied. β‐FNA pretreatment of brain homogenates in Tris‐HCl buffer reduced the amount of [3H]‐DAMGO binding. This was principally due to a reduction in the maximal number of binding sites measurable. However, approximately 30% of sites labelled by 1 nm [3H]‐DAMGO were insensitive to 1 μm β‐FNA. Similar findings were obtained when the alkylation was performed in brain homogenates prepared in Krebs solution buffered with HEPES. β‐FNA pretreatment of whole myenteric‐plexus longitudinal muscle strips caused an increase in the IC50 values of μ‐agonists, but not of κ‐agonists. However, the binding of [3H]‐DAMGO to homogenates of myenteric‐plexus longitudinal muscle was not altered by pre‐incubation with β‐FNA in Tris‐HCl buffer. On the other hand when the pretreatment was carried out in whole tissue in Krebs solution, or in homogenates in the presence of NaCl and Gpp(NH)p, a marked reduction in [3H]‐DAMGO binding was observed. These results suggest that a low affinity form of the μ‐opioid receptor is the physiologically relevant site for β‐FNA alkylation in the myenteric‐plexus and that differences exist between μ‐receptor systems in guinea‐pig myenteric plexus and brain. 1991 British Pharmacological Society