Alkylation with beta-funaltrexamine suggests differences between mu-opioid receptor systems in guinea-pig brain and myenteric-plexus.

1. The effects of pre-incubation with beta-funaltrexamine (beta-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. 2. beta-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 microM beta-FNA. Similar findings were obtained when the alkylation was performed in brain homogenates prepared in Krebs solution buffered with HEPES. 3. beta-FNA pretreatment of whole myenteric-plexus longitudinal muscle strips caused an increase in the IC50 values of mu-agonists, but not of kappa-agonists. However, the binding of [3H]-DAMGO to homogenates of myenteric-plexus longitudinal muscle was not altered by pre-incubation with beta-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. 4. These results suggest that a low affinity form of the mu-opioid receptor is the physiologically relevant site for beta-FNA alkylation in the myenteric-plexus and that differences exist between mu-receptor systems in guinea-pig myenteric plexus and brain.