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Alkylation with beta-funaltrexamine suggests differences between mu-opioid receptor systems in guinea-pig brain and
1Department of Chemistry, University of Technology, Loughborough, Leics.
Abstract:
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.
Insights
Beta-funaltrexamine (beta-FNA) selectively alkylates mu-opioid receptors in guinea-pig brains and myenteric plexus. This study reveals differences in mu-receptor systems between these tissues, impacting drug binding and therapeutic potential.
Area of Science:
- Pharmacology
- Neuroscience
- Molecular Biology
Background:
- Opioid receptors are crucial for pain modulation and gastrointestinal function.
- Beta-funaltrexamine (beta-FNA) is a known irreversible antagonist of opioid receptors.
- Understanding the selectivity and effects of beta-FNA is vital for opioid research.
Purpose of the Study:
- To investigate the impact of beta-funaltrexamine (beta-FNA) on [3H]-[D-Ala2, MePhe4, Gly-ol5]enkephalin ([3H]-DAMGO) binding.
- To compare the effects of beta-FNA on mu-opioid receptors in guinea-pig brain and myenteric plexus.
Main Methods:
- Radioligand binding assays using [3H]-DAMGO.
- Pre-incubation of tissue homogenates and whole tissue strips with beta-FNA.
- Analysis of binding site density and affinity.
Main Results:
- Beta-FNA reduced [3H]-DAMGO binding in brain homogenates, primarily by decreasing the number of binding sites.
- In the myenteric plexus, beta-FNA increased IC50 values for mu-agonists but not kappa-agonists.
- The binding of [3H]-DAMGO to myenteric plexus homogenates was unaffected by beta-FNA unless specific conditions (NaCl, Gpp(NH)p) were present.
Conclusions:
- A low-affinity form of the mu-opioid receptor is the primary target for beta-FNA in the myenteric plexus.
- Significant differences exist in mu-opioid receptor systems between the guinea-pig myenteric plexus and brain.
- These findings have implications for understanding opioid receptor heterogeneity and drug targeting.