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Comparison of the endogenous heptapeptide Met-enkephalin-Arg6-Phe7 binding in amphibian and mammalian brain
M Wollemann1, J Farkas, G Tóth
1Institute of Biochemistry, Biological Research Center of the Hungarian Academy of Sciences, Szeged.
Abstract:
In previous communications [4, 38] we published that [3H]Met-enkephalin-Arg6-Phe7 (MERF) binds to opioid (kappa2 and delta) and sigma2 sites in frog and rat brain membrane preparations, however no binding to kappa1 sites could be established. In the present paper we compare the frog, rat and guinea pig brain membrane fractions with respect to their MERF binding data. No qualitative differences were found between the three species but specific binding of labelled MERF was maximal in frog brain and lowest in guinea pig brain, which corresponds to their kappa2 opioid receptor distribution. The naloxone resistant binding was also present in all investigated species and varied from 25% in frog and guinea pig cerebrum, to 50% in rat cerebrum and cerebellum, but no naloxone inhibition was found in guinea pig cerebellum where no kappa2 opioid receptors have been found. The presence of sigma2-like receptor was demonstrated in each investigated membrane fraction with displacement experiments using (-)N-allyl-normetazocine as competitor of tritiated MERF. It was shown that this site was responsible for 60-80% of [3H]MERF binding. The remaining part of the naloxone resistant labelled MERF binding could be displaced only with endogenous opioid peptides as met-enkephalin, dynorphin and beta-endorphin. The eventual physiological role of multiple MERF receptors is discussed.
Insights
[3H]Met-enkephalin-Arg6-Phe7 (MERF) binds to opioid and sigma2 receptors across frog, rat, and guinea pig brains. Binding affinity varied by species, correlating with kappa2 opioid receptor distribution, suggesting diverse physiological roles.
Area of Science:
- Neuroscience
- Pharmacology
- Biochemistry
Background:
- Previous studies identified [3H]Met-enkephalin-Arg6-Phe7 (MERF) binding to opioid (kappa2, delta) and sigma2 receptors in frog and rat brain membranes.
- No binding to kappa1 opioid receptors was previously established for MERF.
Purpose of the Study:
- To compare MERF binding characteristics in frog, rat, and guinea pig brain membrane fractions.
- To investigate the species-specific distribution and characteristics of MERF binding sites.
Main Methods:
- Comparative analysis of MERF binding data across frog, rat, and guinea pig brain membrane preparations.
- Displacement experiments using (-)N-allyl-normetazocine to identify sigma2-like receptor involvement.
- Assessment of naloxone resistance and displacement with endogenous opioid peptides.
Main Results:
- No qualitative differences in MERF binding were observed across the three species.
- Specific MERF binding was highest in frog brain and lowest in guinea pig brain, correlating with kappa2 opioid receptor distribution.
- Naloxone-resistant binding, attributed to sigma2-like receptors, accounted for 60-80% of total MERF binding and was present in all species investigated, except for guinea pig cerebellum.
Conclusions:
- MERF exhibits binding to multiple receptor types, including kappa2 opioid and sigma2-like receptors, across different mammalian and amphibian species.
- The observed variations in MERF binding affinity correlate with kappa2 opioid receptor distribution, suggesting species-specific functional roles.
- The presence and contribution of sigma2-like receptors to MERF binding are significant, warranting further investigation into the physiological implications of these multiple receptor interactions.
