Genetic studies at the receptor level: investigations in human twins and experimental animals

In receptors, as in enzymes, quantitative as well as qualitative genetic variation may exist. Studies in inbred strains of mice have shown for various receptors that the receptor density as determined by Bmax values is under genetic control. In healthy adult twins we have shown that the density of alpha-adrenoceptors on platelets is also influenced by genetic factors, since monozygotic twins were much more similar to one another than dizygotic twins. However, Bmax values are up-regulated and down-regulated by endogenous neurotransmitters and pharmacologically active agents. Thus, receptor densities are under considerable regulatory influences. Bmax values therefore reflect regulatory mechanisms rather than innate characteristics of the receptor protein. In another twin study we failed to find evidence for a genetic influence on the density of imipramine-binding sites on platelets. Since qualitative variation (polymorphism) is well known in enzymes, it may also apply to receptors. Qualitative differences in the receptor protein within one species would be of particular interest because of possible functional implications. As a first approach we examined central benzodiazepine receptors by photoaffinity labelling and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A comparison of fish, frog, chicken, mouse, rat and calf led to the detection of variation between species. Investigations in five inbred mouse and rat strains have not so far revealed genetic variation in benzodiazepine receptors. Nevertheless variation may be detectable by more sensitive methods such as peptide mapping after limited proteolysis or two-dimensional electrophoresis.