Binding of a drug or other ligand to a receptor can evoke or prevent a cellular response. Most receptors are localized in the cell membrane, others in the cytoplasm or in the nucleus. Receptor classification occurs according to the effector mechanism and the time between ligand binding and cellular response. The first group involves the ion channels and transport proteins. Binding of a ligand opens the ion channel very quickly (within milliseconds) and specific ions can go in or out down their concentration gradient. Transport proteins can let enter or release certain molecules (e.g. nutrients) upon binding of a ligand. G-protein coupled receptors are membrane receptors with seven transmembrane helices which are in connection with a G-protein.
Their effect can be established within seconds. Peptide hormones (LH, ACTH) and neurotransmitters (dopamine, serotonin) usually bind to G-protein coupled receptors. Kinase receptors have one transmembrane helix and specific sites for autophosphorylation. Phosphorylation of other enzymes evokes the response. The effector time till tissue response takes several minutes. Growth factors and insulin bind to these tyrosine kinase receptors. Nuclear receptors are localized in the cytoplasm or nucleus. After binding to the receptor, the receptor-complex interacts with the DNA and initiates gene transcription. The process between ligand binding and protein synthesis can take hours or days. Steroid hormones act through nuclear receptors.