Addiction to opioid drugs (opioids) represents one of the most severe forms of drug abuse. Opioid dependence develops as a consequence of changes in the central nervous system (CNS) induced by prolonged exposure to opioid drugs, e.g., morphine or heroin. With the help of biochemical techniques, we study the effect of long-term morphine administration on the function and density of opioid receptors (OR) in the forebrain cortex of rat and the problem of the development of drug addiction. We utilize also cell lines transfected by particular type of opioid receptor. Within the OR studies, we investigate the effect of monovalent cations and cholesterol on delta opioid receptor (δ‑OR). Techniques we use for our research include density gradient fractionations (sucrose, Percoll), radioligand binding studies, electrophoretic methods (SDS-PAGE, 2D-Elfo) a confocal fluorescent microscopy techniques (CLSM, FRAP, RICS).