Opioid receptor binding to intact brain cells

This model system was designed to investigate opioid receptor binding in a viable, intact cell system closely resembling in vivo cellular conditions. Cells were mechanically dissociated and sieved from adult rat brains, and iso-osmotic physiological buffer was used, thus avoiding mechanical and chemical disruption of cellular integrity due to homogenization or the use of non-physiological buffers, respectively. Using the opioid antagonist [³H]naloxone as the radioligand our studies show saturable binding, with a B(max) of 200.1 ± 6.3 fmoles/mg protein (mean ± SEM), and a K(d) of 1.38 ± 0.2 nM. Stereospecificity of [³H]naloxone binding to the intact cells, demonstrated by displacing the radioligand with the opioid agonist levorphanol and its inactive stereoisomer dextrorphan, yielded average K(i) values of 14.26 ± 2.42 nM and 16.37 ± 0.30 μM, respectively (n = 3). [³H]Naloxone binding was also linear over a wide range of tissue concentrations. This model will be useful for studying the binding of opioids or other classes of drugs to brain tissue in a relatively more physiologically relevant system.