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Abstract
The molecular basis underlying the stress-induced increment in the density of central benzodiazepine receptor from chick forebrain, observed previously at 4°C, was studied from a biophysical perspective. The thermal dependence of [ 3 H] flunitrazepam binding to the central benzodiazepine receptor and the supramolecular organization were studied in forebrain membranes from chicks submitted to partial water immersion. The equilibrium dissociation constants increased with temperature in membrane from both control and stressed chicks. The heat capacity values in control samples ( j C p, CON ) were significantly less negative than j C p STR. Changes in j H and j S between 4-37°C were greater in stressed chicks compared to control; however, the binding was exothermic and driven by enthalpy in both conditions. At 4°C, the receptor density (B max ) was higher in stressed chicks compared to control. Such a difference was lost irreversibly upon temperature elevation, possibly due to the hysteresis between the heating and cooling behaviour of B max, CON and the constancy in B max, STR. The fluorescence anisotropy of diphenylhexatriene was higher in control samples with respect to stressed chicks below 10°C. A temperature-induced increment in protein intrinsic-fluorescence was observed only in control, and was quenched by acrylamide more easily at 4°C than at 25°C. A higher microviscosity at 4°C in control favoured more external localizations of integral proteins; at higher temperatures, tryptophan residues moved to hydrophobic membrane-regions. Changes in the membrane-organization towards more fluid states favoured the accessibility of benzodiazepine to the central benzodiazepine receptor, expressed by the higher values of B max found in stressed samples at low temperatures with respect to control samples.