Mechanism of gamma-aminobutyric acid/benzodiazepine receptor turnover in neuronal cells: evidence for nonlysosomal degradation.

In a previous report we described the use of flunitrazepam as a photoaffinity label to monitor the turnover of the gamma-aminobutyric acid/benzodiazepine receptor complex in primary brain and spinal cord cell cultures [Science (Wash. D. C.) 226:857-860 (1984)]. In the present communication we have extended our studies on the kinetics of receptor turnover and have examined the mechanism of receptor degradation. There are approximately 60,000 irreversible binding sites per neuron, and photolabeling is stereospecific. Photolabeling does not demonstrably alter the kinetics of degradation, and a complete rate equation relating the kinetic constants for degradation to receptor number is described. The rapid phase of degradation is slowed at low temperature (Q10 = 5, which corresponds to an apparent energy of activation of 25 kcal/mol) and by inhibitors of ATP production (sodium azide, 2-deoxyglucose, and 2,4-dinitrophenol). The fast phase for the degradation of photolabeled receptor is not affected by lysosomotropic agents (methylamine, ammonium chloride, and chloroquine) or by elimination of horse serum and chick embryo extract from the growth medium. In contrast, overall protein degradation is inhibited by methylamine and enhanced in serum-free medium. The results suggest that the gamma-aminobutyric acid receptor complex is degraded through an energy-dependent nonlysosomal pathway.