Downregulation of [3H]Ro5-4864 binding sites after exposure to peripheral-type benzodiazepines in vitro.

Peripheral-type benzodiazepine (BZD) binding sites undergo a rapid and pronounced downregulation after exposure to these compounds in vitro. Friend erythroleukemia cells were incubated with micromolar concentrations of BZD after which they were washed thoroughly and the binding of the specific peripheral-type BZD radioligand [3H]Ro5-4864 was determined. Exposure to the peripheral-type BZD Ro7-3351 decreased the number of [3H]Ro5-4864 binding sites from 324 to 41 fmol/10(6) cells with no change in affinity. Downregulation appears to require active cellular processes because it is blocked when exposure to BZD is at 4 degrees C rather than at 37 degrees C. Furthermore, whereas [3H]Ro5-4864 binding is decreased substantially in membrane preparations made from downregulated cells, it is not altered when membrane preparations from control cells are exposed to BZD. The time course of downregulation is quite rapid, as it occurs within minutes. In contrast, the return of sites requires days and there is a close relationship between return of sites and growth of new cells. The ability of BZDs to downregulate correlates more closely with affinity for the peripheral-type site than with biological activity. The ability to undergo downregulation is characteristic of receptors and its occurrence suggests that peripheral-type BZD binding sites are functional receptors.