Modulation of specific [3H]phenytoin binding by benzodiazepines.

We have shown that diazepam (ED50 2.4 microM), flunitrazepam (ED50 10.2 microM) and Ro5-4864 (ED50 5 microM) are able to enhance both total and specific [3H]phenytoin binding. Picrotoxin (IC50 1.43 microM) and chloride, either NaCl or KCl (IC50 42.4 microM) inhibit both the increase in total and specific binding of [3H]phenytoin, Ro15-1788 does not. The optimum time for this enhancement was 3-4 hours. While the ED50's for the benzodiazepines are high their order of potency suggests that an involvement of both the "peripheral type" benzodiazepine receptor and the GABA-chloride ionophore complex is likely. Clonazepam (IC50 23 microM), oxazepam (IC50 12 microM) chlordiazepoxide (IC50 35 microM) and Ro8682-10, a convulsant benzodiazepine (IC50 16 microM) all inhibit both total and specific [3H]phenytoin binding. These effects were not blocked by chloride ions, picrotoxin or Ro 15-1788, and reached equilibrium within 45 minutes. This order of potency also parallels that for the "peripheral' benzodiazepine receptor in rat brain. These data suggest the presence of a micromolar benzodiazepine receptor site which may play a role in the control of CNS excitability. Nitrazepam, medazepam, bromazepam and the tetralobenzodiazepines U38335, U42794, U43434, and U37834 had no effect on total or specific [3H]phenytoin binding nor on the actions of the other benzodiazepines described in concentrations up to 50 microM.