Augmentation of GABA-induced chloride current in frog sensory neurons by diazepam.
The effect of diazepam (DZP) on the GABA-induced macroscopic and microscopic Cl- current was investigated in isolated frog sensory neurons using both 'concentration-clamp' and patch-clamp techniques. At concentration range between 10(-9) and 10(-4) M, DZP itself evoked no response but potentiated time- and dose-dependently the subthreshold GABA responses, though at high DZP concentrations beyond 10(-5) M the potentiation ratio decreased. The potentiation effect was long-lasting and desensitized slowly over the course of several 10 minutes after washing-out of DZP. DZP potentiated GABA response without shifting the GABA reversal potential. The entire GABA dose-response curve was shifted in a parallel manner to the left by adding DZP without changing cooperatively: the Hill slope was 2.0. The potentiation of GABA response by DZP did not depend on either inward or outward direction of the Cl- current but slightly on the membrane potential. The time constants of activation of desensitization of GABA-gated Cl- current consisted of fast and slow components, respectively. The slow components were concentration-dependent, and significantly changed in the presence of DZP, while DZP had little effects on fast components. In the 'inside-out' configuration, the addition of DZP activated GABA-receptor ionophore complexes under subthreshold without changing the single Cl- channel conductance. It is concluded that DZP may act at a site to modulate GABA binding, in which DZP increases GABA binding affinity and also affects the kinetics of GABA-gated Cl- channels, indicating that DZP has dual action on the GABA-induced responses.
Yakushiji
T
T
Research Laboratories, Yoshitomi Pharmaceutical Industries, Ltd., Japan.
Tokutomi
N
N
Akaike
N
N
D000818:Animals; D002712:Chlorides / Q000502:physiology*; D003975:Diazepam / Q000494:pharmacology*; D005727:Ganglia, Spinal / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials; D009475:Neurons, Afferent / Q000187:drug effects / Q000502:physiology*; D011892:Rana catesbeiana / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
D002712:Chlorides; D005680:gamma-Aminobutyric Acid; D003975:Diazepam