Demonstration of peripheral benzodiazepine receptor-subtypes in the rat phrenic nerve diaphragm preparations using flumazenil

ABSTRACT

The effect of graded cumulative concentrations of the benzodiazepine [bd] receptor agonists, diazepam and midazolam, on the height of diaphragmatic twitches of rat phrenic nerve diaphragm preparations was investigated both in the absence and presence of the central bd receptor antagonist, flumazenil. The statistical analysis of the results demonstrated that the two bd agonists investigated in equimolar concentrations ranging 0.02-0.32 mM had different effects on twitch tension. Diazepam elicited a biphasic response consisting of an increase of twitch tension by 14% and 24.5% at a lower concentration [0.02 and 0.04 mM, respectively] and an inhibition by 21%, 60% and 96% at higher concentrations [0.08, 0.16 and 0.32 mM, respectively]. On the other hand, midazolam induced an inhibition of twitch tension by 14%, 20%, 53%, 75% and 99% throughout the whole range of the tested concentrations with no initial stimulatory effect on twitch tensions. Comparing the inhibitory concentration 50 [IC50] of diazepam with midazolam, midazolam was found to be twice potent in eliciting its inhibitory effect [IC50, 0.07 mM versus 0.14 mM for diazepam]. The central bd receptor antagonist, flumazenil [0.1 mM], specifically antagonized the potentiating effect of diazepam on twitch tension, but failed to antagonize the inhibitory effects of both diazepam and midazolam