Affinity of various compounds for benzodiazepine binding sites in rat brain, heart and kidneys in vitro

Influence of flumazenil on the learning-enhancing effect of ambocarb in rats

The effects of flumazenil (Ro 15-1788) and a new beta-carboline, ambocarb (AMB), on learning were investigated using the multichoice maze. The drugs, administered either alone or simultaneously, were injected once a day before training for eight days. AMB, administered alone, improved the performance and decreased the working errors, whilst flumazenil had no effect on performance during its sole administration but weakly prevented the learning-improving effect of AMB. More significantly, flumazenil antagonized the motor activity depressed by AMB. In the study ex vivo, flumazenil decreased and AMB increased the apparent affinity of [3H]flunitrazepam to the central benzodiazepine receptors. Flumazenil reversed the action of AMB on the central benzodiazepine receptors, but failed to reduce significantly the modulative effects of AMB on [3H]muscimol and [35S]t-butylbicyclophosphorothionate ([35S]TBPS) binding. These data indicate that flumazenil, due to its action on the central benzodiazepine receptors, more effectively reverses the inhibition of motor activity than the performance-improving effect of AMB.

Binding of strychnocarpine and related beta-carbolines to brain receptors in vitro

The affinity of strychnocarpine and related beta-carbolines for serotonin, benzodiazepine, tryptamine, opiate and GABA receptors in rat brain was studied. Strychnocarpine showed a low to very low affinity for all the receptors tested. The weak binding to tryptamine receptors might explain part of the tremorigenic effects found earlier in the in vivo studies.

Changes in benzodiazepine receptors of rat brain after long-term treatment with the Ca(2+)-antagonists nifedipine, verapamil, flunarizine and with the calmodulin antagonist trifluoperazine

1. The binding of [3H]flunitrazepam to benzodiazepine receptors in the cerebral cortex and hippocampus (membrane fraction P2) was studied after 13-day oral treatment of male Wistar rats with the Ca(2+)-antagonists nifedipine (20 mg/kg), verapamil (50 mg/kg), flunarizine (10 mg/kg) and with the calmodulin (CaM)-antagonist trifluoperazine (TFP) (3 mg/kg). 2. The changes in the binding characteristics of the benzodiazepine receptors in the frontal cortex were studied in vitro after the addition of nifedipine (10(-6) and 10(-5) M) and verapamil (10(-6) and 10(-5) M). 3. A significant decrease of the binding capacity (Bmax) of [3H]flunitrazepam was established after in vivo treatment with the three Ca(2+)-antagonists as well as after TFP, the decrease being much more pronounced in the hippocampus. 4. Changes in the affinity values (Kd) of [3H]flunitrazepam binding were found in neither of the groups. 5. No data for a direct interaction of nifedipine and verapamil with the brain benzodiazepine receptors were obtained in in vitro experiments.

Benzodiazepines inhibit transport-related oxygen consumption in thick ascending limb

Specific binding sites for benzodiazepines (BZD) have been identified in several nonneuronal tissues including the kidney where they are localized predominantly to the tubular epithelium of the thick ascending limb of Henle's loop (TALH). The physiological function of these nonneuronal (peripheral) BZD-binding sites is undefined, but it has been suggested that they may represent receptors for putative endogenous ligands that may modulate cell function. In the current study, we examined the in vitro effects of diazepam and Ro5-4864, a specific peripheral BZD-receptor agonist, on the oxygen consumption of medullary TALH tubule suspensions of rabbit kidney. Maximal inhibition of total oxygen consumption was achieved at a dose of 5 X 10(-4) M of either agent. On average, diazepam and Ro5-4864 reduced total oxygen consumption by 41 and 44%, respectively. The predominant inhibition was in the ouabain-sensitive component of oxygen consumption, which suggests that BZDs inhibit active sodium-chloride transport in the TALH. To assess whether this inhibition depends on sodium uptake, TALH tubules were pretreated with amphotericin B (2 X 10(-6) M) to enhance sodium uptake and stimulate basal oxygen consumption; subsequent addition of Ro5-4864 (5 X 10(-4) M) still reduced oxygen consumption to a residual value that was not different from that in TALH tubules treated with Ro5-4864 but without pretreatment with amphotericin B. This suggests that BZD inhibition of transport-related oxygen consumption is not caused by diminution of sodium uptake into cells and thus appears to be distinct from the effect of furosemide.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the selectivity, specificity and potency of medetomidine as an alpha 2-adrenoceptor agonist

Medetomidine (4-[1-(2,3-dimethylphenyl)ethyl]-1H-imidazole) was tested for alpha 2-adrenoceptor agonist activity and compared to several reference agents. In binding studies carried out with rat brain membrane preparations, medetomidine showed high affinity for alpha 2-adrenoceptors, as measured by the displacement of [3H]clonidine (Ki 1.08 nM compared to 1.62, 3.20, 6.22 and 194 nM for detomidine, clonidine, UK 14,304 and xylazine, respectively). The affinity of medetomidine for alpha 1-adrenoceptors, as measured by [3H]prazosin displacement, was much weaker, yielding a relative alpha 2/alpha 1 selectivity ratio of 1620 which is 5-10 times higher than that of the reference compounds. Medetomidine caused a concentration-dependent inhibition of the twitch response in electrically stimulated mouse vas deferens with a pD2 value of 9.0 compared to that of 8.6, 8.5, 8.2 and 7.1 for detomidine, clonidine, UK 14,304 and xylazine, respectively. The effect of medetomidine was antagonized by idazoxan. In anaesthetized rats, medetomidine caused a dose-dependent mydriasis which could be reversed by alpha 2-adrenoceptor blockade. In receptor binding experiments and isolated organs medetomidine had no affinity or effects on beta 1-, beta 2-, H1, H2, 5-HT1, 5-HT2, muscarine, dopamine, tryptamine, GABA, opiate and benzodiazepine receptors. Based on these results, medetomidine can be classified as a potent, selective and specific alpha 2-adrenoceptor agonist.

Benzodiazepines stimulate sodium ion transport in frog skin epithelium

Benzodiazepine binding sites are present in a variety of non-neuronal tissues including the kidney where they are localized to distal nephron segments. It is postulated that renal binding sites are involved in modulating ion transport. This study examined the effects of two benzodiazepines on sodium transport in frog skin epithelium, a model system for sodium transport in renal collecting duct. Treatment of short-circuited frog skin with diazepam (a non-selective benzodiazepine agonist) stimulated amiloride-sensitive short-circuit current, reflecting stimulation of active sodium transport. The diazepam response was equally effective with either serosal or mucosal application of the drug. Maximal stimulation of the current (42 +/- 8%) was achieved with 10 microM diazepam (serosal). Short-circuit current was similarly augmented by serosal or mucosal addition of Ro5-4864, a benzodiazepine agonist with selective activity at peripheral (non-neuronal) receptors. The natriferic response to diazepam was additive to that of vasopressin or cyclic AMP suggesting that the mode of action of benzodiazepines is probably distinct from the cyclic AMP pathway. Thus, frog skin appears to be a useful model to examine the epithelial effects of benzodiazepines. Whether stimulation of sodium transport, however, involves peripheral-type benzodiazepine receptors in this tissue requires further studies.