Some properties of brain specific benzodiazepine receptors: new evidence for multiple receptors.

Several new lines of evidence suggest the existence of two or more distinct types of benzodiazepine receptors, in contrast to earlier results suggesting the presence of only one class of receptors. Appropriate thermoinactivation experiments indicate two receptors with different thermostabilities. Several triazolopyridazines, with some of the pharmacological properties of anxiolytics have recently been shown to displace 3H-diazepam and 3H-flunitrazepam with Ki values in the 6 to 100 nanomolar range. These new substances are active in conflict tests in rats and monkeys and prevent metrazol induced seizures in vivo, but strikingly lack the ataxia and sedative properties of the benzodiazepines. Hill analyses of dose-response curves for some of these substances yields Hill coefficients in the range of 0.4--0.6, suggesting that these compounds may be able to discriminate between several types of benzodiazepine receptors.