No abstract

The benzodiazepine (BZ) receptor is coupled with a GABA-receptor chloride-ionophore complex. The BZs augment the GABA-induced increase in chloride conductance, which leads to postsynaptic inhibition. This effect is believed to be responsible for antianxiety, sedative, muscle relaxant, and anticonvulsant effects, but the mechanisms underlying these behavioral effects are poorly understood. Various other sedative-hypnotics, including ethanol and barbiturates, interact with this system, probably contributing to their behavioral effects. We have recently conducted a selective breeding program to develop lines of mice which are diazepam-resistant (DR) and sensitive (DS) (Gallaher EJ, Hollister LE, Gionet SE, Crabbe JC. Psychopharmacology, 93:25-30, 1987); when tested for the duration of rotarod impairment after 20 mg/kg diazepam the DR line was impaired for 71 +/- 13 min compared with 200 +/- 18 min in the DS line. In the current study we tested mice from the DR and DS lines to determine if BZ sensitivity generalized to ethanol. DS mice became ataxic with lower brain ethanol concentrations, and recovered at later times and with lower blood ethanol concentrations, than did DR mice, indicating that sensitivity differences did extend to ethanol. Following a series of sequential doses over 5 to 6 hr DS mice developed minimal rapid tolerance, whereas DR mice developed considerable tolerance. By the end of the day DS mice were therefore much more sensitive to ethanol than were DR mice; this difference was greater in males than in females. High dose ethanol toxicity was studied by assaying brain ethanol concentrations at the cessation of respiration; no differences were found between lines or sexes.

Selective breeding techniques were used to alter allelic frequencies responsible for diazepam sensitivity and resistance. We used the rotarod test to determine the duration of diazepam-induced neurologic deficit in genetically heterogeneous mice. Males were more sensitive than females in the initial population. We then selectively bred for diazepam resistance and sensitivity. A significant difference between the lines was apparent in both sexes after two generations, and divergence has continued over seven generations. Brain benzodiazepine assays indicated that absorption and distribution of diazepam do not differ in the two lines. Differences in brain benzodiazepine concentrations at recovery from ataxia indicated that the two lines differ in central nervous system sensitivity. We found diazepam-induced rotarod impairment to be blocked in a dose-dependent manner by the specific benzodiazepine antagonist Ro 15-1788, indicating that this effect is mediated through BZ receptors. A dose-response curve obtained from generations 6 and 7 indicates a 9- to 14-fold difference in dose required to obtain similar effects in the two lines. These mice are expected to be useful experimental subjects in studies of benzodiazepine mechanisms.

Benzodiazepine (BZ) effects include anxiolyis, sedation, seizure protection, and muscle relaxation; the mechanisms underlying these various effects are not understood. We have recently used the rotarod test in conjunction with selective breeding techniques to develop lines of mice which are diazepam-sensitive (DS) and diazepam-resistant (DR). We review the general methods of selective breeding, along with a description of the DS/DR selection study, and then describe a variety of behavioral and neurochemical studies which have been conducted in an attempt to characterize these mice. We have investigated the effects of other sedative drugs believed to interact with the BZ receptor, including ethanol, pentobarbital, and phenobarbital. We have also tested these mice for seizure threshold and open-field activity. DS and DR mice do not differ in diazepam-induced seizure protection, suggesting that different mechanisms underlie rotarod performance and the anti-convulsant effect. These results provide evidence to support the search for nonsedating anti-convulsants. To determine the neurochemical basis for observed differences, BZ receptor density and chloride flux have been measured. We discuss the interaction between behavioral and neurochemical approaches, and describe a conceptual framework to guide future studies with these unique new animals.