Behavioral effects of the isomers of pentobarbital and secobarbital in mice and rats.

To determine what stereoselective differences there may be in the behavioral effects of the isomers of pentobarbital and secobarbital, the effect of each isomer was determined on the spontaneous motor activity (SMA) and multiple fixed-ratio 30, fixed-interval 600-sec (mult FR30 FI600) responding of mice, and on the variable-interval 60-sec (VI60) responding of rats. The S-(-) isomers of pentobarbital and secobarbital decreased SMA at lower doses than those required for the R-(+) isomers. At moderate to high doses of R-(+)-pentobarbital (30-42.5 mg/kg) and low to moderate doses of S-(-)-secobarbital (5.6-17.5 mg/kg) SMA was increased. An increase in SMA following R-(+)-secobarbital was only observed at 30 mg/kg, and no increases were observed with S-(-)-pentobarbital. No potency differences were observed between the isomers of pentobarbital and secobarbital on the responding of mice under the mult FR30 FI600 schedule over a dose range of 1-30 mg/kg. Increases in FI600 responding were only observed following moderate doses of the S-(-) isomer of pentobarbital (5.6-17.5 mg/kg). In rats responding under the VI60 schedule of food presentation, no qualitative stereoselective differences were observed in the behavioral effects of the isomers of pentobarbital (1-13 mg/kg) and secobarbital (1-13 mg/kg), but small differences in potency were observed. Thus, differences in the effects of the isomers were usually restricted to differences in potency, but in some cases differences in efficacy were observed.