Variable-ratio schedules of timeout from avoidance: effects of anxiolytic drugs

Concurrent performances in rats were studied under conditions where responses on one lever postponed shock on an unsignaled avoidance schedule, and responses on another level produced periods of signaled timeout from avoidance on a variable-ratio schedule. This procedure resulted in relatively high rates of responding on the timeout lever, and provided a baseline which permitted simultaneous evaluation of drug effects on two different types of negative reinforcement (shock postponement vs timeout). Chlordiazepoxide and ethanol selectively increased responding on the timeout lever at low doses, while higher doses decreased responding on both levers. Two 5-HT(1A) agonists, buspirone and 8-OH-DPAT, had different effects. Buspirone decreased responding across all effective doses, but 8-OH-DPAT increased responding on both the timeout and avoidance levers, with greater increases noted in responding maintained by timeout. These results replicate and extend previous findings, and support the notion that traditional anxiolytic drugs like chlordiazepoxide and ethanol may increase the reinforcing properties of escape from an avoidance schedule. Differences between the behavioral effects of buspirone and 8-OH-DPAT may reflect differential activity at the 5-HT(1A) receptor or the dopaminergic properties of buspirone.