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Five male Wistar rats were exposed to a two- component multiple schedule. In one component, signaled by a tone, food pellets were presented on a random-time 120-s schedule. In the other component, food pellets were presented on a random-time 30-s schedule. Pellets were only presented during a 10-s time-in period that alternated with a 50-s time-out period, unless the subject pressed a lever to postpone time-out presentation by 20 s. Response-independent food pellets were never presented within 2 s of this avoidance response. For most subjects avoidance rates were consistently higher when response-independent food pellets were delivered infrequently than when they were delivered more often. The amount of time spent in time-in varied considerably between subjects but was not consistently related to the frequency of response-independent pellet presentation. Once stable response rates were established subjects were intraperitoneally injected with different doses of chlordiazepoxide (1, 3, 10, 17, or 30 mg/kg) or buspirone (0.1, 0.3, 1.0, 1.7, 3.0, or 4.2 mg/kg). Low doses of chlordiazepoxide either did not affect or slightly increased avoidance response rates, whereas higher doses (10 mg/kg and up) produced a dose-dependent decrease in avoidance responding. The time subjects spent in the presence of stimuli associated with the availability of response-independent food either did not change or increased slightly after the lower doses of chlordiazepoxide, while it decreased dose dependently following the higher doses. Low doses of buspirone increased avoidance rates in subjects first exposed to chlordiazepoxide, but did not alter rates in the remaining subjects. Intermediate doses of buspirone decreased avoidance rates more in the component with the lower frequency of pellet presentation, higher doses further decreased response rates. The amount of time spent in the presence of stimuli associated with pellet presentation was minimally affected by the lower doses of buspirone, but decreased dose dependently following the higher doses. The results of this experiment add further credence to the notion that the behavioral effects of drug administration may depend on nonpharmacological variables including, but not limited to, the nature of the consequent event. |
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