Variable-interval schedules of timeout from avoidance: effects of chlordiazepoxide, CGS 8216, morphine, and naltrexone

Assessing the role of effort reduction in the reinforcing efficacy of timeout from avoidance

Rats responded on concurrent schedules of shock-postponement or deletion (avoidance) and timeout from avoidance. In Experiment 1, 3 rats' responses on one lever postponed shocks for 20 s and responses on a second lever produced a 1-min timeout according to a variable-interval 45-s schedule. Across conditions, a warning signal (white noise) was presented 19.5 s, 16 s, 12 s, 8 s, or 4 s before an impending shock. Raising the duration of the warning signal increased both avoidance and timeout response rates. Timeout responding, although positively correlated with avoidance responding, was not correlated with the prevailing shock rate. In Experiment 2, 3 rats' responses on one lever deleted scheduled shocks according to a variable-cycle 30-s schedule and responses on a second lever produced a 2-min timeout as described above. After this baseline condition, the avoidance lever was removed and noncontingent shocks were delivered at intervals yoked to the receipt of shocks in the baseline sessions. Timeout responding decreased when the avoidance lever was removed, even though the shock-frequency reduction afforded by the timeout remained constant. These results suggest that a key factor in the reinforcing efficacy of timeout is suspension of the requirement to work to avoid shock, rather than the reduction in shock frequency associated with timeout.

Reductions in shock frequency and response effort as factors in reinforcement by timeout from avoidance

Rats' presses on one lever canceled shocks programmed after variable cycles, while presses on a second lever occasionally produced a 2-min timout during which the shock-delection schedule was suspended and its correlated stimuli removed. These concurrent schedules of avoidance and timeout were embedded in a multiple schedule whose components differed, within and across conditions, in terms of the programmed shock rate associated with the shock-deletion schedule. Analyses based on the generalized matching law suggest that the reduction in the response requirement correlated with termination of the avoidance schedule was a more important factor in the reinforcing effectiveness of timeout than was shock-frequency reduction, at least in 2 of 3 rats. After training in each condition, responding on the timeout lever was extinguished by withholding timeouts in both components over seven sessions. Resistance to extinction varied directly with the rates of both shock-frequency reduction and avoidance-response reduction experienced during training. Although reduction in response effort appeared to dominate shock-frequency reduction in the maintenance of responding, neither factor had a clear advantage in predicting the course of extinction.

Extinction of responding maintained by timeout from avoidance

The resistance to extinction of lever pressing maintained by timeout from avoidance was examined. Rats were trained under a concurrent schedule in which responses on one lever postponed shock on a free-operant avoidance (Sidman) schedule (response-shock interval = 30 s) and responses on another lever produced 2 min of signaled timeout from avoidance on a variable-ratio 15 schedule. Following extended training (106 to 363 2-hr sessions), two experiments were conducted. In Experiment 1 two different methods of extinction were compared. In one session, all shocks were omitted, and there was some weakening of avoidance but little change in timeout responding. In another session, responding on the timeout lever was ineffective, and under these conditions timeout responding showed rapid extinction. The within-session patterns produced by extinction manipulations were different than the effects of drugs such as morphine, which also reduces timeout responding. In Experiment 2 shock was omitted for many consecutive sessions. Response rates on the avoidance lever declined relatively rapidly, with noticeable reductions within 5 to 10 sessions. Extinction of the timeout lever response was much slower than extinction of avoidance in all 4 rats, and 2 rats continued responding at baseline levels for more than 20 extinction sessions. These results show that lever pressing maintained by negative reinforcement can be highly resistant to extinction. The persistence of responding on the timeout lever after avoidance extinction is not readily explained by current theories.

Avoidance of time-out from response-independent food presentation: effects of chlordiazepoxide and buspirone

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.

The effects of cocaine on behavior maintained by timeout from avoidance

Rats were trained on concurrent schedules under which responses on one lever postponed shock (avoidance) and responses on the other lever produced brief (2-min) periods of signaled timeout from avoidance. For 6 rats, timeout from avoidance was programmed on a variable-interval 45-s schedule that generally resulted in rates that were lower than those on the avoidance lever. For another 6 rats, timeout was arranged on a variable-ratio 15 schedule that produced higher baseline rates. Cocaine (3 to 40 mg/kg) produced large, dose-dependent increases in behavior maintained by timeout in both groups of rats. Avoidance responding was also generally increased by cocaine, but the increases were of lesser magnitude. Increases in response rates were seen across a broad range of doses on behavior maintained by either interval or ratio schedules, an outcome that was unexpected on the basis of most studies of cocaine on food-maintained behavior. These results were similar to those of previous studies of the effects of amphetamine on behavior maintained by timeout from avoidance and suggest that stimulant drugs affect behavior maintained under a shock-postponement schedule differently than they affect behavior maintained by timeout from avoidance.

Effects of chlordiazepoxide and cocaine on concurrent food and avoidance-of-timeout schedules

Five rats were trained on a concurrent schedule in which responses on one lever produced a food pellet on a random-interval 30-s schedule during 10 s of food availability associated with distinctive exteroceptive stimuli. Responses on another lever postponed for 20 s the presentation of a 50-s timeout, during which all stimuli were extinguished and the schedule contingencies on the food lever were suspended. The response rates maintained by the random-interval schedule exceeded those maintained by the avoidance contingency, but both provided a stable baseline to assess the behavioral effects of different drugs. Low doses of cocaine hydrochloride (1 and 3 mg/kg) did not affect food-reinforced responding or avoidance response rates. Intermediate doses (5.6, 10, and 13 mg/kg) produced a dose-dependent decrease in food-maintained and avoidance response rates, and both types of responding were virtually eliminated after administration of the highest doses (17 and 30 mg/kg) of cocaine. Low doses of chlordiazepoxide (1 and 3 mg/kg) increased food-maintained and avoidance response rates, and both rates decreased systematically after 10 and 30 mg/kg of this drug. The effects of cocaine and chlordiazepoxide on response rates maintained by avoidance of timeout from food presentation were unlike those reported when subjects responded to avoid shock presentation. The results of this experiment thus provide evidence to suggest that the effects of drug administration on avoidance behavior may be a function of the nature of the consequent event to be avoided.

Variable-ratio schedules of timeout from avoidance: effects of d-amphetamine and morphine

Rats were trained on concurrent schedules in which pressing one lever postponed shock and pressing the other lever produced periods of signaled timeout from avoidance on variable-ratio schedules. These procedures generated high rates of timeout-reinforced responding and provided a baseline for studying the effects of drugs on behavior maintained by different types of negative reinforcement (shock postponement vs. timeout). Morphine (2.5 to 10.0 mg/kg) reduced behavior maintained by timeout at doses that increased or had no effect on avoidance responding. In contrast, d-amphetamine (0.125 to 2.0 mg/kg) produced large increases in timeout responding at doses that had minimal effect on avoidance in rats trained on variable-interval and variable-ratio schedules. Thus, the event-dependent effects of morphine, observed in previous studies in which timeout responding was maintained at low rates by interval schedules, were replicated with high timeout rates maintained by variable-ratio schedules. The effects of d-amphetamine could also be described as "event dependent" because timeout responding was stimulated more than avoidance regardless of the maintenance schedule or baseline rate.

Variable-interval schedules of timeout from avoidance: effects of anxiolytic and antipsychotic drugs in rats

Concurrent performances were studied in rats under conditions where responses on one lever postponed shock on a Sidman avoidance schedule and responses on another lever produced periods of signaled timeout from avoidance on a variable-interval schedule. Chlorpromazine decreased rates of responding on both the timeout and avoidance levels to about the same extent. The effects of chlordiazepoxide and CGS 9896 depended upon the event maintaining responding. Both drugs increased responding on the timeout lever at doses that concurrently decreased responding on the avoidance lever. Thus, the novel anxiolytic CGS 9896 produced effects that closely resembled those of the benzodiazepine anxiolytic, chlordiazepoxide. Like chlorpromazine, buspirone decreased both avoidance and timeout responding. Despite the documented anxiolytic properties of buspirone, its actions here were unlike those of the other anxiolytic drugs tested. Nonetheless, the differentiation between drugs obtained with the timeout from avoidance procedure indicates its utility for behavioral pharmacology.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.